__ nourishes almost the enitre living world directly/indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes: __ and __.
photosyn.
autotrophs
heterotrophs
_ are self feeders and produce their organic molecules from CO2 and other inorgainic materials obtained from the environment. They are the __ of the biosphere. Plants are __, organisms that use light as a source of energy to synthesize organic substances.
autotrophs
producers
photoautotrophs
Compare and contrast cellular resp. and photosyn.
CR: energy released from sugar when electrons associated with H are transported by carriers to oxygen, forming water as a by-product; electrons lose potential energy as they fall down the electron chain toward more electroneg. oxygen, and the mitochondrion harnesses that energy to synthesize ATP
Photo: reverses direction of electron flow; water split and electrons are transferred along w/ H+ from the water to carbon dioxide, reducing it to sugar. Because the electrons increase in potential energy as they move from water to sugar, this processes requires energy and is endergonic; energy boost provided by light
A graph plotting a pigment's light absorption versus wavelength is called an ___, which provide clues to the relative effectiveness of different wavelengths for driving __, since light can perform work in __ only if it is absorbed.
absorption spectrum
photosynthesis
chloroplast
__ participates directly in the light reactions
__ is the accessory pigment
__ is a group of accessory pigments
chlorophyll a
chlorophyll b
carotenoids
The spectrum of __ suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while __ is the least effective color. This is confirmed by an __ for photosynthesis, which profiles the relative effectiveness of different wavelengths of radiation in driving the process.
chlorophyll a
green
action sprectrum
__ is prepared by illuminating chloroplasts with light of dif. colors and then plotting wavelength against some measure of photosynthetic rate, such as CO2 comsumption or O2 release; it was first demonstrated by a German botanist in 1883 named Theodor W. Engelmann (used bacteria to measure rates of photosyn. in filamentous algae
action spectrum
Accessory __ with different absorption spectra are also photosynthetically important in chloroplasts and broaden the sprectrum of colors that can be used for photosynthesis.
pigments
a photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy
chlorophyll a
an accessory photosynthetic pigment that transfers energy to chlorophyll a
chlorophyll b
What does a slight difference in chlorophyll a and b result in?
- What is the outcome
absorb at slightly dif. wavelengths in the red and blue parts of the spectrum
- chlorophyll a: blue green
- chlorophyll b: olive green
hydrocarbons taht are various shades of yellow and orange because they absorb violet and blue-green light
carotenoids
__ may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some of them seems to be __.
-How?
Carotenoids
photoprotection
- Some carotenoids absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact w/ oxygen, forming reactive oxidative molecules that are dangerous to the cell
__ similar to the __ ones in chloroplasts have a __ in the human eye. These and related molecules, often found in health food products, are valued as "__", compounds w/ antioxidant properties. Plants can synthesize all the antioxidants they require, but humans and other animals must obtain some of them from their diets.
Carotenoids
photoprotective
photoprotective
phytochemicals
What exactly happens when chlorophyll and other pigments absorb light?
the colors corresponding to the absorbed wavelengths dissapear from the spectrum of the transmitted and reflected light, but energy cannot disappear.
Whnen a molecule absorbs a __ of light, one of the molecules electrons is ___. When the electron is in its normal orbital, the pigment molecule is said to be in its ___. What does absorption of a photon do?
The only __ absorbed are those whose energy is exactly __ to the energy difference between the __ and an __, and this energy difference varies from one kind of molecule to another Thus a particular compound absorbs only __ corresponding to specific __, which is why each pigment has a unique __.
-photon
-elevated to an orbital where it has more potential energy
-ground state
-boosts an electron to an orbital of higher energy and the pigment molecule is then said to be in an excited state
- photons
-equal
- ground state
- excited state
-photons
-wavelengths
-absorption spectrum
Once absorption of a __ raises an electron from the __ to the __, the electron cannot remain there long. The __, like all high-energy states, is __. Generally, when isolated pigment molecuels absorb light, their excited electrons do what?
