2.5.1 The Cell Surface Membrane

Question 1

What is the fluid mosaic model?

Answer 1

• Diagram of membranes
• Describes them as ‘fluid’
• Phospholipids and proteins can move around via diffusion (phospholipids move sideways within own layers)
• Lots of types of proteins within bilayer, although some fixed
• ‘Mosaic’ because scattered pattern of protein looks like mosaic from above

Question 2

Phospholipids notes

Answer 2

• Tails form hydrophobic core
• Phospholipid bilayer acts as barrier to most water-soluble substances (non-polar fatty acid tails prevent polar molecules or ions from passing through membrane)
• So sugars, amino acids and proteins cannot leak out of cell and unwanted water soluble molecules can’t get in
• Can be chemically modified to act as signalling molecules by moving within bilayer to activate other molecules, or by being hydrolysed to release small water-soluble molecules binding to specific receptors in cytoplasm

Question 3

Cholesterol notes

Answer 3

• Increases fluidity of membrane
• Stops it from becoming too rigid at low temperatures
• Stops phospholipids from packing too close together
• Stops it from becoming too fluid at high temperatures
• Binds to hydrophobic tails of phospholipids stabilising them and packing them closer
• So increases mechanical strength and stability of membranes

Question 4

Glycolipids and Glycoproteins notes

Answer 4

• Contain carbohydrate chains on surface enabling them to act as receptor molecules
• Act as signalling receptors for hormones and neurotransmitters
• Receptors in endocytosis
• Receptors in adhesion and stabilisation (forms hydrogen bonds with water molecules surrounding cell)
• Antigens for cell to cell recognition

Question 5

Transport proteins notes

Answer 5

• Hydrophilic channels allowing ions and polar molecules to travel through membrane
• Channel proteins
• Carrier proteins (change shape to transport substance across membrane)
• Specific to particular ions or molecules