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← Theoretical A

Q31 — Photorespiration Pathway

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Calvin Cycle, Photorespiration & C4/CAM Biochemistry

During the Calvin Cycle, carbon dioxide is fixed by RuBisCo, an enzyme in plants. However, RuBisCo can also catalyze reactions with oxygen, which disrupts the photosynthesis.

Photorespiration is a process of recovering ribulose-1,5-bisphosphate (RuBP, the RuBisCo substrate) from products of RuBisCo oxygenation. Recent studies suggest it plays a significant role in NADH generation, managing the metabolic toxin 2-phosphoglycolate (2-PG), and connecting photosynthesis to nutrient assimilation and abiotic stress response pathways.

Schematic of the photorespiration pathway across the chloroplast, peroxisome, and mitochondrion, with enzymes color-coded by the effect of their overexpression on pathway rate (dark green speeds it up, orange has no effect, magenta slows it down). Figure 1. A schematic of photorespiration. When overexpressed, the enzyme in dark green (square) increases the rate of reactions, the enzyme in orange (stars) does not affect it, and the enzyme in magenta (circle) slows it down. GDC = Glycine decarboxylase. C = chloroplast, P = peroxisome, M = mitochondrion.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. If GDC was inhibited, the cell could still recover the RuBP lost to photorespiration through gluconeogenesis.
B. Rising atmospheric carbon dioxide alone should increase need for photorespiration.
C. If all shown peroxisome enzymes were overexpressed, photorespiration would take less time per cycle compared to the wild type.
D. C4 plants rely less on photorespiration than C3 plants.
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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)