Deactivation of enzymes above `40^(⁰)C` affects the ____________ of photosynthesis and decreases the rate of photosynthesis

Options

A. light reaction
B. dark reaction
C. photolysis of water
D. excitation of chlorophyll

Answer

To answer the question, "Deactivation of enzymes above `40^(⁰)C` affects the ____________ of photosynthesis and decreases the rate of photosynthesis," we need to analyze the role of enzymes in the process of photosynthesis.

### Step-by-Step Solution:

1. **Understanding Photosynthesis**: Photosynthesis occurs in two main stages: the light reactions and the dark reactions (Calvin cycle). The light reactions convert solar energy into chemical energy (ATP and NADPH), while the dark reactions use that chemical energy to fix carbon dioxide into glucose.

2. **Role of Enzymes**: Enzymes are biological catalysts that speed up chemical reactions. In the context of photosynthesis, enzymes are crucial for the dark reactions, particularly the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), which catalyzes the first step of the Calvin cycle.

3. **Effect of Temperature on Enzymes**: Enzymes have an optimal temperature range in which they function effectively. Temperatures above this range can lead to enzyme denaturation, where the enzyme's structure is altered, resulting in decreased activity.

4. **Identifying the Affected Process**: Since the question specifies the deactivation of enzymes due to high temperatures, we focus on the dark reactions (Calvin cycle) where RuBisCO is essential for carbon fixation. If RuBisCO is deactivated, the Calvin cycle cannot proceed effectively, leading to a decrease in glucose production and overall photosynthesis rate.

5. **Conclusion**: Therefore, the correct answer to the question is that the deactivation of enzymes above `40^(⁰)C` affects the **dark reaction** of photosynthesis and decreases the rate of photosynthesis.

### Final Answer:
The correct answer is **dark reaction**.

---