Metabolic processes in living organisms are catalyzed by :

Options

A. enzymes
B. nucleotides
C. nucleosides
D. lipids

Explanation

### Step-by-Step Solution:

1. **Understanding Metabolism**:
- Metabolism refers to the set of chemical reactions that occur in living organisms to maintain life. It includes processes that convert food into energy, synthesize necessary compounds, and eliminate waste.

2. **Functions of Metabolism**:
- There are three main purposes of metabolism:
- **Conversion of food to energy**: This energy is essential for running cellular processes.
- **Building blocks for biomolecules**: Metabolism converts food into building blocks for proteins, lipids, nucleic acids, and carbohydrates.
- **Elimination of waste**: Metabolic processes help in getting rid of waste products generated during these reactions.

3. **Role of Enzymes**:
- Enzymes are biological catalysts that speed up metabolic reactions. They lower the activation energy required for reactions to occur, making it easier for organisms to grow, reproduce, and respond to their environment.

4. **Types of Metabolic Reactions**:
- Metabolic reactions can be classified into two types:
- **Catabolic reactions**: These involve the breakdown of complex molecules into simpler ones, releasing energy.
- **Anabolic reactions**: These involve the synthesis of complex molecules from simpler ones, requiring energy.

5. **Conclusion**:
- Given the role of enzymes in facilitating metabolic processes, the correct answer to the question is **Option A: Enzymes**.