Chapter 8: Photosynthesis

synthesis of NADPH and ATP

second main process of light-dependent reactions occurs on the thylakoid membrane electrons obtained from the splitting of water are used for the synthesis of NADPH e- pass through an electron transfer system consisting of a series of electron carriers that are arranged in a chain e- carriers include the same types that act in mitochondrial electron transfer—cytochromes, quinones, and iron-sulfur centers e- as carrier for energy (bc excited) and e- carrier to carry e- - e- derived from water flow through phytostem II, becoming excited in P680 - then transfer to primary acceptor - e- flows "downhill" through plasto-quionene pool into chyrochrome complex - as e- flow here, they lose free energy and generate a H+ gradient across the membrane - then pass to photo stem I via plastocyanan (e- carrier) - excited again - passed to NADP+ (final acceptor) to be reduced to NADPH (by the enzyme NADP+ reductase) overall electron transfer system and chemiosmosis generate ATP and NADHP (+ H+) - to be used in light-independent reactions @ storm A GOOD video of this in engage: 8.2 "Electron flow from water..." - video title: Noncyclic pathway of electron flow

stomata

small pores in the surface of the leaves (particularly the undersurface) and stems CO2 needed for photosynthesis enters here O2 and H2O produced exits rom here ** side: water and minerals required for photosynthesis are absorbed by the roots and transported to cells containing chloroplasts through tubular conducting cells

photophosphorylation

synthesis of ATP coupled to the transfer of electrons energized by photons of light analogous to oxidative phosphorylation in mitochondria - except that in chloroplasts light provides the energy for establishing the proton gradient

energy flow

As the pathway of energy flows from the Sun >> plants (primary producers) >> animals >> decomposers the organic molecules made by photosynthesis are broken down into inorganic molecules again, the chemical energy captured in photosynthesis is released as heat energy. Because the reactions capturing light energy are the first step in this pathway, photosynthesis is the vital link between the energy of sunlight and the vast majority of living organisms.

CAM plants

CAM = crassulacean acid metabolism carbon dioxide is taken up and stored during the night to allow the stomata to remain closed during the daytime, decreasing water loss live in regions that are hot and dry during the day and cool at night stomata on their fleshy leaves or stems open only at night to minimize water loss - when open, O2 generated by photosynthesis is released and CO2 enters include succulents, cactus family, lily family, and orchid family

chloroplast structure

Chloroplasts from individual algal and plant groups differ in structural details chloroplasts of plants and green algae are formed from three membranes that enclose three compartments inside the organelles - outer membrane covers the entire surface of the organelle. - inner membrane lies just inside the outer membrane. - intermembrane compartment between the outer and inner membranes stroma: fluid within the compartment formed by the inner membrane thylakoids: Within the stroma is the third membrane system, the thylakoid membranes, which form flattened, closed sacs - these as within the inner membrane thylakoid lumen: space enclosed within the thylakoids

electron transfer system in mitochondria vs chloroplasts

Comparing the linear pathway with the mitochondrial electron transfer system reveals that the pathway from the plastoquinones through plastocyanin in chloroplasts is essentially the same as the pathway from the ubiquinones through cytochrome c in mitochondria. The similarities between the two pathways indicate that the electron transfer system is an ancient evolutionary development that became adapted to both photosynthesis and oxidative phosphorylation.

summary of e- flow from H2O

Electrons derived from splitting water are used for the synthesis of NADPH and ATP. The electrons flow first through photosystem II, and then pass through part of the electron transfer system to photosystem I releasing energy that is used to create an H+ gradient across the membrane. flow of electrons through the electron transfer system leads to the generation of an H+ gradient across the inner membrane >> synthesis of ATP from ADP and by the ATP synthase

How many molecules of carbon dioxide must enter the Calvin cycle for a plant to produce a sugar containing 12 carbon atoms? How many ATP and NADPH molecules would be required to make that molecule?

