In the absence of ribosomes ER appear smooth and are called____

Answer

**Step-by-Step Solution:**

1. **Understanding the Endoplasmic Reticulum (ER):** The endoplasmic reticulum is a complex network of membranes found in eukaryotic cells. It plays a crucial role in the synthesis and transport of proteins and lipids.

2. **Identifying Ribosomes:** Ribosomes are small organelles that are the sites of protein synthesis. They can either be free-floating in the cytoplasm or attached to the endoplasmic reticulum.

3. **Differentiating Between Rough and Smooth ER:**
- When ribosomes are attached to the endoplasmic reticulum, it is referred to as **Rough Endoplasmic Reticulum (RER)** due to its rough appearance under a microscope.
- In contrast, when ribosomes are absent from the endoplasmic reticulum, it appears smooth and is called **Smooth Endoplasmic Reticulum (SER)**.

4. **Naming the Smooth ER:** The absence of ribosomes in this type of ER leads to its designation as **Smooth Endoplasmic Reticulum (SER)**.

5. **Conclusion:** Therefore, the answer to the question is that in the absence of ribosomes, the endoplasmic reticulum appears smooth and is called **Smooth Endoplasmic Reticulum (SER)**.

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