Plasma membrane

• Membranes keep components in the cell, control movement in and out of the cell and allow a cell to change shape.
• Phospholipids are lipids made from two fatty acid tails attached to a phosphate head. When exposed to water they form a micelle or bilayer.
• The phosphate head is polar and hydrophilic, while the fatty acid tails are non-polar and hydrophobic.
• In structures, the hydrophilic heads face the water, and the hydrophobic tails point inwards away from the water.
• Plasma membranes are made up of intrinsic and extrinsic proteins, glycoprotiens and cholesterol.
• Intrinsic proteins are carrier and channel proteins. They transport substances, that can't diffuse across the membrane such as large polar molecules.
• Extrinsic proteins may be free on the membrane surface or bound to an Intrinsic protein. They can act as receptors for hormones or are involved in cell recognition. Many are glycoproteins.
• Cholesterol is important in controlling membrane fluidity. It's positioned between phospholipds. The more cholesterol, the less fluid and permeable the membrane. It stops the membrane becoming too fluid.
• Glycoproteins are intrinsic proteins with an attached carbohydrate chain. They play a role in cell signalling, cell adhesion and acting as receptors for neurotransmitters and hormones.
• Small non-polar molecules e.g O2 and CO2 rapidly diffuse across a membrane. Small polar molecules e.g H2O and urea, diffuse across, but much slower. Charged particles (ions) are unlikely to diffuse across a membrane, even if they are very small.