USABO Plant Physiology and Photosynthesis 3

Question 1

Rubisco is described as having 'dual catalytic specificity.' Which statement correctly describes this property?

Accepted Answer

It can catalyze both carboxylation and oxygenation of RuBP

Answer

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes carboxylation of RuBP producing 3-PGA for the Calvin cycle, and oxygenation of RuBP initiating the wasteful photorespiration pathway.

Question 2

In non-cyclic photophosphorylation, which molecule serves as the ultimate electron donor?

Accepted Answer

Water (H2O)

Answer

Water is oxidized at the oxygen-evolving complex of PS II, donating electrons that flow through the entire photosynthetic electron transport chain to ultimately reduce NADP+.

Question 3

How does cyclic photophosphorylation differ from non-cyclic photophosphorylation?

Accepted Answer

It produces only ATP without net generation of NADPH or O2

Answer

In cyclic photophosphorylation, electrons from excited PS I cycle back through the cytochrome b6f complex to generate a proton gradient for ATP synthesis, but no NADP+ is reduced and no water is split.

Question 4

How many molecules of CO2 must be fixed by the Calvin cycle to produce one net molecule of glucose (C6H12O6)?

Accepted Answer

6

Answer

Six CO2 molecules are fixed (one per carbon in glucose), requiring 18 ATP and 12 NADPH in three full turns of the Calvin cycle to produce one glucose molecule.

Question 5

The P680 designation for the Photosystem II reaction center chlorophyll refers to what property?

Accepted Answer

It absorbs light most efficiently at a wavelength of 680 nm

Answer

P680 denotes the special pair of chlorophyll a molecules in the PS II reaction center whose absorption maximum lies at 680 nm, making it the highest-energy reaction center in oxygenic photosynthesis.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO Plant Physiology and Photosynthesis 3