Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- The plasma membrane is selectively permeable, so a cell can keep different solute concentrations inside and outside: concentration gradients. This is how cells and organisms keep their solute and water balance.
- Diffusion is net movement from high to low concentration caused by random motion. At equal concentrations, movement continues both ways but there is no net movement (dynamic equilibrium).
- Passive transport (simple diffusion, facilitated diffusion, osmosis) moves substances down their gradient with no direct metabolic energy.
- Active transport needs a direct input of energy (usually ATP) and a specific membrane protein. It can move substances from low to high concentration.
- Endocytosis takes material in by folding the membrane inward to form a vesicle. Exocytosis releases material when a vesicle fuses with the membrane. Both need energy and move large molecules, particles or large amounts.
Key relationships
| Passive | Active | Endocytosis / exocytosis | |
|---|---|---|---|
| Direction | down the gradient only | can go against it | into / out of the cell |
| ATP used directly? | no | yes | yes |
| Protein needed? | only for facilitated diffusion | yes (pump) | cargo travels in vesicles |
| Example | O₂ into a respiring cell | mineral ions into root hairs | phagocytosis of bacteria; insulin release |
| Calculation | How |
|---|---|
| Inside : outside ratio | inside concentration ÷ outside concentration; above 1 for a substance still entering suggests active uptake (uncharged solutes) |
| Rate | change in concentration ÷ time |
| Percentage change | (new − old) ÷ old × 100 |
Assumptions behind the numbers
- Comparing concentrations alone works for uncharged solutes. For ions, the charge across the membrane also matters (Topic 2.8).
- A large volume of outside solution is assumed to keep the outside concentration constant.
- Inhibitor results assume the inhibitor only blocks ATP production and does not damage the membrane.
Mistakes to avoid
- Saying diffusion stops at equilibrium; only net movement stops.
- Saying any process with a protein uses energy; facilitated diffusion is passive.
- Defining active transport as “against the gradient”; the definition is direct energy input.
- Saying vesicle cargo passes through the bilayer.
- Treating a change of temperature as a clean test of ATP dependence.
- Claiming active transport from evidence that a substance merely entered a cell.
Quick self-check
- Inside 6.0, outside 1.5 mmol L⁻¹ for an uncharged solute that is still entering. Ratio and conclusion? (6.0 ÷ 1.5 = 4.0; above 1, so active transport is needed.)
- An inhibitor cuts uptake from 4.2 to 1.4 units. Percentage change? ((1.4 − 4.2) ÷ 4.2 × 100 ≈ −67%, a fall of about two-thirds.)
- Why does a white blood cell need ATP to engulf a bacterium? (Endocytosis uses energy to fold the membrane, form the vesicle and move it.)
Next: practice questions.