Download MEMBRANE PERMEABILITY ! membranes are highly impermeable

MEMBRANE PERMEABILITY
! membranes are highly impermeable to ions and polar
molecules
! bilayer core is hydrophobic, so these species must shed
their water of hydration to get across
! dehydration is energetically very unfavourable (like a
high activation energy barrier for a chemical reaction)
! species such as Na+, Cl!, sugars, amino acids, cannot
cross membranes at a significant rate
! small uncharged molecules, such as glycerol and
ethanol, cross slowly
! hydrophobic molecules, and gases such as O2, CO2 , N2 ,
cross rapidly
! water crosses membranes the fastest of all (not really
understood why)
ION GRADIENTS ACROSS MEMBRANES
! the ionic composition of the cytosol is very different from
that of extracellular fluids
! large ion concentration gradients exist across plasma
membrane
! gradients also exist across organelle membranes
e.g. endoplasmic reticulum lumen has high Ca2+ (1-5 mM)
! generated by membrane transporters
TYPES OF MEMBRANE TRANSPORT
1. Simple diffusion
! compounds move from high c to low c
free energy of solution of concentration c
G = G0" + RT ln c
! free energy change takes place when moving a molecule
from c1 (high) to c2 (low)
c1
c2
ΔG = RT ln c2 - RT ln c1 = RT ln (c2 /c1 )
if c1 > c2, ln (c2/c1) is !ve, so ΔG is !ve
! diffusion occurs spontaneously from high to low
concentration (down gradient), no energy input required
! equilibrium is reached when c1 = c2
2. Membrane channels
! when open, forms doughnut-like pore through which
solutes flow rapidly by diffusion
! always move from high c to low c (down gradient)
! transport rate # substrate concentration, not saturable
! ΔG !ve, spontaneous, no energy required
! animal cells have many ion channels; highly selective,
only let specific ions pass through
! channels open and close in response to specific
stimuli
3. Membrane transporters
! no continuous pore through membrane
! like enzymes; they transport their substrates one
molecule at a time
! rate of transport is saturable at high substrate
concentrations (like enzyme catalysis)
! specific binding site(s) for substrate(s) exists
! substrate binds to site on one side of membrane,
conformational change takes place, and site now
opens to other side of membrane, releasing substrate
Passive transporters
! transport their substrates DOWN a concentration
gradient; ΔG !ve, spontaneous
! e.g. passive glucose transporter, found in all cells,
moves glucose from blood into cytosol - GLUT1 protein
Active transporters
! transport their substrates UP a concentration gradient;
ΔG +ve, energy input required
! often called "pumps" (pump substrate uphill)
! many active transporters of ions are powered by ATP
hydrolysis; large family of ion pumping ATPases
! generate ion gradients across membranes
MEMBRANE POTENTIAL
! when transporter moves a charged species across the
membrane, get a charge imbalance across the
membrane; membrane potential Δψ (in Volts)
! typical cell, Δψ = !60 mV, negative inside
! when Δψ exists, free energy of charged species is
different on each side of membrane
ΔG = zF Δψ
F = Faraday constant
(96.48 J V-1mol-1)
z = unit charge on solute
for transport of a charged species:
ΔG = RT ln (c2/c1) + zF Δψ
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favourable
free energy released
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unfavourable
extra free energy required