What shows the existence of the plasma membrane
1) Cell swelling (hypotonic) and shrink 2) Escape of cell contents if mechanically ruptured
3) Compartmentalisation needed for biochemical activity of cells.
Describe the structure of the plasma membrane:
- Lipid molecules make up 50% of the mass of most animal cell membranes
- Are asymmetrically distributed
- Are amphiphilic: hydrophobic and hydrophilic regions
How do lipid molecules maintain favourable position?
- They spontaneously aggregate to keep their hydrophobic tails inside and hydrophilic heads to water
What does aggregation style depend on?
Depends on shape:
- Cone shaped - Lipid molecules (single chained) form micelles
- Cylinder shape - Lipid molecules (double chained) form bilayers
What are the 2 types of phospholipids?
Glycerol-based (glycolipid) - Phosphoglyceride Sphingosine based (sphingolipid)
What are the main phosphoglycerides in the plasma membrane?
- Phosphatidylcholine
- Phosphatidylethanolamine
- Phosphatidylaserine
- Phosphatidyllinositol
State the phospholipid structure
- Polar head (usually choline, ethanolamine, serine or inositol)
- Glycerol or sphingosine
- Lipid tail(s)
How are glycolipids formed?
By the addition of carbohydrate group(s) to lipids
What are glycospingolipids?
Combination of glycerol and spingosine based.
Example: Cerebrosides & gangliosides
Structure of glycolipid
- Carbohydrate head group
- Lipid backbone
- Glycerol or sphingosine based
Example of glycolipid?
Blood group antigens
What do sterols have large amounts of?
Cholesterol
What are the functions of Sterols?
- Affects PM fluidity
- Increased permeability properties of PM
- Maintains stability and integrity of PM
Describe the 3 types of movement a phospholipid molecule can carry out?
- Rotation about its long axis
- Lateral diffusion by exchanging places with near molecules in the same layer
- Transverse diffusion (flip-flop) from one monolayer to the other (rare)
What happens to membrane fluidity when temperature is altered?
Temp increases - MF (lateral diffusion) increases
Temp decreases - MF decreases
What buffers fluidity changes over a temperature range?
Cholesterol - Supports membrane fluidity
How can composition of phospholipids affect fluidity?
- Shorter chain length: tails do not interact as often
- Cis-double bonds - produce kinks in HC chains. Difficult to pack together.
What are the 3 classes of membrane proteins?
- Integral
- Peripheral
- Lipid anchored
Examples of membrane proteins
- Transporters
- Receptors
- Cell-cell interaction proteins
- Enzymes
Describe integral proteins
Embedded within bilayer
Alpha helix areas are 20 amino acids long
Hydrophobic segments have affinity for hydrophobic interior of bilayer
Hydrophilic regions extend outward from membrane into aqueous phase
Describe peripheral proteins
Located on surface of PM
Attached through ionic interactions (not intrinsic)
Describe lipid anchored proteins
Modified proteins attached to glycolipid
Lipid part of glycolipid anchor is embedded in hydrophobic region of the membrane
Describe lipid rafts
Clusters of lipids and proteins within membrane
Rich in cholesterol and saturated fatty acid tails.
Associated with signalling units. May be transient.
