4.1 Structure of Cell Surface Membranes

%%Hydrophilic head%%, attracted to water, %%point to outside of cell surface membrane%%
%%Hydrophobic tail%%, repelled by water, %%on inside of cell surface membrane%%
%%Lipid soluble substances are able to pass through the membrane%% i.e. enter and leave the cell
%%Water soluble substances are not able to enter or exit the cell%%
Phospholipids make the membrane %%flexible and self-sealed%%

Proteins are interspersed in the phospholipid bilayer
Some proteins occur on the surface of the cell membrane and do not extend the full length, used for %%mechanical support or in conjunction with glycolipids, as receptors for hormones%%
Other proteins span the full membrane, such as %%carrier of channel proteins which allow certain molecules to pass through%%
Function of proteins in membrane is to… * Provide %%structural support%% * Act as %%channel transporting water soluble substances%% through the membrane * Allow %%active transport across membrane through carrier proteins%% * Form %%cell surface receptors%% for identifying cells * %%Help cells adhere together%% * Act as %%receptors%% e.g. for hormones

Add %%strength%% to membranes
%%Very hydrophobic so prevent loss of water%% and dissolved ions by the cell
%%Pull together fatty acid tails to prevent movemen%%t and other molecules so membrane is the correct rigidity
Make membrane %%less fluid at high temps%%

Carbohydrate covalently bonded to a lipid
Carbohydrate portion sticks out end of membrane and act as a %%receptor for specific chemicals%%
Help %%maintain stability%% of membrane
Help %%cells attach to 1 another and form tissues%%

Act as %%cell surface receptors%%, specifically for hormones and neurotransmitters
Help %%cells attach to 1 another and form tissues%%
%%Allow cells to recognise one another%%

In general most molecules do not diffuse through the membrane as they are… * %%Not soluble in lipids%% and therefore cannot pass through the phospholipid layer * %%Too large%% to pass through the channels in the membrane * Of the %%same charge as the charge of the protein channels%% so they are repelled * %%Polar%% so have difficultly passing through the non-polar hydrophobic tails

Fluid: individual phospholipid molecules %%can move relative to each other, fluid as membrane is constantly changing in shape%%
Mosaic: %%proteins embedded%% vary in shape, size and function