Exchange of Materials
Plasma membrane separates the contents of cells from their environments, controlling exchange of materials such as nutrients and waste products between the two.
They also enable separate compartments to be formed inside the cells in which specialized metabolic processes such as photosynthesis and aerobic respiration can take place.
Membrane also acts as receptor sites for recognizing hormones, neurotransmitter and other chemicals either from the external environment or from other parts of the organism.
Partially PermeablePlasma membrane is partially permeable, as glucose, amino acids, fatty acids and glycerol ions can diffuse slowly through them. Movement of liquid is regulated by osmosis and pinocytosis. Solid materials are transported inside the cell by phagocytosis.
Channel Proteins and Carrier Proteins
The proteins within a membrane determine most of its specific functions. Certain plasma membrane proteins are involved in the passage of molecules through the membrane. Some of those have a channel through which a substance simply can move across the membrane, others are carriers that combine with a substance and help it to move across the membrane.
Some proteins in the plasma membrane are receptors. Each type of receptor has a shape that allows a specific molecule to bind it. The binding of a molecule can cause the proteins to change its shape and bring about an intracellular response.
Some plasma membrane proteins have an enzymatic function and carry out metabolic reactions directly. For example the microvillus on epithelial cells, lining some parts of the gut, contains digestive enzymes in their cell surface.
Antigens are glycoproteins. The glycoproteins may have different shapes, so each cell can have its own specific marker. For example foreign antigen can be recognized and attacked by the immune system.
They are involved in cell to cell recognition, and may act as receptor sites for chemical signal.
The proteins present on membrane takes part in energy transfer system e.g. respiration in mitochondria and photosynthesis in chloroplast.
CholesterolIt acts like a plug. It reduces the escape or entry of polar molecules through the cell membrane.