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.
