Membrane structure and function Flashcards
Common features of a membrane
- Non covalent assemblies(held by a lot of weak forces)
- Asymmetric
- Specific proteins that carry out distinctive functions
- Fluid
- Two molecules thick: closed boundaries
Electrically polarised
Lipid composition
• Hydrophobic tail
• Hydrophilic head
• Phospholipids and glycolipids are amphipathic(Have both hydrophobic and hydrophilic ends
• Readily form a bilayer in aqueous media
• Form lipid bilayer(liposomes)
○ Have clinical uses such as delivering drugs
Factors affecting fluidity of membrane(High temperature)
§ Increases fluidity
§ The fatty acid tails of the phospholipid become less rigid
§ Also makes the membrane more fluid
Factors affecting fluidity of membrane(Low temperature)
Fluidity decreases
Factors affecting fluidity of membrane(Cholesterol)
§ Cholesterol molecules slip in-between the phospholipids.
§ At low temperature, cholesterol cause fluidity to increase because the cholesterol molecule causes distance between phospholipids to increase.
§ At high temperatures, cholesterol will cause phospholipids to come in closer and bond with the cholesterol therefore fluidity decreases.
§ Increased cholesterol content leads to a decrease in membrane fluidity as hydrogen bonds form between cholesterol and phospholipid making the membrane more rigid.
§ In spur Cell anaemia, the cholesterol content increases by 25-65% causing the RBC to look abnormal
Factors affecting fluidity of membrane(Chain length)
§ Increased length increases rigidity as there are more bonds
§ Therefore less fluid
Factors affecting fluidity of membrane(Degree and extent of saturation)
§ The more saturated, the more rigid
§ Unsaturated chains contain a double bond which causes a kink in the chain, which decreases the interactions between the chains and increases fluidity
§ Straight chains have more interactions between chains therefore less fluid
Which orientation of movement is ideal for phospholipids?
Lateral movement is more rapid compared to transverse. Transverse movement is slow and requires enzymes
Membrane synthesis lipids
• Phospholipids are manufactured on the cytosolic surface of the ER and enzymes deposit these phospholipids on to the cytosolic side of the bilayer
• Scramblases remove randomly phospholipid from one half of the lipid bilayer and insert it in the other half
○ This ensures the phospholipids are evenly distributed between each monolayer
• Flippase enzymes remove specific phospholipids from the bilayer facing the exterior side and flip them into the monolayer that faces the cytosol
• Floppase moves phospholipids from the extracellular side to the cytosolic side.
These enzymes maintain the asymmetric arrangement of the bilayer
What is apoptosis?
programmed cell death
What happens to the contents of the cell during apoptosis?
The contents aren’t released
Does apoptosis stimulate any response?
No apoptosis doesn’t stimulate any response including inflammation
What does the cell undergoing apoptosis release on its membrane and what enzymes does that activate/inactivate?
• Phosphatidylserine on outside of cell: “Eat me signal”
Caspases enzymes inactivate Flippase and activate the scramblase by cleavage
Integral membrane protein
- Transmembrane region is often an alpha helix and is usually hydrophobic
- Strong non covalent bonds
- Can pass through membrane single or multiple times
- Protein is interlinked with membrane so very difficult to extract
Peripheral membrane protein
- Located on the extracellular side
- Associated by non covalent bonds
- Loosely associated with membrane therefore easier to extract
- Can be associated with lipids or proteins
