Photosynthesis – Photorespiration: NEET MCQs with Answers

Explanation: RuBisCO is the most abundant enzyme in the world, and photorespiration does not occur in C₄ plants because their CO₂-concentrating mechanism keeps RuBisCO working as a carboxylase. Hence both statements are correct.

Explanation: When RuBisCO binds O₂ instead of CO₂, RuBP is broken into one molecule of the three-carbon phosphoglycerate — the only product that can re-enter the Calvin cycle — together with a two-carbon phosphoglycolate. Two molecules of a 3-C compound would be formed only by carboxylation.

Explanation: Photorespiration is the oxidation of a photosynthetic intermediate that yields no ATP or NADH₂ and fixes no CO₂. The substrate of the photorespiratory (C₂) cycle proper is the 2-carbon compound glycolic acid (glycolate), which is formed from phosphoglycolate after RuBP is oxygenated by RuBisCO.

Explanation: Photorespiration is light-induced oxidation of photosynthetic intermediates. It is stimulated whenever the O₂ : CO₂ ratio at the RuBisCO active site is high — that is, high oxygen concentration, low CO₂ concentration, high light intensity, high temperature and ageing of the leaf.

Explanation: Photorespiration is characteristic of C₃ plants, not C₄ plants. Reported by Dicker and Tio (1959) in tobacco, it is a light-dependent oxygenation of RuBP in which O₂ is consumed and CO₂ released. C₄ plants avoid it by using the Hatch–Slack pathway to concentrate CO₂ around RuBisCO.

Explanation: There are two oxygen-consuming steps. The first is in the chloroplast stroma, where RuBP takes up O₂ under the oxygenase activity of RuBisCO. The second is in the peroxisome, where the glycolate transferred from the chloroplast is oxidised using O₂ to form glyoxylate, releasing H₂O₂ as a by-product.

Explanation: The higher photosynthetic rate of C₄ plants is due mainly to their negligible photorespiration. The C₄ pump concentrates CO₂ in the bundle sheath cells, so RuBisCO there operates almost entirely as a carboxylase. In C₃ plants excess O₂ converts RuBP carboxylase into RuBP oxygenase, raising glycolate synthesis and wasting fixed carbon.

Explanation: Photorespiration is a metabolic pathway spanning three organelles. Oxygenation of RuBP begins in the chloroplast; the glycolate produced moves to the peroxisome where it is oxidised to glyoxylate and then transaminated to glycine; glycine then enters the mitochondrion where CO₂ is released. Mnemonic: Cells Produce Magic — Chloroplast, Peroxisome, Mitochondria.

Explanation: Both C₃ and C₄ plants operate the Calvin cycle, glycolysis and respiration. What distinguishes them is photorespiration: it causes a continuous wasteful loss of already-fixed carbon in C₃ plants, whereas C₄ plants, by virtue of Kranz anatomy and the C₄ pump, largely avoid it.

Explanation: When RuBisCO binds O₂ instead of CO₂, RuBP is cleaved into one molecule of 3-carbon PGA (phosphoglycerate) and one molecule of 2-carbon phosphoglycolate. The 2-carbon product is later salvaged and converted back to PGA through the photorespiratory cycle.

Explanation: Statement I is correct: RuBisCO has a greater affinity for CO₂, but when O₂ concentration is relatively high the enzyme acts as an oxygenase, so net CO₂ fixation falls. Statement II is wrong on both counts — mesophyll cells of C₄ plants show no photorespiration at all because they lack RuBisCO, and bundle sheath cells also avoid it because Kranz anatomy keeps CO₂ concentrated around RuBisCO.

Explanation: Both groups photosynthesise, transpire and respire, so none of those distinguishes them. What separates them is that C₃ plants lose a substantial part of their fixed carbon through photorespiration while C₄ plants have effectively eliminated it, which is why C₄ plants perform better in hot bright conditions.

Explanation: C₄ plants largely avoid photorespiration because their CO₂-concentrating mechanism keeps CO₂ high around RuBisCO in the bundle sheath. Photorespiration occurs in light, is characteristic of C₃ plants and begins in the chloroplast.

Explanation: Glycolate formed in the chloroplast passes to the peroxisome, where it is oxidised to glyoxylate and transaminated to glycine; the glycine moves to the mitochondrion, where two molecules condense to form serine with the release of carbon dioxide. The serine then returns through the peroxisome to the chloroplast.

Explanation: Oxygen is consumed twice: first in the chloroplast stroma when RuBisCO oxygenates RuBP, and again in the peroxisome when glycolate oxidase converts glycolate to glyoxylate with the production of hydrogen peroxide. The mitochondrial step releases carbon dioxide but consumes no oxygen.