Question

PEP cases have an advantage compared to rubisco. The advantage is that
A. Rubisco fixes carbon dioxide only in $C_4$ plants, but PEP does it both in $C_3$ and $C_4$ plants
B. PEP case is present in both mesophyll and bundle cells, but rubisco is not
C. Rubisco is subject to photorespiration, but PEPcase is not
D. PEPcase conserves energy but rubisco does not

Answer 1

Hint: Phosphoenolpyruvate carboxylase, an acronym for which is PEPCase, is an important enzyme in $C_4$ plants. In $C_4$ and CAM metabolism, a photosynthetic form is present as both a dimer and a tetramer. Non-photosynthetic PEPcs are also found in comparison.

Complete answer:
In hot and dry conditions, when the plant closes or partially closes its stomata to decrease water loss from the leaves, carbon dioxide concentrations within the leaf fall. Under these circumstances, photorespiration is likely to occur in plants that use Rubisco as the primary carboxylation enzyme, because when the concentrations of carbon dioxide are low, Rubisco adds oxygen to RuBP. However, PEP carboxylase does not use oxygen as a substrate, and it has a greater carbon dioxide affinity than Rubisco does.

Thus, in decreased carbon dioxide conditions, it has the potential to fix carbon dioxide, such as when the stomata on the leaves are only partially open. As a result, when compared to $C_3$ plants, $C_4$ plants lose less water at similar photosynthesis rates. In dry and warm conditions, this explains why $C_4$ plants are preferred.

Thus, Like RUBISCO, carbon dioxide is 'fixed' onto an organic molecule by PEPCase. PEPCase, unlike RUBISCO, does not react to oxygen. As can be seen from its lower Km value, it also has a greater affinity for $CO_2$ than RUBISCO does.

Hence, the correct answer is option (C)

Note: To join the $C_3$ or the Calvin cycle, $C_4$ plants are plants that cycle carbon dioxide to 4-carbon sugar compounds. In climatic conditions that are hot and dry and generate a lot of energy, the $C_4$ plants are very productive. Grain, sorghum, sugarcane, millet, and switchgrass are examples of $C_4$ species. $C_4$ anatomical and biochemical adaptations, however, require extra plant energy and resources than $C_3$ photosynthesis, so $C_3$ plants are usually more photosynthetically effective and active in cooler environments.