Conversion: __ to __
In isolation, some pigments, including __, emit light as well as heat after absorbing __. As excited electrons fall back to the __, __ are given off. This afterglow is called __. If a solution of chlorophyll isolated from chloroplasts is illumiated, it will fluoresce in the red-orange part of the spectrum and also give off __.
photon
ground state
excited state
excited state
unstable
drop back down to the ground state orbital in a billionth of a second, releasing energy as heat
light energy
heat
chlorophyll
photons
ground state
photons
fluorescence
heat
Chlorophyll molecules excited by the __ produce very different results in an intact __ than they do in __. In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into __.
absorption of light energy
chloroplast
isolation
photosystems (PS)
__ is composed of a protein complex called a __ surrounded by several __.
photosystem
reaction- center complex
light- harvesting complexes.
a complex of proteins associated with a special pair of chlorophyll a molecules and a primary electron acceptor. Located centrally in a PS, this complex triggers the light reactions of photosyn. Excited by light energy, the pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain
reaction- center complex
a complex of proteins associated with pigment molecules (including chlorophyll a, b and carotenoids) bound to proteins that captures light energy and transfers it to reaction- center pigments in a PS
light- harvesting complex
The # and variety of pigment molecules enable a ___ to harvest light over a larger surface and a larger portion of the spectrum than any single pigment molecule alone could. Together, these ___ act as an antenna for the ___.
When a pigment molecule absorbs a __, the energy is transferred from pigment molecule to pigment molecule within a __ until it is passed into the ___.
The __ contains a molecule capable of accepting electrons and becomiing reduced; it is called the ___.
The pair of __ in the __ are special because their molecular environment- their location and the other molecules with which they are associated- enables them to use the energy from __ not only to _(do what)__, but also to __(do what)__ to a different molecule- the __
photosystem
light-harvesting complexes
reaction- center complex
photon
light- harvesting complex
reaction- center complex
reaction- center complex
primary electron acceptor
chlorophyll a
reaction- center complex
light
boost one of their electrons to a higher energy level
transfer it
primary electron acceptor
What is the first step of the light reactions?
WHen does the primary electron acceptor capture the electron?
the solar-powered transfer of an electronf rom the reaction- center chlorophyll a pair to the primary electron acceptor
as soon as the chlorophyll electron is excited to a higher energy level
Isolated chlorophyll __ because there is no __, so electrons of __ drop right back to the ground state.
fluoresces
electron acceptor
photoexcited chlorophyll
In a chloroplast, the __ represented by the excited electron is not lost. Thus, each __- a reaction- center complex surrounded by light- harvesting complexes- functions in the chloroplast as a unit. It converts __ to __, which will ultimately be used for the synthesis of sugar.
potential energy
photosystem
light energy
chemical energy
The __ is populated by two types of _ that cooperate in the light reactions of photosynthesis. They are called __ and __.
Which one functions first in the reactions?
Each has a characteristic __- a particular kind of __ next to a special pair of __ molecules associated with specific proteins.
thylakoid membrane
photosystems
PS II
PS I
PS II
reaction- center complex
primary electron acceptor
chlorophyll a
The reaction- center chlorophyll a of PS II is known as __.
Why?
P680
this pigment is best at absorbing light having a wavelength of 68 nm (red part of spectrum)
THe chlorophyll a at the reaction- center complex of PS I is called __.
Why?
P700
because it most effectively absorbs light of wavelength 700 nm (far- red part of spectrum).
These two pigments, __ and __, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoind membrane affects the ___ in the two pigments and accounts for the slight differences in their light- absorbing properties.
P680
P700
electron distribution
What are the two flows for the light reactions?
Which one uses both photosystems?
Linear electron flow
cyclic electron flow
linear electron flow
Light drives the synthesis of __ and __ by energizing the two __ embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane called __, and it occurs during the light reactions.
ATP
NADP
photosystems
linear electron flow
Explain the first step of linear electron flow.
1- A photon of light strikes a pigment molecule in a light- harvesting complex, boosting one its electrons to a higher energy leve. As this electron falls back to its ground state, an electron in a nearby pigment mlecule is simultaneously raised to an excited satte. Thie process continues, with the energy being relayed to other pigment molecuels until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction- center complex. It excites an electron in this pair of chlorophylls to a higher energy state.
Explain the second and third steps of linear electron flow.
2- This electron is transferred from the excited P680 to the primary electron. P680 is now P680+ (missing electron)
3- a) An enzyme catalyses the spiltting of a water molecule= two electrons/ two H+ ions
b) electrons supplied one by one to P680+ pair, each electron replacing one transferred to the primary electron acceptor
c) oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2.
__ is the strongest biological oxidizing agent known; its electron "hole" must be filled. THis greatly facilitates the trasfer of electrons from the split water molecules
P680+
Explain the fourth and fifth steps of linear electron flow.
4- Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an ETC, (the components similar to thos of the ETC in cell. resp.) The ETC between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plstocyanin (Pc).
