L11. Transport across cell membrane Flashcards
what do membrane transport proteins facilitate
the movement of water soluble molecules
what does the rate of diffusion depend on
molecular size and solubility
rate of diffusion - molecules ranked from highest to lowest diffusion
- small nonpolar molecules (all pass through)
- small uncharged, polar molecules
- larger uncharged polar molecules
- ions (none pass through)
ion concentrations - Na+
- sodium
- low inside cell
- high outside cell
ion concentrations - K+
- potassium
- low inside cell
- high outside cell
ion concentrations - Ca2+
- calcium
- really low inside cell
- high outside cell
explain the two components of an electrochemical gradient
- electro: membrane potential
- chemical: concentration gradient
- they can either work together or oppose each other
two components of an electrochemical gradient - what happens as concentration gradient and membrane potential work together
increases solute movement
two components of an electrochemical gradient - what happens as concentration gradient and membrane potential oppose each other
the electrochemical driving force is decreased
what solutes can transporters and channels move across the membrane
inorganic ions and small, polar organic molecules
explain how transporters can move molecules
- taking a molecule from one side and physically move it to another side via conformational changes
- can be used with active or passive transport
explain how channels can move molecules
- they do not stay open
- only uses passive transport
- faster then transporters
explain active transport
- molecules are moved against the concentration gradient (less -> more)
- requires energy (not only ATP)
- only transporters can do this
explain the types of transporters
- uniport
- symport
- antiport
explain passive transport
- molecules move down their concentration gradient (more -> less)
- occurs spontaneously (no energy needed)
- can be transporter- or channel-mediated
types of transporters - uniport
- only moves one molecule
- can be against or toward the gradient
types of transporters - symport
- type of coupled transport
- both molecules go in the same direction
types of transporters - antiport
- type of coupled transport
- molecules to in opposite directions
types of transporters - what is coupled transport
using the movement of one molecule going toward the gradient to pay for the movement of another molecule moving against the gradient
what are the three mechanisms of active transport
- coupled pump
- ATP-driven pump
- light driven pump
passive transport - explain the conformational change in transporters
- outward-open
- occluded
- inward-open
passive transport with transporters - outward-open
binding sites are exposed on the outside of the cell
passive transport with transporters - occuluded
both sides are closed and binding site is not accessible
passive transporter with transporters - inward-open
binding sites are exposed on the inside of the cell
explain the Na+/K+ pump
- it uses ATP hydrolysis to pump Na+ out of cells and K+ into cells
- concentration gradient of Na+ along with the membrane potential creates an electrochemical gradient
explain the Ca2+ pump
- pumps and Ca2+ from the cytosol into the lumen of the sarcoplasmic reticulum
- uses ATP hydrolysis
what is the glucose/Na+ symporter
- uses the Na+ gradient as an energy source made by the Na+/K+ pump
- imports glucose against the gradient into the cytosol
- it uses active transport and it is a symport
what are the two glucose transporters that enable gut epithelial cells to transfer glucose
- Na+ -driven glucose symport
- passive glucose uniport
two glucose transporters - Na+ -driven glucose symport
- uses Na+ gradient as an energy source
- takes up glucose actively, creating high concentrations of glucose in cytosol
- it faces the gut lumen (apical domain)
two glucose transporters - passive glucose uniport
- release of glucose down its concentration gradient for use by other tissues
- faces the extracellular fluid (basal and lateral domains)
how do transmembrane pumps work - animal cells
the Na+/K+ pump (ATPase) establishes a Na+ gradient that facilitates transport by symports
how do transmembrane pumps work - plant cells
use H+ gradient created by the H+ pump that facilitates transport by symports
how are ion channels selective
- ions are surrounded by a shell of water that will be shed during passage through the channel
- passage is also narrow so only ions of appropriate charge and size can go through
how are ion channels gated
- they are not continuously open
- they are gated by specific stimuli
how are ion channels gated
- voltage-gated
- ligand-gated (extracellular or intracellular ligand)
- mechanically-gated
membrane potential - when is it zero
when there is an exact balance of charges on each side of the membrane
membrane potential - when is it non-zero
when K+ leak channels cause K+ to leave the cell, creating an imbalance in the membrane
how does the [K+] gradient and K+ leak channels aid in generating membrane potential
- K+ is transported into the cell via Na+/K+ pump
- but as soon as K+ leak channels open, K+ leaves the cell and negative ions don’t cross
- this causes a charge imbalance that gives rise to membrane potential
how does the [K+] gradient and K+ leak channels create the resting membrane potential
- the imbalance from the [K+] gradient and the K+ leak channels stops K+ from leaving the cell
- the membrane potential will keep K+ inside the cell balanced to the K+ moving down its gradient
- this causes positive and negative ions to be balanced creating the resting membrane potential
typical neuron - what are dendrites
provide enlarged surface area to receive signals from other neurons
typical neuron - what are axons
conduct electrical signals from cell body
typical neuron - what are nerve termini
serve as passage of the message to other target cells
explain the action potential
- it is mediated by voltage gated Na+ channels
- stimulation causes depolarization
- if the stimulation is large enough, the Na+ channels open
- Na+ influx depolarizes the membrane even further
- this then causes more Na+ channels to open and creates the action potential
action potential - what happens to it with distance
gets weaker
action potential - what is depolarization
the membrane potential becoming less negative and instead becomes more positive
explain the Na+ channels as the message is propogating
- the electrochemical driving force for Na+ is 0 bc the membrane potential and [Na+] gradient is equal and opposite
- the channels become will become inactive on a “timer”
Na+ channels during message propagation - inactive conformation
- the channel will be closed even when the membrane is depolarized
- the channel remains inactive until the membrane potential is resorted to a negative value
how do K+ channels aid in the action potential
- K+ channels also open in response to depolarization (slower than Na+)
- they will stay open for as long as the membrane is depolarized
- K+ leaving helps bring membrane back to resting
- quicker than K+ leak channels
- Na+/K+ pumps will then restore the ion gradient
how are electrical signals converted to chemical ones
- neurotransmitters convert electrical signal to a chemical one
- they are stored in synaptic vesicles
electrical -> chemical signal - what happens as the action potential reaches a nerve terminal
- it is relayed at a synapse
- the synaptic vesicle that stores the neurotransmitters then fuse with the plasma membrane
- this forces the neurotransmitters to go into the synaptic cleft
electrical -> chemical signal - voltage gated Ca2+ channels
- when the action potential reaches a nerve terminal, it also opens Ca2+ channels
- influx of Ca2+ triggers the fusion of vesicle and plasma membranes
electrical -> chemical signal - what happens on the synaptic cleft of the postsynaptic target cell
- neurotransmitters bind to their receptors
- this causes a change in membrane potential in that cell
- if the signal is large enough, it will propagate an action potential
electrical -> chemical signal - what happens when a action potential is triggered in the target cell
- the neurotransmitter is quickly removed from the synaptic cleft via an enzyme or re-uptake
- this limits the signal duration
acetylcholine and the neuromuscular junction - overall structure
- 5 transmembrane protein subunits
- form a transmitter-gated aqueous pore
- the pore is lined by 5 transmembrane alpha helices
- has 2 acetylcholine binding sites
acetylcholine and the neuromuscular junction - closed conformation
pore is blocked by hydrophobic amino acid side chains in the gate region
acetylcholine and the neuromuscular junction - open conformation
- acetylcholine (released by a motor neuron) binds to both binding sites
- this triggers a conformational change and the hydrophobic side chains moves apart, opening the gate
- membrane is then depolarized
