Photorespiration Which Competes with Photosynthesis Is Reduced in Plants That Fix CO2 by the C4 Pathway

Photosynthesis is always accompanied by photorespiration—a process that takes place in light, consumes O2, and converts ribulose 1,5-bisphosphate in part to CO2. As Figure 8-43 shows, rubisco catalyzes two competing reactions: the addition of CO 2 to ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate and the addition of O2 to form one molecule of 3-phosphoglycerate and one molecule of the two-carbon compound phosphoglycolate. Photorespiration is wasteful to the energy economy of the plant: it consumes ATP and O2, and it generates CO2. It is surprising, therefore, that all known rubiscos catalyze photorespiration. Probably the necessary structure of the active site of rubisco precluded evolution of an enzyme that does not catalyze photorespiration.

In a hot, dry environment, plants must keep the gasexchange pores (stomata) in their leaves closed much of the time to prevent excessive loss of moisture. This causes the CO 2 level inside the leaf to fall below the Km of rubisco for CO 2. Under these conditions, the rate of photosynthesis is slowed and photorespiration is greatly favored. Corn, sugar cane, crabgrass, and other plants that can grow in hot, dry environments have evolved a way to avoid this problem by utilizing a two-step pathway of CO2 fixation in which a CO 2-hoarding step precedes the Calvin cycle. The pathway has been named the C4 pathway because [14C] CO2 labeling showed that the first radioactive molecules formed during photosynthesis in this pathway are four-carbon compounds, such as oxaloacetate and malate, rather than the three-carbon molecules that begin the Calvin cycle (C3 pathway).

The C4 pathway involves two types of cells: mesophyll cells, which are adjacent to the air spaces in the leaf interior, and bundle sheath cells, which surround the vascular tissue (Figure 8-44a). In the mesophyll cells of C4 plants, phospho-enolpyruvate, a three-carbon molecule derived from pyru-vate, reacts with CO2 to generate oxaloacetate, a four-

Vascular bundle

M FIGURE 8-44 Leaf anatomy of C4 plants and the C4 pathway. (a) In C4 plants, bundle sheath cells line the vascular bundles containing the xylem and phloem. Mesophyll cells, which are adjacent to the substomal air spaces, can assimilate CO2 into four-carbon molecules at low ambient CO2 and deliver it to the interior bundle sheath cells. Bundle sheath cells contain abundant chloroplasts and are the sites of photosynthesis and sucrose synthesis. Sucrose is carried to the rest of the plant via the phloem. In C3 plants, which lack bundle sheath cells, the Calvin cycle operates in the mesophyll cells to fix CO2. (b) The key enzyme in the C4 pathway is phosphoenolpyruvate carboxylase, which assimilates CO2 to form oxaloacetate in mesophyll cells. Decarboxylation of malate or other C4 intermediates in bundle sheath cells releases CO2, which enters the standard Calvin cycle (see Figure 8-42, top).

carbon compound. The enzyme that catalyzes this reaction, phosphoenolpyruvate carboxylase, is found almost exclusively in C4 plants and unlike rubisco is insensitive to O2. The overall reaction from pyruvate to oxaloacetate involves the hydrolysis of one phosphoanhydride bond in ATP and has a negative AG. Therefore, CO2 fixation will proceed even when the CO2 concentration is low. The oxaloacetate formed in mesophyll cells is reduced to malate, which is transferred, by a special transporter, to the bundle sheath cells, where the CO2 is released by decarboxylation and enters the Calvin cycle (Figure 8-44b).

Because of the transport of CO2 from mesophyll cells, the CO 2 concentration in the bundle sheath cells of C4 plants is much higher than it is in the normal atmosphere. Bundle sheath cells are also unusual in that they lack PSII and carry out only cyclic electron flow catalyzed by PSI, so no O2 is evolved. The high CO2 and reduced O2 concentrations in the bundle sheath cells favor the fixation of CO2 by rubisco to

▲ FIGURE 8-45 Schematic diagrams of the two vascular systems—xylem and phloem—in higher plants, showing the transport of water (blue) and sucrose (red). (a) Water and salts enter the xylem through the roots. Water is lost by evaporation, mainly through the leaves, creating a suction pressure that draws the water and dissolved salts upward through the xylem. The phloem is used to conduct dissolved sucrose, produced in the leaves, to other parts of the plant. (b) Enlarged view illustrates the mechanism of sucrose flow in a higher plant. Sucrose is actively transported from mesophyll cells into companion cells, form 3-phosphoglycerate and inhibit the utilization of ribu-lose 1,5-bisphosphate in photorespiration.

In contrast, the high O2 concentration in the atmosphere favors photorespiration in the mesophyll cells of C3 plants (pathway 2 in Figure 8-43); as a result, as much as 50 percent of the carbon fixed by rubisco may be reoxidized to CO2 in C3 plants. C4 plants are superior to C3 plants in utilizing the available CO 2, since the C4 enzyme phosphoenolpyruvate carboxylase has a higher affinity for CO2 than does rubisco in the Calvin cycle. However, one phosphodiester bond of ATP is consumed in the cyclic C4 process (to generate phosphoenolpyruvate from pyruvate); thus the overall efficiency of the photosynthetic production of sugars from NADPH and ATP is lower than it is in C3 plants, which use only the Calvin cycle for CO2 fixation. Nonetheless, the net rates of photosynthesis for C4 grasses, such as corn or sugar cane, can be two to three times the rates for otherwise similar C3 grasses, such as wheat, rice, or oats, owing to the elimination of losses from photorespiration.

and then moves through plasmodesmata into the sieve-tube cells that constitute the phloem vessels. The resulting increase in osmotic pressure within the phloem causes water carried in xylem vessels to enter the phloem by osmotic flow. Root cells and other nonphotosynthetic cells remove sucrose from the phloem by active transport and metabolize it. This lowers the osmotic pressure in the phloem, causing water to exit the phloem. These differences in osmotic pressure in the phloem between the source and the sink of sucrose provide the force that drives sucrose through the phloem.

Loss of water by transpiration

Upward water movement in acellular xylem

Absorption by root cells

Photosynthesis CO2

Photosynthesis CO2

Sucrose and water movement in cellular phloem

Sucrose and water movement in cellular phloem

Upward water movement in acellular xylem

Absorption by root cells

High osmotic^

pressure

Low osmotic pressure

High osmotic^

pressure

Low osmotic pressure

Mesophyll cell (photosynthetic source of sucrose)

Chloroplast

Companion cell

• Plasmodesmata

Sieve plate Sieve-tube cell

Root cells ("sink" of sucrose)

Phloem vessel

Xylem vessel

Phloem vessel

Mesophyll cell (photosynthetic source of sucrose)

Chloroplast

Companion cell

• Plasmodesmata

Sieve plate Sieve-tube cell

Root cells ("sink" of sucrose)

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