test2.txt

Any multicelled heterotroph that ingests other organisms or their tissues, develops through a series of embryonic stages, and is motile during part or all of the life cyle. Most species have epithelial tissues and extracellular matrixes.

Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.

Manipulation of the reproduction of a species as by breeding practices. Only individuals of a captive population that display a valued trait are allowed to reproduce, the goal being to increase the trait�s magnitude and frequency over the generations.

Domain of prokaryotic species; the first kinds of cells that formed after life originated. Collectively, bacteria are the most metabolically diverse organisms. Most kinds are chemoheterotrophs.

[Gk. bios, life, + sphaira, globe] All regions of Earth�s waters, crust, and atmosphere in which organisms live.

Smallest unit that still displays the properties of life; it has the capacity to survive and reproduce on its own.

All populations of all species in a habitat.

Type of heterotroph that feeds on the tissues of other organisms as its source of carbon and energy.

In experimental tests, a group used as a standard for comparison against one or more experimental groups.

[L. dis�, to pieces] One of the prokaryotic or fungal heterotrophs that obtains carbon and energy by breaking down wastes or remains of organisms. The collective action of decomposers helps cycle nutrients to producers in ecosystems.

Of complex multicelled species, a series of stages from formation of gametes, then fertilization, and on through embryonic and adult forms.

Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.

Array of organisms, together with their environment, interacting by a flow of energy and cycling of materials.

With respect to life�s levels of organization, a new property that emerges through interactions of entities at lower levels, none of which displays the property; e.g., living cells that emerge from �lifeless� molecules.

A capacity to do work.

A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.

Domain of eukaryotic species; all �protists,� plants, fungi, and animals.

[L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.

A group of objects or individuals that display or are exposed to the variable under investigation. Test results for this group are compared against the results for a control group.

A test that simplifies observation in nature or the laboratory by manipulating and controlling conditions under which observations are made.

Kingdom of fungi.

[L. genus, race or origin] A grouping of species more closely related to one another in morphology, ecology, and history than to others at the same taxonomic level.

[Gk. homo, same, + stasis, standing] State in which physical and chemical aspects of internal environment (blood, interstitial fluid) are being maintained within ranges that are tolerable for cell activities.

An explanation of a phenomenon, one that has the potential to be proven false by experimental tests.

Transmission, from parents to offspring, of genes that underlie the traits characteristic of their species.

A case of one species (the mimic) closely resembling another (its model) in form, behavior, or both.

Theoretical explanation of any object or event that has not been or cannot be directly observed.

[L. mutatus, a change, + -ion, act, result, or process] Heritable change in DNA�s molecular structure. Original source of new alleles and life�s diversity.

Microevolutionary process; the outcome of differences in survival and reproduction among individuals of a population that differ in the details of their heritable traits.

A set of organs that are interacting chemically, physically, or both in a common task.

Body structure with definite form and function made of more than one tissue.

A multicelled photoautotroph, most with well-developed roots and shoots (e.g., stems, leaves), as well as photosynthetic cells that include starch grains as well as chlorophylls a and b, and polysaccharides such as cellulose, pectin, and lignin in cell walls. The primary producers on land.

A statement, based on a hypothesis, about what you expect to observe in nature; the �if-then process.�

An autotrophic organism.

[L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.

Informal name for all structurally simple eukaryotes, which are now being classified as monophyletic groups.

Sensory cell or a specialized ending of one that detects a particular kind of stimulus.

Any asexual or sexual process by which a parent cell or organism produces offspring.

A time-tested, widely accepted intellectual framework used to interpret a broad range of observations and data about some aspect of nature. Tested rigorously but is still open to tests, revision, and tentative acceptance or rejection.

[L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.

[L. stimulus, goad] A specific form of energy that activates a sensory receptor able to detect it; e.g., pressure.

Any standardized or innovative means by which a prediction based on a hypothesis might be disproved; often requires designing and conducting experiments, making observations, or developing models.

Of multicelled organisms, a group of cells and matrixes interacting in the performance of one or more tasks.

Of experimental tests, a specific aspect of an object or event of interest that may differ over time and among individuals. A single variable is directly manipulated in an experimental group.

[L. acidus, sour] Any water-soluble substance that releases hydrogen ions (H+) in water, yielding a pH below 7.0.

The smallest unit of an element that still retains the element�s properties.

The number of protons in the nucleus of atoms of a given element.

Any water-soluble substance that releases hydroxyl ions in water to yield a pH greater than 7.0. Also the nitrogencontaining component of a nucleotide.

A weak acid and the salt that forms when it dissolves. The two work as a pair to counter slight shifts in pH.

A union between the electron structures of two or more atoms.

A capacity to resist rupturing when placed under tension (stretched).

Molecule consisting of two or more elements in proportions that do not vary, as they can in mixtures.

[L. con, together, + valere, to be strong] A sharing of one or more electrons between two atoms. In a polar covalent bond, the atoms share electrons unequally; in a nonpolar covalent bond, each atom gets an equal share of the electrons.

Negatively charged subatomicparticle. Electrons occupy orbitals around the atomic nucleus.

Fundamental form of matter that cannot be degraded to a simpler form by ordinary means. All atoms of an element have the same atomic number.

Process of conversion of a liquid to a gas; requires energy input.

A weak attraction that has formed between a covalently bonded hydrogen atom and an electronegative atom taking part in another covalent bond.

Free (or unbound) proton; one hydrogen atom that lost its electron and now bears a positive charge (H+).

[Gk. philos, loving] A polar molecule that dissolves easily in water; e.g., glucose.

[Gk. phobos, dreading] A nonpolar molecule that resists dissolving in water; e.g., oil.

Atom having an unequal number ofprotons and electrons; it carries a positive or negative electric charge.

Ions interacting through the attraction of their opposite charges.

One of two or more atoms of the same element (same number of protons) that differ in their number of neutrons.

Sum of protons and neutrons in the nucleus of an element�s atoms.

Two or more types of molecules intermingled in proportions that can and usually do vary.

Two or more covalently bonded atoms of the same or different elements.

Type of subatomic particle in the nucleus of all atoms except hydrogen; has mass but no charge.

Tabular arrangement of elements in order of their increasing atomic number.

Measure of the H+ concentration of a solution. pH 7 is neutral.

Positively charged subatomic particle in the nucleus of all atoms.

Natural, inevitable process by which an atom emits energy as subatomic particles and x-rays as its unstable nucleus spontaneously breaks apart; transforms one element into another in a predictable time span.

Any isotope that has an unstable nucleus.

Any compound that releases ions other than H+ and OH- in solution.

Model for how electrons are distributed in an atom; all of the orbitals are shown as a nested series of shells.

[L. solvere, to loosen] Any substance dissolved in a solution.

Any fluid (e.g., water) in which one or more substances are dissolved.

A clustering of water molecules around molecules or ions of a solute by positive and negative interactions.

Measure of molecular motion.

Any substance with a radioisotope attached; researchers can track it after delivering it into a cell, a multicelled body, ecosystem, or some other system.

Organic compound having one or more hydroxyl groups that dissolves easily in water; e.g., ethanol.

A small organic compound with a carboxylic acid group, an amino group, and a characteristic side group (R); monomer of polypeptide chains.

Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.

Any molecule of carbon, hydrogen, and oxygen typically in a 1:2:1 ratio. Main kinds are monosaccharides, oligosaccharides, and polysaccharides. They serve as structural materials, energy stores, and transportable energy forms.

An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.

Type of chemical reaction in which two molecules become covalently bonded as a larger molecule; water often forms as a by-product.

Disruption of hydrogen bonds and other interactions holding a molecule in its three-dimensional shape, which thereby changes. Increases in temperature, shifts in pH, and detergents can cause it.

[Gk. di, two, + sakcharon, sugar] A carbohydrate composed of two sugar monomers.

Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.

A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.

Type of lipid with one, two, or three fatty acid tails attached to a glycerol head.

Organic compound having a carboxyl group and a backbone of as many as thirty-six carbon atoms; saturated types have single bonds only; unsaturated types include one or more double covalent bonds.

An atom or a group of atoms with characteristic properties that is covalently bonded to the carbon backbone of an organic compound.

Highly branched polysaccharide of glucose monomers; the main storage carbohydrate in animals.

Protein with linear or branched oligosaccharides covalently bonded to it.

[Gk. haima, blood, + L. globus, ball] A hemecontaining protein produced by red blood cells; carries most of the oxygen in blood.

Organic compound with only hydrogen bonded to its carbon backbone.

[L. hydro, water, + Gk. lysis, loosening] A cleavage reaction; an enzyme splits a molecule, then the components of water (�OH and �H) are attached to the fragments.

One of the nonpolar hydrocarbons; e.g., a fat, oil, wax, sterol, phospholipid, or glycolipid. Cells use as storage forms of energy and building blocks.

A protein complexed with cholesterol, triglycerides, or phospholipids that were absorbed from the small intestine.

Any small molecule that is a repeating subunit in a polymer; e.g., the sugar monomers of starch.

[Gk. monos, alone, single, + sakcharon, sugar] A simple sugar.

Single-stranded or doublestranded molecule of nucleotides joined at phosphate groups; e.g., DNA, RNA.

Small organic compound with a five-carbon sugar, a nitrogen-containing base, and a phosphate group. Functions as coenzymes or monomers of nucleic acids.

Short-chain carbohydrate of two or more covalently bonded sugar monomers; e.g., sucrose and other disaccharides.

Any carbon-based molecule that also incorporates atoms of hydrogen and, often, oxygen, nitrogen, and other elements; e.g., fats, proteins.

The chemical bond formed between the carboxyl groups and amino groups of neighboring amino acids, constituting the primary linkage of all protein structures.

A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.

