Under electron microscope it is a bilayered structure. This structure is delicate and elastic. It has some ability to repair itself.
Models of Cell Membrane Structure:
There are two models of cell membrane structure.
(i) Unit Membrane Model: According to this model the cell membrane is composed of lipid bilayer and sandwiched between two protein layers. This structure is present in all cell organelles (e.g. mitochondria, chloroplast etc.)
(ii) Fluid Mosaic Model: According to the fluid mosaic model, the cell membrane consists of a double layer of phospholipid molecules, known as a lipid bilayer. It has proteins and other molecules. The name fluid mosaic is used because the bilayer is a very fluid structure and it contains a ‘mosaic’ of protein molecules.
Cell membranes contain phospholipids 20-40%, proteins 60-80%, cholesterol and polysaccharides.
1. It is about 7 nm thick.
2. The basic structure is a phospholipid bilayer.
3. The hydrophilic phosphate heads of the phospholipids face outwards. The result is the aqueous environment inside and outside the cell.
4. The hydrocarbon tails face inwards and create a hydrophobic interior.
5. Most protein molecules float about in the phospholipid bilayer forming a fluid mosaic pattern.
6. The proteins stay in the membrane because they have regions of hydrophobic amino acids which interact with the fatty acid tails to exclude water.
7. Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoproteins and glycolipids respectively.
8. Membranes also contain cholesterol. Like unsaturated fatty acids cholesterol disturbs the close packing of phospholipids and keeps them more fluid. This can be important for organisms living at low temperatures when membranes can solidify. Cholesterol also increase flexibility and stability of membranes. Without it’ membranes break up.
9. The two sides of a membrane differ in composition and function.
Functions of Membranes:
1. The phospholipid bilayer provides the basic structure of membranes. It also restricts entry and exit of polar molecules and ions.
2. Channel protein and carrier proteins are involved in the selective transport of polar molecules and ions across the membrane.
3. Some proteins act as enzymes for example the epithelial cells on some parts of the gut contain digestive enzymes.
4. Some proteins act as receptor molecules for chemical signaling between cells.
5. Some proteins act as Antigens – these act as cell identity markers.
6.Glycolipids and glycoproteins help cells to recognize each other — allowing the immune system to tell the difference between body cells and invading bacteria.
7. Energy transfer: In Photosynthesis and Respiration proteins take part in the energy transfer systems.
TRANSPORT ACROSS THE CELL SURFACE MEMBRANE
Diffusion and Facilitated Diffusion:
Diffusion is the movement of molecules or ions from a region of their high concentration to a region of their low concentration. The process is passive (does not require energy and happens spontaneously).
Two factors affect the rate of diffusion:
1. Difference in concentration between point A and point B: the steeper the gradient, the faster the rate of diffusion.
2. The greater the surface area of a membrane through which diffusion is taking place, the greater the rate of diffusion.
Some substances enter and leave cells much faster than you would expect if only diffusion occurred. We now know that some membrane proteins facilitate the diffusion of some substances across the cell membrane. Two types protein are responsible for facilitated diffusion.
Specific carrier proteins take particular substances from one side of the membrane to the other.
Ion channels are proteins that open and close to control the passage of selected charged articles.
Channel proteins have a fixed shape.
Osmosis is the passage of water molecules from a region of their high concentration to a region of their low concentration through a partially permeable membrane. We can say that it is a form of diffusion in which only water molecules move.
Active transport is the energy-consuming transport of molecules or ions across a membrane against a concentration gradient. Movement is usually in one direction only, unlike diffusion which is reversible. The energy is supplied in the form of ATP made in respiration. Without respiration, active transport is therefore impossible.
Endocytosis and Exocytosis:
Endocytosis and exocytosis are active processes involving the bulk transport of materials through membranes, either into cells (endocytosis) or out of cells (exocytosis).
Endocytosis occurs by an enfolding or extension of the cell membrane to from a vesicle. It is of two types.
1. Phagocytosis (‘cell eating’)-material taken up is in solid form. Cells specializing in the process are called phagocytes and are said to be phagocytic. For example, some white blood cells take up bacteria by phagocytosis. The sac formed during uptake is called a phagocytic vacuole.
2. Pinocytosis (‘cell drinking’)- material taken up is in liquid form. The vesicles formed are often extremely small, in which case the process is known as micropinocytosis and the vesicles as micropinocytotic vesicles.
Exocytosis is the reverse process of endocytosis. Waste materials may be removed from cells, such as solid, undigested remain from the pancreas is achieved in this way. Plant cells use exocytosis to export the materials needed to form cell walls.