Question

A scientist is using a species of green algae to study the electron transport chain in photosynthesis. He uses a laser to inactivate all of chlorophyll A in the algae but leaves chlorophyll B intact (case 1). In a separate experiment, the scientist applies a chemical to the algae that inhibit PS I but do not affect PS II (case 2).
Which of the following results is expected?
A)in case 1, only PS I completely loses function; in case 2, no proton gradient is produced.
B)In case 1, only PS I completely loses function; in case 2, a proton gradient is still produced.
C)In case 1, both PS I and PS II completely lose function; in case 2, no proton gradient is produced.
D)In case 1, both PS I and PS II completely lose function; in case 2, a proton gradient is still produced.

Answer 1

Answer:

D) In case 1, both PS I and PS II completely lose function; in case 2, a proton gradient is still produced.

Explanation:

The light dependent reaction of photosynthesis, which produces the ATP and NADPH needed in the light independent stage of the process, includes complexes of proteins and pigments called PHOTOSYSTEMS. These photosystems (I and II) are key to the functionality of the light dependent reactions in the thylakoid.

The major pigment present in both photosystems is CHLOROPHYLL A, which absorbs light energy and transfers electrons to the reaction center. Chlorophyll B is only an accessory pigment meaning it can be done without. Hence, if all of the chlorophyll A is inactivated in the algae but leaves chlorophyll B intact as in case 1, both PS I and PS II will lose their function because Chlorophyll A is the major pigment that absorbs light energy in both photosystems.

In case 2, if PS I is inhibited and PS II is unaffected, a PROTON GRADIENT WILL STILL BE PRODUCED because the splitting of water into protons (H+) and electrons (e-) occurs in PSII. Hence, H+ ions can still be pumped into the inner membrane of the thylakoid in order to build a proton gradient even without the occurrence of PS I.