Cell membranes, transport and signalling.

• The roles of membranes:
• seperating cell contents from the outside environment
• seperating cell components from the cytoplasm
• cell recognition and signalling
• holding metabolic pathways together
• regulating transport of materials in and out of the cell

Phospholipids:

The head is hydrophillic and the tail is hydrophobic. Phospholipids form a bilayer because there is water surrounding them, the heads point out towards the water and the tails point inwards. In this state the phosopholipid molecules can move freely, just like the molecules in a fluid, hence the name fluid membrane.

Just like cells, membranes are specialised too...

• The plasma membranes of a cell in a growing plant contain receptors to detect the molecules that regulate growth
• Muscle cell membranes contain alot of channel protiens to allow rapid uptake of glucose to provide energy for muscle contraction
• The membranes of white blood cells contain special protiens that enable the cells to recognise foreign cells and particles.
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Permeability:

All membranes are permeable to water because water diffuses through the cell membrane via osmosis. Some membranes are1000 times more permeable to water because they contain aqua porins (protien channels that allow water molecules through them)

Parts of the cell membrane:

• Extrinsic and intrinsic protiens (extrinsic partly embedded…