The Cellular Level of OrganizationChapter # 3

•The basic unit of life

• •Every living organism
• is composed of cells

• •Cells vary in structure
• and function.

•Cell structure:

–Plasma membrane

–Cytoplasm

Nucleus

• •Plasma membrane
• separates the cell from its surrounding environment

• •Plasma membrane is
• composed from a double layer of phospholipid molecules

–Amphipathic

–Heads are hydrophilic (water loving, charged, phosphate ends of molecule)

–Tails are hydrophobic (water-fearing, non-polar, lipid portion in the center)

• –Cholesterol
• (~20%)

•Helps to maintain membrane fluidity

–Integral proteins

•Most are transmembrane proteins that extend into and through the lipid bilayer.

–Peripheral proteins

•Attached to inner/outer surface of membrane

•Not as firmly embedded in the membrane

–Glycoproteins

• •Proteins that have carbohydrates attached to them
• Serve as identification markers

• –Ion
• channels

• –Selective
• carriers/transporters

–Receptors/ligands


–Enzymes

–Linkers

• •Anchor
• proteins in the plasma membrane

• –Cell
• Identity markers

–Do not require energy expenditure

• –Transport of a given substance
• across the plasma membrane is affected by number of factors:

•Concentration Gradient

•Temperature

•Mass of the diffusing substance

•Surface area

•Diffusion distance

• –Molecule diffuse directly through the lipid bilayer (or through channel) from an area of high concentration to
• the area of low concentration

•E.g. O2, CO2, H2O

–Channel mediated passive transport from an area of high concentration to the area of low concentration

•Permeability affected by the opening or closing of membrane channels

•Channels are specific-allow only one type of solute to pass through

•E.g. Movement of Na+ ions into nerve cells during a nerve impulse

• –Carrier
• structure attract a particle to a binding site, changes its shape and then releases the particle to the other site of the membrane

–E.g. transport of glucose

–Diffusion of water through a selectively permeable membrane

•Membrane is permeable to water, but not to other solutes

•Aquaporins are water channels that permit rapid osmosis

•Water pressure that develops as a result of osmosis is called osmotic pressure

• –Concentrations
• of solutes equal on both sides of the membrane

–Higher pressure

–Lower pressure

–Require the expenditure of metabolic energy of the cell

• •Eg. Ca
• pump, Na/K pump

•Metabolic substrate is ATP

–Transport by Pumps:

•Transport of solute against its concentration gradient

• –Substance
• leaves or enters the cell without moving through the plasma membrane

–Endocytosis


•Phagocytosis

–Large particles are engulfed by the plasma membrane, and enter the cell in vesicles that have pinched off from the membrane

Inside the cell vesicles fuse with lysosomes that digest the particles (e.g. microorganism).

–Pinocytosis

•“Cellular drinking”

•Membrane invaginates and pinches off small droplet of dissolved substance in a pinocytic vesicle. The content of the vesicle is then released in the cytosol.

–Materials move out of the cell by the fusion of vesicles with the plasma membrane

–Require energy supplied by ATP

•Thick, gel-like substance inside the cell

•Composed of many organelles suspended in the watery cytsol.

• •Two components
• –Cytosol
• •Intracellular fluid
• •Constitutes 55% of total cell volume

• –Organelles
• Specialized structures

• •Cytoskeleton is an internal framework of the
• cell

–Three types of structural proteins:

•Microfilaments or “Cellular muscles”

–Thin, twisted strands of protein molecules that usually lie parallel to the long axis of the cell

–Are capable of shortening the cell

•Intermediate filaments

–Form supportive framework

–Attach cells to one another

•Microtubules

–Protein subunits arranged

–Functions: movement (e.g. vesicles within the cell, the movement of chromosomes during cell division)

•Processing and sorting area for protein synthesis by ribosomes

• •Ribosomes are small, non-membrane-bound organelles, composed of protein and ribosomal RNA,
• that are responsible for protein synthesis

•No ribosomes

•Synthesizes certain lipids and carbohydrates

•Creates membranes for use through out the cell

•Discovered by Camillo Golgi in 1898

•Consist of tiny sacks or cisternae stacked on one another

•Located near the nucleus

•Process and package protein molecules

•Made of microscopic membranous sacks that pinched off from Golgi apparatus

•Contain digestive enzymes

–Digest defective cell parts

–Digest particles that have been trapped in the cell

•Mitochondria = power house of the cell

•Bean-shaped organelle with a smooth outer membrane and a folded inner membrane

• •Converts digested nutrients into useable
• energy for the body

–ATP

•Inherited through the maternal lineage

•Carry their own DNA

•Nucleus consist of a nuclear envelope that surrounds nucleoplasm

–Nuclear envelope has nuclear pores that allow molecules to enter and exit the nucleus

• • Nucleus contains DNA, a hereditary molecule
• that determines structure and function of the cell

–Chromatin

•non-dividing cells

–Chromosomes

•early stages of cell division

•Darker area of chromatin in the nucleus

•Produce ribosomal RNA and assembles ribosomes

–Somatic Cell division

–Nuclear division

•the production of two identical cells, each with the same number and type of chromosomes as the original cell

–Reproductive Cell division

•A two step process

•Number of chromosomes in the nucleus is reduced by half