Birefringence can pinpoint the presence of certain chemicals in both the membrane and the cell by the way in which light is doubly refracted through them. Cell membranes are essential for the life of a cell. They determine what gets in and out of it. Cell membranes are only about one hundred angstroms thick. They consist of lipids and proteins organized as tiny leaflets, two molecules thick, or as tightly packed globules.
Cells receive some substances by osmosis.
Simple sugars usually pass through the pore-studded membrane when the concentration of sugar molecules outside a cell is greater than that within it. In some instances, active transport helps molecules and ions to penetrate the cell membrane despite the resistance of electrochemical forces. These pumps may be chemical compounds that react with substances outside the cell and carry them across the cell membrane when their concentration gradients are not otherwise great enough to permit their entry. At the inner edge of the membrane, next to the cytoplasm, the carrier pump probably dissociates from the transported substance, which is then absorbed by the cytoplasm for metabolic use. Active transport processes in the membrane permit cells to regulate food absorption, hormone and enzyme secretion, and the transfer of genetic material from the cell nucleus to the ribosomes, where protein synthesis occurs.