NEET Practice Questions (MCQs) with Answers & Solutions

Elimination of a source of CO₂ would block the action of RUBISCO, which required CO₂ as a subtrate. There would be no further production of 3-phosphoglycerate (3PG).

The remaining reactions of the Clavin Cycle are not block by the absence of CO₂. Intermediates in the cyclic pathway would be converted to ribulose-1,5-bisphosphate (RuBP) and the level of RuBP would rise. 

Read page 223, XI NCERT

Read page 222, XI NCERT

The proton gradient will lead to chemiosmosis and phosphorylation of ADP into ATP. 

Page 222, XI NCERT

In Clavin cycle, CO₂ acceptor molecule is RuBP or RuDP. The enzyme catalyzing this reaction is RuBP carboxylase/oxygenase (Rubisco). As Clavin cycle takes in only one carbon (as CO₂) at a time, so it takes six turns of the cycle to produce a net gain of six carbons )i.e., hexose or glucose). In this cycle, for formation of one mole of hexose sugar (Glucose), 18 ATP and 12 NADPH₂ are used. For 6 molecules of CO₂ it needs 18 ATP and 12 NADPH₂ , molecules so for one molecule of CO₂ it needs 3 ATP and 2 NADPH₂ molecules. 

Phosphoenol Pyruvate (PEP) is found in the mesophyll cell, which accepts the atmospheric CO₂ in C₄ -plants and converts it to oxaloacetate — a C₄ compound. It is the first stable compound of C₄ -plants.

Concept Enhancer C₄ -plants possess special adaptation anatomy in their leaves to cope up the photorespiratory losses. There are dimorphic chloroplast present in them—agranal in bundle sheath cells and granal in mesophyll cells.

In C₄ -plants, the initial fixation of CO₂ occurs in mesophyll cells. The primary acceptor of CO₂ is Phosphoenol Pyruvate (PEP). It combines with CO₂ in the presence of enzyme PEP carboxylase to form the first stable product, i.e., oxaloacetic Acid (OAA). C₃ plants lack PEP_caboxylase enzyme. They posses RuBisCO enzyme. This enzyme can work as both carboxylase (fixation of CO₂) and oxygenase (fixation of O₂). RuBisCO has much greater affinity for CO₂ than for O₂ and the binding is competitive. At higher temperature, its ffinity for CO₂ decrease and it works as oxygenase.

Therefore, at higher temperature photosynthesis decrease in C₃ -plants, while in C₄ -plants increases. 

Photorespiration is the process which makes a different between the C₃ and C₄ plants. In this process, there is a continuous loss of carbon fixed in the form of CO₂. It occurs due to the high O₂ content, high temperature conditions in which RuBP carboxylase starts working as RuBP oxygenate and normal photosynthesis does not occur. 

Diffusion of water vapour and CO₂ are independent process. Their diffusion depends on the difference in their partial pressure in the atmosphere as well as inside the leaves.