Photosynthesis MCQs for NEET — Botany Questions with Answers

Photorespiration in C3 plants starts with the formation of phosphoglycolate from ribulose-1,5-bisphosphate (RuBP) when oxygen instead of carbon dioxide binds to the enzyme RuBP carboxylase/oxygenase (RuBisCO). This phosphoglycolate is then dephosphorylated to glycolate, initiating the photorespiratory pathway.

The Hill reaction is a light-driven oxidation of water that occurs in the presence of an artificial electron acceptor like potassium ferricyanide or benzoquinone. It was discovered by Robert Hill in 1937 and is used to study the light-dependent reactions of photosynthesis in isolation.

Cyclic photophosphorylation is a process pf photophosphorylation in which an electron expelled by the excited photocentre is returned to it after passing through a series of electron carriers. Cyclic photophosphorylation is performed by photosystem I only. The electron is circulated with in the photosystem and the phosphorylation occurs due to cyclic flow of electron. The excited electron does not pass on to NADP⁺ but it is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H⁺.

Clavin pathway occurs in all photosynthetic plants which is CO₂ fixation cycle. Carbon dioxide combines with ribulose- 1, 5, biphosphate (RuBp) to produce a transient intermediate compound. The intermediate compound splits up immediately in the presence of water to form the two molecules of 3-phosphoglycerate or 3-PGA. It is the first stable product of photosynthesis.

Photosystem II mostly occurs in the appressed regions of granal thylakoids.

C4 plants are more efficient than C3 plants because they have little photorespiration while in C3 plants more than half of photosynthetic carbon gets lost in photorespiration. C4 plants are more efficient in picking up CO₂, even in low CO₂ concentration because of high affinity of CO₂ with PEP (Phosphoenol pyruvic acid).

Photorespiration is the light dependent process of oxygenation of ribulose biphosphate (RuBP) and release of carbon dioxide by photosynthetic organs of a plant. Normally photosynthetic organs do the reverse in the light, i.e., uptake of CO₂ and relase of O₂. At high temperature, RuBP carboxylase functions as oxygenase and instead of fixing carbon dioxide (C₃ cycle), oxidises ribulose 1, 5-biphosphate to produce a 3-carbon phosphoglyceric acid and a 2 carbon phosphoglycolate. 

Photorespiration does not produce energy or reducing power. Rather, it consumes energy. Further, it undergoes the work of photosynthesis. There is 25% loss of fixed CO₂. Therefore, photorespiration is a highly wasteful process. This happens only in case of C₃ plants. C₄ plants have overcome the problem of photorespiration. 

The C₄ plants are adapted to dry tropical regions and have greater productivity of biomass. They have special type of leaf anatomy known as Kranz anatomy. In this type of anatomy the bundle sheath celss form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. 

In C₄ plants, initial fixation of carbon dioxise occurs in mesophyll cells. The primary acceptor of CO₂ i.e., phosphoenol pyruvate (PEP) combines with CO₂ in presence of PEP carboxylase or PEPcase to form oxaloacetic acid, which is the first stable carbon dioxide fixation product in C₄ pathway.

Like other alkali metals, potassium hydroxide combines with CO₂ as such :-

2KOH + CO₂ → K₂CO₃ + H₂O