Large molecule of multiple linked monomers.

Three or more amino acids linked by peptide bonds.

[Gk. polus, many, + sakcharon, sugar] Straight or branched chain of covalently bonded monomers of the same or different kinds of sugars; e.g., cellulose, starch, and glycogen.

Organic compound consisting of one or more polypeptide chains. Diverse kinds have structural, functional, and regulatory roles in all organisms.

Ribonucleic acid. Any of a class of single-stranded nucleic acids involved in gene transcription and translation; some RNAs show enzyme activity.

Any lipid consisting of a rigid backbone of four fused carbon rings.

A lipid with three fatty acid tails attached to a glycerol backbone.

A lipid with long-chain fatty acids attached to an alcohol other than glycerol.

Protein monomer of microfilaments that functions in contraction, cell division, and reinforcing or reconfiguring the shape of a cell or its contents.

Complex of adhesion proteins that anchors cells to each other and to extracellular matrixes.

Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.

Domain of prokaryotic species; the first kinds of cells that formed after life originated. Collectively, bacteria are the most metabolically diverse organisms. Most kinds are chemoheterotrophs.

An organelle that started out as a centriole, the source of a 9+2 array of microtubules in a cilium or flagellum. It remains below the finished array.

Smallest unit that still displays the properties of life; it has the capacity to survive and reproduce on its own.

A dynamic mesh of crosslinked cytoskeletal elements just underneath the plasma membrane and attached to it.

Of a tissue, any molecular structure that connects adjoining cells physically, chemically, or both at their plasma membranes.

All organisms consist of one or more cells, the cell is the smallest unit of organization still displaying the properties of life, and life�s continuity arises directly from growth and division of single cells.

Of many cells (not animal cells), a semirigid but permeable structure that surrounds the plasma membrane; helps a cell retain its shape and resist rupturing.

In many mature, living plant cells, an organelle that stores amino acids, sugars, and some wastes; when it enlarges during growth, it forces the cell to enlarge and increase its surface area.

A barrel-shaped structure that arises from a centrosome and organizes newly forming microtubules into a 9+2 array inside a cilium or flagellum.

Organelle of photosynthesis in plants and algae. Two outer membranes enclose a semifluid interior, the stroma. A third membrane forms a compartment inside that functions in ATP and NADPH formation; sugars form in the stroma.

All of the DNA molecules and associated proteins in a nucleus.

In eukaryotic cells, a linear DNA double helix with many histones and other proteins attached. See also Bacterial chromosome.

A motile structure with a 9+2 array of microtubules that projects from the plasma membrane of certain eukaryotic cells. Modified cilia, such as those of hair cells, have sensory functions.

A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.

All cell parts, particles, and semifluid substances between the plasma membrane and the nucleus or nucleoid.

In a eukaryotic cell, the dynamic framework of diverse protein filaments that structurally support, organize, and move the cell and internal structures. Prokaryotic cells have a few similar protein filaments.

Endoplasmic reticulum, Golgi bodies, and transport vesicles concerned with modification of many new proteins, lipid assembly, and their transport within the cytoplasm or to the plasma membrane for export.

ER. Organelle that extends from the nuclear envelope through cytoplasm. Ribosomes coat the cytoplasmic side of rough ER, which modifes many new polypeptide chains in its lumen. Membrane lipids are assembled, fatty acids are broken down, and some toxins are inactivated in the lumen of smooth ER.

[Endo�, within + symbiosis, living together] An intimate, permanent ecological interaction in which one species lives and reproduces in the other�s body to the benefit of one or both.

Of many eukaryotic cells, a long, whip-like motile structure with an inner 9+2 array of microtubules. Prokaryotic flagella do not have this array and are not whiplike; they rotate like a propeller.

Cylindrical arrays of proteins in the plasma membrane of adjoining cells; they pair up as open channels for rapid flows of ions and small molecules.

Organelle of endomembrane system; its enzymes modify many new polypeptide chains, assemble lipids, and package both inside vesicles for secretion or for use inside cell.

Cytoskeletal element that mechanically strengthens some cells.

Gluelike polymer deposited in secondary cell walls; makes some plant parts stronger, more waterproof, and less vulnerable to attacks.

Structural basis of all cell membranes; mainly phospholipids arranged tail-to-tail in two layers, with hydrophilic heads of one dissolved in cytoplasmic fluid and heads of the other in extracellular fluid.

Vesicle filled with enzymes that functions in intracellular digestion.

The thinnest cytoskeletal element; consists of actin subunits that function in cell contraction, movement, and structural support.

Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.

A type of accessory protein that interacts with microfilaments or with microtubules to move cell structures or the whole cell; e.g., myosin.

An ATP-energized motor protein that moves cell components on cytoskeletal tracks. Interacts with actin in sarcomeres to bring about contraction.

A double membrane that is the outer boundary of the nucleus.

The portion of a prokaryotic cell where DNA is physically organized but not enclosed in a membrane.

[L. nucleolus, tiny kernel] In an interphase nucleus, a mass of material from which RNA and proteins are assembled into the subunits of ribosomes.

Large organelle with an outer envelope of two pore-ridden lipid bilayers that separates eukaryotic chromosomes from the cytoplasm.

One of the membrane-bound compartments that carry out specialized metabolic functions in eukaryotic cells; e.g., a nucleus, mitochondria.

Enzyme-filled vesicle that breaks down amino acids, fatty acids, and toxic substances such as ethanol.

A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.

Outer cell membrane; the structural and functional boundary between cytoplasm and extracellular fluid.

A plant cell junction that connects the cytoplasm of adjoining cells.

The first thin, pliable wall of young plant cells.

[L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.

A dynamic lobe of membraneenclosed cytoplasm; functions in motility and phagocytosis by amoebas, amoeboid cells, and many white blood cells.

The site of polypeptide chain synthesis in all cells. An intact ribosome has two subunits of rRNA and proteins.

A rigid, permeable wall inside the primary wall of many plant cells; forms after the first growing season.

The semifluid matrix between the thylakoid membrane system and two outer membranes of a chloroplast where sucrose, starch, cellulose, and other end products of photosynthesis are built.

A relationship in which the volume of an object increases with the cube of the diameter, but the surface area increases with the square.

An array of many strands of fibrous proteins collectively joining the sides of cells that make up an epithelium; the array prevents solutes from leaking between the cells.

A small, membrane-bound sac in the cytoplasm; different sacs transport or store substances or hold enzymes that digest their contents.

Pumping of a specific solute across a cell membrane against its concentration gradient, through the interior of a transport protein. Requires energy input, as from ATP.

Of multicelled species, a plasma membrane protein that helps cells stick together in tissues and to extracellular matrixes such as basement membrane.

Large microbial populations that anchored themselves to epithelium, rocks, or other surfaces by their own secretions.

The mass movement of one or more substances in the same direction, most often in response to pressure.

Active transport protein; pumps calcium ions across a cell membrane against their concentration gradient.

A membrane protein that helps form an open channel between the cytoplasm of adjoining cells.

Difference in the number of molecules or ions of any one substance between two adjoining regions.

Net movement of like ions or molecules from a region where they are most concentrated to an adjoining region where they are less concentrated; they move down their concentration gradient.

A difference in electric charge between adjoining regions.

Cell uptake of substances by forming vesicles from patches of plasma membrane. Three modes are receptor-mediated endocytosis, phagocytosis, and the bulk transport of extracellular fluid.

Fusion of a cytoplasmic vesicle with the plasma membrane; as it becomes part of the membrane, its contents are released to extracellular fluid.

A cell membrane has a mixed composition (mosaic) of lipids and proteins, the interactions and motions of which impart fluidity to it.

Pressure exerted by a volume of fluid against a cell wall, membrane, or some other structure that contains it; also called turgor pressure.

Of two fluids, the one with the higher solute concentration.

Of two fluids, the one with the lower solute concentration.

Any fluid having the same solute concentration as another fluid to which it is being compared.

Structural basis of all cell membranes; mainly phospholipids arranged tail-to-tail in two layers, with hydrophilic heads of one dissolved in cytoplasmic fluid and heads of the other in extracellular fluid.

Diffusion of water across a selectively permeable membrane from a region where the water concentration is higher to a region where it is lower.

The amount of pressure which, when applied to a hypertonic fluid, will stop osmosis from occurring across a semipermeable membrane.

Diffusion of a solute across a cell membrane, through the interior of a transport protein.

[Gk. phagein, to eat] �Cell eating,� a common endocytic pathway by which various cells engulf food bits, microbes, and cellular debris.

A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.

Difference in pressure between two adjoining regions.

One of a class of glycoproteins or glycolipids that project above the plasma membrane and that identify a cell as nonself (foreign) or self (belonging to one�s own body tissue).

Built-in capacity of a cell membrane to prevent or allow specific substances from crossing it at certain times, in certain amounts.

Cotransporter that, when energized, actively transports sodium out of a cell and helps potassium passively diffuse into it at the same time.

Relative solute concentrations of two fluids.

Membrane protein that passively or actively assists specific ions or molecules into or out of a cell. The solutes move through the protein�s interior.

Minimum amount of energy required to start a reaction; enzyme action lowers this energy barrier. Reactions differ in the amount required.

Chemically stable crevice in an enzyme where substrates bind and a reaction can be catalyzed repeatedly.

Adenosine diphosphate. A nucleotide with an adenine base and two phosphate groups.

Any enzyme or cofactor that helps neutralize free radicals before they damage tissues.

Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.

How a cell regenerates its ATP supply. ADP forms when ATP gives up a phosphate group, then ATP forms as ADP binds to inorganic phosphate or a phosphate group split from a molecule.

Energy released as weak bonds form between a substrate, enzyme, and any cofactor.

