(1) Compartmentalization and selective permeability
The plasma membrane can selectively control the entry and exit of substances. Nutrients (water, oxygen, amino acids, sugars, lipids, etc.) can enter the cell easily, while other substances cannot. The metabolic wastes and secretions, such as antibodies and hormones, can be discharged to the outside in time, while the enzymes and DNA in the cells are not released. This property of membrane compartmentalize the cell from the external environment, guaranteeing a relatively stable and independent interior. Of course, some toxic lipid-soluble organics, such as benzene or short chain alkanes, enter the cell easily; some viruses, bacteria can also invade the cell to make people sick.
(2) The plasma membrane is involved in material and information exchange between cells
①Glucose, amino acids enter the cell with the assistance of proteins. The processes that consume energy are called active transport and those that do not consume energy are called passive transport.
② The receptors on the membrane selectively bind to extracellular chemicals to trigger the specific life activities of cell. For example, when blood glucose concentration rises, insulin secreted by the pancreas travels throughout the body in the bloodstream and binds to receptors of target cells surface. These cells begin to accelerate glucose metabolism or convert it into glycogen and fat.
③ The membranes of two cells come into contact to recognize their identity. Cancerous cell produces special glycoprotein in their membrane, such as alpha fetoprotein and carcinoembryonic antigen which immune system recognize to destroy cancer cells.
④ A channel is formed between two adjacent cells to exchange information and material. For example, neighboring cells of advanced plants have plasmodesmata throught which sucrose, hormones, enzymes and RNA can enter from one cell to another.
(3) Some other functions
① Some enzymes are anchored in the cell membrane to catalyze biochemical reactions. In particular, prokaryotes have more enzymes on their cell membranes. The lack of organelle makes many functions to be completed by cell membrane instead.
② Cells of animals and plants often adhere together to form tissues. Tight junctions, desmosomes and gap junctions are all the result of protein interactions on the cell membrane surface.
③ Proteins embedded in the membrane are used to anchor the cell to the extracellular matrix. One end of these proteins are linked to the cytoskeleton and the other end covalently cross-linked to molecules on the extracellular matrix.