Explain how substances cross cell membranes by diffusion, osmosis, facilitated diffusion and active transport.
Why it matters
The cell membrane decides what gets in and what stays out. That matters at the bench every day. Red blood cells burst if you dilute a blood sample in pure water. Cultured cells shrink if their medium is too salty. Drugs only work if they can reach their target inside or on the cell. Understanding membrane transport tells you why lab solutions are made isotonic (the same overall solute concentration as the cell) and why the exact recipe of a buffer matters.
The core idea: a selective barrier
The plasma membrane is a phospholipid bilayer. Each phospholipid has a water-loving (hydrophilic) head and two water-fearing (hydrophobic) tails. In water they line up in two layers, heads out and tails in. Proteins float in this layer, which is why it is called the fluid mosaic model.
The oily middle is the key. Small, uncharged molecules such as oxygen and carbon dioxide slip through easily. Water crosses slowly on its own and much faster through channel proteins called aquaporins. Ions (Na⁺, K⁺, Cl⁻) and larger polar molecules such as glucose cannot cross the oily core without help from proteins.
Passive transport: no energy needed
Passive transport moves substances down their concentration gradient, from where they are more concentrated to where they are less concentrated. The cell spends no ATP.
- Simple diffusion: molecules spread out through the bilayer by random motion. Example: oxygen moving from the blood into a muscle cell.
- Facilitated diffusion: a channel or carrier protein provides a path, but the direction is still downhill. Example: glucose entering red blood cells through the GLUT1 carrier.
- Osmosis: the diffusion of water across a membrane that solutes cannot cross. Water moves toward the side with more dissolved solute (less free water).
Osmosis in practice: tonicity
Tonicity describes a solution compared with the inside of a cell.
| Outside solution | Water moves | Animal cell (red blood cell) | Plant cell |
|---|---|---|---|
| Hypotonic (fewer solutes outside) | into the cell | swells and may burst (lysis) | becomes firm (turgid); the wall stops it bursting |
| Isotonic (equal) | no net movement | normal shape | limp (flaccid) |
| Hypertonic (more solutes outside) | out of the cell | shrinks and crinkles (crenation) | membrane pulls away from the wall (plasmolysis) |
Worked example. Physiological saline is 0.9% sodium chloride, which is roughly isotonic with human blood. That is why it is used to dilute blood cells for counting and as a base for many cell-washing buffers. Mix red cells with distilled water instead and they swell and lyse within minutes, releasing haemoglobin: the solution turns clear red. Mix them with 3% salt and they crenate. A lab that sees unexpected lysis in a sample checks the diluent first.
Active transport: energy needed
Active transport moves substances against their gradient, from low to high concentration. It needs energy, usually ATP.
The best-known example is the sodium-potassium pump (Na⁺/K⁺-ATPase). For each ATP it uses, it pushes 3 Na⁺ out of the cell and brings 2 K⁺ in. This keeps sodium low and potassium high inside cells, which nerves and muscles depend on. It is also why a blood sample left unseparated for too long can show a falsely high potassium result: as the cells run out of energy, the pumps slow and potassium leaks out of the cells into the serum.
Cells also move large particles in bulk:
- Endocytosis: the membrane folds inward to take material in. Phagocytosis ("cell eating") is how white blood cells engulf bacteria.
- Exocytosis: vesicles fuse with the membrane to release their contents, as in the antibody example from the last lesson.
Comparing the methods
| Method | Direction | Needs a protein? | Needs ATP? | Example |
|---|---|---|---|---|
| Simple diffusion | down gradient | no | no | O₂, CO₂ |
| Facilitated diffusion | down gradient | yes (channel or carrier) | no | glucose via GLUT1, water via aquaporins |
| Osmosis | water toward more solute | sometimes (aquaporins) | no | red cells in water |
| Active transport | against gradient | yes (pump) | yes | Na⁺/K⁺ pump |
| Endo/exocytosis | bulk | membrane vesicles | yes | phagocytosis, secretion |
Common mistakes
- Saying water moves toward more water. In osmosis water moves toward the side with more solute.
- Thinking facilitated diffusion uses energy. It uses a protein but no ATP; the gradient drives it.
- Mixing up hypo- and hypertonic. "Hypo" means under: fewer solutes outside, so water rushes in.
- Forgetting the plant cell wall. Plant cells in pure water do not burst; the wall pushes back.
Quick recap
- The phospholipid bilayer lets small nonpolar molecules through and blocks ions and large polar ones.
- Passive transport (diffusion, facilitated diffusion, osmosis) runs down the gradient with no ATP.
- Active transport pumps against the gradient using ATP; the Na⁺/K⁺ pump is the classic example.
- Lab solutions are made isotonic (such as 0.9% saline) so cells keep their shape.