Figure 5.12 Noncyclic photophosphorylation.
the use of NADP instead of NAD as in respiration. NADP is used preferentially when the reducing power is to be used for synthesizing compounds, not just for transporting electrons.) The sequence of events takes about 5 ms. The overall reaction for noncyclic photo-phosphorylation is
12H2O + 48 hv + 12 NADP + 12 ADP + 12 P,- ) 6O2 + 12NADPH2 + 12 ATP
Equation (5.55), as written, involves two electrons, each of which requires two photons. The protons produced by photolysis of water, plus additional protons pumped by the electron transport system (not shown), wind up in the thylakoid lumen. The che-miosmotic gradient across the thylakoid membrane can be as much as 3 pH units, with an electric potential of up to 100 mV. About one ATP is produced per NADPH2. Overall, then, noncyclic photophosphorylation requires four photons to produce one ATP and one NADPH2.
Noncyclic photophosphorylation produces NADPH2 and ATP in about equal amounts; but glucose production requires additional ATP, and the cell has other uses for ATP as well. To get more ATP, plants are able to exploit cyclic photophosphorylation, similar to bacteria, which does not produce oxygen or NADPH2. It does this by a sort of "short circuit'' that shunts electrons excited by photosystem I back to the electron transport chain of photosystem II (see the dotted line in Figure 5.12). The process is controlled by NADPH2 levels; high levels hinder the flow of electrons out of photosystem I, forcing them into the other pathway. Thus, the cell can control somewhat independently the relative amounts of NADPH2 and ATP that it produces.
The energy of a mole of photons, E, is related to the frequency, v (or wavelength, l) by hc
l where h is Planck's constant and c is the speed of light. The two peak wavelengths shown in Figure 5.11 are 430 and 650 nm. The corresponding energy levels are 67 and 43.5 kcal/ mol, respectively. Four moles of photons of light energy must be absorbed by each of the two photosystems (eight photons in all) to produce one molecule of O2, two of NADPH2, and two of ATP. To produce a six-carbon sugar by the dark reactions, the cell will need 12 NADPH2 and 18 ATP. This can be satisfied most efficiently by 48 photons in noncyclic photophosphorylation (making 12NADPH2 and 12ATP) and 12 photons in cyclic photo-phosphorylation (six more ATP), for a total of 60 photons. Assuming that a conservative 40 kcal/mol of photons gives a total of 2400 kcal/mol glucose formed since the standard Gibbs free energy of formation of glucose is 686 kcal/mol, the potential efficiency is 29% (based on photons absorbed). The actual efficiency is less, due largely to a wasteful side reaction called photorespiration, described below.
Dark Reactions The conversion of CO2 to glucose is called CO2 fixation. It is accomplished by two pathways. The first is a series of 12 reactions called the Calvin cycle (Figure 5.13). The cycle starts in the stroma of the chloroplast when CO2 combines with a pentose (five-carbon sugar) to form two three-carbon acids:
ribulose-1,5biphosphate + CO2 ) 2,3-phosphoglycerate (5.57)
Note that this step may be considered to be the actual point of carbon fixation. Yet it does not require energy from light or even ATP. But ATP and the reducing agent NADPH2 are needed to ensure a supply of the ribulose to keep this reaction going. Several
subsequent reactions use two NADPH2 and two ATP to produce phosphoglyceraldehyde (PGAL, or glyceraldehyde-3-phosphate). Most of the PGAL goes on to be converted back into the ribulose, to keep the cycle going. This requires another ATP.