5- THe exergonic "fall" of electrons to a lower energy level provides energy for ATP synthesis. As electrons pass through the cytochrome complex, the pumping of protons builds a proton gradient that is subsequently used in chemiosmosis
Explain the sixth step of linear electron flow.
6- Meanwhile, light energy was transferred via light- harvesting complex pigments to the PS I reaction- center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. THe photoexcited electron was then transferred to PS I's primary electron acceptor, creating an electron "hole" in the P700+- which is now called P700+. In other words, P700+ can now act an an electron acceptor, accepting an electron that reaches the bottom of the ETC from PS II.
Explain the seventh step in linear electron flow.
7- Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second ETC through the protein ferredoxin (Fd). (THis chain doesn't create a proton gradient and thus doesnt produce ATP)
Explain the eighth and last step of linear electron flow.
8- THe enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. This molecule is at a higher energy level than water, and its electrons are more readily available for the reactions of the Calvin cycle than were those of water
The light reactions use solar power to generate __ and __, which provide __ and __, respectively, to the carbohydrate- synthesizing reactions of the __.
ATP
NADPH
chemical energy
reducing power
In certain cases, photoexcited electrons can take an alternative path called __, which uses __ but not __.
cyclic electron flow
PS I
PS II
In the Cyclic electron flow, electrons cycle back from __ to the __ and from there continue on to a __ in the __.
ferredoxin
cytochrome complex
P700 chlorophyll
PS I reaction- center complex
There is no production of __ and no release of __ in the cyclic electron flow. However, __ is generated.
NADPH
oxygen
ATP
Several photosynthetic bacteria appear to have PS I and not PS II. FOr them, __ is the sole means of generating ATP in photosynthesis.
cyclic electron flow
__ can occur in both photosystems as well.
cyclic electron flow
What is the benefit of carrying out cyclic electron flow?
cyclic electron flow may be photoprotective, protecting cells from light-induced damage
can grow well where light is intense
TRUE OR FALSE
Whether ATP synthesis is driven by linear or cyclic electron flow, the actual mechanism is the same.
TRUE
Compare:
mitochondria
chloroplasts
Both:
-generate ATP by the same mechanism: chemiosmosis
- an ETC assembled in the membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative.
---> In this way, ETCs transform redox energy to a proton-motive force.
-built into membrane is an ATP synthast complex that couples the diffusion of H+ ions down their gradient to the phosphorylation of ADP
---> Some electron carriers are very similar
- ATP synthases very much alike
Contrast:
mitochondria
chloroplasts.
- differences between oxidative phosphorylation and photophosphorylation
Mitochondria: high- energy electrons dropped down the transport chain are extracted from organic molecues (which are then oxidized)
Chloroplasts: source of electrons is water; do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain
SUMMARY: Mitochondria uses chemiosmosis to transfer chem. energy from food molecules to ATP, whereas chloroplasts transform light energy into chem. energy in ATP
What are the similarities in the spatial organization of chemiosmosis in chloroplasts and mitochondria?
Mitochondria: mitochondrial membrane pumps protons from matrix out to the intermembrane space, which then serves as reservoir for H+ ions
Chloroplasts: thylakoid membrane pumps protons from stroma to the thylakoid space, serving as H+ reservoir.
What is a similarity in the two organelles concerning ATP synthase?
Mitochondria: protons diffuse down concentration gradient from intermembrane space through ATP synthase to matrix, driving ATP synthesis
Chloroplasts: ATP synthesized as H+ diffuse from the thylakoid space back to stroma through ATP synthase, forming ATP in stroma, where it is used to help drive sugar synthesis during the Calvin cycle
The proton gradient, or pH gradient, across the __ is substantial. When chloroplasts in an experimental setting are illuminated, the pH in the __ drops to about 5 (pH more __) and the pH in the __ increases to about 8 (pH more __).
This gradient of __ pH units corresponds to a thousandfold difference in H+ concentration.
thylakoid membrane
thylakoid space
acidic
stroma
basic
NADPH and ATP are produced on the side of the membrane facing the __, where the Calvin cycle reactions take place.
stroma
If in a laboratory, you turned the lights off, what happens?
pH gradient is abolished.
TRUE OR FALSE
Each thylakoid has numerous copies of its molecues and molecular complexes.
TRUE
Summarize the light reactions.
Electron flow pushes electrons from water, where they are at a low state of potential energy, ultimately to NADPH, where they are stored at a high state of potential energy.
The light-driven electron current generates ATP.
