Proteins ( export proteins) that are to be used outside the cell are synthesized

Options

A. in mitochondria
B. on SER
C. on RER
D. on free ribosomes

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks where export proteins, which are used outside the cell, are synthesized.

2. **Analyzing the Options**:
- **Option 1: Mitochondria**: Mitochondria are known as the powerhouse of the cell and are primarily involved in ATP production. They do not synthesize proteins, so this option is incorrect.
- **Option 2: Smooth Endoplasmic Reticulum (SER)**: The smooth endoplasmic reticulum is involved in lipid synthesis and the metabolism of xenobiotic compounds. It does not synthesize proteins, so this option is also incorrect.
- **Option 3: Rough Endoplasmic Reticulum (RER)**: The rough endoplasmic reticulum has ribosomes on its surface and is involved in protein synthesis. However, it primarily synthesizes proteins for use within the cell, not for export outside the cell. Therefore, this option is incorrect as well.
- **Option 4: Free Ribosomes**: Free ribosomes are suspended in the cytoplasm and are responsible for synthesizing proteins that are exported outside the cell. This option is correct.

3. **Conclusion**: The correct answer is that export proteins are synthesized on **free ribosomes**.

### Final Answer:
Export proteins that are to be used outside the cell are synthesized on **free ribosomes**.

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