Cell Membranes

PLASMA MEMBRANES (CELL SURFACE MEMBRNAES):

- Barrier between the Cell  and its Environment 

- Partially Permeable, hence it allows substances to enter and  leave the cell 

- Allows recognition of other cells and cell communication 

MEMBRANES WITHIN CELLS:

- Organelle membranes divide the cell into different compartments hence they are a barreir between the organelle and the cytoplasm, this makes different functions more efficient

- Form Vesicles to transport substances around the cell

- Partially  Permeable so they control what enters and leaves the cell 

- Contents of the membrane and the rest of the organelle also have barriers

- Membranes within cells can be site of chemical  reactions

PHOSPHOLIPIDS:

- Hydrophilic  head hence attracted to water, Hydrophobic  head hence it repels water 

- Phospholipids automatically arrange themselves into a bilayer, with their heads facing outwards on each side

- Centre of the bilayer is Hydrophobic, so it doesn't allow water  soluble substances (like ions) through it,the Hydrophobic centre acts as a barrier to the dissolved  substances, (Big fat-soluble substances like fat-soluble vitamins can dissolve in the bilayer and pass directly  through)

CHOLESTEROL:

- Gives the membrane Stability

- Present in all cell membranes (apart from bacterial  cell membranes)

- Cholesterol fits between phospholipids, binding to the Hydrophobic  tail

- The binding of cholesterol to the tails causes the tails to pack  closer  together

- Due to this membranes are less Fluid and More Rigid

PROTEINS:

- Control which substances enter and leave the cell

- Some form channels which allow small charged particles through

- Carrier Proteins transport molecules/ions across the membrane via Active Transport and Facilitated Diffusion 

- They act as receptors in cell signaling

- The molecule (hormone) binds to the protein which triggers a chemical reaction inside of the cell 

GLYCOLIPIDS & GLYCOPROTEINS:

- Both of the above components act as receptors for messenger molecules

- They form hydrogen bonds with surrounding water molecules (so they are polar molecule)

- Hydrogen Bond forming helps to stabilise the membrane

They are also:

                      - sites where drugs, hormones, and antibodies bind

                      - act as receptors for cell signaling

                      - antigens (for cell surface molecules involved in the immune response)