S2) Membrane Proteins & Cell Signalling Flashcards
The movement of ions and hydrophilic molecules across a membrane is mediated.
Provide reasons for this
- Maintenance of intracellular pH
- Maintenance of intracellular ionic composition
- Regulation of cell volume
- Extrusion of waste products of metabolism
What is passive transport?
Passive transport is a movement of molecules across a cell membrane, down a concentration gradient, without energy input
When does passive transport occur?
Occurs when non-polar molecules can enter and, therefore, diffuse across the hydrophobic domain of lipid bilayers
How does the rate of passive transport change with the concentration gradient?
The rate of passive transport increases linearly with increasing concentration gradient
What is osmosis?
Osmosis is a process by which molecules of water tend pass across a semipermeable membrane from a less concentrated solution into a more concentrated one
What are permeability coefficients?
- Permeability coefficient is the speed at which any substance transverses a lipid bilayer
- Low for ions and hydrophilic molecules, higher for water molecules
What is facilitated transport?
Facilitated transport is a saturable process of spontaneous passive transport of molecules or ions across a cell’s membrane via specific transmembrane integral proteins
What is active transport?
Active transport is the movement of ions or molecules, against a concentration gradient (low-high) across a cell membrane, requiring energy
What is secondary active transport?
- Secondary active transport is when the transport of one substance is linked to the concentration gradient for another substance via a co-transporter
- The primary energy source, e.g. hydrolysis of ATP, is used indirectly
Identify and describe two examples of secondary active transport
- Na+- Ca2+-exchange – inward flow of sodium down its concentration gradient drives outward flow of Ca2+ up its concentration gradient (antiport)
- Na+- H+- exchange – inward flow of sodium down its concentration gradient leads to cell alkalization by removing H+ (antiport)
Active transport is employed to overcome unfavourable chemical or electrical gradients.
How does it work?
- Movement of the transported ion or molecule must be coupled to a thermodynamically favourable reaction
- Free energy to drive active transport can come either directly or indirectly from the hydrolysis of ATP, electron transport or light
What is uniport?
Uniport: one solute molecule alone is transported from one side of the membrane to the other
Symport and antiport are two forms of co-transport.
Distinguish between them
- Symport: the transfer of one solute molecule depends on the simultaneous transfer of a second solute in the same direction
- Antiport: the transfer of one solute molecule depends on the simultaneous transfer of a second solute in the opposite direction
general function of membrane
- continuous
- highly slectively permeable
- control enclosed chemical environment
- recognition of signalling molecules
- signal generation in response to stimuli
examples of areas on the membrane with different functions
there are specific bits on the bilayer
- inetraction with basment membrane
- interaction with adjacent cells
- secretion
- transport
- synapse
composition of the membrane
40% lipid
60% proteins
examples of some phospholipid head groups
choline, amines, amino acids, sugar
names of sugar containing lipids
- cerebrosides - head group single sugar monomer
- gangliosides - multiple sugar monomer (neurosystem)
bilayer water interface
typs of phospholipid movement
flexion (tails move apart and together)
rotation
allows for lateral diffusion
flip flop (rare) phsopholipd from bottom moves to top
what does cis bonds do
casues unsaturated fatty acids and allows for flexibility
need polyunssachrated fat in diet to provide this cis bond
role of cholesterol
solid structure with a tail
stabilises membrane properties iver range of temps or conditions
how does cholesterol attach to the membrane
- it forms a bond with the phospolipid bilayer
and locks the sructure so reduces the motion of the motion and reduces the fluidity
how can cholesterol oppose itself
- reduces fluidity by attaching itself
- increases fluidity by sepertaing the phosphlipids further
therefore it stabilises the structure
lipid rafts
- cholesterol rich structures
- sphingolipids with saturated fatty acids and cholesterol
- acts as a region in the membrane where signalling molecules can scaffold together to make transfer of info efficient
- contains receptors and signalling proteins
what does cross linking of signalling receptors do
increases their affinity for rafts
what does rast clustering do
amplify the signal by bringing signal components together
evidence of proteins in membrane
functional
- F.D
- ion gradients
- specificity of cell response
biochemical
- membrane fractation + gel electrophoresis
- freeze fracture
electrophoresis
highlights presence of protiens with diffeent bands showing
freeze fracture
- membrane frozen
- crystal broken
- membrane split in half
- shadow the sections to find the structre and you can see shadows where there are holes in the structre is where protein was and you can see proteins in the structure.
three modes of motion
- confrontational
- rotational
- lateral
- NO flip flopping of proteins (their movement is quite restrained)
2 classifications of membrane proteins
- peripheral
- integral
peripheral membrane protein
- bound to surface
- via electostatic and hydrogen bonds
- affected by ph
types of amino acids found in the membrane
- small
- hydrophobic
- plar (unchargarged)
integral membrane protein
- interacts with hydrophobic
- cant be removed by ph or ionic strenght
- removed by detergents that mess with the peripheral layer so move out
what does it mean by membrane protein topology
the orientation of the protein in the bilayer
- sometimes the N terminal is inside the cell, the body is in the membrane and the C terminal sticks outside the cell
association of protein in bilayer
cytoskeleton
- spectrin lattice adhered through attachment proteins
- forms a lattice via transmembrane protein anchors
- look at workbook
hereditary spherocytosis anemia
- spectin depleted so little cytoskeleton
- eryrthrocytes round up
- less resistant to lysis
- cleared by spleen
hereditary elliptoccytosis
defect in spectrin
cant form hetertetrameters
form fragile ellptoid cells
how are secreted proteins syntheisised?
- ribsome translates rna and adds amino acid and starts in cytoplasm
- have N terminus of polypeptide, sequence of hydrophobic amino acids
- the amino acids are recognised by signal recognnition protein (SRP) and they bind
- protien held against ribosome so its unable to carry on elongate
- SRP recognised on membrane by docking protein and brings whole complex along
- then SRP dissocates and leaves a free N terminal
- syntheis continues and protein is made on the membrane, end is cleaved by peptidase and then protein in released into the ER Lumen
membrne protein biosynthesis
- same as secreted but once protein gets to the ER surface
- hydrophobic part of protein forms an interaction with membrane so acts as a stop transfer sequence
- protein locked in membrane
