Transport Flashcards
State 5 features of membrane transporters
Integral membrane proteins Channels or carriers Specific or selective Regulated Passive or active
Types of transporter
Voltage-gated
Ligand-gated
Mechanically gated
3 examples of passive transporters
Glucose transporter, Anion exchanger (Cl, HCO3) facilitates chloride shift
4 features of active transporters
Use metabolic energy to transport splutters (ATP)
Can transports compounds against a concentration gradient
Can establish concentration gradients
Most require hydrolysis of ATP
Types of ATPase transporters
P-type: catalyse auto phosphorylation of a conserved aspartate residue, hydrolysis of the intermediate is required for transport.
V-type: vacuolar, not phosphorylated during transport process, creates acidic environ,net in cytoplasmic vesicles
F- type: couples transport of ATP synthesis (H-ATPase in ETC)
3 examples of ABC transporters
Use energy of ATP hydrolysis to transport substrates across the cell membrane
MDR protein
TAP transporter
CFTR
Membrane potential
Charge difference across the membrane
Ion concentrations in typical cell
K+: 160mM in, 5mM out
Na+: 10mM in, 150mM out
Cl-: 5mM in, 115mM out
Ca2+: 0.2uM in, 2mM out
Refractory periods
Absolute: membranes refractory to stimulation during action potential due to the inactivation of Na+ channels
Relative: membrane becomes more responsive as it repolarises because inactive Na+ channel enters the closed state, however open K+ channels cause hyperpolarisation of the membrane. A strong depolarisation (stimulus) is required to produce an action potential.
Why are refractory periods necessary
Prevents irreversible depolarisation of the membrane
Enables directionality of conduction of nerve impulses
Frequency of action potentials determines ‘strength’
Role of myelination
Axons of neurons covered by glia (oligodendrocytes in CNS, Schwann cells in PNS) which insulate the nerve fibres forming a myelin sheath. Gaps in the sheath (nodes of ranvier) allow action potentials to be conducted at a much faster speed (saltatory conduction).
NB. Multiple sclerosis is an autoimmune disease where T-cells attack myelin sheath. Demyelination causes loss of sensation, muscle spasms, ataxia, dysphagia, fatigue and pain
Neurotransmitter release
Arrival of action potential to the nerve terminal depolarises it and opens VG-Ca2+ channels
Ca2+ enters the cell and promotes the fusion of vesicles containing neurotransmitter with the presynaptic membrane and the neurotransmitter is released into the synaptic cleft via exocytosis.
Role of transporters in Sensory perception
Change of environmental energy into nerve action potentials.
Stimulus causes cell membrane to become more permeable to positive ions, leading to a localised depolarisation (generator potential). The strength if the stimulus determines the size of the generator potential (graded response). Action potentials produced at a frequency related to the strength of the generator potential.
What is the difference between depolarisation and hyperpolarisation?
Depolarisation: decrease in membrane potential, cytoplasm becomes less negative due to the flow of Na+ (and Ca2+) ions into the cell through open channels
Hyperpolarisation: increase in membrane potential. The cytoplasm becomes more negative due to the opening of K+ and Cl- channels which allow ions to flow out of the cell.
What is the difference between a channel and a carrier?
Channels open large pores in the membrane which allow molecules to flow through freely and rapidly. They are faster and less specific than carriers.
Carriers undergo a conformational change when bound to its substrate that then cause it to flip to the other side of the membrane and release the substrate into the cell.
Give four examples of Na+ co-transporters
Na+-glucose symporter in intestinal epithelial cells. Transports glucose from the intestinal lumen into epithelial cells.
Na+/H+ antiporter regulates cell volume becaus the influx of Na+ causes water to enter the cell by osmosis, also regulates cell pH as efflux of H+ ions increases the pH of the cytosol.
Na+/Ca2+ antiporter in cardiomyocytes helps maintain a low intracellular [Ca2+]
Na-dependent anion exchanger helps regulate cell pH (Na+/HCO3 in, H+/Cl out)
Compounds which can cross the cell membrane
lipohillic molecules e.g. cholesterol, steroid hormones
water
small uncharged polar molecules (CO2, urea, ethanol)
small hydrophobic solutes (O2)
Describe how an action potential is produced
Membrane depolarisation opens voltage-gated Na+ channels. Na+ enters the cell and depolarises the membrane.
Depolarisation opens more Na+ channels, if the threshold of -50mV is reached there is a large influx of Na+ ions through open channels.
Import of Na+ decreases as the action potential apporaches Ena. The chemical potential of Na+ is balanced by the membrane potential. Na+ channels become inactivated.
Membrane depolarisation opens VG-K+ channels (inward recifying channels). K+ leaves the cell and the membrane becomes less more negative, causing hyperpolarisation of the membrane.
Different functions of the cell membrane
Anchorage
Signalling (cell surface receptors)
Barrier (separates extracellular and intracellular environment)
Hydrophobic spaces (molecules and ions require transport)
Outline secretory and endocytic pathways
Secretory pathway: modified proteins and lipids from the golgi are packaged into secretory vesicles and transported to the cell surface where the are excreted by exocyosis.
Endocytic pathway: extracellular material is taken into the cell via invagination of clathrin-coated pits which form a vesicle (endosome). Vesicle fuses with a lysozome and the material is degraded.
Role of Ca2+-ATPases in mammalian cells
Ca2+-ATPases are present on the plasma membrane and on the SR/ER or cells.
Help maintain a low cytoplasmic concentration of Ca2+ by actively pumping Ca2+ out of the cell or into intracellular stores.
Role of Na+/K+-ATPase
Helps maintain Na+ and K+ gradients. 3Na+ ions pumped out of the cell and 2K+ ions pumped in.
Define symport and antiport
Symport: two compounds simultaneously transported across a cell membrane in the same direction, one compound being transported down a concentration gradient, the other against a gradient.
Antiport: transport of two compounds across a membrane in opposite directions, one down its concentration gradient and one against it.
How is the resting membrane potential established
The resting membrane potential of appox -70mV
The Na+/K+ gradients are maintained by the action of Na+/K+-ATPase
The cell membrane also contains K+ leak channels which are constantly open, the membrane is therefore more permeable to K+ ions.
What is equilibrium potential?
The membrane potential at equilibrium for a particular ion.
How do movements of K, Na, Cl and Ca affect the membrane potential?
When K+ moves out, cells become hyperpolarised
When Na+ and Ca2+ move into the cell, causing depolarisation
When Cl- moves into the cell, it becomes hyperpolarised
Describe how pacemaker potentials are produced
Action potential opens Na+ channels, Na+ rapidly enters the cell causing depolarisation.
K+ channels open (inward rectifying) and there is a transient decline in membrane potential.
Depolaristion of the cell activates VG-Ca2+ channels on the cell membrane and Ca2+ enters the cell. This produces a plateau as Ca2+ moves in and K+ continues to move out.
Ca2+ channels close, and K+ channels remain open, repolarising the cell.
What is the role of the anion exchanger in RBCs
Bohr effect.
CO2 from respiring tissues enters RBCs and is converted to H+ and HCO3- by carbonic anhydrase. H+ ions bind to Hb, at an allosteric site, promoting the release of oxygen. HCO3- is transported out of the cell into the plasma via the anion exchanger.
