Photosynthesis Energy Conversion 5

Question 1

What evidence shows that the oxygen released during photosynthesis comes from water rather than CO2?

Accepted Answer

Isotope labeling with heavy oxygen (18O) in H2O produced 18O2 gas

Answer

When plants were supplied with H218O, the 18O appeared in the O2 released; when 18O was in CO2, only unlabeled O2 was produced, proving water is the electron donor.

Question 2

How does the redox potential (Em) difference between water oxidation and NADP+ reduction determine the minimum photon energy needed?

Accepted Answer

The Em gap of ~1.14 V means each photon must supply at least that much energy to drive electrons uphill

Answer

Electrons must be driven from the low-energy oxidized water (+0.82 V) to NADP+ (−0.32 V), a span of ~1.14 V, setting the minimum energy that photons absorbed by PSII and PSI must collectively provide.

Question 3

In CAM plants, how is the temporal separation of carboxylation and decarboxylation related to energy conversion?

Accepted Answer

CO2 is fixed at night into malate, stored in vacuoles, then released by day for the Calvin cycle, reducing water loss without sacrificing carbon fixation efficiency

Answer

CAM plants open stomata at night (lower water loss) to fix CO2 into malate, which is stored until daylight, when decarboxylation releases CO2 directly to rubisco inside closed stomata.

Question 4

What limits photosynthesis under very high light intensities (photoinhibition)?

Accepted Answer

Excess excitation energy damages the D1 protein of PSII, reducing electron transfer efficiency

Answer

At light saturation, over-excitation of PSII generates reactive oxygen species that damage the D1 reaction-center protein, causing photoinhibition that reduces the quantum yield of O2 evolution.

Question 5

Why is the synthesis of one glucose molecule said to require approximately 48 photons?

Accepted Answer

Fixing 6 CO2 needs 12 turns of the Calvin cycle, each requiring 4 photons (2 per photosystem × 2 photosystems)

Answer

Each CO2 fixed requires ~8 photons (4 for PSII + 4 for PSI) in non-cyclic flow; fixing 6 CO2 for one glucose therefore demands ~48 photons, consistent with measured quantum requirements.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Energy Conversion 5