Two of every 12 PGALs formed are converted to another triose, which leaves the chloroplast for the cytoplasm. There they can be converted to glucose or other carbohydrates in the second pathway. The enzymes for forming glucose are the same as the ones in glyco-lysis, acting in reverse. In fact, PGAL is one of the key intermediates in glycolysis. PGAL can also be converted into glycerol and fatty acids for fats, or into amino acids for proteins. Overall, since each turn of the cycle incorporates a single CO2, it takes six cycles to produce one molecule of glucose. The overall stoichiometry for the Calvin cycle is
6CO2 + 12NADPH2 + 18 ATP ) C6H12O6 + 12NADP + 18 ADP + 18 P. + 6 H2O
Strangely, most plants have a wasteful side reaction, called photorespiration, which uses the same enzyme as reaction (5.57). The reaction occurs during hot, dry conditions when leaf pores close to conserve water. CO2 is depleted inside the leaf and O2 builds up. The oxygen competes with the CO2 for the enzyme, producing a side reaction with the ribulose-1,5 biphosphate. Waste products are formed instead of carbohydrates, and ATP is consumed. There seems to be no benefit to the plant. This is a major drain on biological productivity for the world's plants and can reduce the net efficiency of photosynthesis below 1%.
A minority of plants (about 100 species are known) have developed a mechanism to tilt the balance back toward normal CO2 fixation. This has occurred in plants that grow in hot, arid regions. Closing the stomates to limit water loss also limits CO2 entry to the leaf. In these plants, instead of forming the three-carbon phosphoglycerate in reaction (5.57), cells near the outside of the leaf use a different reaction that forms the four-carbon oxaloace-tate, which is then reduced by NADPH2 to the four-carbon malate. The malate diffuses to other cells in the leaf interior, where it reacts to form pyruvate, CO2, and NADPH2. This has the net effect of transporting CO2 and NADPH2 to the inner cells at a higher concentration, where they enter the Calvin cycle. Twelve additional ATPs are used per glucose, but the net efficiency increases because photorespiration is relatively low. Plants that do this include the important food crops sorghum, corn, and sugarcane. Because they form a four-carbon product with CO2, they are called C4 plants. The more common plants without this ability are called C3 plants.
Actual maximum photosynthetic efficiencies of the C4 crops sugarcane (Saccharum officinale), sorghum (Sorghum vulgare), and corn (Zea mays) have been measured to range from 2.5 to 3.2%; for the C3 crops alfalfa, sugar beet, and the alga Chlorella, the range is 1.4 to 1.9%. C3 plants can fix 15 to 40 mg of CO2 per dm2 of leaf surface per hour, while C4 plants can fix 40 to 80 mg/dm2 per hour. C4 plants lose much less water by transpiration than do C3 plants. The optimum daytime temperature for growth of C4 plants is higher: 30° to 35°C vs. 20° to 25°C for C3 plants. Examples of C3 crop grasses are wheat (Triticum aestivum), rye (Secale cereale), oats (Avena sativa), and rice (Oryza sativa).
A third strategy for photosynthesis is carried out by succulents such as the jade plant and some cacti. These are called CAM (for ''crassulacean acid metabolism'') plants. CAM plants fix carbon at night with the stomata (pores in the leaves) open. During the heat of the day the stomata close, and the Calvin cycle obtains its carbon from the nighttime storage. Note that C4 and CAM plants have all the photosynthetic machinery of C3 plants: namely, the photosystems and the Calvin cycle. However, they have additional mechanisms that enable them to utilize CO2 more efficiently.
The difference between C3 and C4 plants has caused concern about the effect of the global CO2 increase that is under way. Fossil fuel consumption has caused CO2 in the atmosphere to increase from 280 ppmv in preindustrial times to 356 ppmv in 1993. C3 plants are more sensitive to low CO2 levels. Experimental results have verified that they can benefit more from an increase than C4 plants. This has the potential to cause ecological changes as the competitive balance between plant species changes. It is also feared that some of the C4 food plants could face increased competition from C3 weeds. On the other hand, the lawn grasses Kentucky Bluegrass (Poa pratensis) and creeping bent (Agrostis tenuis) are C3 plants, whereas crabgrass (Digitaria sanguinalis) is a C4 plant, ordinarily giving it an advantage during hot, dry summers.
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