Equipment of thylakoid membrane converts light energy to chem energy stored in ATP and NADPH w/ oxygen as a byproduct.
How is the Calvin cycle similar to the citric acid cycle?
a starting material is regenerated after molecules eneter and leave the cycle
The Calvin cycle is __, building carbohydrates from smaller molecuels and consuming energy.
anabolic
__ enters the Calvin cycle in the form of __ and leaves in the form of __.
carbon
CO2
sugar
The Calvin cycle spends __ as an energy source and consumes __ as reducing power for adding high-energy electrons to make the sugar.
ATP
NADPH
The carbohydrate produced directly from the Calvin cycle isn't actually __, but a __- carbon sugar called ___.
glucose
three
glyceraldehyde-3-phosphate (G3P)
For the net synthesis of one molecule of __, the cycle must take place __ times, fixing three molecules of __.
G3P
three
CO2
__ refers to the initial incorporation of CO2 into organic material.
carbon fixation
The calvin cycle is divided into three phases: __, __, __
carbon fixation
reduction
regeneration of the CO2 acceptor (RuBP)
Phase 1 of the Calvin cycle: __: the Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five carbon sugar named __. The enzyme that catalyzes this first step is __, or __. (This is the most abundant protein in chloroplasts and is also said to be the most abundant protein on Earth.) The product of the reaction is a __- carbon intermediate so unstable that it immediately splits in half, forming two molecules of __ (for each CO2 fixed).
Carbon fixation
ribulose biphosphate (RuBP)
RuBP carboxylase
rubisco
6
3-phosphoglycerate
Phase 2 of the Calvin cycle: __
Each molecule of __ receives an
additional phosphate group from ATP, becoming __. Next, a pair of electrons donated from __ reduces __, which also loses a phosphate group, becoming __. Specifically, the electrons from __ reduce a __ on __ to the __ group of __, which stores more potential energy. __ is a sugar- the same three- carbon sugar fromed in glycolysis by the splitting of glucose. (For every __ molecules of CO2 that enter the cycle, there are __ molecules of __ formed.) But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate. The cycle began with __ worth of carbohydrate in the form of three molecules of the five-carbon sugar __. Now there are __ carbons' worth of carbohydrate in the form of __ molecules of__. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of __.
Reduction
3-phosphoglycerate
1,3- bisphosphoglycerate
NADP
1,3-bisphosphoglycerate
G3P
NADPH
carboxyl
1,3 - bisphosphoglycerate
aldehyde
G3P
G3P
three
six
G3P
15
RuBP
18
six
G3P
RuBP
Phase 3 of the Calvin cycle: __.
In a complex series of reaction, the carbon skeletons __ molecules of __ are rearranged by the last steps of the Calvin cycle into three molecules of __. To accomplish this, the cycle spends three more molecules of __. The __ is now prepared to receive CO2 again, and the cycle continues.
Regeneration of the CO2 acceptor (RuBP)
5
G3P
RuBP
ATP
RuBP
Fo the net synthesis of one G3P molecule, the Calvin cycle consumes a total of __ molecules of ATP and __ molecules of NADPH. THe __ regenerate the ATP and NADPH.
9
6
light reactions
The __ spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.
Neither the light reactions nor the Calvin cycle alone can make sugar from CO2.
__ is an emergent property of the intact chloroplast, which integrates the two stages of photosynthesis.
G3P
Photosynthesis
An important example of tradeoffs in plants is __.
the compromise between photosynthesis and the prevention of excessive water loss from the plant
The CO2 required for photosynthesis enters a leaf via __, the pores through the leaf surface. However, they are also the main avenues of __, the evaporative loss of water from leaves.
stomata
transpiration
What is a wasteful technique that plants use?
photorespiration
In most plants, initial fixation of carbon occurs via __. Such plants are called __ because the first organic product of carbon fixation is a three-carbon compound, __.
C3 plants
3-phosphoglycerate
Rice, wheat, and soybeans are __ that are important in agriculture. When their stomata partially close on hot, dry days, __ plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle. In addition, __ can bind O2 in place of CO2. As CO2 becomes scarce within the air spaces of the leaf, __ adds O2 to the Calvin cycle instead of CO2. The product splits, and a two-carbon compound leaves hte chloroplasts. __ and __ rearrange and split this compound, releasing CO2. The process is __ because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration). However, unlike normal cell resp., __ generates no __; in fact, it consumes it. And unlike photosynthesis, __ produces no sugar. In fact, _ decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing CO2 that would otherwise be fixed.
