Cell Membranes are composed by two layers of phospholipid, which form a bilayer. Phospholipids are made up of a polar, hydrophilic area containing phosphate group bonded to glycerol and a non-polar, hydrophobic area containing fatty acids. Hydrophilic (water-loving) areas point inwards and the ‘heads’ appear on the outside of the membrane where water is present. Hydrophobic areas (water-hating) point outwards and the ‘tails’ orientate inside the double layer, away from water. Different types of proteins present in the bilayer:
1) Integral Protein: embedded in the bilayer.
2) Peripheral Protein: attached on the surface.
Transport across membranes:
The movement of substances down a concentration gradient from an area of high concentration to an area of lower concentration without the need of energy to be used. Examples of passive transport are diffusion and osmosis.
1) Diffusion: many molecules pass into and out of the cells by diffusion. No energy is required and movement occurs by way of a simple concentration gradient. Diffusion occurs where the membrane is fully permeable to the substance or where the protein channels in the membrane are large enough for it to pass through. For example, carbon dioxide molecules build up in cells because of respiratory activity, so it begins to diffuse through the plasma membrane to an area of lower concentration.
2) Facilitated Diffusion: when molecules are large, they are often transported across membranes through this process. An integral protein in the membrane forms a channel so that the substance particles can pass through them into or out of the cell.
3) Osmosis: the passive movement of water across a partially permeable membrane from an area of lower solute concentration (water here is of higher concentration) to an area of higher solute concentration (water here is of lower concentration).
The movement of cells against the concentration gradient. This means that they are pumping something (usually ions) from areas of lower to higher concentration. To move these substances into the cell against concentration gradient, the cell must use metabolic energy released from the breakdown of ATP.
Exocytosis and Endocytosis:
Cells often have to transport large chemical molecules or material in the bulk across the plasma membrane. This is done by cells releasing and taking in materials in vesicles. Export is called…