Membrane Transport Flashcards
How does a voltage clamp work?
- electrode in axon and out of axon
- measure the voltage
How does a patch clamp work?
- study native membrane
- study changes occurring, the potential across the membrane
How does a pure membrane measurement work?
- Can incorporate single protein into bilayer
- Study of opening + closing of channels in real time (minuscule changes)
Why should we have transport across membranes?
- uptake of nutrients , cofactors & minerals
- removal of toxic waste
- move biomolecules from one compartment to another ( such as in glycolysis, moving products from cytoplasm to the mitochondria)
- control movement of signalling molecules/ ions between compartments and cells
- production of ATP by H+ gradient in mitochondria
What are the different types of membrane transport?
- passive diffusion (diffusion of solutes down a concentration gradient, no proteins involved)
- facilitated diffusion (diffusion of solution down a conc gradient, proteins involved)
- active transport (requires energy, solute moves against conc gradient, proteins involved)
What is Fick’s law?
- diffusion is directly proportional to the conc gradient across the membrane
- J = -D change con/ change position
How does permeability affect diffusion across a membrane?
- water, hydrophobic molecules and small, uncharged polar molecules are able to cross straight though
- large and charged/ polar molecules and ions cannot
What is Graham’s Law?
rate of effusion or diffusion of a gas is inversely proportional to the square root of its molecular weight
- the bigger the molecule, the less likely to diffuse
- more hydrophobic, more likely to diffuse
What is effusion?
The process in which gas escapes through a small hole
What experiment was used to conclude that membrane associated proteins must be involved with facilitated diffusion?
- isolate membranes from RBC and measure permeability constant for different solutes
- experiment undertaken in phospholipid membranes alone all conform to ideality
- experiment with RBC membranes & protein denaturant mean that all solutes now conform to ideality
What is facilitated diffusion?
- movement down a conc gradient, requiring proteins
What is the facilitated diffusion kinetics equation? (Hint: similar to MM)
J = Jmax[S] / Km + S
What factors affect facilitated diffusion?
- temp
- pH
- protein denaturing agents
- solute specificity (e.g Glucose transporter)
What are the methods of facilitated diffusion?
- using selective channels
- using carriers
What are ionophores?
a substance which is able to transport particular ions across a lipid membrane in a cell
- penetrates through membrane and releases
What are the types of selective channels?
- non-gated
- ligand gated
- electrically or potential gated
- stress gated
Describe the characteristics of non-gated selective channels? Inc example
- always open
- selectively through charge and site
- e.g. aquaporins
- highly selective
- only let water in
- impermeable to charged molecules
Describe the characteristics of ligand gated selective channels? Inc example
- ligand alters conformation allowing molecule to go through
- e.g. nicotinic acetylcholine receptors (nAChR)
- binding site at the top
- conformational change opens iron pore
- ACh/ nicotine is the ligand
- once bound it twists, allows Na/K channels to open
Describe the characteristics of potential gated selective channels? Inc example
- change in potential (such as in axons) opens the gate
- e.g. Voltage gated sodium channels (NavMs)
- channel domain in the middle
- changes in charge causes domain to change position and pivot in (mechanical change)
Describe the characteristics of stress gated selective channels? Inc example
- under pressure, conformational change occurs and gate opens
- e.g. MscL
- lots of water intake thins the membrane due to osmotic pressure
- molecule stretches out, opening of pore and release of pressure
How do specific plasma membrane carriers work?
- binding of one side causes conformational change and opens on the other side
- never opens up on both sides at the same time
How do co-transporters work?
- antiporters and symporters utilise the energy of movement of one solute down its conc gradient to move another solute against its gradient
What are the 2 types of co-transporters?
Symporters
- both solutes go in the same direction
Antiporters
- solutes go in the opposite direction
How does primary active transport occur?
- requires energy in the form of ATP
- transports solute against the concentration gradient by directly hydrolysing ATP
Why do pumps and co-transports transport solutes at a slower rate than channels?
Pumps, co-transporters and carriers have to undergo numerous conformational steps in order to transport the solute
- channels are a pore and so allow fast movement
How does group translocation work?
- multicomponent system
- example is the PEP group translocation phosphotransferase system
- utilises energy from glucose influx through the membrane to translocate other solutes
How can ion gradients be used to transport other molecules? Glucose example, Amino acid example, Sugar example.
- using co-transporters
- glucose uptake into mammalian cells using Glucose/Na+ co-transporter
- amino acids uptake into cells using a.a./Na+ co-transporter
- sugar uptake into bacteria cells such as in the lac operon using Lactose/H+ co-transporter
How does the lactose permease co-transporter work?
- proton binds first
- lactose binds after
- flips and causes conformational change
- release of the lactose first
- when releasing the H+, the protein changes back
What is the structure of the sodium potassium pump? (Na+/K+ ATPase)
Tetramer α2 β2. The α subunits have 10 α-helices. There is an A domain (Actuated domain) that sends the signal for conformation change. There is a P domain (Phosphorylated domain) where binding occurs. There is a N domain (Nucleotide binding domain)
What happens during the action of the sodium potassium pump work?
- Active transport
- Each ATP hydrolysis= 3 Na+ pumped out and 2K+ pumped inside.
What is the affinity like in the E1 and E2 conformations of the sodium potassium pump?
E1: (inside face) - high affinity sites for Na+ - low affinity sites for K+ E2: (outside face) - high affinity sites for K+ - low affinity sites for Na+
How does the sodium potassium pump work?
The pump exists in 2 major conformations, E1 and E2. During E1: (inside face) 1. Binding of ATP opens up for 3Na binding 2. Phosphorylation occurs (ATP ------> ADP + P) - causes conformational change - stimulates the release of 3Na outside During E2: (outside face) 3. Exchange Na for 2K - stimulates hydrolysis of Pi - release of Pi 4. Release of 2K inside Back to the beginning
What does Ca2+ regulate?
- neurotransmitter function
- hormone function
- muscle contraction
- cell growth / differentiation
- cell death
How does Ca2+ regulate heart muscle contraction?
- A high conc of Ca2+ outside means that the calcium moves in to the heart cell
- leading to contraction of the cell - Na/Ca exchange stops contraction to happen all the time as calcium leaves the cell
- Ca leave the cell and Na goes into the cell - Na/K pump releases Na out of the cell to maintain the gradient (low Na in cell) so that step 2 can occur
How do poisons/ drugs increase heart rate?
Drugs inhibit step 3 (Na/K pump) so that Na+ stays in the cell
- this means that the conc. gradient will not be as steep anymore
- Ca2+ is able to stay in the cell and contract the heart for longer
How does the Ca2+ ATPase protein work?
The pump exists in 2 major conformations, E1 and E2. E1 State in endoplasmic reticulum 1. Add ATP - Opens up E1 state - High affinity for 2Ca2+ 2. Binding of 2Ca2+ - Makes ATP------>ADP+P E2 state: 3. Conformational change 4. Release of 2Ca2+ CYCLE