C3 plants x2
rubisco x2
peroxisomes
mito.
photorespiration x2
ATP
photorespiration
__ is evolutionary baggage- a metabolic relic from a much earlier time when the atmosphere had less O2 and more CO2 that it does today. In the ancient atmosphere that prevailed when rubisco first evolved, the inability of the eenzyme's active site to exclude O2 would have made little dierece. The hypothesis suggest that modern __ retains some of its chance affinity for O2, which is now so concentrated in the atmosphere that a certain amount of __ is inevitable.
photorespiration
rubisco
photorespiration
__, in some cases, plays a protective role in plants.Plants that are impaired in their ability to carry out __ are more susceptible to damage induced by excess light.
__ acts to neutralize the otherwise damaging products of the __, which build up when a low CO2 concentration limits the progress of the __.
photorespiration x3
light reactions
Calvin cycle
In many plants, __ drains away as much as 50% of the carbon fixed by the __.
Asheterotrophs that depend on __ in chloroplasts for food, __is wasteful.
photorespiration
Calvin cycle
carbon fixation
photorespiration
__ and __ are two important photosynthetic adaptions that fix carbon in different ways.
CAM
C4
__ are so named because they preface the Calvin cycle with an alternate mode of __ that forms a four- carbon compound as its first product. (Sugarcane and corn)
C4
carbon fixation
A unique leaf anatomy is correlated with the mechanism of __. In __ plants, there are two distinct typles of photosynthetic cells: __ and __.
C4 photosyn.
C4
bundle-sheath cells
mesophyll cells
__ are arranged into tightly packed sheaths around the veins of the leaf.
bundle- sheath cells
Between the bundle sheath and the leaf surface are more loosely arranged __.
mesophyll cells.
The Calvin cycle is confined to the chloroplasts of the __. However the cycle is preceded by incorportaion of CO2 into organic compounds in the __.
bundle-sheath cells
mesophyll cells
What is the C4 pathway?
1- In mesophyll cells, the enzyme PEP carboxylase adds carbon dioxide to PEP
2- After the C4 plant fixes carbon from CO2, the mesophyll cells export their four-carbon compound (conveys the atoms of the CO2) into a bundle-sheath cell via plasmodesmata.
3-In bundle-sheath cells, CO2 (by four-carbon compound-oxaloacetate) is released and enters the CC.
__ is an enzyme tht adds CO2 to phosphoenolpyruvate (PEP), forming the four carbon product oxaloacetate; has a much higher affinity for CO2 than does rubisco and no affinity for O2 and can fix carbon efficiently when rubisco cannot- that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to fall and O2 cincentration to ruse.
PEP carboxylase
The same reaction regenerates __, which is transported to mesophyll cells. There, ATP is used to convert __ to __, allowing the reaction cycle to continue; this ATP can be thought of as the 'price' of concentrating CO2 in the __.
To generate this extra ATP, __ carry out __. In fact these cells contain __ but no __, so __ is their only photosynthetic mode of generating ATP.
pyruvate
pyruvate
PEP
bundle-sheath cells
cyclic electron flow
PS I
PS II
cyc. electron flow
In effect, the __ of a C4 plant pump CO2 into the __, keeping the CO2 concentration in the __ high enough for __ to bind carbon dioxide rather than oxygen. The cyclic series of reactions involving __ and the regeneration of __ can be thought of as a CO2- concentrating pump that is powered by ATP.
C4 photosyn. minimizes __ and enhances sugar production. The adaptation is especially advantageous in hot regions with intense sunlight, where stomata partially close during the day.
mesophyll cells
bundle sheath
bundle-sheath cells
rubisco
PEP carboxylase
PEP
photorespiration
An adaption that has evolved in many succulent (water-storing) plants)- pineapple, cacti, etc.
CAM plants
WHat do CAM plants do?
open their stomata during the night and close them during the day
- at night they take in CO2 and incorporate it into a variety of organic acids
The mesophyll cells of __ store the organic acids they make during the night in their vacuoles until morning, when the stomata close. During the day, when the light reactions can supply __ and _ for the Calvin cycle, CO2 is released from the organic acids made the night before to become incorporated into sugar in the chloroplasts.
CAM plants
ATP
NADPH
Compare and contrast the CAM and C4 plants.
Similar: carbon dioxide is first incorporated into organic intermediates before it enters the Calvin cycle.
Difference: C4- Initial steps of carbon fixation are separated structurally from the Calvin cycle
CAM: two steps occur at separate times, but w/in same cell.
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