Membrane bilayer & proteins (1) Flashcards Preview

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Flashcards in Membrane bilayer & proteins (1) Deck (27)
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What is the function of a membrane bilayer?

- Continuous, highly selective permeability barrier
- Controls enclosed chemical environment
- Communication
- Recognition (signalling, adhesion, immune surveillance)
- Signal generation in response to stimuli


What are the dry mass weight proportions of a membrane's composition?

- Lipid: 40%
- Protein: 60%
- Carbohydrate: 1-10%


What proportion of a membrane bilayer's weight is water?

- 20%


Outline the structure of a phospholipid.

- 2 FA chains: may have kinks
- Glycerol molecule
- Interchangeable head


What is the glycerol and FA chains together called?

- Phosphatidyl backbone


What do the heads to the phospholipid have to be? Give some examples.

- Polar
- Choline/serine/ethanolamine/inositol.


What causes the kink in the FA chain?

- Cis double bond.


What are glycolipids?

- Sugar containing lipids base on sphingomyelin backbone


What are the different types of glycolipids?

- Cerebrosides: Head group sugar monomer
- Gangliosides: Oligosaccharides head group


What four types of phospholipid motion?

- Flexion (dynamic vibration)
- Rotation
- Lateral diffusion
- Flip-flop (rare as hydrophobic through hydrophilic environment)


What is the structure of cholesterol?

- Polar head
- Rigid planar steroid ring structure
- Non-polar hydrocarbon tail


How does cholesterol join to other phospholipid molecules?

- Via beta-OH group


What is the role of cholesterol in membranes?

- Maintains integrity to prevent crystalline islands
- Reduces phospholipid packing: increases fluidity
- Reduces phospholipid chain motion: decreases fluidity (helpful at high temps as reduces vibration so less damage)


What is the evidence for proteins in membranes?

- Facilitated diffusion
- Ion gradients
- Specificity of cell responses

- Membrane fractionation and gel electrophoresis
- Freeze fracture


What are the motions of proteins in a bilayer?

- Conformational change
- Rotational
- Lateral
- NOT flip-flop


Why can't proteins in membranes flip-flop?

- The energy requirement to move such a large hydrophobic molecule through a hydrophilic environment is too high
- Too destructive to move


What restrictions of membrane protein mobility is there?

- Aggregates (greater mass harder to move)
- Tethering (cell wall)
- Interactions with other cells


How are there restrictions on mobility for proteins in bilayer?

- Lipid mediated effects
- Proteins tend to separate out into fluid phase/cholesterol poor region
- Association with extra-membranous proteins.


What are peripheral membrane proteins?

- Bound to the surface
- Electrostatic and H bond interactions
- Removed by changes in pH/ionic strength


What are integral membrane proteins?

- Interact extensively with hydrophobic domains of lipid bilayer
- CAN'T be removed by pH/ionic strength
- Removed by non-polar interactions (detergents/organic solvents)


What are the characteristics of transmembrane domains?

- Alpha helical
- R groups of AA are hydrophobic


How can proteins be locked into place so they all face in the same direction?

- Post-translational lipid modifications.


What bands do erythropoietin proteins contain, why and where?

- Band 3
- Glycophorin
- Flexibility
- Adhered to inside face of cellular membrane


What is hereditary spherocytosis?

- Spectrin decreased by 40-50%
- Erythrocytes become more rounded
- Become less resistant to lysis
- Cleared by spleen


What is hereditary elliptocytosis?

- Defect in Spectrin molecule
- Unable to form heterotetramers
- Fragile elliptoid cells


Outline secreted protein biosynthesis.

- SRP from docking protein recognises ribosomes with signal sequence.
- Ribosomes with signal sequence pass replicated protein strands through the membrane to the ER lumen via signal sequence receptors.
- Replicated protein joins to signal peptidase to prevent further replication.
- Protein detaches from ribosome to ER lumen
- Ribosomes separate and return to cytoplasm.


How are membrane proteins biosynthesis?

- After replicated protein is passed through membrane, signal sequence receptors close the pore.
- Ribosomes continue to replicate then signal peptidase stops further replication.
- Protein is trapped between ER lumen and cytoplasm.