Non-secretory proteins are synthesized by

Options

A. ER-bound ribosomes
B. Free ribosomes
C. Polysomes
D. Endosomes

Answer

### Step-by-Step Solution:

1. **Understanding Protein Classification**:
- Proteins can be classified into two categories: secretory and non-secretory proteins.
- Secretory proteins are those that are secreted out of the cell, while non-secretory proteins remain inside the cell.

2. **Identifying Non-Secretory Proteins**:
- Non-secretory proteins are synthesized within the cell and do not exit due to the lack of compatible transporter proteins.
- This means that specific transporter proteins are required for secretory proteins to leave the cell, which is not applicable for non-secretory proteins.

3. **Role of Ribosomes in Protein Synthesis**:
- Proteins are synthesized by ribosomes.
- Ribosomes can either be attached to the endoplasmic reticulum (ER) or be free in the cytoplasm.

4. **Endoplasmic Reticulum (ER) and Protein Synthesis**:
- The rough endoplasmic reticulum (RER), which has ribosomes attached to it, is responsible for synthesizing secretory proteins.
- In contrast, free ribosomes in the cytoplasm are responsible for synthesizing non-secretory proteins.

5. **Evaluating Options**:
- **Option 1: ER bound ribosomes** - These synthesize secretory proteins, so this option is incorrect.
- **Option 2: Free ribosomes** - These synthesize non-secretory proteins, making this option correct.
- **Option 3: Polysomes** - These are multiple ribosomes attached to mRNA and are involved in translation, not specifically in synthesizing non-secretory proteins, so this option is incorrect.
- **Option 4: Endosomes** - These are involved in sorting and trafficking proteins and lipids, not in their synthesis, making this option incorrect.

6. **Conclusion**:
- The correct answer is that non-secretory proteins are synthesized by **free ribosomes** in the cytoplasm.

### Final Answer:
Non-secretory proteins are synthesized by **free ribosomes**.

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