Unit 5: Membrane Transport and Membrane Potential
How a cell decides what crosses its membrane and how, and how those transport decisions add up to a voltage sitting across every living cell.
8 topics · 33 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Simple Diffusion Across a Lipid Bilayer
- How it worksHow does a molecule's size and polarity decide whether it can cross the lipid bilayer unassisted?
- Worked exampleFind which of two molecules crosses a membrane faster by simple diffusion: one with a partition coefficient of 10 or one with a partition coefficient of 0.01.
- Common mistakeWhy isn't diffusion across a membrane driven by the membrane at all, rather than by the molecule's own random motion?
- Going furtherWhat happens to a small nonpolar molecule's diffusion rate when the membrane's thickness doubles?
Your turn1 of 2
Two molecules are compared for how fast they cross a lipid bilayer by simple diffusion: one with an oil-water partition coefficient of 10 (favoring the lipid phase), the other with a partition coefficient of 0.01 (favoring the water phase). Which crosses the membrane faster?Osmosis and Tonicity
- How it worksHow does water move across a membrane toward the side holding more dissolved solute?
- Worked exampleFind the direction water moves between two solutions separated by a membrane, one at 300 milliosmolar and one at 150 milliosmolar.
- Common mistakeWhy isn't osmosis the movement of solute molecules rather than water molecules?
- Going furtherWhat happens to a plant cell's turgor pressure when it's placed in a hypertonic solution?
Your turn1 of 2
A membrane separates two solutions: one at 300 milliosmolar and one at 150 milliosmolar. In which direction does water move across the membrane?Facilitated Diffusion Through Channels and Carriers
- How it worksHow does a channel protein let ions rush through a water-filled pore down their gradient?
- How it worksHow does a carrier protein change shape to shuttle glucose across the membrane one molecule at a time?
- Common mistakeWhy doesn't facilitated diffusion require any energy input from the cell?
- Worked exampleFind the transport rate in micromoles per minute for a carrier with a Vmax of 60 and a Km of 10 millimolar at a glucose concentration of 10 millimolar.
Your turn1 of 2
A glucose carrier has a Vmax of 76 micromoles per minute and a Km of 14 millimolar. Find its transport rate at a glucose concentration of 28 millimolar.Aquaporins and Water Channel Specificity
- How it worksHow does an aquaporin's narrow pore let water through single file while still blocking protons?
- Common mistakeWhy isn't water crossing a membrane always fast enough without any dedicated channel?
- Going furtherWhat happens to a kidney cell's water reabsorption when its aquaporin channels are removed from the membrane?
Your turn1 of 2
A student argues that dedicated water channels like aquaporins are unnecessary, since water is a small polar molecule that should already cross the lipid bilayer fast enough on its own. Why is this argument wrong for cells like those in the kidney?The Sodium-Potassium Pump and Primary Active Transport
- How it worksWalk me through how the sodium-potassium pump uses one ATP to move three sodium ions out and two potassium ions in.
- How it worksHow does the pump change shape between two conformations to move ions against their own gradient?
- Common mistakeWhy isn't active transport just a faster version of diffusion through the same kind of channel?
- Worked exampleFind how many ATP molecules a cell hydrolyzes per second if its sodium-potassium pumps move 3 million sodium ions per second, at 3 sodium ions per ATP.
- Going furtherWhat happens to a cell's resting membrane potential when its sodium-potassium pump is poisoned with ouabain?
Your turn1 of 2
A cell's sodium-potassium pumps move 6.72 million sodium ions out of the cell per second, at 3 sodium ions moved per ATP hydrolyzed. Find how many million ATP molecules the cell hydrolyzes per second.Secondary Active Transport: Symport and Antiport
- How it worksHow does the sodium gradient built by the sodium-potassium pump get spent to drag glucose in through a symporter?
- How it worksHow does an antiporter move two substances in opposite directions within a single transport cycle?
- Common mistakeWhy isn't secondary active transport considered passive just because it doesn't hydrolyze ATP directly?
- Ties togetherHow does the sodium gradient that powers glucose symport in the gut trace back to the very same pump that sets the resting membrane potential?
Your turn1 of 2
A glucose symporter moves glucose into a cell against its concentration gradient without directly hydrolyzing any ATP. A student concludes this transport must be passive, since it doesn't use ATP directly. Why is this wrong?Ion Channels and Gating
- How it worksHow does a voltage-gated channel sense a change in membrane potential and flip open?
- How it worksHow does a ligand-gated channel open only when a specific neurotransmitter binds it?
- Worked exampleFind the current through a single ion channel with a conductance of 20 picosiemens and a driving force of 100 millivolts.
- Common mistakeWhy isn't a gated channel simply open or shut at random regardless of the cell's state?
- Going furtherWhat happens to a channel's behavior when it enters an inactivated state right after opening?
Your turn1 of 2
Find the current through a single ion channel with a conductance of 29 picosiemens and a driving force of 105 millivolts.The Resting Membrane Potential and the Nernst Equation
- How it worksHow does the resting membrane's much higher permeability to potassium than sodium set the resting potential?
- Worked exampleFind the equilibrium potential for potassium using the Nernst equation, given 140 millimolar inside and 5 millimolar outside at 37 degrees C.
- Common mistakeWhy isn't the resting membrane potential equal to potassium's equilibrium potential down to the last millivolt?
- Going furtherWhat happens to a neuron's resting potential when extracellular potassium concentration is doubled?
Your turn1 of 2
Find the equilibrium potential for potassium using the Nernst equation at 37 degrees C, given a potassium concentration of 145 millimolar inside the cell and 8 millimolar outside.