Question

what best describes the relationship between the two sets of reactions of photosynthesis, which are the light - dependent reactions and the light - independent reactions?
a. each set produces sugars, and by working together they produce more sugars than they would produce separately.
b. each set produces half of a sugar molecule, and later the two halves are joined together.
c. only the light - dependent reactions produce sugars, but they depend on products of the light - independent reactions.
d. only the light - independent reactions produce sugars, but they depend on products of the light - dependent reactions.

Explanation:

To solve this, we analyze photosynthesis reactions:

• Light - dependent reactions (LDR) produce ATP, NADPH, and \(O_2\) (no sugars).
• Light - independent reactions (Calvin cycle, LIR) use ATP, NADPH (from LDR) and \(CO_2\) to make sugars (glucose). So LIR depends on LDR’s products, and only LIR makes sugars.

Now, evaluate options:

• Option A: LDR doesn’t make sugars. Eliminate.
• Option B: Neither set “produces half a sugar molecule” in this way. Eliminate.
• Option C: LDR doesn’t make sugars; LIR depends on LDR’s products. Eliminate.
• Option D: Correct. LIR makes sugars and needs LDR’s products (ATP, NADPH).

Answer:

D. Only the light - independent reactions produce sugars, but they depend on products of the light - dependent reactions.