[BIOL 1100] - Midterm Exam Guide - Everything you need to know! (14 pages long)

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BIOL 1100
MIDTERM EXAM
STUDY GUIDE
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01-30: Membrane Potentials and Transport
Monday, January 30, 2017
3:00 PM
Membrane Transport
Cell concentrations
o Concentrations in 100mM range for the ions
Diffusion
o Small, non-polar, uncharged, lipophilic substances are able to pass through lipid bilayer
o Some small polar molecules (water)
Ion channels
o Allow small charged molecules to pass through and takes water with it
Ions actually pass through faster than diffusion
o Selectivity filter
Narrow region of the channel's pore where the choice is made which ion will go
through
Which ion goes through? Depends on size of ion, size of hydrated ion, net charge,
charge density, dehydration E (how easy it is to strip off the water molecules solvating
the ion)
o Gating
Ligand-gated: when ligand binds, goes from closed to open conformation and allows
ion to go through
Some ligands are EC (neurotransmitters, hormones), some are IC (G-protein,
Ca2+, cAMP, lipids)
Voltage-gated: change in voltage opens the channel (most by positive voltage)
Particularly important in nerve impulses, muscle contraction (very rapid)
Direction of ion flow depends on electrochemical gradient
Light-gated (algae)
Temperature-gated: respond to different temperature ranges
TRP (transient receptor potential) channels
Some respond to hot temperatures and also respond to capsaicin
Some respond to cold temperatures and also to menthol
Stretch-gated: respond to the stretch of the membrane
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Important in regulating the cell volume, sensing mechanical stimuli including
sound waves, muscle contraction
o Permeability/selectivity
Can have K+, Na+, Ca2+, Cl- selective
Non-selective cation channels (let in K+, Na+, Ca2+, Mg2+, H+ with similar
permeability, but no Cl-)
Generally activated by excitatory potentials --> when you open non-selective
cation channels, depolarize the cell
o Structure
Sometimes there are classes of channels that aren't based on function or gating,
sometimes they are just classified by primary structure (cAMP and voltage gated look
similar)
o Most of the time, channels are gated by more than one thing - are classified by what is most
frequently/effectively gating them under physiological conditions
Ex. cAMP-gated channels are also slightly dependent on voltage
Ex. Ca2+-activated K+ channels
Need Ca2+ around in order to be sensitive to voltage
Carriers
o Allow larger molecules to pass through
Much slower than ion channels because both sides never open at the same time
o Facilitated diffusion: transport of one molecule through a carrier
o Coupled carriers: can transport multiple substances at a time
Cotransporters/symporters: transport in same direction
Exchange carrier/counter-transport/antiporter: transport molecules in opposite
direction
Pumps (primary active transport)
o Carriers + ATP to transport molecules against their concentration gradients
Hydrolyze ATP to form ADP + Pi, the energy from the phosphate bond is used to
transport the molecules
Also called ATPase
Ex. Na+/K+ pump
For a lot of functions you need to have high sodium outside and low sodium
inside, so a lot of energy goes to these pumps to maintain the concentrations
o Secondary active transport
Doesn't use ATP directly but uses an ion gradient that is maintained by active
transport
Ex. Ca2+ is mM outside and uM inside (on average - has a very non-uniform
distribution in the cell)
Cell works really hard to keep calcium really low because it's an important
secondary messenger through:
Calcium-binding proteins
Calcium pumps (pump to outside and organelles like the mitochondria,
SR)
Na+/Ca2+ exchange carriers: let Na+ in and Ca2+ out
Na+ coupled carriers
Do NOT directly use ATP, uses favorable Na+ concentration gradient to drive
calcium across a huge gradient
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