Double unit membrane is absent in

Options

A. Ribosomes
B. Nucleus
C. Plastids
D. E.R.

Answer

To solve the question "Double unit membrane is absent in," we need to analyze the provided options: ribosomes, nucleus, plastids, and endoplasmic reticulum. Let's go through each option step by step.

### Step-by-Step Solution:

1. **Understanding Double Unit Membrane**:
- A double unit membrane refers to a structure that has two lipid bilayers. This is characteristic of certain organelles in eukaryotic cells.

2. **Analyzing Ribosomes**:
- Ribosomes are known as membrane-less organelles. They are composed of ribosomal RNA (rRNA) and proteins.
- Their primary function is protein synthesis.
- Since ribosomes do not have any membrane, they do not possess a double unit membrane.

3. **Analyzing Nucleus**:
- The nucleus is a membrane-bound organelle that contains the cell's genetic material (DNA).
- It is surrounded by a double membrane known as the nuclear envelope, which separates the contents of the nucleus from the cytoplasm.
- Therefore, the nucleus does have a double unit membrane.

4. **Analyzing Plastids**:
- Plastids, such as chloroplasts, are found in plant cells and are involved in processes like photosynthesis.
- They are also surrounded by a double membrane structure.
- Thus, plastids do have a double unit membrane.

5. **Analyzing Endoplasmic Reticulum (ER)**:
- The endoplasmic reticulum is an extensive network of membranes within the cell.
- It exists in two forms: rough ER (with ribosomes) and smooth ER (without ribosomes).
- Both forms of ER are membrane-bound and thus have a double unit membrane.

6. **Conclusion**:
- Among the options provided, ribosomes are the only organelles that do not have a double unit membrane.
- Therefore, the correct answer is **ribosomes**.

### Final Answer:
**Ribosomes**