For each carbon atom that is released from the Calvin cycle in a carbohydrate molecule, one carbon dioxide molecule must enter the cycle. Therefore, to produce a molecule containing 12 carbon atoms, 12 molecules of carbon dioxide must enter the cycle

How are carbon fixation and the Calvin cycle different in plants and CAM plants?

In C4 plants, carbon fixation and the Calvin cycle occur in different cell types, mesophyll cells and bundle sheath cells, respectively. In CAM plants, carbon fixation and the Calvin cycle occur at different times, at night and during the day, respectively.

comparing carbon fixation processes

In each case, carbon fixation produces the four-carbon oxaloacetate, which is processed to generate CO2 that feeds into the Calvin cycle (C3). In C3 plants, carbon fixation takes place in the mesophyll cells In C4 plants, carbon fixation and the Calvin cycle occur in different cell types: - carbon fixation by the C4 pathway takes place in mesophyll cells, while the - Calvin cycle takes place in bundle sheath cells. In CAM plants, carbon fixation and the Calvin cycle occur at different times (but both in mesophyll cells): - carbon fixation by the C4 pathway takes place at night, - the Calvin cycle takes place during the day.

sites of photosynthesis

In eukaryotes, the photosynthetic reactions take place in the chloroplasts of plants and algae. In cyanobacteria, the reactions are distributed between the plasma membrane and the cytosol.

In which organelle does photosynthesis take place in plants? Where in that organelle are the two stages of photosynthesis carried out?

In plants, photosynthesis takes place in the chloroplast. The light-dependent reactions are carried out on the thylakoid membranes and stromal lamellae. The light-independent reactions are carried out in the stroma.

What is the C4 pathway, and how does it enable C4 plants to circumvent photorespiration?

In the C4 pathway, carbon fixation involves the reaction of CO2 with phosphoenolpyruvate (PEP) to produce a four-carbon molecule, oxaloacetate. The oxaloacetate is reduced to malate by electrons transferred from NADPH, and malate then is oxidized to pyruvate in a reaction releasing CO2, which is used in the rubisco-catalyzed first step of the Calvin cycle. In plants, carbon fixation and the Calvin cycle occur in different cell types: carbon fixation in mesophyll cells, and the Calvin cycle in bundle sheath cells. This alternative method of carbon fixation minimizes photorespiration.

What is the difference between the linear electron flow pathway and the cyclic electron flow pathway?

In the linear electron flow pathway, electrons run through the entire set of photosystems and electron carriers, producing both NADPH and ATP. In the cyclic electron flow pathway, electrons flow cyclically around photosystem I; photosystem II is not involved. The cycle of electrons is through the cytochrome complex and plastocyanin to photosystem I, to ferredoxin, but then back to the cytochrome complex rather than on to NDAP+ reductase. Only ATP is produced by this pathway.

light absorption by photostems

Light energy in the form of photons is absorbed by the pigment molecules of the antenna complex the absorbed light energy is conducted to the P680 or P700 molecules in the reaction center the absorbed light is converted to chemical energy when an excited electron from the special chlorophyll a molecule is transferred to a primary acceptor, which is also in the reaction center that high-energy electron is passed out of the reaction center and out of the photosystem to the electron transfer system

alternative methods for carbon fixation

Many plant species that live in hot, dry environments have evolved alternative processes of carbon fixation that minimize photorespiration and, therefore, its negative effect on photosynthesis

distribution of oxygen molecules in products of photosynthesis

O2 comes from the splitting of water demonstrated experimentally in 1941 by Samuel Ruben and Martin Kamen used a heavy isotope of oxygen, 18O, to trace the pathways of the atoms through photosynthesis. - A substance containing heavy 18O can be distinguished readily from the same substance containing the normal isotope, 16O. - When a photosynthetic organism was supplied with water containing 18O, the heavy isotope showed up in the given off in photosynthesis. - However, if the organisms were supplied with carbon dioxide containing 18O, the heavy isotope showed up in the carbohydrate and water molecules assembled during the reactions—but not in the oxygen gas ** H2O >> O2 ** CO2 >> glucose and H2O

who uses photosynthesis

PLANTS occurs also in some groups of bacteria. - photosynthetic electron transfer system components are embedded in membranes, but there are no chloroplasts like those of plants. Cyanobacteria, the only prokaryotes that produce oxygen by photosynthesis, contain both photosystems II and I and normally carry out the light-dependent reactions using the linear electron flow pathway. - are also capable of using the cyclic electron flow pathway involving photosystem I photosynthesis is also carried out by some archaea.

