Flow of electrons in non-cyclic photophosphorylation is

Options

A. from PS I to PS II
B. bidirectional
C. unidirectional
D. both (2) and (3)

Answer

To understand the flow of electrons in non-cyclic photophosphorylation, we can break it down into several key steps:

### Step-by-Step Solution:

1. **Initiation of Non-Cyclic Photophosphorylation**:
- Non-cyclic photophosphorylation occurs during the light-dependent reactions of photosynthesis, specifically in the thylakoid membranes of chloroplasts. This process requires light energy.

2. **Water Splitting**:
- The process begins with the photolysis of water (H₂O). Light energy is absorbed by chlorophyll, which leads to the splitting of water molecules into oxygen (O₂), protons (H⁺), and electrons (e⁻).
- Reaction: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

3. **Electron Transport through Photosystem II**:
- The electrons released from water are then transferred to Photosystem II (PSII). Here, the absorbed light energy excites these electrons, raising them to a higher energy level.

4. **Electron Transport Chain**:
- The excited electrons are passed along a series of proteins known as the electron transport chain (ETC). This chain includes plastoquinone, cytochrome b6f, and plastocyanin.
- As the electrons move through the ETC, they lose energy, which is used to pump protons into the thylakoid lumen, creating a proton gradient.

5. **Photosystem I**:
- The electrons eventually reach Photosystem I (PSI), where they are re-excited by light energy absorbed by PSI. This further energizes the electrons.

6. **Formation of NADPH**:
- The high-energy electrons from PSI are then transferred to NADP⁺, along with protons (H⁺), to form NADPH, a crucial energy carrier for the Calvin cycle.
- Reaction: NADP⁺ + 2 e⁻ + 2 H⁺ → NADPH + H⁺

7. **ATP Formation**:
- The proton gradient created during the electron transport is utilized by ATP synthase to convert ADP and inorganic phosphate (Pi) into ATP through chemiosmosis.

### Summary of Electron Flow:
- The flow of electrons in non-cyclic photophosphorylation is unidirectional and linear, starting from water, passing through Photosystem II, moving through the electron transport chain, reaching Photosystem I, and ultimately leading to the formation of NADPH and ATP.