(i) Ribosomes are the protein factories that synthesize proteins using genetic information.
1. Ribosomes are protein factories of cell and were first observed as dense particles in electron micrograph of a cell by scientist Palade in 1953.
2. Ribosomes lack membranous covering around them and are made up of Ribosomal RNA and proteins.
3. In a eukaryotic cell, ribosomes are present in mitochondria, plastids (in plant cells) and in cytosol.
4. Ribosomes are either found attached to outer surface of Rough Endoplasmic Reticulum and nuclear membrane or freely suspended in cytoplasm.
5. Both are of 80S type. Each ribosome is made up of two subunits- a large (60S) and a small (40S) subunit.
6. Bound ribosomes generally produce proteins that are transported outside the cell after processing in ER and Golgi body. e.g. Bound ribosomes of acinar cells of pancreas produce pancreatic digestive enzymes.
7. Free ribosomes come together and form chains called polyribosomes for protein synthesis.
8. Free ribosomes generally produce enzymatic proteins that are used up in cytoplasm, like enzymes required for breakdown of sugar.
9. Both types of ribosomes (bound and free) can interchange position and function.
10. Number of ribosomes is high in cells actively engaged in protein synthesis.
(ii) Sphaerosomes:
a. These are found mainly in cells involved in synthesis and storage of fats. For e. g. endosperm of oil seeds.
b. The membrane of sphaerosome is half unit membrane i.e. this membrane has only one phospholipid layer.
(iii) 1. Totipotency (totus – entire, potential – power) is the capacity or the potential of living nucleated cell, to differentiate into any other type of cell and thus, can form a complete new organism.
2. A cell is totipotent as it has the entire genetic information of the organism stored in its nucleus.
3. Embryonic animal cells are totipotent and are termed as stem cells.
4. Stem cells are used in curing many diseases. Therefore, they have great potential for medical applications.
