Question

The principal function of the light-dependent reactions of photosynthesis is to generate

• A. NADPH and ATP
• B. NADH and ATP
• C. FADH2 and ATP
• D. NADP and ATP

Hint:

The light reactions turn solar energy into two key chemical currencies: "ATP" (the energy source) and "NADPH" (the reducing power). These are then spent in the dark reactions to convert inorganic \(\text{CO}_2\) into glucose.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Photosynthesis is divided into light-dependent reactions (which occur on the thylakoid membranes) and light-independent reactions (Calvin-Benson cycle in the stroma).
The light-dependent reactions capture solar energy and convert it into stable chemical energy stored in molecular bonds.

Step 2: Detailed Explanation:
During the light-dependent reactions of photosynthesis:
1. Chlorophyll molecules absorb light energy, driving electrons from water molecules through Photosystem II, the Cytochrome \(b_6f\) complex, and Photosystem I.
2. The splitting of water (photolysis) releases oxygen (\(\text{O}_2\)) and generates a proton gradient across the thylakoid membrane.
3. This proton gradient drives the synthesis of ATP by the transmembrane ATP synthase complex (photophosphorylation).
4. Simultaneously, at the end of the electron transport chain, ferredoxin-NADP\(^+\) reductase (FNR) transfers electrons to \(\text{NADP}^+\) to form NADPH.
These two molecules, ATP (chemical energy) and NADPH (reducing power), serve as the vital inputs for the carbon-fixation reactions (Calvin cycle) to reduce carbon dioxide into sugars.
Other nucleotides like NADH and \(\text{FADH}_2\) are primary electron carriers in respiration (mitochondria), not photosynthesis.

Step 3: Final Answer:
The principal function of the light reactions is to generate NADPH and ATP, corresponding to option (A).