Cell Membranes and Transport

• The fluid mosaic model: phospholipid bilayer, proteins, cholesterol, glycolipids and glycoproteins.
• Roles of membrane components in stability, fluidity, permeability, transport, signalling and recognition.
• Cell signalling: ligand secretion, transport, binding to receptors.
• Simple and facilitated diffusion, osmosis, active transport, endocytosis, exocytosis.
• Practical work with Visking tubing, agar and plant tissue.
• Surface area to volume ratio calculations and the agar block investigation.
• Estimating the water potential of plant tissue; effects of osmosis on plant and animal cells.

The fluid mosaic model

The cell surface membrane is a phospholipid bilayer about 7 nm thick with proteins scattered in it.

• Phospholipids arrange themselves so the hydrophilic phosphate heads face the watery cytoplasm on one side and the watery tissue fluid on the other, while the hydrophobic fatty acid tails point inwards, away from water. This forms spontaneously in water.
• Fluid: phospholipids and many proteins can move sideways within their layer.
• Mosaic: proteins are scattered through the bilayer like tiles in a mosaic.
• Intrinsic (transmembrane) proteins span the bilayer: regions with hydrophobic R groups sit among the tails; hydrophilic regions face the water on each side. Extrinsic (peripheral) proteins sit on the surface, held by hydrophilic interactions.
• Cholesterol molecules lie between phospholipids in both layers, their hydrophobic part among the tails and the –OH group near the heads.
• Glycolipids and glycoproteins carry carbohydrate chains on the outer surface only, facing the tissue fluid; together these form the glycocalyx.

What each component does

From one cell to a specific response in another

1. Secretion ligand
2. Transport
3. Binding