The following points highlight the five important physiological functions of plasma membrane. The functions are: 1. Permeability 2. Osmosis 3. Diffusion or Passive Transport 4. Active Transport 5. Endocytosis and Exocytosis.
Physiological Function # 1. Permeability:
The plasma membrane is a thin, elastic membrane around the cell which usually allows the movement of small ions and molecules of various substances through it. This nature of plasma membrane is termed as permeability.
Physiological Function # 2. Osmosis:
The plasma membrane is permeable to water molecules. To and fro movement of water molecules through the plasma membrane occurs due to the differences in the concentration of the solutes on its either side. The process by which the water molecules pass through a membrane from a region of higher water concentration to the region of lower water concentration is known as osmosis (Gr., osmos = pushing).
The process in which the water molecules enter into the cell is known as endosmosis, while the reverse process which involves the exit of the water molecules from the cell is known as exosmosis.
Physiological Function # 3. Diffusion or Passive Transport:
When molecules of two kinds are placed together they tend to mix with each other by a process known as diffusion. The diffusion of certain solutes or substances takes place through the plasma membrane. Such diffusing solute particles require no energy for the diffusion process through the plasma membrane. The diffusion of ions through the plasma membrane depends on the concentration and electrical gradients.
Physiological Function # 4. Active Transport:
When the molecules or ions move through the plasma membrane from low concentration to higher concentration, they require energy for such movement. The energy is provided by adenosine triphosphate (ATP) which is produced by the oxidative phosphorylation in the mitochondria. The active transport of the molecules occurs in nerve cells and kidney cells.
Through the pores of plasma membrane, some chemical compounds such as urea, formamide and glycerol could pass actually. Brachet (1957) has shown that sometimes large molecules of certain proteins penetrate the cell.
Physiological Function # 5. Endocytosis and Exocytosis:
The plasma membrane participates actively in the ingestion of certain large-sized foreign or food substances. The process by which the foreign substances are taken in and digested is known as endocytosis. In the process of exocytosis, the cells which have secretory function such as pancreatic cells pass out their enzymatic secretions outside the cell.
According to the nature of the food or foreign substances, the endocytosis may be classified into two types, viz.:
(i) Pinocytosis
(ii) Phagocytosis
(i) Pinocytosis:
When the ingestion of fluid material in bulk takes place by the cell through the plasma membrane, the process is known as pinocytosis.
(ii) Phagocytosis:
Sometimes the large-sized solid food or foreign particles are taken in by the cell through the plasma membrane. The process of ingestion of large-sized solid substances by the cell is known as phagocytosis.