Q33 — Xanthophyll Cycle and Photoprotection

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Xanthophylls are yellow-orange colored and fat-soluble photoprotective pigments. This is likely an adaptation to the unforgiving sunlight in dry ecosystems. As excess light leads to the accumulation of protons in the thylakoid’s lumen, this lower pH triggers a transformation of precursors (Pre-Z) into zeaxanthin (Z). All of them are xanthophylls.

When protonated, Z changes the conformation of photosystem II to dissipate extra light energy as heat. Reduced ascorbate is a cofactor for de-epoxidase, the enzyme that catalyzes the Pre-Z → Z reactions.

Diagram of the thylakoid membrane showing the stroma, lumen, epoxidase and de-epoxidase enzymes, photosystem II, ascorbate, and the xanthophyll-cycle interconversion between Pre-Z and zeaxanthin. Figure 1. A diagram of the thylakoid membrane, where S – stroma, L – lumen, E – epoxidase, DE – de-epoxidase, PSII – photosystem II, C6H7O6- – ascorbate, red wavy arrows – heat, and green arrows with the plus sign – activation.

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

A. Sun-tolerant plants with the deleted de-epoxidase gene will exhibit shade preference instead.

B. A temperate steppe shrub population will have higher xanthophyll leaf content as compared to a forest understory shrub of the same species.

C. The hot and sunny subtropical climate would lead to higher total xanthophyll concentrations in thylakoids compared to a colder sunny climate.

D. If you have Vitamin C/ascorbate deficiency, you should eat leaves from the top of the canopy rather than the bottom.

← Previous: Q32 Next: Q34 →

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)