Factors Determining The Size Of a Cell !
Cells are the structural and functional unit of all living organisms. There is a vast variety of cells in the living system . But all cells share certain fundamental structures and properties. A cell have a Cytosol, a plasma membrane and a nucleus or nucleoid. Cytosol is a highly concentrated solution of enzymes, RNA molecules and other suspended components. Plasma membrane is a lipid bilayer that separates the cell content from the surrounding. All cells at least some part of their life have a nucleus or nucleoid which contains the genetic material.
There is a great variation in the size of different types of cell. The bacterial cells are about 1 to 2 micrometre long , whereas the plant and animal cells are about 5 micrometre to 100 micrometre in diameter.
What factors determine the size of a cell?
Cell requires various biomolecules like lipids, proteins and nucleic acids for its activity. These biomolecules give rise to various supramolecular structures like chromatin, ribosomes various enzymes which are essential for a cell to live. Minimum number of biomolecules required for sustainment of the cell determines how small a cell can be. Mycoplasmas are the smallest known bacterial cells. The diameter of these cells is about 300 nm. The ribosomes (protein synthesizing machinery) have a diameter of 20 nm. So theoretically, mycoplasma cell can only contain about 3000 ribosomes and cell should have space for other biomolecules as well.
The famous E.coli contains about 15000 ribosomes and thousands of different types of enzymes and other biomolecules in it. So the number of biomolecules required by a cell to live determines how small a cell can be.
Now let’s see which factors determine how large a cell can be. Cell requires various solute molecules like oxygen to carry out metabolism. Consider an aerobic bacterial cell which requires oxygen to carry out metabolism. Cell generate energy by transferring electrons from fuel ( various carbohydrates, lipids and protein molecules) to oxygen. Bacterial cell obtain oxygen from surrounding by diffusion. These type of bacterial cells are classified as aerobic organisms.
If the cell is small then its surface area to volume ratio will be high that is a large surface area will be there for a particular amount of volume. In this condition oxygen can easily diffuse from the surrounding into the cell and can reach to every part of cytoplasm. But if the size of the cell is large then the surface area to volume ratio will be low . And the rate of diffusion of oxygen into the cell will not match the oxygen requirement of the metabolism.
So there is a limit of size of cell at which the rate of diffusion of Oxygen will match with the requirement of the metabolism and that is what determines how large a cell can be. Animal cells and plant cells have transporting systems for oxygen and other solute molecules and that’s why they can have a size up to 100 micrometer. Bacterial cells don’t have a transporting system for oxygen so that’s why its size is limited to about 1 to 2 micrometers.
“Size – one of the most important parameter of the cell which makes life possible !”