In a biologically active cell many substances enter and many others go out of the cell. However, if the entry and exit is unregulated, it is deleterious to the cell. Hence plasma membrane regulates and decides which substances are to be allowed and which should not.
This unique property of the cell membrane called selective permeability or differential permeability is responsible for most of the physiological activities of the cell.
All the membranes have certain special substances (on their surface) called receptor molecules which mediate and control the message from one to another. The receptor molecules vary in different membranes.
These molecules combine with substances (ligands) of different types and generate signals (in the form of cyclic AMP) directed to the interior of the cell. The best studied ligands are the hormones and neurotransmitters.
4. Interaction between cells:
It is the responsibility of the plasma membrane as the living boundary between cells to effectively coordinate actions between cells since an organism consists of different types of cells and many a time the actions have to be coordinated to achieve harmony.
5. Energy transformations:
Membranes are intimately associated with the inter conversion of energy. For instance, in green plants the light energy has to be converted into chemical energy.
Similarly the chemical energy stored in carbohydrates has to be converted into energy currency i.e. ATP molecules. The machinery for energy capture and conversion is hidden in the membranes of mitochondria and chloroplasts.
Locus for biochemical functions:
Membranes provide the structural framework for the location of various enzymes. The scaffolding of the membrane interconnecting the various organelles helps in the coordination of activities. Such an effective coordination of activities is best illustrated in the mitochondrial membrane where the various components of the electron transport chain work together as they are located in an orderly fashion.