PS I is inactive at

Options

A. 550 nm
B. 680 nm
C. 690 nm
D. 780 nm.

Answer

To determine at which wavelength Photosystem I (PS I) is inactive, we need to analyze the provided options in relation to the optimal functioning wavelength of PS I.

### Step-by-Step Solution:

1. **Understand the Functioning of PS I**:
- Photosystem I is a crucial component of the photosynthetic process in plants. It is primarily responsible for the light-dependent reactions of photosynthesis.

2. **Identify the Optimal Wavelength for PS I**:
- PS I is maximally active at a wavelength of approximately 700 nanometers (nm). This means it can effectively function at wavelengths equal to or less than 700 nm.

3. **Evaluate the Given Options**:
- **Option A: 550 nm** - This wavelength is less than 700 nm, so PS I will be active here.
- **Option B: 680 nm** - This wavelength is also less than 700 nm, meaning PS I will be active.
- **Option C: 690 nm** - This wavelength is still less than 700 nm, so PS I will remain active.
- **Option D: 780 nm** - This wavelength is greater than 700 nm, indicating that PS I will be inactive at this point.

4. **Conclusion**:
- Since PS I is inactive at wavelengths greater than 700 nm, the correct answer to the question "PS I is inactive at" is **780 nm**.

### Final Answer:
**PS I is inactive at 780 nm.**

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