All cells are enclosed by a thin, film like Cell membranes, plasma lemma, biological membranes or biomembranes. The plasma membrane is a semipermeable (not everything can pass through) boundary between the cell and its environment. It is made up of
lipids, proteins and carbohydrates. Chemically a biomembrane consists of lipids(20-40%), proteins(50-75%) and carbohydrates (1-5%).
Plasma membranes hold the semi fluid protoplasmic contents.
2. Protection: It protects the animal cell from injury. Plant cell possess wall over the plasma membrane for extra protection and support.
3. Individuality: It maintains form and individually of the cell. Anchoring of the cytoskeleton to provide shape to cell.
4. Selective Permeability: Cell membranes allow entry to only selected substances. They are impermeable to others. It allows some solute particles(1-15A0) to pass through it readily along with all solvents.
5. Organelles: Cell membrane delimit a number of subcellular structures or organelles like nucleus, mitochondria, plastids, Golgi apparatus, endoplasmic reticulum, microbodies etc.
6. Compartmentalisation: Cell membrane separate the cells from their external environment and cell organelle from cytosol. They help the cells and their organelles to have their own microenvironments, structural and functional individuality.
7. Cell Recognition: With the help of glycolipids and glycoproteins, cell membranes are able to differentiate similar cells from dissimilar ones, foreign substances and tissue from familial materials. Cell recognition is useful for tissue formation and defence against microbes.
8. Antigens: Cell membranes possess antigens which determine blood grouping, immune response, acceptance or rejection of a transplant.
9. Microvilli: They are membrane evaginations which are specialised for increasing absorption.
10. Sheaths: Plasma membrane forms sheaths around cilia and flagella.
11. Cytoplasmic bridges: Plasmodesmata and gap junctions form cytoplasmic bridges between adjacent cells.
12. Endocytosis: Bulk intake of materials or endocytosis occurs through development of membrane vesicles.
13. Exocytosis: It is reverse of endocytosis that provides for throwing out of waste products and secretory materials.
14. Impulse transmission: Nerve impulses travel through the plasma membrane of nerve cells.
15. Cell metabolism: Cell membranes controls cell metabolism through selectively permeability and retentivity.
16. Electron transport: Electron transport chains of mitochondria and chloroplasts are located along the membranes.
17. Osmosis: It is caused by the presence of of special water channels in the membranes.
18. Carrier proteins: They occur in the cell membranes and control active transport.
19. Membrane enzymes: About 30 enzymes are located in the membranes for performing various biochemical reactions like synthesis and hydrolysis of ATP by ATPase.
20. Membrane Receptors: Membrane possess receptors for hormones, neurotransmitters, immunoglobulins and several other biochemicals.
21. Cell movements: Undulation and pseudopodia are cell membrane phenomenon involved in cell movement. Amoeba, macrophages and WBCs move with the helps of temporary organelles like pseudopodia. Some mammalian cells such as fibroblasts can move over a solid surface by wave like undulations of the plasma membrane.
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