Question

Which one of the following options represents the products of light reactions of photosynthesis in plants?

• A. \(O_2\)/NADH/\(H_2\)
• B. \(O_2\)/NADP\(^+\)/\(CO_2\)
• C. \(O_2\)/NADPH/ATP
• D. \(O_2\)/NADP\(^+\)/ATP

Hint:

Match each product to its source: water splitting gives \(O_2\), electron transport reduces NADP+ to NADPH, and the proton gradient makes ATP.

Answer 1

The Correct Option is C

Step 1: Recall what the light reactions of photosynthesis actually do.
The light reactions take place in the thylakoid membrane of the chloroplast. Light energy captured by photosystems II and I drives the splitting of water and the flow of electrons down an electron transport chain.

Step 2: Track each product formed.
Splitting water (photolysis) at photosystem II releases oxygen as a byproduct: \(2H_2O \rightarrow O_2 + 4H^+ + 4e^-\). The electrons that flow down the chain, together with protons, are used at the end of the chain to reduce \(NADP^+\) to \(NADPH\) by the enzyme ferredoxin-NADP+ reductase. The proton gradient built across the thylakoid membrane during electron transport drives ATP synthase, which converts \(ADP + P_i\) into \(ATP\), a process called photophosphorylation.

Step 3: Confirm the three products.
So the three outputs of the light reactions are \(O_2\), \(NADPH\), and \(ATP\). These two energy carriers, \(NADPH\) and \(ATP\), are exactly what the Calvin cycle (the dark/light-independent reactions) needs to fix \(CO_2\) into sugar.

Step 4: Rule out the other options.
Option (A) lists \(NADH\) and \(H_2\), but photosynthesis reduces \(NADP^+\) to \(NADPH\), not \(NAD^+\) to \(NADH\), and no free \(H_2\) is released. Option (B) lists \(NADP^+\) and \(CO_2\) as products, but \(NADP^+\) is a reactant that gets reduced, and \(CO_2\) is consumed (not produced) during the Calvin cycle, not the light reactions. Option (D) keeps \(NADP^+\) instead of its reduced form \(NADPH\), which is the actual usable product carried forward to the Calvin cycle.

Final Answer:
The light reactions produce \(O_2\), \(NADPH\), and \(ATP\). \[ \boxed{O_2 / NADPH / ATP} \]