C4 Pathway & Photorespiration - NEET MCQ

When CO₂ is added to PEP, the first stable product synthesised is oxaloacetate.The beginning of this process takes place in the mitochondrial matrix, where pyruvate molecules are found. A pyruvate molecule is carboxylated by a pyruvate carboxylase enzyme, activated by a molecule each of ATP and water. This reaction results in the formation of oxaloacetate. NADH reduces oxaloacetate to malate.

PEP carboxylase catalyzes carboxylation of phosphoenolpyruvate (PEP, the primary acceptor of CO₂) to form four carbon compound oxaloacetate in mesophyll cells of C₄ plants. Initial carboxylation reaction of C₄ cycle takes place in mesophyll cell wherein oxaloacetate is converted into the intermediate malate. Malate is transported to bundle-sheath cell and is decarboxylated to produce pyruvate and CO₂. Rubisco present in bundle sheath cells then carries out fixation of released carbon dioxide. Maize is a C4 with characteristic kranz anatomy consisting of mesophyll and bundle sheath cells. Thus, the correct answer is option A.

Kranz anatomy occurs in the chloroplast of C4 plants such as sugarcane, maize (Zea mays), sorghum etc. It has two types of photosynthetic cell arranged in a specific manner. These are bundle sheath cells that surround the vascular centres, and mesophyll cells that, in turn, surround the bundle sheath cells. Rubisco is localised only within internalised bundle sheath cells which helps in fixation of carbon dioxide.
So, the correct answer is option B.

• The first step of the Calvin pathway is the carbon dioxide fixation step.
• The carbon dioxide acceptor molecule RuBP combines with carbon dioxide to give 2 molecules of 3PGA with RuBisCO as the catalyst.

This process occurs when the concentration of oxygen increase and carbon dioxide decrease and its substrate is glycolate. RubisCO, catalase (CAT) and GOX, and GDC are the main enzymes in chloroplasts, leaf peroxisomes, and mitochondria from mature leaves, respectively, and support the major role of the photorespiratory C₂ cycle in leaf metabolism.
So the correct option is 'Mitochondria, peroxisomes and chloroplasts'.

The leaves of C₄ plants have a characteristic anatomy consisting of concentric bundle sheath and mesophyll cell layers around the vascular bundles. These two distinct cell layers possess chloroplasts which have either structural or size dimorphism.

Photorespiration is a biochemical process in plants in which, especially under conditions of water stress, oxygen inhibits the Calvin cycle, the carbon fixation portion of photosynthesis. The three organelles participates in photorespiration are chloroplast, mitochondria and peroxisomes. Thus, option A is correct.
Organelles such as vacuoles, Golgi bodies, rough endoplasmic reticulum and dictyosomes does not involve in photorespiration. Thus, other options are wrong.

• The given reaction is that of the first step of the Calvin cycle, carbon dioxide fixation.
• RuBP combines with carbon dioxide to form two molecules of 3PGA. RuBisCO is an enzyme that catalyses the reaction.

The Bundle sheath cells are rich in RuBisCo while lacks PEPCase.

- RuBisCo: Enzyme responsible for carbon fixation in the Calvin cycle.
- PEPCase: Enzyme involved in C4 photosynthesis, found in mesophyll cells.

In C4 plants, Bundle sheath cells possess RuBisCo for the Calvin cycle, while PEPCase is located in mesophyll cells where initial carbon fixation occurs. This spatial separation enhances efficiency by concentrating CO2 for RuBisCo in Bundle sheath cells.