Test: C4 pathway & Photorespiration (June 21) - 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.

Stomata usually open when leaves are transferred from darkness to light. However, reverse-phase stomatal opening in succulent plants has been known. CAM plants such as cacti and Opuntia ficus–indica achieve their high water use efficiency by opening their stomata during the cool, desert nights and closing them during the hot, dry days. Signal transduction pathway for stomatal opening by blue light photoreceptors including phototropins and the carotenoid pigment zeaxanthin has been suggested. Blue light regulated signal transduction pathway on stomatal opening could not be applied to CAM plants, but the most possible theory for a nocturnal response of stomata in CAM plants is photoperiodic circadian rhythm.

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.

One key biochemical innovation of agaves and cacti is the Crassulacean Acid Metabolism (CAM), a water-efficient type of photosynthesis, which allows plants to fix carbon dioxide during the night, in this way preventing evaporation during the day.