Fluorescent light formed when certain organisms convert chemical bond energy to photon energy.

Any metabolic pathway by which one or more organic compounds are synthesized.

Potential energy in the bonds between atoms in molecules.

No net change in concentrations of reactants and products in a reversible chemical reaction.

An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.

A metal ion or a coenzyme that assists an enzyme in catalysis by accepting or donating electrons or functional groups.

Any of the stepwise series of metabolic reactions that break down organic compounds.

Array of enzymes and other molecules in a cell membrane that accept and give up electrons in sequence; operation of chain releases the energy of the electrons in small, usable increments.

A chemical reaction that requires a net energy input and converts more stable reactants into less stable products; not spontaneous.

A capacity to do work.

Measure of how much and how far a concentrated form of energy has been dispersed after an energy change.

A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.

Any chemical reaction with a net energy loss.

Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.

Mechanism by which a change that results from some cellular activity triggers responses that decrease or shut down the activity.

Energy cannot be created or destroyed.

Any unbound molecular fragment with an unpaired electron.

Explanation of how some enzymes work; their shape changes and fits a bound substrate more closely, and the tension destabilizes substrate bonds so that they can break.

A substance formed between the start and end of a metabolic pathway.

1,000 calories of heat energy; amount needed to raise the temperature of 1 kilogram of water by 1�C. Standard unit of measure for food�s caloric content.

Energy of motion.

A stepwise sequence of enzyme-mediated reactions.

All the controlled, enzyme-mediated chemical reactions by which cells acquire and use energy as they synthesize, store, degrade, and eliminate substances.

Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.

Nicotinamide adenine dinucleotide phosphate. A phosphorylated nucleotide coenzyme; after it accepts electrons and H+, abbreviated NADPH2.

Transfer of electrons between reactant molecules.

Enzyme-mediated transfer of a phosphate group to an organic compound.

A object�s capacity to do work owing to its position in space or the arrangement of its parts.

A substance remaining at the end of a reaction.

Substance that enters a reaction.

Energy tends to flow from concentrated to less concentrated forms.

A reactant molecule that is specifically acted upon by an enzyme.

Membrane protein that passively or actively assists specific ions or molecules into or out of a cell. The solutes move through the protein�s interior.

Range of wavelengths that a given type of pigment can absorb.

A pigment that absorbs and transfers light energy to a photosystem; extends the range of light wavelengths for photosynthesis; e.g., a carotenoid.

One of a class of accessory pigments that reflect red to blue light.

A type of membrane-bound active transport protein that also catalyzes the formation of ATP.

[Gk. auto, self, and trophos, feeder] An organism that synthesizes its own food from simple inorganic compounds in its environment with energy captured from the sun or from oxidizing inorganic substances; e.g., a photoautotroph or chemoautotroph.

Type of plant in which threecarbon PGA is the first stable intermediate to form after carbon fixation.

Type of plant in which four-carbon oxaloacetate is the first stable intermediate to form after initial carbon fixation; in these plants, carbon is fixed twice, in two different types of photosynthetic cells.

Cyclic reactions that form sugar and regenerate RuBP in the second stage of photosynthesis. The reactions require carbon (from carbon dioxide). They use energy from ATP and hydrogens and electrons from NADPH, both of which form in the first stage.

Type of plant that conserves water by opening stomata only at night, when it fixes carbon by repeated turns of the C4 pathway; stands for crassulacean acid metabolism.

Process by which any autotrophic cell incorporates carbon atoms into a stable organic compound. Different cells get carbon dioxide from the air or dissolved in water.

One of a class of accessory pigments in photosynthesis that reflect red, orange, and yellow light. One kind, beta-carotene, is a precursor of vitamin A.

[Gk. chloros, green, + phyllon, leaf] In plants and algae, a pigment that is a receptor for the photon energy required to start photosynthesis; it absorbs mainly violet and red light and reflects or transmits green light.

An accessory pigment that absorbs mainly blue and orange light.

Organelle of photosynthesis in plants and algae. Two outer membranes enclose a semifluid interior, the stroma. A third membrane forms a compartment inside that functions in ATP and NADPH formation; sugars form in the stroma.

Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.

Oldest photosynthetic pathway. Photon energy forces electrons out of membrane-bound photosystems to transfer systems, which return them to the photosystems. Electron flow across the membrane sets up H+ gradients that drive ATP formation.

All wavelengths of photon energy from gamma rays less than 10�5 nanometers long to radio waves more than 10 kilometers long.

Array of enzymes and other molecules in a cell membrane that accept and give up electrons in sequence; operation of chain releases the energy of the electrons in small, usable increments.

[Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.

First stage of photosynthesis. Pigments trap photon energy, which is transduced to ATP chemical energy. In a noncyclic pathway, a reduced coenzyme, NADPH, also forms.

Second stage of photosynthesis. Involves carbon fixation and cyclic reactions that form sugars and regenerate an organic compound that is the cycle�s entry point. ATP from the first stage delivers energy that drives the reactions. NADPH from the first stage donates electrons and hydrogen building blocks. The carbon and nitrogen come from CO2.

[L. non, not, + Gk. kylos, circle] The light-dependent reactions of photosynthesis that produce both ATP and NADPH; its oxygen by-product is the basis of Earth�s oxygen-rich atmosphere.

Phosphoglycerate. During glycolysis, the intermediate that results after ATP has formed by substrate-level phosphorylation; also the first stable intermediate of the Calvin�Benson cycle of photosynthesis.

Phosphoglyceraldehyde. During glycolysis, the intermediate that gives up electrons and hydrogen to form NADH. During turns of the Calvin�Benson cycle, two PGALs form one sugar; rearrangements of ten others regenerate a compound that is the entry point for the cycle.

[Gk. photos, light, + -lysis, breaking apart] Reactions that split water molecules, which release electrons for the noncyclic pathway of photosynthesis; oxygen is a by-product.

Unit of electromagnetic energy; has wave-like and particle-like properties.

The process by which photoautotrophs capture sunlight energy and use it in the formation of ATP and NADPH, then in the formation of sugars from carbon dioxide and water. ATP gives up energy that drives the sugar-building reactions, and NADPH donates electrons and hydrogen building blocks.

In photosynthetic cells, a cluster of membrane-bound pigments and other molecules; it converts light energy to chemical energy.

One of a class of accessory pigments in cyanobacteria and red algae that reflects red to blue light.

Any light-absorbing molecule.

At a photosystem�s center, a special pair of chlorophyll a molecules; the center loses electrons on absorption of photon energy, thereby initiating the lightdependent reactions of photosynthesis.

RuBP carboxylase. Carbon-fixing enzyme of the C3 photosynthesis pathway.

Ribulose bisphosphate. A five-carbon organic compound; the entry point for the Calvin�Benson cycle, which regenerates it.

A gap between two plumped guard cells that lets water vapor and gases diffuse across the epidermis of a leaf or primary stem; diffusion stops when the cells lose water and collapse.

The semifluid matrix between the thylakoid membrane system and two outer membranes of a chloroplast where sucrose, starch, cellulose, and other end products of photosynthesis are built.

A chloroplast�s inner membrane system, often folded as flattened sacs, that forms a continuous compartment in the stroma. In the first stage of photosynthesis, pigments and enzymes in the membrane function in the formation of ATP and NADPH.

The distance between the crests of two successive wavelike forms of energy in motion.

One of a class of accessory pigments in photosynthesis that reflects yellow to orange light.

Coenzyme A bound to a twocarbon fragment from pyruvate, which it transfers to oxaloacetate for Krebs cycle.

[Gk. aer, air, + bios, life] Oxygen-requiring pathway of ATP formation in mitochondria: from glycolysis, to Krebs cycle and electron transport phosphorylation. Typical net energy yield: 36 ATP per glucose molecule.

An anaerobic ATPforming pathway using pyruvate and NADH from glycolysis. NADH transfers electrons to an intermediate, acetaldehyde, forming ethanol. Occurs in cytoplasm only. The net yield is 2 ATP from glycolysis; the steps remaining only regenerate NAD+.

[Gk. an, without, + aer, air] Of some bacteria and archaeans, ATP formation by way of a flow of electrons through transfer chains in the plasma membrane to a final electron acceptor that is not oxygen.

Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.

A type of membrane-bound active transport protein that also catalyzes the formation of ATP.

An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.

Final stage of aerobic respiration; electron flow through electron transfer chains in inner mitochondrial membrane sets up H+ concentration and electric gradients that drives ATP formation. Oxygen accepts electrons at the end of the chain.

Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.

Breakdown of glucose or another organic compound to two pyruvates. First stage of aerobic respiration, fermentation, or anaerobic electron transfer. Oxygen has no role in glycolysis, which takes place in the cytoplasm of all cells. Two NADH form. Net yield: 2 ATP per glucose molecule.

The second stage of aerobic respiration in which many coenzymes form as pyruvate from glycolysis is fully broken down to CO2 and H2O. Two ATP also form. Occurs only in mitochondria.

One of the anaerobic pathways of ATP formation. NADH from glycolysis donates hydrogen and electrons to pyruvate, converting it to three-carbon lactate, and regenerating NAD+. The net energy yield is 2 ATP (from glycolysis).

Double-membraned organelle of ATP formation; only site of the second and third stages of aerobic respiration.

Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.

A fourcarbon compound with roles in metabolism; e.g., the point of entry into the Krebs cycle.

Phosphoglyceraldehyde. During glycolysis, the intermediate that gives up electrons and hydrogen to form NADH. During turns of the Calvin�Benson cycle, two PGALs form one sugar; rearrangements of ten others regenerate a compound that is the entry point for the cycle.

Three-carbon compound that forms as an end product of glycolysis.

Direct, enzyme-mediated transfer of a phosphate group from a substrate to another molecule.

