Svedberg unit, number of r-RNA in ribosome

Answer

### Step-by-Step Solution:

1. **Understanding Svedberg Unit**:
- The Svedberg unit (S) is a measure of the rate of sedimentation of particles in a centrifuge. It is defined as the time in seconds it takes for a particle to sediment under the influence of centrifugal force.
- 1 Svedberg (1 S) is equal to 10^-13 seconds.

2. **Ribosome Structure**:
- Ribosomes are the cellular machinery responsible for protein synthesis. They consist of ribosomal RNA (rRNA) and proteins.
- Ribosomes are classified based on their sedimentation coefficients, which are expressed in Svedberg units (S).

3. **Types of Ribosomes**:
- Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S. Additionally, mitochondrial and chloroplast ribosomes are also 70S.
- The "S" value reflects the size and density of the ribosome, which is influenced by the rRNA and protein composition.

4. **Number of rRNA in Ribosomes**:
- In prokaryotic ribosomes (70S), there are three types of rRNA: 16S, 23S, and 5S.
- In eukaryotic ribosomes (80S), there are four types of rRNA: 18S, 28S, 5.8S, and 5S.

5. **Conclusion**:
- The Svedberg unit is a crucial measure in understanding ribosome structure and function. The number of rRNA molecules varies between prokaryotic and eukaryotic ribosomes, with prokaryotes having three and eukaryotes having four.