BIOL214 Exam 2 - transport across cell membranes

why are lipid bilayers impermeable to most molecules?

they block ions and most uncharged polar molecules

what determines the rate of solute diffusion across a protein-free lipid bilayer?

the solute’s size and solubility

what two types of proteins move molecules across membranes?

transporters and channels

solute concentrations inside the cell 

low Na+ concentration 

solute concentration outside the cell

high Na+ concentration

passive transport

movement down a concentration gradient without energy input, can occur by simple diffusion or facilitated transport 

active transport

movement against a concentration gradient via pumps —> requires energy

osmosis

net movement of water across a semipermeable membrane down its concentration gradient 

aquaporins  

greatly speed water transport; each monomer forms a narrow pore for single water molecules

how do aquaporins block ions and protons

central asparagine residues exclude hydrated ions and prevent proton conduction

what experiment showed aquaporin function?

frog oocytes expressing aquaporins swelled and burst in hypotonic solution; controls did not 

how do passive transporters work?

move solutes along electrochemical gradients via conformational changes *ex: glucose transporter)

what are the three classes of pumps?

gradient-driven, ATP-driven, and light-driven pumps

Na+/K+ pump

uses ATP to pump 3 Na+ out and 2 K+ in, maintaining low Na+ inside and high K+ inside 

how fast does on Na+/K+ pump cycle repeat?

~10 ms

what is the Na+ gradient compared to?

Water behind a dam; it stores potential energy to drive other processes

SERCA Ca2+ pump

pumps Ca2+ into the sarcoplasmic reticulum, keeping cytosolic Ca2+ low

what type of pump is SERCA?

A P-type ATPase

what are the 3 cytosolic domains of SERCA

N (nucleotide-binding), P (phosphorylation), A (actuator) 

uniport

moves one solute across the membrane

symport

moves two solutes in the same direction

antiport

moves two solutes in the opposite directions

how is glucose actively imported into animal cells?

by a Na+/glucose symport using the Na+ electrochemical gradient

what ion gradient drives active transport in plants, fungi, and bacteria?

the H+ gradient

Selectivity filter

makes ion channels selective

what causes the resting membrane potential?

K+ concentration gradient + K+ leak channels

patch-clamp recording

technique that measures ion channel activity

what stimuli control channel opening/closing?

mechanical stress, ligand binding (extracellular or intracellular), or voltage change

what ion channels enable hearing? 

mechanically gated channels in stereocilia of hair cells 

what generates an action potential?

depolarization opening voltage-gated Na+ channels

what is the Na+ channel cycle during an action potential?

closed —> open —> fast inactivated —> reset after repolarization

to what voltage does the membrane depolarize during an action potential?

+40 mV (from -80 mV)

why do action potentials propagate in one direction?

inactivated Na+ channels prevent backward spread

what do voltage-gated Ca2+ channels in nerve terminals do? 

convert electrical signals into chemical signals by triggering neurotransmitter release? 

what are the two types of neurotransmitters?

excitatory and inhibitory

optogenetics

light-gated ion channels that transiently activated or inactivates neurons in living animals