Cell Membrane Structure
Phospholipid Bilayer:
- CF: caused by a faulty transport protein in the surface membranes of epithelial cells
- Basic structure of the bilayer is two layers of phospholipid
Phospholipid Molecule:
- Only has two fatty acids; negatively charged phosphate group replaces the third fatty acid
- Phosphate head is polar (one end slightly negative and the other positive) HYDROPHILIC
- The fatty acid tails are non-polar HYDROPHOBIC
- Phospholipids arrange themselves to avoid contact between the hydrophobic tails and the water
- They may form a layer with the tails out of the water or form spherical clusters called micelles or form a bilayer
Phospholipid Bilayer:
- Cells; filled with aqueous cytoplasm and surrounded by aqueous tissue fluid
- Cell surface membrane; adopts most stable arrangement
Fluid Mosaic Model:
- Phospholipid bilayer also contains proteins, glycoproteins, cholesterol, and glycolipids
- Glycolipids; lipid molecules with polysaccharides attached
- Glycoproteins; protein molecules with polysaccharides attached
- Some proteins span the membrane whilst others are only on the outer/inner layer
- Membrane proteins have hydrophobic areas (these are inside the bilayer)
- Some proteins are fixed whilst other move around in the fluid (fluid mosaic model)
Evidence:
- Old structure; three layer protein-lipid sandwich based on election micrographs (A)
- However this structure did not allow the hydrophilic heads to be in contact with water and it does not allow hydrophobic tails to stay away from water
- Phosphate heads are more dense
Experiments:
- Showed two different types of protein; one removed from membrane easily by increasing ionic strength of solution, other could only be removed by adding detergents.
- Supported fluid mosaic model; integral membrane proteins are fully embedded in phospholipids whilst those on the outside were loosely attached
- Additional evidence; freeze fracture electron microscopy studies
- Frozen membrane sections were fractured along the weak point between the lipid layers and the inner fractured surface coated in heavy metal
- Scanning electron microscopy revealed a smooth mosaic-like surface (lipid tails) interspersed by much larger particles (integral proteins)
- Experiments carried out using labelled molecules that only attach to other specific molecules
- Showed membranes as asymmetric (the outside of the membrane is different to the inside)
Experiment; fusing mouse cells with human cells:
- A specific membrane protein was labelled in each cell type
Light microscope used to follow where points moved. |
- Mouse; green fluorescent
- Human; red fluorescent
- Immediately after fusion the coloured labels remained in their respective halves
- After 40 minutes at 370C there was a complete intermixing of proteins
- Only possible by diffusion through membrane; proved fluidity of membrane
Unsaturated Phospholipids:
- The more unsaturated; the more fluid the membrane
- ‘kinks’ in hydrocarbon tail of unsaturated phospholipids prevent them packing tight together (making more movement)
Protein function:
- Enzymes
- Carrier and channel proteins
- Glycoproteins and glycolipids have important roles in cell-to-cell recognition as receptors