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Biological membranes Typeit
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A membrane controls what enters and leaves a cell. It is partially permeable and consists of a bilayer, which only substances can pass through, and a range of other biological substances interspersed throughout and bound to the surface of it. molecules restrict the movement of other molecules in the membrane, while glycolipids and attached to the surface of the membrane can act as receptors in cell signalling.
There are two important types of protein that span the membrane. Channel proteins provide a way for hydrophilic molecules to cross the bilayer, and change shape when a specific substance binds to them, allowing it to be transported across the membrane. Both of these processes are part of diffusion. Like simple diffusion, this is a passive process which moves substances their concentration gradient.
In contrast, moves substances from an area of low concentration to an area of high concentration. It requires energy from the hydrolysis of , and also involves carrier proteins.
is the movement of water across a partially permeable membrane. Water moves from an area with a high water potential to an area with a lower water potential, which is important in maintaining the concentration of solutes in animal cells. If the water potential of a cell is much than that of the surrounding solution, water may enter at a rate which causes the cell to swell and burst. Plant cells are protected from bursting by their , which keeps them turgid.
The permeability and stability of membranes can change based on the conditions. Permeability and fluidity increase as increases because phospholipids have more kinetic energy and move around more in the bilayer. The addition of such as ethanol and acetone can also damage the structure of membranes.