Midterm #2: 7-9 pm on Wednesday, November 8, McKenzie 240C
Review Session: 7-9 pm on Thursday, November 2 (tomorrow), Lawrence 115
Membrane design features
Phospholipid has 4 basic parts: lipid tails, glycerol, phosphate, head group
Credit: smc.edu
Phospholipids form bilayers
Credit: Goodsell
Selective permeability: Only some molecules can pass through the bilayer
Which molecules would be able to pass through a bilayer?
Only small, uncharged molecules can diffuse across a membrane
Membranes host proteins and sugars for diverse functions in transport, signaling, and molecular recognition
Credit: https://teaching.ncl.ac.uk/Focus today on integral membrane proteins that move material across it
Why are transport proteins needed?
Permeability: some stuff can't pass the bilayer
Why are transport proteins needed?
Moving a potassium ion ($K^{+}$) across a bilayer is hard because:
Answer: 2
Why are transport proteins needed?
Control. If transport proteins only open and close in certain circumstances, this leads to regulated behavior.
Selectivity: the case of aquaporin
Water can get across a bilayer, but it's slow
There are cases where this needs to go faster and be controlled. Where might this be?
Kidneys, "epithelial" cells exposed to environment...
How would you build something that would letter water through but not these other molecules?
Answer: make it so only water can fit and be satisfied.
Water goes single-file and has to pass by positively charged residues in the channel.
How does aquaporin filter the following molecules?
Chloride ion is much larger than water
Selectivity can also involve an allosteric conformational change
Can this protein move glucose against a concentration gradient?
No. Can only go with a gradient
Channels and carriers move molecules with a concentration gradient
Pumps move molecules against concentration gradients
How does a cell move something against a concentration gradient?
Channels and carriers move molecules with a concentration gradient
In active transport, an unfavorable reaction (going against a concentration gradient) is coupled to a favorable reaction ($ATP \rightarrow ADP+P_{i}$).
$\Delta G_{transport} = \Delta G_{conc} + \Delta G_{ATP}$
Coupling between $ATP \rightarrow ADP + P_{i}$ and transport is allosteric conformational change
How do signals cross meters in our body when even motor proteins would take hours to get that far?
Na+/K+ pumps create electrical potential across the bilayer
What happens if I open a Na+ specific channel (not pump)?
What happens if I open a Na+ specific channel (not pump)?
Na+ ions will rush in, potential difference will drop
Gated ion channels respond to changes in electrical potential (voltage)
Gated ion channels respond to changes in electrical potential (voltage)
Gated ion channels respond to changes in electrical potential (voltage)
Action potential propagates along the nerve
Action potential propagates along the nerve
Resting
Action potential propagates along the nerve
Propagation
After signal, Na+/K+ pumps restore action potential