Optimizing Carbon Fixation in C4 Plants

Pathway where carbon dioxide captured by phosphoenolpyruvate is not re-released?

The pathway where carbon dioxide captured by phosphoenolpyruvate (PEP) is not re-released is called the C4 pathway or C4 photosynthesis. C4 plants have evolved this pathway to improve their carbon fixation efficiency in environments with high temperatures and limited water availability.

In C4 plants, such as corn, sugarcane, and some grasses, the initial carbon fixation step occurs in the mesophyll cells of the leaf through the enzyme PEP carboxylase. Phosphoenolpyruvate (PEP) is a three-carbon molecule that captures CO2 and forms a four-carbon compound oxaloacetate (OAA) in the presence of PEP carboxylase. This reaction takes place in specialized anatomical structures called bundle sheath cells

Unlike in C3 plants, where the enzyme responsible for capturing CO2 is RuBisCO, C4 plants have an additional set of reactions that prevent the release of CO2 during the initial carbon fixation step. The four-carbon compound OAA is then converted into malate or aspartate, which can be shuttled into the bundle sheath cells

Within the bundle sheath cells, malate or aspartate is decarboxylated, releasing the CO2 that was originally captured by PEP carboxylase. This CO2 is then used in the Calvin cycle, where it is assimilated into sugars in a high concentration of CO2, resulting in a more efficient carbon fixation process

The C4 pathway essentially spatially separates the initial carbon fixation and the Calvin cycle, enabling the plants to concentrate and efficiently use CO2 in high temperatures and low CO2 environments. This adaptation allows C4 plants to minimize water loss through open stomata since they do not need to keep them open as long as C3 plants to capture sufficient CO2

In summary, the C4 pathway is a specialized carbon fixation strategy in which carbon dioxide captured by phosphoenolpyruvate (PEP) is not re-released. This pathway allows certain plants to optimize their photosynthetic efficiency under conditions of high temperature and limited water availability

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