Unit 5: Membranes and Transport
Why a bilayer builds itself, what actually crosses it, and how a cell spends energy to move things the wrong way.
10 topics · 35 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.
Amphipathic Lipids and Bilayer Self-Assembly
- How it worksHow does a phospholipid's water-loving head and oily tails force it into a two-layered sheet inside water?
- How it worksWalk me through how the hydrophobic effect, not any new bond, drives an entire bilayer to self-assemble.
- Common mistakeWhy isn't a lipid bilayer held together by strong covalent bonds between neighboring phospholipids?
- Going furtherWhat happens to a lipid bilayer's shape once it gets torn open while sitting in water?
Your turn1 of 2
A student sees a lipid bilayer stay intact and assumes neighboring phospholipids must be held together by strong covalent bonds linking one molecule to the next, the way amino acids are linked in a protein chain. What actually holds the bilayer together?Membrane Fluidity, Cholesterol, and Fatty Acid Saturation
- How it worksHow does cholesterol wedged between phospholipids keep a membrane from getting too fluid or too stiff?
- Going furtherWhat happens to a membrane's fluidity as its fatty acid tails become more unsaturated?
- Common mistakeWhy isn't a cell membrane a fixed, unchanging structure once it's built?
Your turn1 of 2
A student pictures a cell membrane as a fixed structure, permanently set once it is built, similar to a solid wall. Why is this picture wrong?Membrane Proteins: Integral and Peripheral
- How it worksHow does an integral membrane protein arrange its hydrophobic and hydrophilic amino acids to sit stably across a bilayer?
- Common mistakeWhy isn't every membrane protein embedded all the way through the bilayer?
- Going furtherWhat happens to a membrane protein's function once it gets extracted from the lipid environment it depends on?
Your turn1 of 2
An integral membrane protein sits stably wedged inside the oily bilayer core, yet most of the protein is not oily at all. A student wonders how a mostly hydrophilic molecule can sit permanently embedded in a hydrophobic environment. What is the actual arrangement?Simple Diffusion Across a Membrane
- How it worksHow does a molecule's size and polarity determine whether it can dissolve through the membrane's hydrophobic core?
- Worked exampleFind the net direction of diffusion for a molecule at 5 millimolar inside a cell and 2 millimolar outside, assuming free simple diffusion.
- Common mistakeWhy doesn't simple diffusion across a membrane require the cell to spend any ATP?
Your turn1 of 2
A small nonpolar molecule diffuses freely across a membrane. It sits at 14 millimolar inside the cell and 13 millimolar outside. Find the concentration gradient driving diffusion, defined as outside concentration minus inside concentration.Facilitated Diffusion Through Channels and Carriers
- How it worksHow does a carrier protein change shape to shuttle a polar molecule across the membrane without ever forming an open pore?
- Going furtherWhat happens to facilitated diffusion's rate once every carrier protein in the membrane is already occupied?
- Ties togetherHow does a carrier protein's shape-changing shuttle resemble the induced-fit binding an enzyme uses on its own substrate?
Your turn1 of 2
Glucose cannot cross a lipid bilayer on its own, so a carrier protein moves it down its concentration gradient instead, without ever forming an open channel through the membrane. A student assumes the carrier must work exactly like a channel, just with a different name. How does a carrier protein actually move glucose across?Ion Channel Gating
- How it worksHow does a voltage-gated channel's charged sensor segment move in response to a change in membrane voltage?
- How it worksWalk me through how a channel's narrow selectivity filter lets one ion species through while blocking others of a similar size.
- Common mistakeWhy isn't an open ion channel just a nonspecific hole punched through the membrane?
- Going furtherWhat happens to an ion channel's behavior when a toxin locks its gate permanently open?
Your turn1 of 2
A voltage-gated sodium channel opens within a fraction of a millisecond after the membrane depolarizes slightly. A student wonders how the channel protein itself can sense a change in voltage at all, since voltage seems like an abstract electrical property rather than something a molecule could physically detect. How does the channel actually sense it?Active Transport and the Sodium-Potassium Pump
- How it worksHow does the sodium-potassium pump use one ATP hydrolysis to shove three sodium ions out and pull two potassium ions in?
- Worked exampleFind how many ATP molecules the sodium-potassium pump uses to move 90 sodium ions out of a cell.
- Common mistakeWhy isn't active transport just diffusion running a little slower?
- Going furtherWhat happens to a cell's ion gradients once the sodium-potassium pump is poisoned and stops running?
Your turn1 of 2
The sodium-potassium pump moves 3 sodium ions out of a cell for every 1 ATP it hydrolyzes. Find how many ATP molecules the pump uses to move 176 sodium ions out of a cell.Electrochemical Gradients and Membrane Potential
- How it worksHow do a chemical concentration gradient and an electrical charge gradient combine into one electrochemical gradient?
- Worked exampleFind potassium's equilibrium potential using the Nernst equation, given 140 millimolar inside and 5 millimolar outside at 37 degrees C.
- Going furtherWhat happens to a membrane's voltage when a channel selective for a different ion suddenly swings open?
- Common mistakeWhy isn't membrane potential caused by a bulk imbalance of charge spread through the whole cell?
Your turn1 of 2
Find potassium's equilibrium potential using the Nernst equation at 37 degrees C, given a concentration of 149 millimolar inside the cell and 8 millimolar outside.Osmosis and Water Balance Across Membranes
- How it worksHow does water move across a membrane toward the side with more dissolved solute, even though water itself isn't being pumped?
- Worked exampleFind which direction water flows when a cell at 300 milliosmolar is placed into a solution at 500 milliosmolar.
- Going furtherWhat happens to a plant cell's shape when it's placed in a solution far saltier than its own cytoplasm?
Your turn1 of 2
A cell at 324 milliosmolar is placed into a solution at 312 milliosmolar. Find the osmotic concentration difference, defined as outside minus inside, that determines the direction of net water flow.Endocytosis and How Viruses Enter a Cell
- How it worksHow does a patch of membrane bud inward and pinch off to swallow material during endocytosis?
- How it worksWalk me through how receptor-mediated endocytosis selectively captures one type of particle out of everything around a cell.
- Going furtherWhat happens when a virus like HIV hijacks a cell's own membrane-fusion machinery to get its genetic material inside?
- Ties togetherHow does a virus binding its target receptor use the same shape-matching chemistry as a hormone binding its own receptor?
Your turn1 of 2
A cell needs to bring in one specific type of particle out of many different molecules present in the surrounding fluid, far too large to pass through any channel. A student assumes the cell must simply engulf everything nearby indiscriminately. How does receptor-mediated endocytosis actually achieve selectivity?