Ribosomes differ from cell to cell/species to species in

Options

A. types of membranes
B. types of rRNA
C. `Mg^(++)` concentration
D. all of these

Answer

### Step-by-Step Solution:

1. **Understanding Ribosomes**: Ribosomes are essential cellular structures responsible for protein synthesis. They are found in both prokaryotic and eukaryotic cells.

2. **Ribosomal Structure**: Ribosomes consist of two subunits. In prokaryotic cells, the ribosomal subunits are 50S and 30S, while in eukaryotic cells, they are 60S and 40S.

3. **Identifying Differences**: The question asks how ribosomes differ from cell to cell and species to species. We need to analyze the options provided:
- **Types of membranes**: Ribosomes are not membrane-bound organelles; hence, this option is not applicable.
- **Types of rRNA**: Ribosomes are composed of ribosomal RNA (rRNA) and proteins. The types of rRNA can vary between different species and cell types.
- **Mg²⁺ concentration**: The concentration of magnesium ions (Mg²⁺) is generally consistent across different species and does not account for the differences in ribosomes.

4. **Evaluating Options**:
- Since ribosomes are not membrane-bound, the types of membranes option is incorrect.
- The types of rRNA are indeed different across species, making this a valid option.
- The Mg²⁺ concentration is also not a differentiating factor.

5. **Conclusion**: The primary difference in ribosomes from cell to cell and species to species lies in the types of rRNA they contain. Therefore, the correct answer is the types of rRNA.

### Final Answer:
Ribosomes differ from cell to cell/species to species in the **types of rRNA**.