which of the following structure is present only in prokaryotic cell ?

Options

A. Plasmid
B. Nucleus
C. Mitochondria
D. Ribosomes

Answer

To determine which structure is present only in prokaryotic cells, we will analyze each of the given options: plasmid, nucleus, mitochondria, and ribosome.

### Step-by-Step Solution:

1. **Understanding Plasmid**:
- Plasmids are small, circular, double-stranded DNA molecules that are separate from chromosomal DNA.
- They are found in many bacteria (which are prokaryotes) and can replicate independently of the chromosomal DNA.
- **Conclusion**: Plasmids are present only in prokaryotic cells.

2. **Analyzing Nucleus**:
- The nucleus is a membrane-bound organelle that contains the cell's genetic material.
- It is a defining feature of eukaryotic cells.
- Prokaryotic cells do not have a true nucleus; instead, their genetic material is located in a region called the nucleoid, which is not membrane-bound.
- **Conclusion**: The nucleus is not present in prokaryotic cells.

3. **Examining Mitochondria**:
- Mitochondria are membrane-bound organelles known as the powerhouse of the cell, responsible for ATP production.
- They are only found in eukaryotic cells.
- **Conclusion**: Mitochondria are not present in prokaryotic cells.

4. **Considering Ribosomes**:
- Ribosomes are the sites of protein synthesis and are not membrane-bound.
- They are present in both prokaryotic and eukaryotic cells, but the size differs (70S in prokaryotes and 80S in eukaryotes).
- **Conclusion**: Ribosomes are present in both prokaryotic and eukaryotic cells.

### Final Answer:
The structure that is present only in prokaryotic cells is **plasmid**.