Ribosomes are essential for protein synthesis but they are present in mitochondria and plastids , the sites of respiration and photosynthesis . What is the role of ribosomes in those organelles ?

Options

A. Ribosomes transport ATP formed in respiration and photosynthesis to cytoplasm through ER
B. Subunits of some required proteins are synthesised in these organelles
C. Ribosomes transport RNA and DNA to cytoplasm
D. All the above correct

Answer

### Step-by-Step Solution:

1. **Understanding Ribosomes**: Ribosomes are cellular structures responsible for protein synthesis. They can be found in various locations within the cell, including the cytoplasm and within certain organelles.

2. **Types of Ribosomes**: In eukaryotic cells, ribosomes can be classified into two types:
- Free ribosomes, which float in the cytoplasm.
- Membrane-bound ribosomes, which are attached to the endoplasmic reticulum (ER).

3. **Ribosomes in Mitochondria and Plastids**: Mitochondria and plastids (like chloroplasts) are known as semi-autonomous organelles because they can produce some of their own proteins independently of the nuclear DNA.

4. **70S Ribosomes**: The ribosomes found in mitochondria and plastids are of the 70S type, which is different from the larger 80S ribosomes found in the cytoplasm of eukaryotic cells. The "S" refers to the Svedberg unit, which measures the rate of sedimentation during centrifugation.

5. **Role of Ribosomes in Mitochondria**: In mitochondria, ribosomes synthesize proteins that are essential for the organelle's function in cellular respiration. These proteins include components of the electron transport chain and enzymes involved in metabolic pathways.

6. **Role of Ribosomes in Plastids**: In plastids, particularly chloroplasts, ribosomes are responsible for synthesizing proteins necessary for photosynthesis, including components of the photosynthetic machinery.

7. **Conclusion**: The primary role of ribosomes in mitochondria and plastids is to synthesize specific proteins that are crucial for the functions of these organelles—energy production in mitochondria and photosynthesis in plastids.