When does photorespiration occur? What are the reactions of photorespiration, and what are the energetic consequences of the process?

Photorespiration uses oxygen and releases CO2. It occurs when oxygen concentrations are high relative to CO2 concentrations. In that condition, rubisco acts as an oxygenase rather than a carboxylase, catalyzing the combination of RuBP with O2 rather than CO2. The toxic products formed by this reaction cannot be used in photosynthesis and are eliminated from the plant as CO2. Photorespiration uses energy to salvage the carbons from phosphoglycolate, which greatly reduces the efficiency of energy use in photosynthesis. This can be seen in the reduced growth of plants grown under photorespiration conditions.

What is the reaction that rubisco catalyzes? Why is rubisco the key enzyme for producing the world's food, and how is it the key regulatory site of the Calvin cycle?

Rubisco catalyzes a reaction combining carbon dioxide with RuBP to form two molecules of 3-phosphoglycerate (3PGA). Rubisco, an enzyme unique to photosynthetic organisms, is the key enzyme for producing the world's food because it is responsible for carbon dioxide fixation, a process that ultimately provides organic molecules for most of the world's organisms. Rubisco is the key regulatory site of the Calvin cycle for the following reason: - During the daytime, sunlight powers the lightdependent reactions, and the NADPH and ATP produced by those reactions stimulate rubisco, which, in turn, keeps the Calvin cycle running. - In darkness, however, NADPH and ATP levels are low, and - as a result, rubisco's activity is inhibited and the Calvin cycle slows down or stops.

Rubisco (RuBP)

Rubisco, RuBP carboxylase/oxygenase, is so named because it is both a carboxylase and an oxygenase - an enzyme that can bind either O2 or CO2 - when CO2 binds, carbon fixed (produces two 3PGA's in phase 1 of Calvin cycle) - when O2 binds, produces one 3PGA and a physophoglycolate 3PGA is used in the Calvin cycle within the chloroplast only in the carbolxylase reaction is carbon fixed in oxegynase reaction, phospohoglyclate is hydrolyzed to glycolate and releases a molecule of CO2 - so no carbon fixation // carbon is lost - this process called photorespiration rubisco has a ten-times greater affinity for CO2 than it does for O2 in oxygen reaction: carbon fixed by the Calvin cycle may be lost because only one three-carbon 3PGA molecule is produced instead of two

What is the difference in function between the chlorophyll a molecules in the antenna complexes and the chlorophyll a molecules in the reaction centers of the photosystems?

The chlorophyll a molecules in the antenna complexes are normal molecules of the pigment, consisting of a carbon ring structure with a magnesium atom bound at the center and an attached hydrophobic side chain. - These chlorophyll a pigments absorb light. The chlorophyll a molecules in the reaction centers have modified light absorption properties that result from interactions with particular proteins of the photosystems. The two special chlorophyll a molecules of photosystem II are P680; those of photosystem I are P700. These pigment molecules capture light energy from the antenna complex pigments in the form of an excited electron that is passed to a primary acceptor molecule. - That electron is passed to the electron transfer system.

How is NADPH made in the linear electron flow pathway?

The making of NADPH begins when electrons derived from water splitting are pushed to higher energies by light absorption in photosystem II. The high-energy electrons pass to a primary acceptor in photosystem II and then down an electron transfer system to P700 in photosystem I, losing energy along the way. Light energy absorbed by photosystem I again excites the electrons, which pass to different electron carriers, ending with ferredoxin. The ferredoxin transfers high-energy electrons to NADP+, which is reduced to NADPH by NADP+ reductase.

How are the reactants and products of photosynthesis and cellular respiration related?

