We have already seen that the cell is the structural and functional unit of living things. There are different types of cells, having different shapes, structures and functions. A typical cell or a generalized cell has been theoretically created so that structures and functions of cell’s components may be easily studied and understood. Almost all cells have the following four characteristics.
1. A surrounding membrane or the plasma membrane.
2. A thick fluid enclosed by this membrane that along with the other cell contents is called protoplasm.
3. Organelles are located within the protoplasm that carry out certain cellular functions.
4. A control center called nucleus that contains the hereditary material DNA.
A generalized structure of a eukaryotic cell is discussed as seen under light and electron microscope. The fine structure of the cell as seen with electron microscope is known as ultra-structure.
|Electron microscopic structure of a plant cell|
Every cell is bounded by plasma membrane Protoplasm is a semi fluid with cell contents, including the cell organelle and the nucleus of the cell. The membrane is also called cell membrane, is ultra-thin, delicate and elastic. It gives shape and mechanical support to the cell. Chemically cell membrane consists of proteins 60-80%, lipids 20-40% and small quantity of carbohydrates.
Fluid Mosaic Model
In 1972 S. Singer and G. Nicolson introduced the fluid mosaic model of membrane structure. This model proposes that the membrane is a phospholipids’ bi-layer in which protein molecules are either partially or wholly embedded. The proteins are scattered throughout the membrane in an irregular pattern that can vary from membrane to membrane.
|Electron microscopic structure of an animal cell|