Definition of Cell membrane: A thin semi permeable membrane that serves as a boundary between the interior of a cell and its extracellular environment and is present in both animal and plant cells.
-Also present in both eukaryotic and prokaryotic cell
Synonym: plasma membrane, plasma lemma, bio membrane
In plant cells, a thick cellulosic cell wall is also present which surrounds the plasma membrane.
Composition: Cell membrane is made up of lipid, protein and small amount of carbohydrate.
Functions of Plasma membrane
1. Protection: It protects the cell from its surroundings or extracellular environment. Plant cell possess wall over the plasma membrane for extra protection and support.
2. Holding cell contents: Plasma membranes hold the semi fluid protoplasmic contents of the cell intact; thus keeping the individuality of the cell.
3. Selective Permeability: Cell membrane allows only selected or specific substances to enter into the cell and are impermeable to others.
- Gases like O2 and CO2 can diffuse rapidly in solution through membranes.
- Small compounds like H2O and methane can easily pass through where as sugars, amino acids and charged ions are transported with the help of transport proteins.
- The size of the molecules which can pass through the plasma membrane is 1-15 A0. This property is responsible for keeping a cell ‘as a cell’, an individual unit.
4. Shape: It maintains form and shape of the cell. It serves as site of anchorage or attachment of the cytoskeleton; thus providing shape to the cell (especially in animal cells without cell wall).
5. Organelles: Cell membrane delimits or covers all sub-cellular structures or organelles like nucleus, mitochondria, plastids, Golgi apparatus, endoplasmic reticulum, microbodies etc. thus protecting them form the surroundings and also helps in maintaining a constant internal environment.
6. Compartmentalization: Cell membrane separate the cells from their external environment and cell organelle from cytosol. It 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 on its surface, cell membranes are able to differentiate similar cells from dissimilar ones, foreign substances and cells own 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 (graft rejection by MHC’s on plasma membrane).
9. Microvilli: They are microscopic finger like projections of plasma membrane present on some cells like intestinal epithelial cells, which are involved in a wide variety of functions, including increasing surface area for absorption, secretion, cellular adhesion etc.
10. Sheaths of cilia and flagella: Cilia and flagella are projections from the cell; made up of microtubules which are covered by an extension of the plasma membrane.
11. Cytoplasmic bridges in plasmodesmata and gap junctions: Plasmodesmata in plant cells and gap junctions in animal cells; meant for intercellular transport and communication, form cytoplasmic bridges between adjacent cells through plasma membrane.
12. Endocytosis and Exocytosis: Bulk intake of materials or endocytosis occurs through development of membrane vesicles or invagination and engulfing by plasma membrane.
Exocytosis: It is reverse of endocytosis that provides for releasing waste products and secretory materials ot of the cells with the help of plasma membrane.
13. Impulse transmission in neurons: The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the plasma membrane of the neuron
14. Cell metabolism: Cell membranes control cell metabolism through selective permeability and retentivity of substances in a cell.
15. Electron transport chain in bacteria: In bacteria; Electron transport chain is located in cell membrane.
16. Osmosis through cell membrane: It is movement of solvent molecules (generally water) from the region of less concentrated solution to the region of high concentrated solution through a semi permeable membrane. Here the semi permeable membrane that helps in osmosis is the cell membrane. Eg: Root cells take up water from the soil by osmosis
17. Carrier proteins for active transport: They occur in the cell membranes and control active transport of substances. Example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer or plasma membrane.
18. Plasma Membrane enzymes: Many enzymes are present on the plasma membrane with wide variety of catalytic activity. Example: Red blood cell plasma membranes contain a number of enzymes such as ATPases, anion transport protein, glyceraldehyde 3-phosphate dehydrogenase, protein kinases, adenylate cyclase, acetylcholinesterase.
19. Cell Membrane Receptors: Receptor on the plasma membrane performs signal transduction, converting an extracellular signal into an intra-cellular signal. Membrane possess receptors for hormones, neurotransmitters, antibodies and several other biochemicals.
20. Plasma membrane assisted 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. Pseudopods are temporary cytoplasmic projections of the cell membrane in certain unicellular protists such as Amoeba. Some mammalian cells such as fibroblasts can move over a solid surface by wave like undulations of the plasma membrane.