The reactions of photosynthesis and cellular respiration are essentially the reverse of one another, with CO2 and O2 being the reactants of photosynthesis and the products of cellular respiration. Phosphorylation reactions involving electron transfer systems are part of each process, namely, photophosphorylation in photosynthesis and oxidative phosphorylation in cellular respiration. G3P is an intermediate in both pathways: in photosynthesis it is a product of the Calvin cycle, and in cellular respiration it is generated in glycolysis in the conversion of glucose to pyruvate. In photosynthesis, G3P is used for the synthesis of sugars and other fuel molecules, and in cellular respiration, it is part of the catabolism of sugars to simpler organic molecules.

What are the two stages of photosynthesis?

The two stages of photosynthesis are: 1. the lightdependent reactions, in which the energy of sunlight is absorbed and converted into chemical energy in the form of ATP and NADPH; and 2. the light-independent (dark) reactions, in which electrons carried by NADPH are used as a source of energy to convert carbon dioxide from inorganic to organic form.

cellular respiration vs photosytnesis in plants

are essentially the reverse of each other - the reactants of one are the products of the other and vice versa photosynthesis confined to tissues containing chloroplasts and cellular respiration taking place in all cells both have key phosphorylation reactions involving an electron transfer system - photophosphorylation in photosynthesis - oxidative phosphorylation in cellular respiration >> followed by the chemiosmotic synthesis of ATP G3P is found in the pathways of both processes - photosynthesis: it is a product of the Calvin cycle and is used for the synthesis of sugars and other organic fuel molecules (used by anabolic pathways) - cellular respiration: it is an intermediate generated in glycolysis in the conversion of glucose to pyruvate (a product of catabolic pathways)

2 types of photostems

carry out diff parts of light-dependent reactions Plants, green algae, and cyanobacteria have two types of these complexes photosystem II functions before photosystem I each with two closely associated components - an antenna complex (also called a light-harvesting complex): aggregate of many chlorophyll pigments and a number of carotenoid pigments that serves as the primary site of absorbing light energy in the form of photons - a reaction center: receives light energy absorbed by the antenna complex in the same photosystem reaction center contains special subsets of chlorophyll a molecules complexed with proteins - called P680

Calvin's experiments

combined 14C carbon and CO2 to trace pathways of light-independent reactions - done in green algae (Chlorella) cells - exposed actively photosynthesizing Chlorella cells to the labeled carbon dioxide - at various times, they removed cells and placed them in hot alcohol, which instantly stopped all the photosynthetic reactions of the algae - extracted radioactive carbohydrates from the cells and used two-dimensional paper chromatography to separate and to identify them chemically through this, were able to reconstruct the reactions of the Calvin cycle

light independent reactions

electrons carried from light-dependent reactions by NADPH provide the reducing power required to fix into carbohydrates and other organic molecules in the light-independent reactions takes place fully in storm ATP generated in the light-dependent reactions supplies additional energy for the light-independent reactions reactions using NADPH and ATP to fix are the Calvin cycle (light independent rxns) use CO2, ATP, and NADPH as inputs Three turns of the cycle results in the synthesis of enough G3P molecules so that one can be released to be used in carbohydrate synthesis For three input molecules of CO2, one of which is used in each of three turns of the cycle, the key product is one molecule of the three-carbon carbohydrate molecule glyceraldehyde-3-phosphate (G3P) 3 turns of the cycle (so 3 mol CO2) >> 1 molecule of the product (G3P) 3 phases: - carbon fixation - reduction - regeneration

