2 - Lipids and Proteins in Membranes Flashcards
What is the function of a membrane?
- Selectively permeable barrier
- Communication
- Control of enclosed environment
- Recognition (signalling molecules)
- Signal generation, e.g electrical
Are all membranes the same?
No, specialised for function, e.g mitochondria different to cell membrane. Different parts of same membrane can be different too
What is the membrane composition (dry)?
- 60% protein
- 40% lipid
- 1-10% carbohydrate
Water makes up 20% wet weight
What is the main property of membrane lipids that makes them suitable for their function?
Amphipathic
What is the structure of a phospholipid?
Head: Small and polar. Can have small attachements, e.g choline
Tail: C14-C24, C16/18 most popular so uniform thickness. Unsaturated bonds, cis, cause kink to increase fluidity
What is the exception to the phospholipid structure?
Sphingomyelin. Not based on glycerol.
How do you make a glycolipid?
Take of phosphocholine head and replace with sugar.
How do lipids arrange themselves in water?
How do phospholipids move in the membrane?
How are membranes kept dynamic?
Cholesterol and cis bonds.
Reduce phospholipid packing
What is the structure of cholesterol and what is it’s function?
Regulate fluidity. Need in diet for membrane integrity.
How do proteins move and how can they be restricted?
- Everything but flip-flop (too large, need lots of KE and wouldnt do function, e.g LGIC wrong way)
- Aggregates
- Bound to cytoskeleton
- Cell adhesion
- High cholesterol areas
What is the evidence for membrane proteins?
- Facilitated diffusion
- Ion gradients
- Specificity of cell responses
- Freeze fracture (heavy metal, snow drift EM, E AND P FACE)
- Fractionation and Gel electrophoresis
What types of proteins are in the membrane and how are they removed?
Peripheral: Associated by H-bonds and electrostatic attractions. Removed by pH or ionic change, e.g salt wash
Integral: Associated with hydrophobic area. Removed by agents that compete for non-polar interactions and destroy membrane, e.g solvent/detergent
What is hydropathy?
Can tell shape of membrane protein, hydrophobic parts in membrane and hydrophilic out
What is toplogy of membrane proteins?
Membrane proteins always orientated one way for efficiency
What does the erythrocyte skeleton consist of?
A rigid lattice of long spectrin molecules adhered to transmembrane proteins by peripheral attachment proteins, preventing lysis
How can you tell what membrane proteins are in the erythrocyte membrane?
Ghost membranes. Only transmembrane are Glycophorin A and Band 3
What is spectrin?
Tetramer protein of 2 A and 2 B. Tethered to cell membrane to maintain integrity
What are two types of haemolytic anaemias due to poor functioning of the cytoskeleton?
- Hereditary Spherocytosis - Lack of spectrin so cells appear spherical. More prone to lysis, cleared by spleen
- Heriditary Elliptocytosis - Defective spectin, heterotetramers cannot form so cells are fragile and rugby ball shaped, lysis
Treatment: Blood transfusion
Why is the membrane asymmetrical?
Different compositions and functions
How are secretory proteins synthesised?
How are proteins for the inside of the cell made?
In the ribosome in the cytoplasm, there is no signal sequence
How are membrane proteins produced?
- Same as secretory proteins
- Hydrophobic stop transfer signal reached
- Ribosome detaches and completes synthesis in cytoplasm
- ER –> Cis Golgi –> Trans Golgi –> Vesicle –> Fuse with membrane, same as before
What determines where N and C terminal are?
Signal Peptidase: N in, C out
No cleavage: C in, N out
What is the role of band 3 and glycophorin A?
Integral proteins that prevent flip flop rotation
What property does spectrin give erythrocytes?
Allows them to be rigid but flexible, holding their shape
What do ankyrin and band 4.1 do?
Attach spectrin to integral proteins, restricting lateral mobility
How do membrane bilayers form in water?
- Liposomes
- Form spontaneously
- Non covalent vdw forces between hydrophobic tails
- Hydrogen bonds between hydrophilic heads
