USABO Plant Physiology and Photosynthesis

Question 1

Which photosystem is responsible for the oxidation of water molecules during the light-dependent reactions of photosynthesis?

Accepted Answer

Photosystem II (P680)

Answer

Photosystem II (P680) catalyzes the light-driven oxidation of water, releasing O2, protons, and electrons that enter the electron transport chain.

Question 2

What is the primary CO2 acceptor molecule in the Calvin cycle that is carboxylated by Rubisco?

Accepted Answer

Ribulose-1,5-bisphosphate (RuBP)

Answer

RuBP is the 5-carbon acceptor molecule that Rubisco carboxylates with CO2 to form two molecules of 3-PGA.

Question 3

In C4 plants, which cell type performs initial CO2 fixation, and which enzyme catalyzes this reaction?

Accepted Answer

Mesophyll cells; PEP carboxylase

Answer

C4 plants use PEP carboxylase in mesophyll cells to fix CO2 into oxaloacetate, concentrating CO2 in bundle sheath cells to suppress photorespiration.

Question 4

Which two products are generated by non-cyclic photophosphorylation (the Z-scheme) in photosynthesis?

Accepted Answer

ATP and NADPH

Answer

Non-cyclic photophosphorylation generates both ATP (via chemiosmosis) and NADPH (via NADP+ reduction at PSI), with water oxidized to release O2.

Question 5

Photorespiration reduces photosynthetic efficiency primarily because it:

Accepted Answer

Releases previously fixed CO2 via the glycolate pathway

Answer

When Rubisco oxygenates RuBP, the products enter the glycolate pathway in peroxisomes and mitochondria, releasing CO2 and consuming ATP without net carbon gain.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO Plant Physiology and Photosynthesis