Unit 3: The Cell Membrane and Transport
How a paper-thin membrane decides exactly what gets into a cell and what gets kept out.
7 topics · 23 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 Phospholipid Bilayer
- How it worksHow do water-fearing tails end up facing each other while water-loving heads face outward?
- Going furtherWhat happens to the bilayer's arrangement if you drop phospholipids into water and let them settle?
- Common mistakeWhy isn't the cell membrane a solid, rigid wall around the cell?
Your turn1 of 2
No construction crew assembles a cell's membrane piece by piece, yet phospholipids reliably form a double layer around every cell. Why does this double-layer arrangement form itself without anyone building it?Selective Permeability
- How it worksHow do embedded proteins let the membrane choose exactly which molecules cross and which get blocked?
- Going furtherWhat happens to a cell's contents if its membrane suddenly let everything through equally?
- Common mistakeWhy isn't the cell membrane just a screen that blocks anything above a certain size?
Your turn1 of 2
Oxygen gas slips straight across a cell membrane in an instant, while a charged sodium ion of similar size cannot cross without help. What actually accounts for this difference?Diffusion
- How it worksHow does random molecular motion move a substance from where it's crowded to where it's scarce?
- Worked exampleFind the direction oxygen diffuses between a cell interior at 2 percent oxygen and blood at 20 percent oxygen.
- Common mistakeWhy doesn't diffusion stop the instant concentrations become equal on both sides?
- Going furtherWhat happens to the rate of diffusion when the temperature of the solution goes up?
Your turn1 of 2
A drop of food coloring dropped into a glass of still water gradually spreads out on its own until the whole glass is evenly tinted, with no stirring at all. Why does this spreading happen without anyone mixing it?Osmosis and Tonicity
- How it worksHow does water cross a membrane toward the side with more dissolved solute?
- Going furtherWhat happens to a red blood cell dropped into pure distilled water?
- Going furtherWhat happens to a red blood cell dropped into a very salty solution instead?
- Common mistakeWhy doesn't osmosis mean water moves toward the side with more water molecules?
Your turn1 of 2
A student says osmosis means water always moves toward whichever side of a membrane has more water molecules. Why is that description backwards from what actually happens?Facilitated Diffusion
- How it worksHow does a channel protein open a pathway for a specific molecule to diffuse through?
- Common mistakeWhy does facilitated diffusion still not require the cell to spend any energy?
- Going furtherWhat happens to the rate of facilitated diffusion once every channel protein is already busy?
Your turn1 of 2
Glucose is too large and too polar to slip through the membrane's fatty interior on its own, yet it enters cells quickly whenever glucose levels outside are high. How does it get across so fast?Active Transport and the Sodium-Potassium Pump
- How it worksHow does the sodium-potassium pump move three sodium ions out for every two potassium ions it brings in?
- Common mistakeWhy isn't active transport just diffusion running a little bit faster?
- Going furtherWhat happens to a cell's ion balance if you block its sodium-potassium pumps with a poison?
Your turn1 of 2
The sodium-potassium pump moves 3 sodium ions out of a neuron for every 2 potassium ions it brings in, spending one ATP per complete cycle. If a neuron's pumps complete 5632 cycles, how many sodium ions total are moved out of the cell?Endocytosis and Exocytosis
- How it worksHow does a cell membrane pinch inward to wrap a particle in its own pocket during endocytosis?
- Common mistakeWhy isn't endocytosis the same process as diffusion just running in the opposite direction?
- Ties togetherHow does exocytosis let a cell release a protein that was built and packaged by the Golgi apparatus?
Your turn1 of 2
A white blood cell engulfs an entire bacterium by wrapping its membrane around it, rather than waiting for the bacterium to slowly diffuse in piece by piece. Why does the cell need to swallow it whole instead?