BCH210H1 Lecture Notes - Lecture 17: Integral Membrane Protein, Facilitated Diffusion, Membrane Transport
25 May 2018
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Lecture 17: Membrane Protein Function
Membrane Transport
• Essential for life
- Nutrients in and garbage out
- Inorganic ions in and out
• Some small uncharged molecules freely diffuse across
- Passive diffusion
• Most require integral membrane protein transporters
- Facilitated transport
- Active transport (using ATP)
Passive Diffusion
• Favors small, uncharged nonpolar molecules
• Diffuse down their concentration gradient (high to low)
• No help from any transporter system
• Simple diffusion
Bilayer Permeability
• Few molecules cross membranes by passive diffusion
- Gases (CO2, N2, O2) → passive diffusion
- Small uncharged (ethanol) → passive diffusion
- Water, urea - - > slow and inefficient but can be diffused
- Large uncharged (glucose), ions (K+, Mg2+, Ca2+, Cl-), charged polar (AAs, ATP) do not pass
Facilitated Diffusion/Transport
• Passive diffusion of most molecules is too slow
• Integral proteins can facilitate diffusion across membrane (faster)
1. In thermodynamically favorable direction (△G < 0) – high to low
2. Display affinity and specificity for transported species
Failitated diffusion displays saturation ehavior
• Facilitated - rates display saturation behavior similar to substrate
binding to enzymes
• Passive - more concentration you have, more increase in the rate
Membrane transport: Channel proteins
• Ion channels are prototypic membrane transporters that facilitate diffusion
• Hallmark features of ion channels
- Selectivity (K+ >> Cl- and even K+ > Na+)
- Conduct ions at very high rate (108s-1)
- Gate (open/close in response to a stimulus)
• Structures need to understand function
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BCH210H1 Full Course Notes
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Document Summary
Inorganic ions in and out: some small uncharged molecules freely diffuse across. Passive diffusion: most require integral membrane protein transporters. Passive diffusion: favors small, uncharged nonpolar molecules, diffuse down their concentration gradient (high to low, no help from any transporter system, simple diffusion. Bilayer permeability: few molecules cross membranes by passive diffusion. Gases (co2, n2, o2) passive diffusion. Water, urea - - > slow and inefficient but can be diffused. Large uncharged (glucose), ions (k+, mg2+, ca2+, cl-), charged polar (aas, atp) do not pass. Integral proteins can facilitate diffusion across membrane (faster: passive diffusion of most molecules is too slow. In thermodynamically favorable direction ( g < 0) high to low: display affinity and specificity for transported species. Fa(cid:272)ilitated diffusion displays (cid:862)saturation (cid:271)ehavior(cid:863: facilitated - rates display saturation behavior similar to substrate binding to enzymes, passive - more concentration you have, more increase in the rate.
