Small and nonpolar slips through. Ions and big polar molecules need a protein. Uphill always costs energy.
You can name any transport mechanism by asking what the cargo is and which way it's going. Small nonpolar molecules slip through the lipids. Ions and large polar molecules need a protein. Uphill, or huge, costs energy.
Key terms
- Simple diffusion — nonpolar molecules such as oxygen, carbon dioxide, and steroids cross the bilayer directly, down their gradient. No protein needed.
- Facilitated diffusion — ions and large polar molecules cross down their gradient through a channel or carrier protein. No ATP.
- Active transport — moving a solute against its gradient, which costs energy. A pump protein uses ATP directly.
- Sodium-potassium pump — for each ATP, it moves 3 Na+ out and 2 K+ in, both against their gradients.
- Cotransport — a protein lets one ion flow down its gradient and uses that flow to carry a second solute up its gradient.
- Endocytosis and exocytosis — vesicles carry large cargo in (the membrane folds inward) or out (a vesicle fuses with it), using energy.
How it works
- Small and nonpolar? It dissolves through the hydrophobic core. Charge is the hardest barrier: an ion drags a shell of water the core won't accept.
- Ion or large polar molecule, moving down its gradient? It takes a channel or a carrier. Still passive, still no ATP.
- Moving against its gradient? Something has to pay. A pump spends ATP directly; a cotransporter spends a gradient that a pump already built.
- Each cycle, the sodium-potassium pump exports one more positive charge than it imports. That helps hold the inside negative, part of the membrane potential.
- Too big for any protein, like a bacterium or a batch of secreted proteins? Vesicles carry it in or out, at an energy cost.
Why it matters
Transport experiments hinge on one move: change the energy supply and watch. Cut ATP with cyanide or by withholding oxygen. If uptake collapses, it was active. If adding ATP back restores it, the carrier was never damaged. Cold is a weak test, because it slows every process, diffusion included. And a solute stacked up inside, far above the outside level, got there by active transport.
Exam traps
| Trap | Actually |
|---|
| Cooling slowed uptake, so it must need ATP. | Cold slows everything, diffusion included. To test for ATP, remove it: add cyanide or withhold oxygen. |
| Glucose and ions are small enough to slip through the bilayer. | Size isn't the only filter. Charge and polarity keep them out, so they cross quickly only through proteins. |
| The sodium-potassium pump swaps ions one for one. | Each ATP moves 3 Na+ out and 2 K+ in. One net positive charge leaves per cycle. |
| Cotransport is passive because the protein never touches ATP. | It spends a gradient a pump built with ATP. Moving a solute uphill is still active transport. |
Worked example
A cultured cell line concentrates glucose to 12 times the outside level. When a drug blocks the sodium-potassium pump, the inside : outside ratio falls to 1 : 1, although ATP levels stay normal. When sodium is removed from the outside solution, the ratio also falls to 1 : 1. How does glucose enter these cells?
- A 12 : 1 ratio means glucose is moving against its gradient. Something is paying for that.
- Blocking the pump stops accumulation even though ATP is plentiful. So the glucose transporter isn't spending ATP itself; it depends on the pump's work.
- The pump keeps sodium high outside and low inside. Removing outside sodium also stops accumulation, so glucose entry rides on sodium flowing in.
Cotransport: sodium flows in down its gradient and carries glucose in against glucose's gradient. The sodium-potassium pump pays for it by using ATP to rebuild the sodium gradient.
The energy source is ATP, spent one step removed. The near-miss is a glucose pump that uses ATP directly, but ATP was plentiful and accumulation still stopped. Facilitated diffusion is out too: a passive carrier can bring glucose only to 1 : 1, never 12 : 1.