Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A molecule’s structure (size, polarity, charge) decides whether it can cross the hydrophobic core of the bilayer. Charge and polarity matter more than size.
- Facilitated diffusion moves ions and large polar molecules through transport proteins, down their concentration gradient, with no energy input.
- Channel proteins form a hydrophilic pore; they are fast, specific and often gated. Carrier proteins bind a specific molecule and change shape; they are slower.
- Na⁺ and K⁺ need channel proteins. Because ions carry charge, their movement can make one side of the membrane more negative: the membrane becomes polarized.
- Aquaporins are channels that move large quantities of water quickly. Without them, water crosses the bilayer only slowly.
Key relationships
| Substance | Route | Example |
|---|---|---|
| small nonpolar | simple diffusion through the bilayer | O₂, CO₂ |
| ions | channel proteins | K⁺ leaving through K⁺ channels |
| large polar | carrier proteins | glucose into red blood cells |
| water in bulk | aquaporins | kidney collecting ducts, root cells |
| Data pattern | Meaning |
|---|---|
| rate ∝ concentration (straight line) | simple diffusion; no proteins to saturate |
| rate levels off at high concentration | carriers or channels saturated: facilitated diffusion (or active transport) |
| uptake not affected by blocking ATP | passive: simple or facilitated diffusion |
| more transport proteins | higher maximum rate; direction unchanged |
Assumptions behind the numbers
- At saturation, the rate is assumed proportional to the number of transport proteins.
- When a protein route is blocked, the remaining rate is assumed to come from the bilayer route.
- Concentration comparisons ignore charge; for ions, the charge across the membrane also matters.
Mistakes to avoid
- Saying facilitated diffusion uses ATP because a protein is involved.
- Saying channels “pump” ions.
- Explaining ion exclusion by size instead of charge.
- Saying water can only cross through aquaporins.
- Expecting facilitated diffusion to keep speeding up as concentration rises.
- Doubling the whole rate when only the protein route is doubled.
Quick self-check
- Why does K⁺ leaving a cell make the inside negative? (K⁺ carries positive charge out; the negative ions it balanced cannot follow.)
- Concentration doubles from 20 to 40 units; rate rises from 3.0 to 3.3. Percentage rise and meaning? ((3.3 − 3.0) ÷ 3.0 × 100 = 10%; the carriers are close to saturation.)
- Is water movement through aquaporins active? (No. It is passive; aquaporins only speed it up.)
Next: practice questions.