The cell membrane
Every cell has a thin skin called the cell membrane. It is made of two layers of phospholipids (fat-like molecules), with proteins floating in it. Scientists call this the fluid mosaic model: the layer can move like oil, and proteins are dotted in it like tiles.
The membrane is selectively permeable. This means it lets some substances through and stops others.
- Pass easily: small, uncharged molecules such as oxygen, carbon dioxide and (slowly) water.
- Need help: charged ions (Na⁺, K⁺) and bigger molecules like glucose.
- Cannot pass on their own: very big molecules like proteins and starch.
Diffusion and facilitated diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Concentration means how crowded the particles are. Diffusion is passive: the cell spends no energy.
Examples: oxygen moves from the air sacs of the lungs into blood; carbon dioxide moves from leaf air spaces into leaf cells.
What makes diffusion faster?
- A bigger difference in concentration (steeper gradient)
- Higher temperature (particles move faster)
- A larger surface area (e.g. folded villi in the gut)
- A thinner membrane / shorter distance
Facilitated diffusion
Ions and glucose cannot slip through the oily middle of the membrane. They pass through channel proteins or carrier proteins. They still move from high to low and still use no energy. It is diffusion with a door.
Osmosis and tonicity
Osmosis is the diffusion of water through a partially permeable membrane, from a dilute solution (more water) to a more concentrated solution (less water). The dissolved substance (solute) cannot cross, so water moves instead.
We compare the solution outside a cell with the inside:
| Outside solution | Water moves | Animal cell | Plant cell |
|---|---|---|---|
| Hypotonic (less solute) | into the cell | swells, may burst | becomes firm (turgid); the wall stops bursting |
| Isotonic (same) | in = out | no change | a bit soft (flaccid) |
| Hypertonic (more solute) | out of the cell | shrinks, wrinkles | membrane pulls from the wall (plasmolysis) |
Living things control their water balance; this is called osmoregulation. Freshwater fish make a lot of dilute urine; a freshwater single-celled organism like Paramecium pumps out extra water with a contractile vacuole.
Active transport
Active transport moves particles from low to high concentration, against the gradient. It needs carrier proteins (pumps) and energy from ATP, made in respiration.
Examples:
- Root hair cells take in mineral ions (like nitrate) from soil that is less concentrated than the root.
- Gut cells absorb the last bits of glucose from food.
- The sodium–potassium pump in nerve cells pushes 3 Na⁺ out and 2 K⁺ in for each ATP.
If a poison stops respiration, active transport stops, but diffusion and osmosis carry on.
Bulk transport and a quick comparison
Very big particles are moved in tiny bubbles called vesicles. This also uses ATP.
- Endocytosis: the membrane folds in and swallows something. A white blood cell eating a bacterium does this (phagocytosis).
- Exocytosis: a vesicle joins the membrane and lets its contents out, as when gland cells release hormones or enzymes made by the Golgi body.
| Diffusion | Facilitated | Osmosis | Active | |
|---|---|---|---|---|
| Direction | high → low | high → low | water: dilute → concentrated | low → high |
| Protein? | no | channel/carrier | sometimes (aquaporins) | pump |
| ATP? | no | no | no | yes |
Key formulas and definitions
- Diffusion: particles move high → low concentration, no ATP
- Facilitated diffusion: high → low through channel/carrier proteins, no ATP
- Osmosis: water moves from dilute to concentrated solution across a partially permeable membrane
- Active transport: low → high, carrier pump + ATP
- Surface area : volume ratio = surface area ÷ volume (smaller cells have a bigger ratio)
- Percentage change in mass = (final − initial) ÷ initial × 100
Worked examples
1. A potato strip of 5.0 g is left in salty water and becomes 4.6 g. Find the percentage change in mass and explain it.
Change = 4.6 − 5.0 = −0.4 g. % change = −0.4 ÷ 5.0 × 100 = −8%. The salty water was hypertonic, so water left the potato cells by osmosis.
2. A cube-shaped cell has sides of 2 µm. Find its surface area to volume ratio.
Surface area = 6 × 2 × 2 = 24 µm². Volume = 2 × 2 × 2 = 8 µm³. Ratio = 24 ÷ 8 = 3 : 1. A 1 µm cube gives 6 : 1, so smaller cells exchange substances faster for their size.
3. Root hair cells take in nitrate ions even when the soil has fewer nitrate ions than the cell. Name the process and what it needs.
Active transport. It moves ions from low to high concentration, so it needs carrier proteins and energy (ATP) from respiration. That is why roots need oxygen.
4. Why does a red blood cell burst in pure water, but an onion cell does not?
Pure water is hypotonic, so water enters both cells by osmosis. The red blood cell has only a membrane, so it swells and bursts. The onion cell has a strong cell wall, so it just becomes firm (turgid).
Common mistakes
- Saying osmosis is the movement of salt or sugar. Osmosis is only the movement of water.
- Saying particles stop moving when concentrations are equal. They keep moving; there is just no net movement.
- Thinking facilitated diffusion needs ATP because it uses a protein. It does not; it is still passive.
- Mixing up hypotonic and hypertonic. Hyper = more solute outside, so water goes out and the cell shrinks.