Nuclear division stage. Sister chromatids of each chromosome are separated from each other and move to opposite spindle poles.

Of eukaryotic cells, a dynamic array of microtubules that moves chromosomes with respect to its two poles during mitosis or meiosis.

A malignant neoplasm; a mass of abnormally dividing cells that can leave their home tissue and invade and form new masses in other parts of the body.

Of eukaryotic cells, a series of events from the time a cell forms until it reproduces. A cycle consists of interphase, mitosis, and cytoplasmic division.

The mechanism of cytoplasmic division in plant cells. After nuclear division, vesicles derived from Golgi bodies deposit the material for a cross-wall that cuts through the cytoplasm and connects to the parent cell wall.

A barrel-shaped structure that arises from a centrosome and organizes newly forming microtubules into a 9+2 array inside a cilium or flagellum.

Of a eukaryotic chromosome, a constricted region having binding sites (kinetochores) for spindle microtubules.

Dense mass of material in the cytoplasm of eukaryotic cells from which microtubules start to grow.

In eukaryotic cells, a linear DNA double helix with many histones and other proteins attached. See also Bacterial chromosome.

The sum of all of the chromosomes in cells of a given type.

Mechanism of cytoplasmic division of animal cells. Just beneath the plasma membrane, a thin band of contractile filaments around the cell midsection contracts and pinches the cytoplasm in two.

[Gk. kinesis, motion] Cytoplasmic division.

Cytokinesis. After nuclear division, a splitting of the parent cell cytoplasm that completes formation of daughter cells.

Of many sexually reproducing species, having two chromosomes of each type, or pairs of homologues, in somatic cells.

Animal cell set aside for sexual reproduction; gives rise to gametes.

A protein that stimulates increases in size; e.g., by inducing mitosis.

Cancer cell of a lineage used in research laboratories around the world.

Type of structural protein that helps organize and condense eukaryotic chromosomes and control access to genes during interphase.

In a eukaryotic cell cycle, the interval between mitotic divisions when a cell grows in mass, roughly doubles the number of its cytoplasmic components, and replicates its DNA.

Type of enzyme that transfers a phosphate-group to an organic molecule.

A mass of protein and DNA in the centromere to which microtubules of the spindle attach.

[Gk. meioun, to diminish] A nuclear division process that halves the parental chromosome number, to a haploid (n) number. Prerequisite to the formation of gametes and sexual spores.

Of meiosis I, stage when all pairs of homologues are positioned at the equator of a bipolar spindle. Of mitosis or meiosis II, the stage when all duplicated chromosomes are positioned at the equator.

Abnormal migration of cancer cells that break away from home tissues and may start colonies in other tissues.

Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.

[Gk. mitos, thread] Type of nuclear division that maintains the parental chromosome number. The basis of growth in size, tissue repair, and often asexual reproduction for eukaryotes.

Mass of cells (tumor) that lost control over the cell cycle.

Small stretch of eukaryotic DNA wound twice around a spool of proteins called histones.

All of the duplicated chromosomes in a cell condense and get attached to a newly forming spindle.

One of the two attached members of a duplicated eukaryotic chromosome.

[Gk. soma�, body] Any body cell that is not a germ cell.

Of meiosis I, a stage when one member of each pair of homologous chromosomes has arrived at a spindle pole. Of mitosis and of meiosis II, the stage when chromosomes typically decondense into threadlike structures and two daughter nuclei form.

Tissue mass of cells dividing at an abnormally high rate. Benign tumor cells stay in their home tissue; malignant ones metastasize, or slip away and invade other places in the body, where they may start new tumors. See also neoplasm.

One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.

Any reproductive mode by which offspring arise from one parent and inherit that parent�s genes only; e.g., prokaryotic fission, transverse fission, budding, vegetative propagation.

Of a eukaryotic chromosome, a constricted region having binding sites (kinetochores) for spindle microtubules.

The sum of all of the chromosomes in cells of a given type.

A genetically identical copy of DNA, a cell, or a multicelled organism.

At prophase I of meiosis, reciprocal exchange of segments between two nonsister chromatids of a pair of homologous chromosomes. Puts novel combinations of alleles in gametes.

Of many sexually reproducing species, having two chromosomes of each type, or pairs of homologues, in somatic cells.

Mature female gamete, or ovum.

Fusion of a sperm nucleus and an egg nucleus, the result being a single-celled zygote.

Haploid cell formed by meiotic cell division of a reproductive cell; required for sexual reproduction.

[Gk. phyton, plant] A haploid multicelled body in which haploid gametes form during the life cycle of plants and some algae.

Unit of heritable information in DNA, transmissable from parents to offspring.

Outcome of any process that puts new genetic information in a DNA molecule; e.g., by crossing over.

Animal cell set aside for sexual reproduction; gives rise to gametes.

The sum of all chromosomes in cells with one of each type of chromosome characteristic of the species; e.g., in a gamete.

[Gk. homologia, correspondence] One of a pair of chromosomes in body cells of diploid organisms; except for a pairing of nonidentical sex chromosomes, a pair has the same size, shape, and gene sequence.

[Gk. meioun, to diminish] A nuclear division process that halves the parental chromosome number, to a haploid (n) number. Prerequisite to the formation of gametes and sexual spores.

A type of immature egg.

Mature secondary oocyte.

One of four cells that form by meiotic cell division of an oocyte but that does not become the ovum.

Production of genetically variable offspring by meiosis, gamete formation, and fertilization.

One of the two attached members of a duplicated eukaryotic chromosome.

Mature male gamete.

A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.

[Gk. phyton, plant] A sporeproducing vegetative body of a plant or multicelled alga that grows by mitotic cell divisions from a zygote.

Method of identifying which self-recognition proteins of types A and B are at the surface of an individual�s red blood cells; the absence of either type is designated O.

One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.

A condition in which a pair of nonidentical alleles that influence two different phenotypes are expressed at the same time in heterozygotes.

Of individuals of a population, a range of small differences in the phenotypic expression of a trait.

Type of experiment that starts with a cross between two truebreeding, homozygous parents that differ in two traits governed by alleles of two genes. The actual experiment is a cross between two of their F1 offspring that are identically heterozygous for alleles of the two genes; e.g., AaBb x AaBb.

Of diploid cells, an allele that masks the phenotypic effect of any recessive allele paired with it.

An interaction among products of two or more gene pairs that influence the same trait.

Unit of heritable information in DNA, transmissable from parents to offspring.

Genetic makeup of an individual; a single gene pair or the sum total of an individual�s genes.

Having a pair of dominant alleles at a gene locus on homologous chromosomes; e.g., AA.

Having a pair of recessive alleles at a gene locus on homologous chromosomes; e.g., Aa.

Individual having a nonidentical pair of alleles for a trait being studied.

Condition in which one allele of a pair is not fully dominant; the heterozygous phenotype is somewhere between both homozygous phenotypes.

All genes on a chromosome.

• A brownish-black pigment.
• monohybrid cross
• An experiment that starts with a cross between two truebreeding, homozygous parents that differ in a trait governed by alleles of one gene. The experiment is a cross between two F1 offspring that are identically heterozygous for the two genes; e.g., Aa x Aa.
• multiple allele system
• Three or more slightly different molecular forms of a gene that persists among the individuals of a population.
• mutation
• [L. mutatus, a change, + -ion, act, result, or process] Heritable change in DNA�s molecular structure. Original source of new alleles and life�s diversity.
• P, F1, F2
• The first and second generation offspring of experimental crosses.
• phenotype
• [Gk. phainein, to show + typos, image] Observable trait or traits of an individual.
• pleiotropy
• A case of alleles at a single gene locus having positive or negative impact on two or more traits.
• polygenic inheritance
• Inheritance of multiple genes that affect the same trait.
• probability
• The odds that each outcome of an event will occur is proportional to the total number of ways in which that outcome can be reached.
• Punnett-square method
• A simple way to predict the probable outcomes of a genetic cross by constructing and filling in a diagram of all possible combinations of genotypes, phenotypes, or both.
• recessive allele
• Allele whose expression in heterozygotes is fully or partially masked by expression of a dominant partner allele. It is fully expressed only in homozygous recessives.
• testcross
• A cross that might reveal the (unknown) genotype of an individual showing dominance for a trait; the individual is crossed with a known homozygous recessive individual.
• true-breeding lineage
• A group consisting of parents and their offspring in which only one version of a trait persists over time.

One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.

A type of chromosome abnormality in which body (somatic) cells have one extra or one less chromosome relative to the parental chromosome number.

Of a sexually reproducing species, any chromosome of a type that is the same in both males and females.

At prophase I of meiosis, reciprocal exchange of segments between two nonsister chromatids of a pair of homologous chromosomes. Puts novel combinations of alleles in gametes.

Loss of a chromosome segment; often leads to genetic disorders. Also the loss of one or more nucleotide bases from a DNA molecule.

Base sequence in DNA that has been repeated two or more times.

Of animals, a new individual that forms by cleavage, gastrulation, and other early stages of development. Of plants, a young sporophyte until germination.

In mammalian development, the stage after all major organ systems have formed until time of birth.

Unit of heritable information in DNA, transmissable from parents to offspring.

A less common or rare version of a heritable trait.

An inherited condition causing mild to severe medical problems.

Outcome of any process that puts new genetic information in a DNA molecule; e.g., by crossing over.

A chromosomal alteration; part of the DNA sequence gets oriented in the reverse direction, with no molecular loss.

Preparation of an individual�s metaphase chromosomes arranged by length, centromere location, and shape.

All genes on a chromosome.

Failure of sister chromatids or homologous chromosomes to move apart in meiosis or mitosis. Daughter cells get too many or too few chromosomes.

