Photosynthesis Energy Conversion 4

Question 1

Which thermodynamic principle explains why photosynthesis requires continuous light energy input?

Accepted Answer

The Second Law of Thermodynamics — converting low-entropy light into high-entropy heat is spontaneous, but the reverse requires energy input

Answer

Photosynthesis builds ordered, energy-rich molecules (glucose) from disordered CO2 and H2O, a decrease in entropy that requires sustained energy input to drive the unfavorable reaction.

Question 2

At what point in the electron transport chain of thylakoids is the proton gradient across the membrane established?

Accepted Answer

At the cytochrome b6f complex and during water splitting on the lumenal side

Answer

Protons accumulate in the thylakoid lumen from water oxidation (2H2O → O2 + 4H+ + 4e−) and from plastoquinol oxidation at the cytochrome b6f complex, building the gradient that drives ATP synthase.

Question 3

Why does isolated spinach chloroplast thylakoid membrane fluoresce red light when illuminated in vitro?

Accepted Answer

Because chlorophyll molecules that cannot transfer energy to a reaction center re-emit photons at longer wavelengths

Answer

When electron transfer is disrupted (e.g., by solvent extraction), chlorophyll cannot pass excitation energy onward, so it relaxes by emitting red fluorescence (~680–740 nm).

Question 4

What is the Emerson enhancement effect, and what does it reveal about photosynthesis?

Accepted Answer

Using two light wavelengths simultaneously produces more O2 than either alone predicts, proving two photosystems work cooperatively

Answer

Emerson found that simultaneous illumination with far-red (>680 nm) and shorter-wavelength light yielded more O2 than either alone, providing key evidence for the two-photosystem model (PSI + PSII).

Question 5

In C4 plants, how does spatial separation of reactions improve energy conversion efficiency?

Accepted Answer

CO2 is concentrated around rubisco in bundle-sheath cells, suppressing photorespiration

Answer

C4 plants fix CO2 into C4 acids in mesophyll cells, then release concentrated CO2 around rubisco in bundle-sheath cells, minimizing the oxygenase reaction and the energy wasted in photorespiration.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Energy Conversion 4