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.

NCERT Topic: CO2 fixation

NCERT Line: "Experiments conducted over a wide range of plants led to the discovery of another group of plants, where the first stable product of CO2 fixation was again an organic acid, but one which had 4 carbon atoms in it. This acid was identified to be oxaloacetic acid or OAA."

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.

NCERT Topic: C4 plants

NCERT Line: "The primary CO₂ acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells."

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.

NCERT Topic: Kranz anatomy

NCERT Line: "The particularly large cells around the vascular bundles of the C plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz' anatomy."

• 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.

NCERT Topic: Carboxylation

NCERT Line: "Carboxylation is the fixation of CO2 into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA."

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'.

NCERT Topic: Chloroplast, mitochondrion and peroxisome

NCERT Line: "Chloroplast, mitochondrion and peroxisome."

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.

C4 plant does not show photorespiration.

NCERT Topic: Photorespiration

NCERT Line: "In C₄ plants photorespiration does not occur."

• 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.

NCERT Topic: The Calvin cycle

NCERT Line: "Carboxylation is the fixation of CO2 into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA."

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.

NCERT Topic: Bundle sheath cells

NCERT Line: "The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase."