Through which of the following substances the photosystem-I passes an electron to NADP during light reactions

Options

A. Cytochrome
B. Ferrodoxim
C. Plastocyanin
D. Plastoquinone

Answer

To answer the question regarding which substance the photosystem-I passes an electron to NADP during light reactions, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context of Photosynthesis**:
- Photosynthesis occurs in two main stages: light reactions and dark reactions (Calvin cycle). The light reactions take place in the thylakoid membranes of chloroplasts and involve the conversion of light energy into chemical energy.

2. **Identify the Role of Photosystem I**:
- Photosystem I (PSI) is a protein-pigment complex that plays a crucial role in the light reactions. It absorbs light energy and uses it to energize electrons.

3. **Electron Flow in Light Reactions**:
- During the light reactions, water molecules are split (photolysis) to release electrons. These electrons are transferred through a series of proteins and carriers.

4. **Role of Ferredoxin**:
- The energized electrons from PSI are transferred to a carrier molecule known as ferredoxin. Ferredoxin is a small iron-sulfur protein that acts as an electron carrier.

5. **Transfer to NADP+**:
- The electrons from ferredoxin are then transferred to NADP+ (nicotinamide adenine dinucleotide phosphate) through the action of an enzyme called NADP+ reductase. This process converts NADP+ into NADPH, which is used in the Calvin cycle.

6. **Conclusion**:
- Based on the flow of electrons during the light reactions, we conclude that the substance through which photosystem-I passes an electron to NADP is **ferredoxin**.

### Final Answer:
The correct answer is **ferredoxin**.