Which Organelle Synthesizes Proteins – The main role of ribosome is its ability to catalyse the formation of peptide bonds between amino acids, so that the amino acids are incorporated into proteins. Ribosomes are dense granules without covering membranes. Ribosomes are found in all cells. E. coli has 10,000 ribosomes, and form about 25% of total mass of bacterial cell. A mammalian cell contains about 10 million ribosomes.
Which Organelle Synthesizes Proteins – Rough ER (RER) is involved in some protein production, protein folding, quality control and despatch. It is called ‘rough’ because it is studded with ribosomes. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). Both types are present in plant and animal cells. The two types of ER often appear as if separate, but they are sub-compartments of the same organelle.
Ribosomes on the rough endoplasmic reticulum are called ‘membrane bound’ and are responsible for the assembly of many proteins. This process is called translation. Certain cells of the pancreas and digestive tract produce a high volume of protein as enzymes. Many of the proteins are produced in quantity in the cells of the pancreas and the digestive tract and function as digestive enzymes.
This is an extensive organelle composed of greatly convoluted but flattish sealed sacs, which are contiguous with the nuclear membrane. These are called membrane bound ribosomes and are firmly attached to the outer cytosolic side of the ER About 13 million ribosomes are present on the RER in the average liver cell. Rough ER is found throughout the cell but the density is higher near the nucleus and the Golgi apparatus.
Proteins are produced for the plasma membrane, Golgi apparatus, secretory vesicles, plant vacuoles, lysosomes, endosomes and the endoplasmic reticulum itself. Some of the proteins are delivered into the lumen or space inside the ER whilst others are processed within the ER membrane itself.
Proteins are subjected to a quality control check and any that are found to be incorrectly formed or incorrectly folded are rejected. These rejects are stored in the lumen or sent for recycling for eventual breakdown to amino acids.