[A]: Hill and Bendall proposed Z-scheme and suggested that photosystems operate in series.
[R]: The light-dependent reactions require the participation of two light gathering units called PS-I and PS-I I.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question regarding the Z-scheme proposed by Hill and Bendall and the role of photosystems in the light-dependent reactions of photosynthesis, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding the Z-Scheme**:
- The Z-scheme is a model that describes the flow of electrons during the light-dependent reactions of photosynthesis. It illustrates how energy from sunlight is captured and used to generate ATP and NADPH.

2. **Identifying Photosystems**:
- There are two main photosystems involved in this process: Photosystem I (PS-I) and Photosystem II (PS-II).
- PS-II is also known as P680 because it absorbs light most efficiently at a wavelength of 680 nm, while PS-I is known as P700 as it absorbs light at 700 nm.

3. **Operation in Series**:
- The Z-scheme demonstrates that these two photosystems operate in series.
- PS-II captures light energy first, which excites electrons that are then transferred to an electron transport chain. This process leads to the production of ATP and NADPH.

4. **Role of Light-Harvesting Complexes**:
- Each photosystem is associated with light-harvesting complexes that contain chlorophyll and other pigments. These complexes capture light energy and funnel it to the reaction centers of the photosystems.

5. **Electron Transport and Water Splitting**:
- The electrons lost from PS-II are replenished by the splitting of water molecules (photolysis), which also releases oxygen as a byproduct.
- The electrons move through the electron transport chain, ultimately reaching PS-I, where they are re-excited by light energy and used to reduce NADP+ to NADPH.

6. **Conclusion**:
- Both the statement about the Z-scheme and the reason regarding the participation of PS-I and PS-II in the light-dependent reactions are true. The Z-scheme effectively illustrates how these two photosystems work in series to facilitate photosynthesis.

### Final Answer:
- **[A]**: True - Hill and Bendall proposed the Z-scheme and suggested that photosystems operate in series.
- **[R]**: True - The light-dependent reactions require the participation of two light-gathering units called PS-I and PS-II.