Chart of connections among individuals related by descent.

A case of somatic cells having three or more of each type of chromosome characteristic of the species.

A paired cross that may identify the role of parental sex on the inheritance of a trait. In the second cross, a trait characteristic of each sex is reversed compared to the original cross.

One of two kinds of homologous chromosomes that, in certain combinations, dictate the gender of the new individual. Also has genes unrelated to sexual traits.

A sex hormone necessary for the development and functioning of the male reproductive system of vertebrates.

Attachment of a piece of a broken chromosome to another chromosome. Also, a mechanism by which organic compounds are conducted in phloem.

Having one extra chromosome in somatic cells; e.g., trisomy 21 (2n + 1).

Of a given gene locus, the allele that occurs normally or with the greatest frequency among individuals of a population.

A type of sex chromosome that influences sex determination; e.g., XX mammalian embryo becomes female; an XY pairing causes it to develop into a male.

Recessive condition in which the responsible, mutated gene is on the X chromosome.

Distinctive chromosome in males or females of many species (not both); e.g., human males XY, females, XX.

One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also refers to a nucleotide having an adenine base component.

One of a class of viruses that infects bacteria.

A genetically identical copy of DNA, a cell, or a multicelled organism.

One of the four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also applies to a nucleotide that contains a cysteine base.

Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.

Type of enzyme that catalyzes the sealing of short stretches of DNA into a continuous strand during replication; also seals strand breaks.

Type of enzyme that catalyzes the addition of free nucleotides to new DNA strands during replication; also proofreads and corrects mismatches.

Any enzyme-mediated process that fixes DNA replication errors or strand breaks.

Process by which a cell duplicates its DNA molecules before it divides into daughter cells.

One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also may refer to a nucleotide that contains a guanine base.

Type of enzyme that catalyzes breaking of hydrogen bonds during DNA replication so the two strands of double helix can unwind from each other.

Small organic compound with a five-carbon sugar, a nitrogen-containing base, and a phosphate group. Functions as coenzymes or monomers of nucleic acids.

A nucleotide base with a double ring structure; e.g., adenine or guanine.

A nucleotide base with a single ring structure; e.g., cytosine, thymine, uracil.

Type of enzymes that repairs nucleotide mismatches in a DNA strand.

[Gk. semi�, half, + L. conservare, to keep] Mechanism by which a DNA molecule is duplicated. The double helix unzips along its length, exposed bases of each strand are a template upon which a new strand is assembled, then each conserved strand and its new partner wind up in a double helix. Two double helixes, each with a parental strand and new strand of DNA, result.

Self-perpetuating, undifferentiated animal cell. A portion of its daughter cells becomes specialized; e.g., red blood cells from stem cells in bone marrow.

A cap of repetitive DNA sequence on the end of a chromosome. Each nuclear division, enzymes digest a bit of it; cells stop dividing when only a nubbin remains.

One of four nitrogencontaining bases in nucleotide monomers of DNA; also applies to a nucleotide with a thymine base component.

Film image of xrays scattered by a crystalline sample; the resulting pattern of streaks and dots can be used to calculate the spacing between the atoms in the crystal lattice.

Event by which the same gene can specify two or more slightly different proteins. All exons in a pre-mRNA transcript of the gene are retained or some are removed and the rest spliced in various combinations for the mature transcript.

A small organic compound with a carboxylic acid group, an amino group, and a characteristic side group (R); monomer of polypeptide chains.

Series of three nucleotide bases in tRNA that can base-pair with mRNA codons.

Linear order of nucleotides that compose a DNA or RNA strand.

Mutation in which one nucleotide is wrongly substituted for another during DNA replication.

Loss of a chromosome segment; often leads to genetic disorders. Also the loss of one or more nucleotide bases from a DNA molecule.

A base sequence in eukaryotic DNA that is part or all of a protein-encoding gene; may or may not be excised from a pre-mRNA during transcript processing.

Small-scale change in the nucleotide sequence of a gene; can result in an altered protein product.

Correspondence between triplets of nucleotides in DNA and mRNA, and specific sequences of amino acids in a polypeptide chain; near-universal language of protein synthesis; mitochondria and a few species have a few variant code words.

A mutation by which one or more bases are introduced into a DNA strand. Also a movable attachment of muscle to bone.

One of the noncoding sequences in eukaryotic genes; it is excised from the pre-mRNA transcripts before translation.

Form of radiation with enough energy to eject electrons from atoms.

mRNA. A single strand of ribonucleotides transcribed from DNA; the only type of RNA that carries proteinbuilding information to ribosomes.

Of a given gene locus, the probability that a spontaneous mutation will happen in a specified interval.

Form of radiation that carries enough energy to boost electrons to higher energy levels but not enough to eject them from an atom.

A series of ribosomes that are all translating the same mRNA molecule at the same time.

Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.

rRNA. A class of RNA that becomes complexed with proteins to form ribosomes; some catalyze assembly of polypeptide chains.

The site of polypeptide chain synthesis in all cells. An intact ribosome has two subunits of rRNA and proteins.

Enzyme that catalyzes transcription of DNA into RNA.

[L. trans, across, + scribere, to write] First stage of protein synthesis. An RNA strand is assembled from nucleotides using a gene region in DNA as a template.

tRNA. One of a class of small RNA molecules that delivers amino acids to a ribosome. Its anticodon pairs with an mRNA codon during translation.

Second stage of protein synthesis. At ribosomes, information encoded in an mRNA transcript guides the synthesis of a new polypeptide chain from amino acids.

Transposable element. A stretch of DNA that jumps spontaneously and randomly to a different location in the genome and may mutate a gene.

One of four nitrogencontaining bases in nucleotide monomers of RNA; also applies to a nucleotide with a uracil base component. Like thymine, uracil can base-pair with adenine.

Model for the genetic basis of flower formation; products of three master genes (A, B, C) control the development of sepals, petals, and stamens and carpels from meristematic tissue.

The attachment of an acetyl group (CH3CO) to an organic compound.

Of the two X chromosomes in the somatic cells of female mammals, the one that has been condensed.

In developing embryos of multicelled organisms, the process by which different cell lineages selectively express a different fraction of their genome and thereby become specialized in their composition, structure, and function.

A gene control mechanism in female mammals in which most genes on one of two X chromosomes in somatic cells are inactivated; ensures that X chromosome genes are expressed at the same levels as in males (XY).

A small sequence in DNA that binds transcription-regulating molecules; enhances transcription rates.

A molecular mechanism that governs if, when, or how a specific gene is transcribed or translated.

One of a class of master genes; helps determine identity of body parts during embryonic development.

[Gk. hormon, stir up] Signaling molecule secreted by one cell that can alter activities of any cell with receptors for it.

One of the noncoding sequences in eukaryotic genes; it is excised from the pre-mRNA transcripts before translation.

An experiment in which a living organism is engineered so that one of its genes does not function.

Attachment of a methyl group to an organic compound; also a common gene control mechanism.

In female mammals, an outcome of random X chromosome inactivation; different patches of tissue are expressing different X-linked alleles.

Control mechanism by which one or more regulatory proteins slow down a cell activity.

Part of an operon; a DNA binding site for a regulatory protein.

Group of bacterial genes together with a promoter�operator DNA sequence that controls their transcription.

In animal embryonic development, the sculpting of specialized tissues and organs from clumps of cells in the proper places, in the proper order by way of embryonic induction.

Of some insects, a chromosome consisting of many parallel copies of the same DNA molecule.

Use of regulatory proteins to promote gene expression.

Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.

Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.

Part of mechanisms that control transcription, translation, and gene products by interacting with DNA, RNA, new polypeptide chains, or proteins such as enzymes.

Type of protein that can block transcription of a prokaryotic gene by binding to an operator.

Outcome of controls over which gene products a cell makes or activates in a specified interval. Basis of cell differentiation.

In a female mammalian embryo, the programmed painting of special RNAs over most of one of the two X chromosomes, which cuts off access to the majority of its genes. Which X chromosome gets painted in each cell is a random event, so tissues of adult female mammals are a mosaic of traits. See also Dosage compensation.

Extremely rapid, robotic method of identifying the nucleotide sequence of a region of DNA. Gel electrophoresis and laser detection of fluorescent tracers are part of method.

DNA synthesized from an mRNA transcript through the use of the enzyme reverse transcriptase.

Any DNA molecule that can accept foreign DNA and that can be replicated inside a host cell.

Microarray of thousands of gene sequences that represents a large subset of a genome; stamped onto a glass plate and used to study gene expression.

Fragment of DNA inserted into a vector such as a plasmid and introduced into a host organism; used to make many copies of a particular segment of DNA.

A way to distinguish one individual from all others based on unique differences in parts of their DNA; fragments cut from an individual�s DNA (RFLPs) have a unique pattern of sizes.

Type of enzyme that catalyzes the sealing of short stretches of DNA into a continuous strand during replication; also seals strand breaks.

Method of separating DNA molecules according to length, or protein molecules according to size and charge. The molecules move apart while migrating through a gel matrix in response to a weak electric current.

Collection of host cells that contain different cloned DNA fragments representing all or most of a genome.

Manipulation of an organism�s DNA, usually to alter at least one aspect of phenotype.

All DNA in a haploid number of chromosomes for a species.

The study of genes and gene function in humans and other organisms.

The transfer of one or more normal or modified genes into a person to correct a genetic defect, or to boost resistance to a disease.

Any basepairing between DNA or RNA strands from different sources.

Polymerase chain reaction. A method to rapidly copy DNA fragments.

A small, circular bacterial DNA molecule having a few genes; replicated independently of the bacterial chromosome.

Short nucleotide sequence that researchers design as an initiation site for synthesis of a DNA strand on a DNA or RNA template.

