Unit 4: Membrane Structure and Biophysics
The plasma membrane as a two-dimensional fluid: how it assembles itself, why its two faces differ, and what actually controls how loose or stiff it is.
7 topics · 25 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.
The Fluid Mosaic Model
- How it worksHow do phospholipids and proteins drift sideways through the membrane while still forming one continuous sheet?
- Common mistakeWhy isn't the plasma membrane a rigid, static barrier that sits unchanged around the cell?
- Going furtherWhat happens to two membrane proteins from different cells when the cells are experimentally fused together?
Your turn1 of 2
Researchers fuse a mouse cell (with fluorescently tagged surface proteins) to a human cell (with a different fluorescent tag) and watch the fused cell's surface over the next hour. What do they observe?The Hydrophobic Effect and Bilayer Self-Assembly
- How it worksHow does the hydrophobic effect push water-fearing tails away from water and into each other?
- How it worksWalk me through how this same self-assembly seals a bilayer's edges into a closed compartment.
- Common mistakeWhy isn't bilayer self-assembly powered by attraction between the phospholipid tails themselves?
- Going furtherWhat happens when phospholipids are shaken into water at a concentration too low to fully cover a surface?
Your turn1 of 2
Phospholipids dropped into water spontaneously assemble into a bilayer. A student explains this by saying the fatty acid tails are strongly attracted to one another, pulling the structure together. What actually drives this self-assembly?Membrane Fluidity and Fatty Acid Saturation
- How it worksHow does a kink from a double bond in a fatty acid tail keep neighboring lipids from packing tightly?
- Worked exampleFind which membrane is more fluid at 20 degrees C: one built from fatty acids with 0 double bonds or one built from fatty acids with 2 double bonds.
- Common mistakeWhy doesn't a membrane's fluidity depend only on temperature and not at all on its lipid composition?
- Going furtherWhat happens to a cold-water fish's membrane lipid composition compared to a warm-water fish's?
Your turn1 of 2
Two model membranes are compared at 20 degrees C: one built entirely from fatty acids with zero double bonds, the other from fatty acids with two double bonds each. Which membrane is more fluid, and why?Cholesterol's Role in Membrane Fluidity
- How it worksHow does cholesterol's rigid ring structure restrain phospholipid tails at high temperature?
- How it worksHow does that same cholesterol molecule keep those same tails from packing too tightly at low temperature?
- Common mistakeWhy isn't cholesterol simply a fluidizing agent, given that it makes membranes more fluid at cold temperatures?
- Ties togetherHow does cholesterol's buffering of fluidity explain why animal membranes tolerate a wider temperature range than a pure phospholipid bilayer?
Your turn1 of 2
Cholesterol makes a membrane more fluid when the membrane is cold, by preventing phospholipid tails from packing into a tight, crystalline arrangement. A student concludes cholesterol is simply a fluidizing agent overall. Why is this an incomplete picture?Membrane Asymmetry Between the Two Leaflets
- How it worksHow do flippases and floppases actively sort specific phospholipids to one leaflet or the other?
- Common mistakeWhy doesn't phosphatidylserine's appearance on a cell's outer leaflet mean nothing biologically?
- Going furtherWhat happens to a dying cell's membrane asymmetry as it moves toward apoptosis?
Your turn1 of 2
Phosphatidylserine is normally restricted to the inner leaflet of a healthy cell's membrane, but it appears on the outer leaflet of a dying cell. A student assumes this is a meaningless side effect of the cell simply falling apart. What is wrong with that assumption?Integral and Peripheral Membrane Proteins
- How it worksHow does a transmembrane protein's stretch of hydrophobic amino acids anchor it permanently inside the bilayer?
- How it worksHow does a peripheral protein attach to a membrane's surface without ever crossing the bilayer?
- Common mistakeWhy isn't a membrane protein's position fixed the moment it's first inserted into the bilayer?
- Worked exampleFind how many amino acids an alpha-helical transmembrane segment needs to span a 4 nanometer bilayer, given about 0.15 nanometers of helical rise per residue.
Your turn1 of 2
Find how many amino acids an alpha-helical transmembrane segment needs to span a 5 nanometer bilayer, given about 0.156 nanometers of helical rise per residue.Lipid Rafts and Membrane Microdomains
- How it worksHow do cholesterol and sphingolipids pack together into a thicker, more ordered raft within the bilayer?
- Common mistakeWhy isn't a lipid raft a fixed structure that sits in the same spot on the membrane at all times?
- Going furtherWhat happens to a raft-associated signaling protein's activity when cholesterol is stripped out of the membrane?
Your turn1 of 2
Lipid rafts are consistently found in similar overall abundance across a cell's membrane over time. A student concludes that any one raft must be a fixed structure sitting in the exact same spot on the membrane at all times. Why is this wrong?