Transport Across the Cell Membrane
There are 5 ways that substances can pass through the partially permeable membrane. Diffusion, Facilitated Diffusion, Osmosis, Endocytosis and Exocytosis, and Active Transport.
Diffusion is the movement of molecules or ions from an area of higher concentration to an area of lower concentration, until they are evenly distributed.
Ions and molecules are always in a state of random movement but if they are highly concentrated in one area there will be a net movement away from that area until equilibrium is reached (until there is a uniform distribution).
The rate of diffusion is affected by:
- The concentration gradient i.e. the greater the difference in the concentration of molecules in two areas, the greater the rate.
- The distance of travel i.e. the shorter the distance between the two areas the greater the rate.
- The surface area of the membrane - the larger the area the quicker the rate.
- The thickness of the membrane - the thinner the membrane the greater the rate.
- An increase in temperature results in an increase in the rate, since there is an increase in molecular energy and therefore movement.
Charged particles or ions and large molecules such as glucose do not readily pass through the cell membrane because they are relatively insoluble in lipid. In the cell membrane protein molecules span the membrane from one side to the other and help such particles to diffuse in or out of the cells.
These proteins are of two types:
Channel proteins - consist of pores lined with polar groups allowing charged ions to pass through. The channel is hydrophilic, which allows water-soluble substances to pass through. As each channel protein is specifis for each type of ion each protein will only let one particular substance throgh. They can also open and close according to the cell.
Carrier proteins - they allow diffusion across the membrane of larger polar molecules such as sugars and amino acids. A particular molecule attaches to the carrier protein at the binding site and causes the carrier protein to change its shape, releasing the molecule through the membrane.
Carrier and channel proteins increase the rate of diffusion along a concentration gradient WITHOUT the need for energy in the form of ATP.
Most cell membranes are permeable to water and certain solutes only. In biological systems osmosis is a special form of diffusion which involves the movement of water molecules only.
Osmosis is the movement of water from a higher water potential to a lower water potential across a partially permeable membrane.