Botany MCQs for NEET — Practice Questions with Answers

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

CO₂ acceptor molecule in C4 plants is phosphoenol pyruvate (PEP). C4 plants are more efficient in picking up CO₂ even when it is found in low connection because of high affinity of PEP. PEP carboxylase (PEPcase) is the key enzyme which catalyses the primary fixation of CO₂. 

In photosystem (PS-I), the photocentre is a special chlorophyll-a molecule called P₇₀₀ . It is active in both red and far-red light, while a very small amount of special form of clorophyll-a called P₆₈₀ constitutes the reaction centre of photosystems-II (PS-II), which is inactive in far-red light (beyond 680 nm).

 C₄ plants have charasteristic leaf anatomy, called 'Kranz anatomy'. 'Kranz' means 'wreath' (ring) and is a reflection of the arrangement of cells. The large sized bundle sheath cells may form several layers around the vascular bundles. These cells are characterised by having a large number of a chloroplast, thick walls inpervious to gaseous exchange and no intercellular spaces. 

Plants that utilise primarily RuBP to fix CO₂ , which results in the formation of a 3 carbon compound (3PGA) are called C3 plants. Hatch and Slack proposed a new pathway in plants for CO₂ fixation via the carboxylation of PEP (Phosphoenol pyruvate). Because the products are 4 carbon compound, the plants are known as C4 plants. 

Certain important mineral elements like Mg, Fe, Mn, Cu, Cl, P are closely associated with key reactions of photosynthesis. Therefore deficiency of these minerals ultimately reduce photosynthesis.