Short nucleotide sequence that has been labeled with a tracer; designed to hybridize with part of a gene or mRNA.

Techniques by which DNA molecules from different species can be cut into fragments, spliced together into cloning vectors, and then amplified to useful quantities.

One of hundreds of proteins that recognize and cut specific base sequences in double-stranded DNA.

A viral enzyme that catalyzes the assembly of free nucleotides into a strand of DNA on an RNA template.

A storage facility where genes of diverse plant lineages are preserved.

One of many copies of short base sequences positioned one after another on a chromosome; used in DNA fingerprinting.

Surgical transfer of an organ from one species to another.

[Gk. analogos, similar to one another] Dissimilar body parts that have become similar in structure, function, or both in lineages that are not closely related but were subjected to similar pressures.

Eon extending from the time that life originated, 3.8 billion years ago, to 2.5 billion years ago.

The modern geologic era, from 65 million years ago to the present.

[Gk. morph, form] Scientific study of comparable external body parts of embryonic stages and adult forms of major lineages.

[L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.

How fossils form over time. An organism or evidence of it gets buried in sediments or volcanic ash; water slowly infiltrates the remains, and metal ions and other inorganic compounds dissolved in it replace the minerals in bones and other hardened tissues.

Recognizable, physical evidence of an organism that lived in the distant past.

Time scale for Earth�s history; major subdivisions correspond to mass extinctions. Dates are now absolute as a result of radiometrically dating.

The unvarying time it takes for half of a quantity of any radioisotope to decay into a more stable form.

Transmission, from parents to offspring, of genes that underlie the traits characteristic of their species.

Line of descent.

Large-scale patterns, rates of change, and trends among lineages.

Era of spectacular expansion in the range of global diversity; lasted from 240 million to 65 million years ago.

Model used to calculate the time of origin of one lineage relative to others; assumes that a group of genes accumulates mutations at a constant rate, measurable as a series of predictable ticks back through time. The last tick stops close to the time the lineage originated.

A pattern of macroevolution. In response to similar environmental pressures, body parts of evolutionarily distant lineages slowly evolve in similar ways and end up being alike in function, appearance, or both.

Pattern of macroevolution. One or more body parts of genetically diverging lineages undergo structural and functional changes from the parts in the common ancestor.

Microevolutionary process; the outcome of differences in survival and reproduction among individuals of a population that differ in the details of their heritable traits.

Any basepairing between DNA or RNA strands from different sources.

Era from 544 million to 248 million years ago; Cambrian through Permian.

Paleozoic supercontinent; the first land plants and animals evolved on it.

Theory that great slabs or plates of Earth�s outer layer float on a hot, semi-molten mantle. All plates are moving slowly and have rafted continents to new positions over time.

Era between 2.5 million to 544 million years ago. An oxygen-rich early atmosphere formed, sparking the Cambrian explosion of biodiversity.

Method of measuring proportions of a radioisotope in a mineral trapped long ago in newly formed rock and a daughter isotope that formed from it by radioactive decay in the same rock. Used to assign absolute dates to fossil-containing rocks and to the geologic time scale.

[L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.

Stacks of sedimentary rock layers, built up by deposition of silt and other materials over time.

Theory that Earth�s surface has changed in slow, uniformly repetitive ways except for expected annual catastrophes, such as big floods. Changed Darwin�s view of evolution; has since been discredited by plate tectonics theory.

[L. adaptare, to fit] Any long-term, heritable aspect of form, function, or behavior that improves an individual�s chances of surviving and reproducing; outcome of natural selection and other microevolutionary processes.

Abundance of one allele relative to others at a gene locus among individuals of a population.

One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.

An outcome of natural selection against homozygotes, so that two or more alleles for a trait are being maintained in the population.

Severe reduction in the size of a population, brought about by intense selection pressure or a natural calamity.

Persistence of two forms of the same trait in a population.

Mode of natural selection by which forms at one end of a range of phenotypic variation are favored.

Mode of natural selection that favors different forms of a trait at both ends of a range of variation; intermediate forms are selected against.

Permanent move of one or more individuals out of a population.

[L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.

Of a population, the loss of all alleles but one at a gene locus; all individuals have become homozygous for the allele.

A form of bottlenecking. By chance, a few individuals that establish a new population differ in allele frequencies relative to the original population.

Microevolutionary process; alleles enter and leave a population by immigration and emigration. Counters mutation, natural selection, and genetic drift, hence reproductive isolation.

Small-scale change in the nucleotide sequence of a gene; can result in an altered protein product.

Change in allele frequencies over generations due to chance alone. Most pronounced effects in small populations.

In theory, a state in which a population is not evolving with respect to a specified gene locus. Compare Hardy�Weinberg rule.

Theoretical baseline for tracking changes in allele frequencies over the generations. Frequencies do not change as long as there is no mutation, the population is infinitely large and isolated from other populations, and all individuals are reproducing equally and randomly.

One or more individuals move and take up residence in another population of its species.

Nonrandom mating among very close relatives that share many identical alleles; may fix harmful alleles.

Mutation having drastic effects on phenotype; usually causes death.

Of a population, a smallscale change in allele frequencies resulting from mutation, genetic drift, gene flow, natural selection, or a combination of them.

A mutation with no effect on phenotype; natural selection thus cannot change its frequency in a population.

[Gk. polus, many,+morphe, form] Persistence of two or more qualitatively different forms of a trait, or morphs, in a population.

All individuals of the same species living in a specified area.

The odds that each outcome of an event will occur is proportional to the total number of ways in which that outcome can be reached.

A notable difference between female and male phenotypes of a population.

A category of natural selection; an outcome of differences in success at attracting mates and reproducing among individuals of a population.

Mode of natural selection; intermediate phenotypes are favored over extremes at both ends of the range of variation.

A macroevolutionary pattern. A burst of genetic divergences from a lineage that gives rise to many species, each able to use a novel resource or to move into a new, or newly vacated, habitat.

A set of different niches that become be filled by a group of species.

[Gk. allos, different, + L. patria, native land] Speciation model. A physical barrier arises and separates populations or subpopulations of a species, ends gene flow, and so favors divergences that result in new species.

A major pattern of speciation. Directional changes in allele frequencies and morphology are confined within a single lineage, and in time a new type differs so much from the ancestral type that it is classified as a separate species.

Definition of a sexually reproducing species as one or more populations of individuals that interbreed under natural conditions, produce fertile offspring, and are reproductively isolated from other such populations.

[Gk. klados-, branch] All species that share a unique trait, being descended from an ancestral species in which the trait first evolved.

One speciation pattern. A lineage branches when one or more of its populations or subpopulations become reproductively isolated, and then genetic divergences result in new species.

Evolutionary tree diagram that depicts relative relatedness among groups. Each branch is monopyletic; it includes only an ancestral species in which a unique trait first evolved and all of its descendants.

A novel feature shared only by descendants of an ancestral species in which it originated.

A treelike diagram in which each branch point represents a divergence from a shared ancestor; each branch is a separate line of descent.

Irrevocable loss of a species.

Microevolutionary process; alleles enter and leave a population by immigration and emigration. Counters mutation, natural selection, and genetic drift, hence reproductive isolation.

An accumulation of differences in the gene pools of two or more populations or subpopulations of a species after gene flow stops entirely; mutation, natural selection, and genetic drift operate independently in each one.

[L. genus, race or origin] A grouping of species more closely related to one another in morphology, ecology, and history than to others at the same taxonomic level.

Addresses the rate of speciation and cites fossil evidence that morphological changes accumulate slowly over great time spans.

One of ever more inclusive groupings of species; e.g., family, order, class, phylum, kingdom.

A chance modification in some body structure or function that gives a species the opportunity to exploit the environment more efficiently or in a novel way; e.g., modifications of the forelimbs of amniotes into diverse legs and wings during radiations into adaptive zones.

Large-scale patterns, rates of change, and trends among lineages.

Catastrophic event or phase in geologic time when families or other major groups are lost.

A set of species that share a derived trait, a novel feature that evolved in one species and is present only in its descendants; all of the evolutionary branchings from a single stem.

[L. nidas, nest] Sum total of all activities and relationships in which individuals of a species engage as they secure and use the resources required to survive and reproduce.

A speciation model. Populations in contact along a common border evolve into new species; hybrids that form in the contact zone are less fit than individuals on either side of it and thereby act as a reproductive isolating mechanism.

A case of somatic cells having three or more of each type of chromosome characteristic of the species.

Addresses the rate of speciation; cites fossil evidence that morphological changes required for reproductive isolation evolve in a relatively brief time span, within the tens to hundreds of thousands of years when two or more populations are diverging from each other.

Any heritable feature of body form, function, or behavior that prevents interbreeding between two or more populations; sets the stage for genetic divergences.

One of the macroevolutionary processes; formation of daughter species from a population or subpopulation of a parent species; the routes vary in their details and duration.

[L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.

[Gk. sym, together, + patria, native land] A speciation model. Occurs inside the home range of a species in the absence of a physical barrier; e.g., by way of polyploidy in flowering plants.

A set of organisms of a given type.

Field of biology that identifies, names, and classifies species.

A classification system that groups all organisms into domains Bacteria, Archaea, and Eukarya.

[Gk. auto, self, and trophos, feeder] An organism that synthesizes its own food from simple inorganic compounds in its environment with energy captured from the sun or from oxidizing inorganic substances; e.g., a photoautotroph or chemoautotroph.

Any prokaryotic cell that makes its own food by oxidizing inorganic substances.

A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.

[Endo�, within + symbiosis, living together] An intimate, permanent ecological interaction in which one species lives and reproduces in the other�s body to the benefit of one or both.

Type of cell that starts life with a nucleus and other membranebound organelles.

[Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.

