CO2 Fixation in Tropical Plants
tropical plants, there is a C4 pathway (Figure 22.18), so named
because it involves four-carbon compounds. The operation of this pathway (also
called the Hatch–Slack pathway)
ultimately leads to the C3(based on 3-phosphoglycerate)pathway of
the Calvin cycle. (There are other C4 pathways, but this one is most
widely studied. Corn [maize] is an important example of a C4 plant,
and it is certainly not conÞned to the tropics.)
When CO2 enters the leaf through pores in the outer cells, it reacts Þrst with phosphoenolpyruvate to produce oxaloacetate and Pi in the mesophyll cells of the leaf. Oxaloacetate is reduced to malate, with the concomitant oxidation of NADPH. Malate is then transported to the bundle-sheath cells (the next layer) through channels that connect the two kinds of cells.
In the bundle-sheath cells, malate is decarboxylated to give pyruvate and CO2. In the process, NADP+ is reduced to NADPH (Figure 22.19). The CO2 reacts with ribulose-1,5-bisphosphate to enter the Calvin cycle. Pyruvate is transported back to the mesophyll cells, where it is phosphorylated to phos-phoenolpyruvate, which can react with CO2 to start another round of the C4 pathway. When pyruvate is phosphorylated, ATP is hydrolyzed to AMP and PPi. This situation represents a loss of two high-energy phosphate bonds, equivalent to the use of two ATP. Consequently, the C4 pathway requires two more ATP equivalents than the Calvin cycle alone for each CO2 incorporated into glucose. Even though more ATP is required for the C4 pathway than for the Calvin cycle, there is abundant light to produce the extra ATP by the light reaction of photosynthesis.
Note that the C4 pathway fixes CO2 in the mesophyll cells only to unfix it in the bundle-sheath cells, where CO2 then enters the C3 pathway. This observation raises the question of the advantage to tropical plants of using the C4 pathway.
The conventional wisdom on the subject focuses on the role of CO2, but there is more to the situation than that. According to the conventional view, the point of the C4 pathway is that it concentrates CO2 and, as a result, accelerates the process of photosynthesis. Leaves of tropical plants have small pores to minimize water loss, and these small pores decrease CO2 entry into the plant. Another point to consider is that the KM for CO2 of phosphoenolpyruvate carboxylase is lower than that of rubisco, allowing the outer mesophyll cells to fix CO2 at a lower concen-tration. This also increases the concentration gradient of CO2 across the leaf and facilitates the movement of CO2 into the leaf through the pores. In tropical areas, where there is abundant light, the amount of CO2 available to plants controls the rate of photosynthesis.
The C4 pathway deals with the situation, allowing tropical plants to grow more quickly and to produce more biomass per unit of leaf area than plants that use the C3 pathway. A more comprehensive view of the subject includes a consideration of the role of oxygen and the process of photorespiration, in which oxygen is used instead of CO2 during the reaction catalyzed by rubisco.
Although the actual biological role of photorespiration is not known, several points are well established. The oxygenase activity appears to be an unavoid-able, wasteful activity of rubisco. Photorespiration is a salvage pathway that saves some of the carbon that would be lost because of the oxygenase activity of rubisco. In fact, the photorespiration is essential to plants even though the plant pays the price in loss of ATP and reducing power; mutations that affect this pathway can be lethal. The principal substrate oxidized in photorespiration is glycolate (Figure 22.20). The product of the oxidation reaction, which takes place in peroxisomes of leaf cells, is glyoxylate. (Photorespiration is localized in peroxisomes.) Glycolate arises ultimately from the oxidative breakdown of ribulose-1,5-bisphosphate. The enzyme that catalyzes this reac-tion is ribulose-1,5-bisphosphate carboxylase/oxygenase, acting as an oxygen-ase (linked to O2) rather than as the carboxylase (linked to CO2) that fixes CO2 into 3-phosphoglycerate.
When levels of O2 are high compared with those of CO2, ribulose-1,5-bisphosphate is oxygenated to produce phosphoglycolate (which gives rise toglycolate) and 3-phosphoglycerate by photorespiration, rather than the two molecules of 3-phosphoglycerate that arise from the carboxylation reaction. This situation occurs in C3 plants. In C4 plants, the small pores decrease the entry not only of CO2 but also of O2 into the leaves. The ratio of CO2 to O2 in the bundle-sheath cells is relatively high as a result of the operation of the C4 pathway, favoring the carboxylation reaction. C4 plants have successfully reduced the oxygenase activity by compartmentation and thus have less need of photorespiration. This is an advantage in the hot climates in which C4 plants are principally found.
In tropical plants, four-carbon compounds are frequently involved in the pathway of CO2 fixation.
This alternative pathway facilitates the movement of CO2 into leaves and prevents water loss.