photorespiration

entire process from the oxygenase reaction of rubisco to the release of CO2 - oxygenase (O2 binds RuPB) reaction produces one 3PGA and one phosphoglycolate - phosphoglycolate is hydrolyzed to its nonphosphorylated derivative, glycolate - broken down in the peroxisomes by reactions that release a molecule of CO2, which can then be used for carbon fixation no carbon is fixed during the oxygenase reaction and there is no carbon gain. That is, in the oxygenase reaction, CO2 is released, a net loss of carbon process that metabolizes a byproduct of photosynthesis extent to which photorespiration occurs in a plant depends on the relative concentrations of O2 and CO2 in the leaves RuBP favors CO2 BUT: at higher concentrations of O2 within the leaf, oxygen acts as a competitive inhibitor of the enzyme, and this favors the reaction of RuBP with O2 rather than with CO2: photorespiration occurs activity occurs more as temperatures rise due to the mechanism used to limit water loss from leaves - stomata close to reduce water loss, but now gas can't get in // CO2 used up For a plant with high photorespiration rates at elevated temperatures, as much as 50% of the carbon fixed by the Calvin cycle may be lost because only one three-carbon 3PGA molecule is produced instead of two

carbon fixation

first phase of the Calvin cycle use 9 ATP, 6 NADPH, and 6 CO2 - energy from light-dep rxns - CO2 from atm involves the cycle's key reaction in which each input molecule of CO2 is added to one molecule of ribulose 1,5-bisphosphate (RuBP) that is cleaved almost immediately to produce two three-carbon molecules of 3-phosphoglycerate (3PGA) This reaction, which fixes into organic form, is catalyzed by the carboxylase activity of the key enzyme of the Calvin cycle, RuBP carboxylase/oxygenase (abbreviated as rubisco). For three turns of the cycle, the three input molecules of (3 carbons) reacting with three molecules of RuBP (15 carbons) produce six molecules of 3PGA (18 carbons) Because the product of the carbon fixation reaction is a three-carbon molecule, the Calvin cycle is also called the pathway, and plants that initially fix carbon in this way are termed C3 plants. PRODUCT: 3PGA

linear electron flow

flow of electrons from H2O to NADP+ in step two of light-dependent reactions of photosynthesis occurs on the thylakoid membrane - e- derived from water flow through phytostem II, becoming excited in P680 - then transfer to primary acceptor - e- flows "downhill" through plasto-quionene pool into chyrochrome complex - as e- flow here, they lose free energy and generate a H+ gradient across the membrane (energy source for ATP synthesis) - then pass to photo stem I via plastocyanan (e- carrier) - excited again - passed to NADP+ (final acceptor) to be reduced to NADPH (by the enzyme NADP+ reductase) overall generates ATP and NADHP - one of each

G3P

formed by 3 turns of the Calvin cycle the starting point for the production of a wide variety of organic molecules More complex carbohydrates such as glucose and other monosaccharides are made from G3P by reactions that, in effect, reverse the first half of glycolysis EX: Glucose is a six-carbon sugar and G3P is a three-carbon sugar, which means that six turns of the Calvin cycle are needed for the synthesis of each molecule of glucose Once produced, the monosaccharides enter biochemical pathways that make disaccharides, polysaccharides, and the other complex carbohydrates found in cell walls. Other pathways manufacture amino acids, fatty acids and lipids, proteins, and nucleic acids. The reactions forming these products occur both within chloroplasts and in the surrounding cytosol and nucleus

C4 plants

have a characteristic leaf anatomy with: - photosynthetic mesophyll cells tightly associated with specialized, - chloroplast-rich bundle sheath cells, which encircle the veins of the leaf for comparison: in a C3 plant leaf, the bundle sheath cells have few chloroplasts and low concentrations of rubisco include crabgrass and several crops important agriculturally, including corn and sugarcane use the C4 pathway for carbon fixation

light absorption

light as form of radiant energy light is absorbed by molecules of green pigments called chlorophylls and yellow-orange pigments called carotenoids Light is absorbed in a pigment molecule by excitable electrons occupying certain energy levels in the atoms of the pigments If an electron in the pigment absorbs the energy of a photon, it jumps to a higher energy level that is farther from the atomic nucleus - ground > excited state after absorbing a photon, an e- in an atom can either: - return to ground state by exiting light - be transferred from the pigment molecule to a nearby e- accepting molecule - energy of the excited e- but not the e- itself is transferred to a neighboring pigment molecule (e- returns to ground state)

