Biology AS F211 (Cell Membranes)

Membranes have several specific roles.....

- The first and primary reason is usually to seperate the contents of the cell from the environment.

- Seperates the cell contents from the cytoplasm.

- cell recognition and signalling

- holds some metabolic pathways open.

- Regulates the transport of materials into and out of cells.

The cell mebranes are partially permeable barriers

Phospholipids - These are what construct the actual mebrane with their hydrophilic (water loving) heads pointing outwards and there hydrophobic tails pointing inwards. When there is movement in the cell tiny gaps appear in this membrane allowing small molecules through, but bigger ones need a channel.

Intrinsic proteins - These proteins are intrinsic because they go all the way through the cell, for example carrier and channel proteins, these proteins allow the transport of larger molecules through the membrane. Channel proteins move bigger molecules through the membrane, but the carrier proteins actively transport large molecules through the membrane.

Glycolipids and Glycoproteins - These sites are created when a carbohydrate chain bonds with either a phospholipid (Glycolipid) or a protein (Glycoprotein). These function as receptors in cell signalling by allowing hormones to bind to them.

Cholesterol - This sits in the phospholipid bilayer and prevents water ions entering the cell through its tiny gaps and this regulates the membranes fluidity.

As temperature increases molecules gain more energy. This causes the phospholipids in the bilayer to move and creates gaps allowing substances which would not normally enter or leave the cell to enter and leave.

Beetroot in hot conditions will release the red pigment found in its cells.

Cells which live in hot environments will have more cholesterol in order to prevent molecules leaving.

Cell signalling -  these are processes which allow cells to co-ordinatw inbetween each other. This can be seen when Hormones bind to the active sites of cells in order to deliver a message.

Certain drugs have been created that are used to block the receptors of certain cells, this happens when the shape of the hormone is complementary to that of the active site.

These are processes which involve no energy input from the cell like diffusion and osmosis which allows materials to move in and out.

This is the movement of large amounts of materials when it is bringing materials in it is endocytosis, and exocytosis when they are being moved out. Energy in the form of ATP is needed to fuse membranes and pinch them off for transport.

Examples....

- Hormones in the pancreas

- In plant cells where some products are moved outside so that they can build the cell wall.

This is an active process.

Exo - out

Endo - in

Pino - Liquid

Phago - Solid

osmosis is the process by which water moves down a water potential gradient in order to reach equilibrium.

The reason it does (Just a bit background info here) all water molecules continuosly the fact that there are more doesnt increase how fast they move and it definetly doesnt specify a direction the reason they move to an area of lower water potential is because the more of them there is the higher the higher the chance that they will move there, they move until they reach equilibrium because this is the point where each side has an equal chance of water moving across to it.

When there is a much lower water potential in the cell water will move in, this will cause the cell to swell, in an animal cell there is no cell wall so the cell will burst and become  Haemolysed. A plant cell will not burst and instead becomes turgid.

If there is a uch lower water potential outside the cell, water will move out. this causes an animal cell to shrivel and become crenated. A plant cell will exerience plamolysis, where the cell membrane moves away from the cell wall.