[L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.

Era between 2.5 million to 544 million years ago. An oxygen-rich early atmosphere formed, sparking the Cambrian explosion of biodiversity.

Presumed stage of chemical evolution that preceded living cells.

Model for a time prior to the evolution of DNA; a self-replicating system chemically evolved in which RNA strands were templates for protein synthesis.

Fossilized remains of domeshaped mats of shallow-water communities, cyanobacterial species especially, that were infiltrated with dissolved minerals and fine sediments. Some are 3 billion years old.

Acquired immune deficiency syndrome. A set of chronic disorders that develops after prolonged infection by HIV has weakened the immune system.

Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.

Rod-shaped prokaryotic cell.

A circular, doublestranded molecule of prokaryotic DNA.

One of a class of viruses that infects bacteria.

Of many cells (not animal cells), a semirigid but permeable structure that surrounds the plasma membrane; helps a cell retain its shape and resist rupturing.

Any prokaryotic cell that makes its own food by oxidizing inorganic substances.

A spherical prokaryotic cell.

Among prokaryotic species, a mode of gene transfer that is possible when one of the cells has an F plasmid. Also a sexual reproductive mode among some single-celled eukaryotes.

A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.

Of certain bacteria, a resting structure enclosing a bit of cytoplasm and the DNA; resists heat, irradiation, drying, acids, disinfectants, and boiling water. It germinates when conditions favor growth and a bacterium emerges from it.

Rapid spread, then subsidence, of a disease within a population.

Bacterium or archaean adapted to an extremely salty habitat.

Bacterium or archaean adapted to a hot aquatic habitat; e.g., a hot spring or hydrothermal vent.6

Invasion and multiplication of a pathogen or parasite in a host. Disease follows if defenses are not mobilized fast enough against the tissue disruptions.

Gross damage to a cell wall, plasma membrane, or both that lets cytoplasm leak out; causes cell death.

A latent period that extends many viral replication cycles. Viral genes are integrated into host chromosome and may remain inactivated through many host cell divisions before being replicated.

A rapid viral replication pathway that ends with lysis of host cell.

Any bacterium or archaean that produces methane gas as by-product of anaerobic reactions.

[Gk. pathos, suffering, + gene�s, origin] A virus, bacterium, fungus, protist, or parasitic worm that infects an organism and multiplies in it, thus causing disease.

Any photosynthetic autotroph; e.g., nearly all plants, most algae, and a few bacteria.

A small, circular bacterial DNA molecule having a few genes; replicated independently of the bacterial chromosome.

A type of protein particle normally in vertebrate nervous systems that turns infectious when its shape changes.

Cell reproduction mechanism of prokaryotic cells only.

A viral enzyme that catalyzes the assembly of free nucleotides into a strand of DNA on an RNA template.

A spiral-shaped prokaryotic cell.

A noncellular infectious agent of DNA or RNA, a protein coat and, in some types, an outer lipid envelope; it can be replicated only after its genetic material enters a host cell and subverts the host�s metabolic machinery.

Rapid, huge increases in algal population sizes after nutrient enrichment of an aquatic habitat.

A type of single-celled eukaryote that has many tiny, membrane-bound sacs just beneath the plasma membrane; e.g., a ciliate, apicomplexan, or dinoflagellate.

A single-celled amoebozoan that moves on pseudopods. All are predatory or parasitic; none forms colonies.

Asexual reproductive mode of certain invertebrates; the body splits spontaneously, then both parts grow what is missing. See also Prokaryotic fission.

A stramenopile; a multicelled marine photoautotroph with an abundance of the pigment fucoxanthin; e.g., kelps.

An alveolate having arrays of cilia at its surface; traditionally called a ciliated protozoan. Most are free-living predators.

A single-celled marine autotroph having calcium carbonate plates; one of the chrysophytes that are abundant in phytoplankton and the leading source of calcium deposits on the seafloor.

Among prokaryotic species, a mode of gene transfer that is possible when one of the cells has an F plasmid. Also a sexual reproductive mode among some single-celled eukaryotes.

[L. contractus, to draw together] Organelle in some single-celled eukaryotes that collects excess water in the cell body and then expels it.

Of many microbes, a resting stage with a secreted cover. Also an abnormal, fluid-filled sac in skin with no opening.

A single-celled photoautotroph with a perforated silica shell, which has two overlapping parts that fit together like a pillbox; one of the chrysophytes.

A single eukaryotic cell with a thick flagellum. Different kinds are colorless heterotrophs or photoautotrophs of aquatic habitats.

Single-celled, predatory eukaryote with a richly perforated shell through which thin pseudopods project.

A colorless flagellate; the only eukaryote with mitochondrial DNA massed inside a mitochondrion almost as long as the cell; e.g., Trypanosoma.

Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.

[Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.

A thin, flexible, protein-rich body covering of some single-celled eukaryotes.

Aquatic community of mostly microscopic autotrophs and heterotrophs.

Of plasmodial slime molds, a multinucleated mass that forms whena single diploid cell undergoes rounds of mitosis without cytoplasmic division.

Informal name for all structurally simple eukaryotes, which are now being classified as monophyletic groups.

A dynamic lobe of membraneenclosed cytoplasm; functions in motility and phagocytosis by amoebas, amoeboid cells, and many white blood cells.

A single-celled predatory eukaryote that has pseudopods projecting from a perforated silica shell

An aquatic, mostly multicelled photoautotroph having an abundance of phycobilins that masks its chlorophyll a.

An algal blood that turns the water near coasts rust-red or brown.

An amoebozoan; one of the free-living, amoebalike cells that also cluster into a migrating mass, differentiate, and form reproductive structures.

A single-celled or multicelled eukaryote with four outer membranes and 2 flagella, one with thin filaments projecting from it.

[Gk. angeion, vessel + spermia, seed] A flowering plant; its egg-containing ovules mature into seeds within closed, protected chambers called ovaries.

Nonvascular land plant. The haploid stage dominates its life cycle, and its sperm require standing water to reach eggs. A moss, liverwort, or hornwort.

A type of photoautotroph once grouped with other green algae but now known to be more closely related to land plants; e.g., desmids, stoneworts.

A nonrenewable energy source that formed more than 280 million years ago from submerged, undecayed, and slowly compacted plant remains.

The joint evolution of two species interacting so closely that a change in the structure, function, or behavior of one exerts selection pressure on the other over the generations.

Reproductive structure of certain seed-bearing plants; has clusters of scales with exposed ovules on their surface.

A type of gymnosperm adapted to conserve water through droughts and cold winters. Cone-producing woody trees or shrubs with thickly cuticled needlelike or scalelike leaves.

Seed leaf; part of a flowering plant embryo. In eudicots, two cotyledons absorb nutrients from endosperm, emerge aboveground as the seed germinates, and transfer nutrients that sustain early growth; photosynthetic before true leaves form. In most monocots, one small cotyledon helps transfer nutrients from endosperm to the embryo, but it remains underground when a seed germinates and is never photosynthetic.

Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.

A gymnosperm of subtropical or tropical habitats; pollen-bearing and seedbearing strobili on separate plants.

Nutritive tissue in the seeds of flowering plants only.

A single eukaryotic cell with a crystalline rod reinforcing a thick flagellum. Different kinds are colorless heterotrophs or photoautotrophs of aquatic habitats.

A seedless vascular plant having fronds that often are divided in leaflets.

A reproductive structure of fertile parts (stamens, carpels) nonfertile parts (sepals, petals), and a receptacle (modified base of floral shoot).

[L. after frui, to enjoy] Mature ovary, often with accessory parts, from a flower.

[Gk. phyton, plant] A haploid multicelled body in which haploid gametes form during the life cycle of plants and some algae.

A deciduous gymnosperm; its ancestors were diverse in dinosaur times.

A type of woody, vinelike or shrubby gymnosperm.

[Gk. gymnos, naked, + sperma, seed] A vascular plant that forms seeds on exposed surfaces of sporeproducing structures; e.g., conifers, cycads.

Bryophyte having a horn-shaped sporophyte attached to a flat gametophyte.

Seedless vascular plant having rhizomes, scale-like leaves, and hollow stems with silica-reinforced ribs.

Gluelike polymer deposited in secondary cell walls; makes some plant parts stronger, more waterproof, and less vulnerable to attacks.

One of the bryophytes.

A type of seedless vascular plant, typically with true leaves, roots, and stems; e.g., a club moss.

One of three major flowering plant groups; e.g., magnolias, avocados.

Haploid meiotic spore in ovary of seed-bearing plants; gives rise to a female gametophyte with egg cell.

Type of walled haploid spore of gymnosperms and angiosperms that gives rise to pollen grains.

Monocotyledon; flowering plant characterized by embryo sporophytes having one cotyledon; floral parts usually in threes (or multiples of three); and often parallel-veined leaves.

Most common kind of bryophyte.

[L. ovum, egg] Of seedbearing plants, an egg-containing female gametophyte surrounded by tissue layers; a mature ovule is a seed.

Compressed, soggy, acidic mat of accumulated remains of peat mosses.

Plant vascular tissue that distributes photosynthetic products through the plant body. Its conducting tubes are interconnecting, living cells assisted by companion cells that help load solutes into the tubes.

[L. pollen, fine dust] A tiny structure that forms from microspores; consists of a sturdy wall around a few cells that will develop into a mature, sperm-bearing, male gametophyte.

Arrival of pollen on a carpel�s stigma in a flower of the same species.

Any agent that delivers pollen grains to the egg-containing structures in flowers of the same species; e.g., wind, water, or birds, bats, and other animals.

A rootlike absorptive structure.

A short absorptive stem that grows underground in a horizontally branching pattern, most often.

The first seedless vascular plants; originated in Gondwana lowlands.