2 phases of photosynthesis

light dependent reactions (first) - energy of sunlight is absorbed and converted into chemical energy in the form of ATP and NADPH - ATP is the main energy source for plant cells (as it is for all types of living cells), and NADPH (nicotinamide adenine dinucleotide phosphate) carries electrons that are pushed to high energy levels by absorbed light light independent reactions - these electrons are used as a source of energy to convert inorganic CO2 to an organic form (CO2 fixation) - CO2 fixation is a reduction reaction; electrons (and protons - H+) are added to CO2 to create a carbohydrate (CH2O) Notice that the ATP and NADPH produced by the light-dependent reactions, along with , are the reactants of the light-independent reactions ADP produced by the light-independent reaction (along with H2O) are reactants for light dependent reactions light-dependent and light-independent reactions thus form a cycle in which the net inputs are H2O and CO2, and the net outputs are organic molecules and O2 each involve multiple reactions

light dependent reaction

light energy is converted to chemical energy, involve two main processes: 1. light absorption; and 2. synthesis of NADPH and ATP.

autotroph

organism that produces its own food using CO2 and other simple inorganic compounds from its environment and energy from the Sun or from oxidation of inorganic substances Autotrophs that use light as the energy source to make organic molecules by photosynthesis are called photoautotrophs. Consumers and decomposers, which need a source of organic molecules to survive, are called heterotrophs

reduction

phase 2 of Calvin cycle reactions raise the energy level of 3PGA by the addition of a phosphate group transferred from ATP and electrons from NADPH ) to produce G3P, another three-carbon molecule The ATP and NADPH used are products of the light-dependent reactions.) For three turns of the cycle, six molecules of 3PGA (18 carbons) produce six molecules of G3P (18 carbons). One of these G3P molecule exits the cycle as a net product of the three turns and is used as the primary building block for reactions synthesizing the six-carbon glucose and many other organic molecules in chloroplasts. The other five molecules of G3P are used to regenerate RuBP in the next phase of the cycle

regeneration

phase 3 of Calvin cycle: the G3P molecules generated by three turns of the cycle that do not exit the cycle are used to produce RuBP First, G3P enters a complex series of reactions that yields the five-carbon sugar ribulose 5-phosphate then, in the final reaction of the cycle, a phosphate group is transferred from ATP to regenerate the RuBP used in the first reaction. For three turns of the cycle, five molecules of G3P (15 carbons) produce three molecules of ribulose 5-phosphate (15 carbons) which then produce three molecules of RuBP (15 carbons)

cyclic electron flow

photosystem I works independently of photosystem II - to produce ONLY ATP (not NADPH or O2) electrons pass through the cytochrome complex and plastocyanin to the P700 chlorophyll a in the reaction center of photosystem I where they are excited by light energy electrons then flow from photosystem I to the mobile carrier ferredoxin, but rather than being used for NADP+ reduction by NADP+ reductase, they flow back to P700 in the cytochrome complex electrons again pass to plastocyanin and on to photosystem I where they receive another energy boost from light energy, and so the cycle continues - !!! means e- not coming from water and thus O2 not produced more H+ is pumped across the thylakoid membranes, driving ATP synthesis in the way already described for the linear (noncyclic) electron flow pathway net result of cyclic electron flow is that light energy is converted into the chemical energy of ATP without the production of NADPH or O2 researchers still debate whether it is a real physiological process

source of e- in photosynthesis

plants, algae, and one group of photosynthetic bacteria (the cyanobacteria), the source of electrons and protons for CO2 fixation is water (H2O), the most abundant substance on Earth. Oxygen generated from the splitting of the water molecule is released into the environment as a by-product of photosynthesis (splitting water) 2 H2O >> 4 H+ + 4 e- + O2 SO plants, algae, and cyanobacteria use three resources that are readily available—sunlight, water, and —to produce almost all the organic matter on Earth, and to supply the oxygen of our atmosphere (photosynthesis) 6 CO2 + 12 H2O >> C6H12O6 + 6 O2 + 6 H2O

