Diffusion, Osmosis and Active Transport
Three ways substances cross the cell membrane. Learn the definitions word-for-word — marks are awarded for exact phrases like net movement, concentration gradient and partially permeable membrane.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (i.e. down a concentration gradient), as a result of their random movement.
- It is passive — no energy from respiration is needed.
- It moves gases and small soluble molecules: oxygen and carbon dioxide across the alveoli, glucose into cells, urea out of liver cells into the blood.
- Particles keep moving in both directions; "net" means more go one way than the other until the concentrations are equal.
Osmosis
Osmosis is the net movement of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.
- A partially permeable membrane lets small water molecules through but not larger solute molecules such as sucrose.
- Osmosis is really just diffusion of water, so it is also passive.
- Never write "semi-permeable" — the specification says partially permeable.
Effects on cells:
| Animal cell (no cell wall) | Plant cell (cell wall) | |
|---|---|---|
| In dilute solution / pure water | Water enters, cell swells and may burst | Water enters, vacuole swells, cell becomes turgid — wall stops bursting |
| In concentrated solution | Water leaves, cell shrinks and crenates | Water leaves, cytoplasm shrinks from wall — cell becomes flaccid, then plasmolysed |
Active transport
Active transport is the movement of particles against a concentration gradient (from a lower to a higher concentration) across a cell membrane, using energy released by respiration.
- Uses carrier proteins in the membrane; needs mitochondria to supply energy.
- Examples you should be able to quote:
- Root hair cells absorbing mineral ions (e.g. nitrate) from very dilute soil water into a more concentrated cytoplasm.
- Glucose reabsorbed from the filtrate into the blood at the proximal convoluted tubule (PCT) of the nephron.
- Salt (sodium ions) absorbed from the large intestine into the blood.
Summary table
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Any small particle/gas | Water only | Ions and molecules e.g. glucose, mineral ions |
| Direction | Down gradient | Dilute → concentrated (water) | Against the gradient |
| Energy from respiration? | No | No | Yes |
| Membrane needed? | Not necessarily | Partially permeable membrane required | Membrane with carrier proteins |
Examiner tip. Two specific traps. (1) In kidney questions, examiners repeatedly report that candidates lose marks for not naming the proximal convoluted tubule (PCT) and not naming active transport — name both explicitly. (2) Read the question stem fully: candidates asked about salt absorption in the large intestine wrote answers about the kidney. And when a question is about movement of substances, use the technical word — say "by diffusion", not "it moves across".