RER is well developed in cells engaged in synthesis of

Options

A. Nucleotides
B. Proteins and lipids
C. Lipids
D. Secretory products

Answer

To answer the question "RER is well developed in cells engaged in synthesis of," we will analyze the options provided and understand the role of the Rough Endoplasmic Reticulum (RER).

### Step-by-Step Solution:

1. **Understanding RER**: The Rough Endoplasmic Reticulum (RER) is characterized by the presence of ribosomes on its surface, which gives it a "rough" appearance. The primary function of RER is the synthesis of proteins.

2. **Analyzing the Options**:
- **A. Nucleotides**: Nucleotides are the building blocks of nucleic acids (DNA and RNA) and are not primarily synthesized in the RER.
- **B. Proteins and Lipids**: While RER is primarily involved in protein synthesis, it also plays a role in the synthesis of certain lipids, particularly those associated with membranes.
- **C. Lipids**: Lipid synthesis primarily occurs in the Smooth Endoplasmic Reticulum (SER), not the RER.
- **D. Secretory Products**: Secretory products often include proteins that are synthesized in the RER and then transported out of the cell.

3. **Conclusion**: The RER is well developed in cells that are actively synthesizing proteins, and it also contributes to the synthesis of some lipids. Therefore, the most appropriate answer from the options provided is **B. Proteins and Lipids**.

### Final Answer:
**B. Proteins and Lipids**

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