A cell membrane functions like your skin, as a barrier between what’s inside and out. And both allow for stuff to cross in a controlled manner. Yet they differ in that stuff (sebum, salt & sweat) mostly flows out of your skin, while an exchange goes both ways through the membrane of cells. How molecules pass in and out of a cell is based on cell membrane composition.
Cell membrane composition is primarily lipids and proteins. Its basic structure is composed of phospholipids, with cholesterol added for stability. This arrangement allows for small or lipid soluble molecules to flow through with little resistance, like oxygen, carbon dioxide and steroids, whereas larger molecules cannot, such as water, glucose, amino acids and ions. So how these molecules gain entry is with the help of proteins that are dispersed throughout, via an energy using mechanism known as active transport. A cell membrane is said to be semipermeable because its composition controls the exchange of substances.
The outside surface of cell membranes also include receptor and antigen molecules. Surface receptors function as communicators for the cell by signaling changes in the outside environment. Antigens monitor for the presence of foreign material and mobilizes immune system antibodies for an invader’s destruction.
Defects in cell membrane components may lead to disease, for instance spherocytosis syndrome, diabetes, sickle cell anemia and cystic fibrosis. Some possible causes for a composition defect to occur include toxins, free radicals, genes and trans fatty acids.