- diffusion is directly proportional to the conc gradient across the membrane - J = -D change con/ change position

Membrane Transport Flashcards by Rhea K

How does a voltage clamp work?

electrode in axon and out of axon

- measure the voltage

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How does a patch clamp work?

study native membrane

- study changes occurring, the potential across the membrane

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How does a pure membrane measurement work?

Can incorporate single protein into bilayer

- Study of opening + closing of channels in real time (minuscule changes)

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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

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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)

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What is Fick’s law?

diffusion is directly proportional to the conc gradient across the membrane
J = -D change con/ change position

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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

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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

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What is effusion?

The process in which gas escapes through a small hole

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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

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What is facilitated diffusion?

movement down a conc gradient, requiring proteins

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What is the facilitated diffusion kinetics equation? (Hint: similar to MM)

J = Jmax[S] / Km + S

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What factors affect facilitated diffusion?

temp
pH
protein denaturing agents
solute specificity (e.g Glucose transporter)

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What are the methods of facilitated diffusion?

using selective channels

- using carriers

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What are ionophores?

a substance which is able to transport particular ions across a lipid membrane in a cell
- penetrates through membrane and releases

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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?

1. A high conc of Ca2+ outside means that the calcium moves in to the heart cell - leading to contraction of the cell 2. 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 3. 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 ```