Question

some photosynthetic bacteria (e.g., purple sulfur bacteria) have only photosystem i, whereas others (e.g., cyanobacteria) have both photosystem i and photosystem ii. no photosynthetic bacteria have only photosystem ii. which of the following might be a possible explanation for this observation?

both photosystems evolved together, and photosystem ii was selected against in some species.

photosynthesis with only photosystem i is more ancestral.

photosystem i is more efficient at photosynthesis than both photosystems i and ii together.

linear electron flow is more primitive than cyclic flow of electrons.

Explanation:

• Option 1: If both photosystems evolved together, it's not clear why only photosystem II would be selected against in some species without a strong selective pressure explanation.
• Option 2: If photosynthesis with only photosystem I is more ancestral, it makes sense that some bacteria (like purple sulfur bacteria) retained the ancestral state (only photosystem I), while others (cyanobacteria) evolved to have an additional photosystem (photosystem II).
• Option 3: Photosystem I alone is not more efficient than both photosystems I and II together. Linear electron flow (which uses both photosystems in plants) produces more ATP and NADPH compared to cyclic electron flow (which mainly involves photosystem I).
• Option 4: Cyclic electron flow (associated more with photosystem I) is considered more primitive than linear electron flow (which involves both photosystems in plants).

Answer:

Photosynthesis with only photosystem I is more ancestral.