Side by side
| Passive transport | Active transport | |
|---|---|---|
| Direction relative to the gradient | Down the concentration gradient, toward even distribution | Against the concentration or electrochemical gradient |
| Energy cost | None; the gradient is a form of potential energy that dissipates as it is used | Energy required, usually as ATP generated by the cell's metabolism |
| Mechanisms | Simple diffusion, facilitated diffusion through channels and carriers, and osmosis | Primary active transport by pumps, and secondary active transport driven by an existing gradient |
| Standard example | Oxygen diffusing into a cell, or water entering by osmosis through aquaporins | The sodium-potassium pump, Na⁺-K⁺ ATPase, in animal cells |
| Stoichiometry of that example | No fixed ratio; each substance follows its own independent gradient | Three Na⁺ out for every two K⁺ in, which makes the pump electrogenic |
| What it achieves for the cell | Lets the cell exchange materials with its surroundings at no metabolic cost | Maintains concentrations of ions and other substances against constant passive leakage |
| Metabolic significance | Continues even in a cell with no ATP supply | A red blood cell uses most of its metabolic energy maintaining its Na⁺ and K⁺ imbalance |
The gradient decides which one you are looking at
The test is simple and it is not about whether a protein is involved. Ask which way the substance is moving relative to its own gradient. If it is moving from higher concentration to lower, the cell pays nothing, because the gradient is potential energy that dissipates as the substance spreads out. That is passive transport, and it covers simple diffusion, facilitated diffusion through channels and carriers, and osmosis. If the substance is moving from lower concentration to higher, something has to supply energy. That is active transport, and the energy comes from ATP generated through the cell's metabolism, either directly or through a gradient another pump already built.
Where students go wrong
The biggest error is using protein involvement as the test. Facilitated diffusion uses membrane proteins and is still passive, because the substance is still moving down its gradient; the protein provides a route, not a push. The second error is treating the relevant gradient as concentration alone. For an ion, both the concentration gradient and the electrical charge across the membrane matter, and the combination is the electrochemical gradient. Potassium is the standard illustration: its electrical gradient drives it into the negatively charged cell interior while its concentration gradient drives it out, so the two can pull in opposite directions.
Where this leads next
The passive half of this page is unpacked in the osmosis and diffusion comparison, and applied to whole cells in the hypertonic and hypotonic page, where tonicity predicts whether a cell swells or shrivels. The active half leads into cellular respiration, because active transport is one of the main things ATP is actually spent on, and a red blood cell devoting most of its metabolic energy to its sodium and potassium balance is the clearest illustration of why cells need a continuous ATP supply rather than a one-off store.
Common mistakes
- Calling facilitated diffusion active because a protein is involved. The substance still moves down its gradient, so no energy is spent.
- Reciting the sodium-potassium pump backwards. It moves three Na⁺ out of the cell for every two K⁺ it moves in.
- Considering only the concentration gradient for an ion. The electrical gradient matters too, and the combination is the electrochemical gradient.
- Assuming active transport only moves small ions. Other mechanisms move much larger molecules across the membrane as well.
Related concepts
- Hypertonic vs hypotonic: Both terms describe the solution outside a cell, compared with the cytoplasm inside it.
- Cellular respiration equation: Aerobic cellular respiration is summarized as C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂, the reverse of the photosynthesis summary.
- Osmosis vs diffusion: Diffusion is the passive movement of any substance from a high concentration area to a low concentration area.