The electron flow from PS-II to NADP via ETS and PS - I follows

Options

A. uphill `rarr` downhill `rarr` uphill `rarr` downhill
B. downhill `rarr` uphill `rarr` downhill `rarr` uphill
C. downhill `rarr` uphill `rarr` uphill `rarr` downhill
D. uphill`rarr` downhill `rarr` downhill `rarr` uphill

Answer

### Step-by-Step Solution:

1. **Understanding the Process**: The question pertains to the flow of electrons during the light reactions of photosynthesis, specifically from Photosystem II (PS-II) to NADP+ via the electron transport system (ETS) and Photosystem I (PS-I). This process is known as non-cyclic photophosphorylation.

2. **Initiation of Electron Flow**: When light strikes the light-harvesting complex associated with PS-II, it excites electrons, causing them to jump to a higher energy level. This is the initial step of electron flow.

3. **Water Splitting**: In PS-II, water molecules are split (photolysis), releasing oxygen and providing electrons to replace those lost by PS-II. This process is crucial as it maintains the electron supply.

4. **Electron Transport Chain (ETS)**: The excited electrons from PS-II are transferred through a series of proteins in the electron transport chain. This transfer involves a series of redox reactions where the electrons lose energy, which is used to pump protons into the thylakoid lumen, creating a proton gradient.

5. **Transition to PS-I**: After moving through the ETS, the electrons reach PS-I. Here, they are re-excited by light energy absorbed by the light-harvesting complex of PS-I.

6. **Formation of NADPH**: The re-excited electrons from PS-I are then transferred to NADP+, reducing it to NADPH. This is the final step in the electron flow.

7. **Understanding the Flow Direction**: The flow of electrons can be described as follows:
- **Uphill**: From PS-II to the primary electron acceptor (due to the energy input from light).
- **Downhill**: Through the ETS (as electrons lose energy).
- **Uphill**: From the ETS to PS-I (as electrons are re-excited).
- **Downhill**: From PS-I to NADP+ (as electrons are transferred to NADP+).

8. **Conclusion**: Therefore, the correct sequence of electron flow from PS-II to NADP+ via ETS and PS-I is uphill, downhill, uphill, and downhill. The correct answer to the question is option number 1.