Membrane Bilayer and permeability Flashcards
Describe membrane composition
40% lipid, 60% protein, 1-10% carbohydrate
Give some general functions of membranes
Continuous, highly selective permeability barrier
Control of enclosed chemical environment
Recognition of signalling molecules, adhesion proteins, immune surveillance
Signal generation in response to stimuli
Describe the structure of phospholipids
Glycerol, 2 fatty acids, phosphate-head group
Range of polar head groups eg. choline, amines, amino acids, sugars
Length of FAs is between C14 and C24, mostly C16-C18
Cis double bond gives kink in unsaturated chains to reduce phospholipid packing, increasing fluidity and reduce ability to form 2D crystals
What movements can phospholipids undergo?
Flexion
Rotation
Flip flop
Lateral diffusion
Describe the structure of glycolipids
Glycolipids are lipids containing a sugar (not glycerol)
In CEREBROSIDES the head group is a sugar monomer
In GANGLIOSIDES the head group is an oligosaccharide
Describe the structure of cholesterol
Has a polar hydrophilic head group
Rigid planar steroid ring structure
Non-polar hydrocarbon tail
What is the function of cholesterol in membranes?
It abolishes the endothermic phase transition phospholipids by forming hydrogen bonds with glycerol and altering chain motion:
It reduces fluidity at high temps (reduced chain motion)
It increases fluidity at low temps (reduced packing)
What is the evidence for protein in membrane bilayers?
Functionally - facilitated diffusion, ion gradients, specificity of cell responses
Biochemically - membrane fractionation, SDS page and electrophoresis
What movements can proteins undergo?
Conformational change
Rotation
Lateral diffusion
(NO FLIP FLOP)
What restricts the movement of proteins?
Aggregation - slows movement
Tethering to BM or cytoskeleton - allows no movement
Interaction with other cells
Lipid mediated effects - proteins tend to separate out into the fluid phase, or cholesterol poor regions
Describe differences between peripheral and integral proteins
Peripheral are bound to surface of membrane by hydrogen bonds and electrostatic interactions. Can be removed by changes in pH or ionic strength.
Integral are embedded completely in the membrane, cannot be removed pH or ionic strength, removed by detergents and organic solvents.
Describe the general structure of an integral protein
R-groups of amino acids in the transmembrane domains are largely hydrophobic
Often alpha-helical
18-22 amino acids span bilayer
Can have a single or multiple transmembrane domains
Describe the mechanism of membrane insertion of integral proteins
When the protein is being translated into the ER lumen the ribosome comes across a highly hydrophobic STOP TRANSFER SIGNAL.
This remains in the ER membrane and the rest of the protein is translated outside of the ER in the cytoplasm.
Proteins with multiple transmembrane domains contain multiple stop and start transfer signals.
Why are hydropathy plots useful?
Can see how many transmembrane domains the protein has
Why is membrane asymmetry important?
Asymmetrical orientation is important for function eg. A receptor for a hydrophilic extracellular messenger molecule must have its recognition site directed towards the extracellular space
Describe the main features of the fluid mosaic model
Fluid - because of the hydrophobic integral components that can move laterally throughout the membrane, meaning it is not solid
Mosaic- because it is made up of many different parts
Describe the structure and function of the erythrocyte cytoskeleton
Holds the shape of erythrocytes, and attachment of integral proteins to the cytoskeleton restricts lateral mobility of the proteins.
It is composed of SPECTRIN and ACTIN molecules, which are attached to the membrane by adapter proteins ANKYRIN (band 3) and GLYCOPHORIN (band 4.1)
Describe Hereditary Sherocytosis
Spectrin depleted by 40-50%
Erythrocytes round up, become more spherical and increased lysis of the cells reduces the lifespan of RBCs
The inability of the bone marrow to compensate leads to haemolytic anaemia
Describe Hereditary Elliptocytosis
Defect in spectrin means that the molecules are unable to form heterotetramers
Erythrocytes round up and become ellptical
Increased lysis and inability of bone marrow to compromise leads to haemolytic anaemia
What properties of solutes affect their movement through the membrane?
Hydrophobic molecules can pass through
Small uncharged polar molecules can pass through
Large uncharged polar molecules cannot pass through
Ions cannot pass through
Distinguish passive diffusion, facilitated diffusion and active transport
Passive: dependent on permeability and concentration/electric potential gradient to provide energy, rate increases linearly with increasing concentration gradient.
Facilitated: permeability for substance increases by incorporating specific protein into bilayer, may be carrier molecules or protein channels
Active transport: movement agains conc/electric gradient requiring energy,
Describe features of channel proteins
May be gated and open and close in response to stimulus.
More than one type of molecule maybe transported: Co-transporters
Uniport= single transported molecule, one direction Symport= two transported molecules, one direction Antiport= two transported molecules, opposing directions
What determines whether movement across the membrane is passive or active?
Whether or not energy is required is determined by the free energy change of the transported species, this is determined by the concentration gradient and electrical gradient across the membrane
NaK-ATPase
Uses ATP to pump 2K in and 3Na out (Antiport) forming the Na and K gradients that are necessary for excitation.
P-type ATPase as ATP phosphorylates aspartate producing a phosphoenzyme intermediate
Alpha subunit- binding sites for ions
Beta subunit- directs pump onto the surface
Drives secondary active transport for control of pH, cell volume, Ca concentration, Na absorption in epithelia, nutrient uptake
