The two steps in photosynthesis are the light-dependent reactions and the Calvin cycle.

Question

Which of the following is true regarding these two steps?
a
The light-dependent reactions use ATP from the Calvin cycle, and the Calvin cycle uses energy from absorbed sunlight.
b
The light-dependent reactions take place in the thylakoid membrane, and the Calvin cycle takes place in the stroma.
c
The Calvin cycle converts water molecules into oxygen gas as a byproduct of its reactions.
d
During the light-dependent reactions, carbon dioxide is fixed to produce sugars that form glucose.

Answer 1

The correct statement regarding the two steps of photosynthesis is:

b: The light-dependent reactions take place in the thylakoid membrane, and the Calvin cycle takes place in the stroma.

• The light-dependent reactions indeed occur in the thylakoid membranes of the chloroplasts, where light energy is converted into chemical energy (ATP and NADPH).
• The Calvin cycle, also known as the light-independent reactions, occurs in the stroma of the chloroplasts, where carbon dioxide is fixed to produce sugars.

The other options are incorrect for the following reasons:

• a: This is inaccurate; the light-dependent reactions produce ATP and NADPH, which are used by the Calvin cycle.
• c: This is incorrect; the Calvin cycle does not convert water into oxygen gas; oxygen is produced as a byproduct of the light-dependent reactions when water is split.
• d: This statement is false; during the light-dependent reactions, carbon dioxide does not get fixed. Instead, it is during the Calvin cycle that carbon dioxide is fixed to produce sugars.