Underground vascular plant structures that absorb water, mineral ions.

A mature ovule.

A practice of cutting and burning trees, then tilling ashes into the soil of a small plot of land. Once called slash-and-burn agriculture.

Aboveground plant parts; e.g., stems, leaves, flowers.

A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.

[Gk. phyton, plant] A sporeproducing vegetative body of a plant or multicelled alga that grows by mitotic cell divisions from a zygote.

A gap between two plumped guard cells that lets water vapor and gases diffuse across the epidermis of a leaf or primary stem; diffusion stops when the cells lose water and collapse.

Of certain nonflowering plants, a cluster of spore-producing structures.

Seedless vascular plant having a branching form and no true roots; e.g., Psilotum.

[Gk. xylon, wood] Of vascular plants, a complex tissue that conducts water and solutes through tubes of interconnected walls of cells that are dead at maturity.

Sexual spore of sac fungi.

Sexual spore of club fungi.

A flagellated intracellular parasite of a fungal group, the microsporidians.

Fungus that produces sexual spores in a club-shaped cell, a basidium.

[L. dis�, to pieces] One of the prokaryotic or fungal heterotrophs that obtains carbon and energy by breaking down wastes or remains of organisms. The collective action of decomposers helps cycle nutrients to producers in ecosystems.

Mode of nutrition; the organism grows in or on organic matter, digests it with secreted enzymes, and absorbs digested bits.

Kingdom of fungi.

Respiratory organ with a thin, moist, vascularized layer for gas exchange.

Fungal filament having chitin-reinforced walls; component of a mycelium.

Mutualism between a fungus and one or more photoautotrophs.

Intracellular fungal parasite of aquatic habitats that forms flagellated spores; belongs to one of the most ancient eukaryotic lineages.

Aboveground reproductive structure produced by many club fungi.

[L. mutuus, reciprocal] A type of symbiotic interaction that benefits both participants.

[Gk. mykes, fungus] Underground mesh of tiny, branching filaments (hyphae); the food-absorbing portion of most fungi.

�Fungusroot.� A form of mutualism between a fungus and young plant roots. Hyphae withdraw some carbohydrates from the plant, which withdraws some absorbed mineral ions from hyphae.

[Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.

Fungus that produces sexual spores in sac-shaped cells; e.g., truffles.

Heterotroph that extracts energy and carbon from nonliving organic matter and so causes its decay.

A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.

[Gk. sym, together, + bios, life, mode of life] An ecological interaction in which one or more individuals interact closely with individuals of a different species for some or all of the life cycle; e.g., mutualism, predation, parasitism.

Type of parasitic or saprobic fungus in which diploid zygotes develop into zygospores, a type of thick-walled sexual spore in a thin, clear covering.

Sexual spore of zygomycetes.

Any multicelled heterotroph that ingests other organisms or their tissues, develops through a series of embryonic stages, and is motile during part or all of the life cyle. Most species have epithelial tissues and extracellular matrixes.

A bilateral invertebrate having a highly segmented body; major groups are polychaetes, oligochaetes, and leeches. Except in leeches, segments have clusters of chitin-reinforced bristles.

Type of invertebrate having a hardened exoskeleton and specialized segments with jointed appendages; e.g., millipedes, spiders, lobsters, insects.

Body plan in which the main axis divides the body into two halvesthat are mirror images of one another.

Of most nervous systems, a major integrating center that receives, processes, and often stores sensory input, and issues coordinated commands for responses.

A dormant shoot, mostly meristematic tissue and often sheathed in small, young leaves. A lateral (axillary) bud forms in a leaf axil; a terminal bud forms at a shoot tip and is the main zone of primary growth.

Venomous predatory arthropod with many segments having paired legs.

[Gk. kephalikos, head] During the evolution of most kinds of animals, the increasing concentration of sensory structures and nerve cells at the anterior end of the body.

Soft-bodied mollusk with a closed circulatory system. Moves by jet propulsion of water from a siphon; e.g., squids, octopuses, chambered nautilus.

Single-celled eukaryote having a microvilli collar around a single flagellum at their anterior end. A sister taxon of animals and fungi.

A type of radial invertebrate having epithelial tissues and a saclike gut. The only animal that makes nematocysts.

Between the gut and body wall, a cavity lined with peritoneum.

A sponge cell having a ring of food-trapping villi around a flagellum.

A tubular digestive system having a mouth at one end and an anus at the other.

Most abundant type of animal tissue. Soft connective tissues differ in the amounts and arrangements of fibroblasts, fibers, ground substance. Adipose tissue, cartilage, bone tissue, and blood are specialized types.

One of the abundant �insects of the seas,� mostly marine arthropods having a hardened, flexible exoskeleton and pairs of jointed appendages.

Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.

[Gk. deuteros, second, + stoma, mouth] A bilateral animal of a lineage characterized in part by events of embryonic development, as when the second indentation to appear on the early embryo�s surface becomes the anus; e.g., an echinoderm or a chordate.

One of the deuterostomes; a radial invertebrate with some bilateral features and calcified spines or plates on the body wall; e.g., sea stars.

[Gk. ecto, outside, + derma, skin] First-formed, outer primary tissue layer of animal embryos; gives rise to nervous tissues and outer layer of the integument.

Inner primary tissue layer of animal embryos; source of the inner gut lining and organs derived from it.

Outermost tissue layer of plants and nearly all animals.

Animal tissue covering external and internal body surfaces. A key innovation that favored larger, more complex bodies; cells started interacting as functional units.

[Gk. skle�ros, hard, stiff] An external skeleton; e.g., a hardened cuticle.

Secretions and other deposits on or between cells of a tissue.

One of the simplest existing animals with organ systems that form from three primary tissue layers.

Distinct cluster of cell bodies of neurons.

Glandular epithelium that lines the gut of many invertebrates.

Primary reproductive organ in animals; produces gametes.

A sac or tube in which food is digested. Also the gastrointestinal tract from the stomach onward.

An individual with male and female gonads.

[Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.

A fluid-filled cavity against which a contractile force can act.

Saclike gut in which both food intake and waste output occur through a single openin.

An immature stage between the embryo and adult in the life cycle of many animals.

One of many small tubes that help insects on land dispose of toxic wastes without losing body water.

Body part of Molluscs. Covers fleshy visceral mass secretes the shell

[Gk. Medousa, one of three sisters in Greek mythology with snake-entwined hair] Of cnidarian life cycles, a free-swimming, bell-shaped stage, often with oral lobes and tentacles.

Primary tissue layers gives rise to many internal organs and part of the integument; pivotal in the evolution of large, complex animals.

Of cnidarians, a gelatinous matrix with scattered cells between the epidermis and gastrodermis; functions as a buoyant, deformable skeleton.

[Gk. meta, change, + morphe, form] Major changes in body form of certain animals. Hormonally controlled growth, tissue reorganization, and remodeling of body parts leads to adult form.

An arthropod with a great many unspecialized segments and paired legs; scavenges decaying plant material.

Only invertebrate with a mantle draped over a soft, fleshy visceral mass; most have an external or internal shell; e.g., gastropods, bivalves, cephalopods.

Periodic shedding of worn-out or too-small body structures. Permits an animal to grow in size or renew parts.

[Gk. nema, thread + kystis, pouch] A fluid-filled, jackin- the-box capsule housed in one of three types of sensory�effector cells in cnidarians. It has a mechanoreceptor projecting above the cell surface and a dischargeable, tubular thread, often with barbs or toxin-drenched. Only cnidarians make nematocysts.

Of some invertebrates, one of many water-regulating units that help control the composition and volume of tissue fluid.

Of bilateral animals, a line of communication, usually paired, that runs parallel with the anterior�posterior axis. In large or long invertebrates, it often has one or more large axons. In chordates, it develops as a hollow, neural tube that gives rise to the spinal cord and brain.

Nervous system of cnidarians and some other invertebrate groups; an asymmetrical mesh of sensory and motor neurons that controls simple movements. It activates epitheliomuscular cells arrayed as sheets or rings in the body wall.

A set of organs that are interacting chemically, physically, or both in a common task.

Body structure with definite form and function made of more than one tissue.

[Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.

The membrane that lines the coelom.

A muscular tube. Invertebrate chordates use theirs in filter-feeding and respiration. In land vertebrates, it is the entrance to the esophagus and trachea.

An asymmetric, soft-bodied animal with two simple tissues around a thin, inner matrix.

Of cnidarians, a type of swimming or creeping larva, usually with a ciliated epidermis.

Vase-shaped, sedentary stage of cnidarian life cycles.

One of many tapeworm body units that bud behind the scolex.

[Gk. proto, first, + stoma, mouth] A bilateral animal of a branching lineage characterized partly by events in embryonic development, as when the first indentation to form on the early embryo�s surface becomes a mouth; e.g., mollusks, annelids, arthropods.

False coelom; a main body cavity incompletely lined with tissue derived from mesoderm.

Animal body plan with four or more roughly equivalent parts around an anterior�posterior axis.

Bilateral, cephalized animal with a false coelom and a crown of cilia.

Bilateral invertebrate with a false coelom and complete digestive system in a cylindrical body. Most are decomposers; many are parasites.

Any secretion from one cell type that can alter the behavior of a different cell that bears a receptor or it; a means of cell communication.

Structurally, the simplest existing animal. Its asymmetrical body has a spiculereinforced matrix in two cell layers (not epithelium). Its phagocytic collar cells trap food from water flowing through pores in its wall.

A drastic twisting of the body, including the visceral mass, as certain molluscan embryos develop.

An air-conducting tube used in respiration. Of land vertebrates, the windpipe.

Of echinoderms, a system of tube feet connected to canals, through which controlled water flow can extend the feet in coordinated ways.