Module 1 - Cells

• Separating cell contents from the outside environment,
• Separating cell components from cytoplasm.
• Cell recognition and signalling.
• Holding the components of some metabolic pathways in place.
• Regulating the transport of materials into or out if cells.
• Keep many large molecules, such as enzymes, within the cell.
• Permit movement by endocytosis and exocytosis.
• Permit recognition by other cells, such as those of the immune system.

The cell membrane is made up of a phospholipid bilayer plus protein. The distance across the membrane is about 7nm. The phospholipid polar heads are hydrophilic i.e. they are attracted to water. This is why they face the exterior of the cell - both these areas are dominated by water. The hydrocarbon tails of the two layers are hydrophobic and held together by weak hydrophibic bonds. Proteins are scattered about the membrane, and transmembrane proteins pass right through. Carbohydrates are attached to protein and lipid, and face the outside of the cell. This structure is called fluid mosaic.

Why is it called fluid mosaic?

• The phospholipids are 'liquid'
• Proteins are like the pieces of 'tile' surrounded by phospholipid, which are like the cement holding everything together.

Phospholipids:

• Form a bilayer that acts as a barrier between cytoplasm and cell exterior.
• Are fluid, so components can move within the membrane.
• Are permeable to non-polar molecules such as oxygen and fatty acids.
• Are impermeable to ions and large polar molecules such as sugars & amino acids.

Cholesterol:

• Stabilises the bilayer by binding to polar heads and non-polar tails of phospholipids.
• Reduces permeability to water, ions and polar molecules.
• Controls fluidity by preventing phospholipids solidifying at low temperatures and becoming too fluid at high temperatures.

Proteins:

• Are transmembrane proteins acting as channels and carriers.
• Are receptors for chemicals made by other cells e.g. hormones.

Glycolipids & Glycoproteins (lipids and proteins with carbohydrate chains attached)

• Are carbohydrate chains only found on the exterior surface of cell membranes.
• Act as receptors for signalling molecules (e.g. hormones) and for drugs.
• Act as cell surface markers that identify the cells to others (also known as cell surface antigens)
• Are involved in 'sticking' cells to one another (call adhesion)