rubisco

the enzyme that catalyzes the first reaction (CO2 fixation) of the Calvin cycle - unique to photosynthetic organisms eight copies each of a large and a small polypeptide, joined together in a 16-subunit structure stimulated by both NADPH and ATP; as long as these substances are available from the light-dependent reactions, the enzyme is active and the light-independent reactions proceed - During the daytime, when sunlight powers the light-dependent reactions, the abundant NADPH and ATP supplies keep the Calvin cycle running. - In darkness, when NADPH and ATP become unavailable, the enzyme is inhibited and the Calvin cycle slows or stops. - Similar controls based on the availability of ATP and NADPH also regulate the enzymes that catalyze other reactions of the Calvin cycle

chlorophyll

the major photosynthetic pigments in plants, green algae, and cyanobacteria absorb photons and transfer excited electrons to nearby electron-accepting molecules, the primary acceptor molecules chlorophyll is oxidized because it loses an electron, and the primary acceptor is reduced because it gains an electron carotenoids: accessory pigments that absorb light energy at a different wavelength than those absorbed by chlorophylls Chlorophylls and carotenoids are bound to proteins that are embedded in photosynthetic membranes main types of chlorophyll are chlorophyll a and chlorophyll b—they differ only in one side group that is attached to a carbon of the ring structure chlorophyll molecule contains a network of electrons capable of absorbing light - at the top cluster In all eukaryotic photosynthesizers, a specialized chlorophyll a molecule passes excited electrons to the primary acceptor. Other chlorophyll molecules, along with carotenoids, act as accessory pigments that pass their energy to chlorophyll a

C4 pathway

the pathway to fix CO2 into oxaloacetate in mesophyll cells and then produce for the Calvin cycle in bundle sheath cells - CO2 that has diffused into the leaf through the stomata initially is fixed in mesophyll cells by combining it with the three-carbon phosphoenolpyruvate (PEP) to produce the four-carbon oxaloacetate - oxaloacetate is then reduced to 4-carbon malate - malate diffuses into bundle sheath cells gets its name because its first product is a four-carbon molecule rather than a three-carbon molecule, as in the C3 pathway carbon fixation reaction producing oxaloacetate is catalyzed by PEP carboxylase - has a much greater affinity for CO2 than rubisco does and, unlike rubisco, it has no oxygenase activity has additional energy requirement - why not used by all plants - for each turn of the C4 pathway, one ATP molecule is hydrolyzed to regenerate PEP from pyruvate - adds an energy requirement of six ATP molecules for each G3P produced by the Calvin cycle hot environments typically receive a lot of sunshine. As a result, the additional ATP requirement can be met easily by increasing the output of the light-dependent reactions

photostems

the sites at which light is absorbed and converted into chemical energy large complex into which the lightabsorbing pigments for photosynthesis are organized with proteins and other molecules embedded in thylakoid membranes and stromal lamellae components: - antenna complex: aggregate of chlorophyll and carotenoid pigments - reaction center (in middle) - primary electron acceptor - pigment molecules (in antennal complex) 2 types: carry out diff parts of light-dependent reactions - photostem I - photostem II

CAM pathway

to fix CO2 into oxaloacetate and then produce CO2 for the Calvin cycle, - both occurring in mesophyll cells, but separated by time of day - CO2 is initially fixed to oxaloacetate in a reaction catalyzed by PEP carboxylase - The CO2 produced by the oxidation of malate is used in the rubisco-catalyzed first step of the Calvin cycle

thylakoids

within inner membrane of choloroplast - surrounded by stroma -light absorption by chlorophylls and carotenoids - electron transfer - ATP synthesis by ATP synthase thylakoid lumen within stack of thylakoids = a granum (grana plural) grana connected by stromal lamella - probably link the thylakoid lumens into a single continuous space within the stroma thylakoid membranes and stromal lamellae house the molecules that carry out the light-dependent reactions of photosynthesis, which include the pigments, electron transfer carriers, and ATP synthase enzymes for ATP production light-independent reactions are concentrated in the stroma