Home › Learning Hub › IGCSE Biology › 3 Diffusion, osmosis, active transport
Topic 3 · 3.1–3.3

Movement into and out of cells

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe diffusion, state where its energy comes from, and investigate the factors that affect it.
  • Describe the roles of water as a solvent; state what osmosis is and investigate it with Visking tubing and plant tissue.
  • Describe osmosis in terms of water potential and explain turgid, turgor pressure, plasmolysis and flaccid EXTENDED.
  • Describe active transport; explain its importance and the role of protein carriers EXTENDED.

📚The biology

Diffusion

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement. The energy comes from the kinetic energy of the random movement of molecules and ions, so it needs no energy from the cell. Some substances diffuse into and out of cells through the cell membrane: oxygen and carbon dioxide in gas exchange, glucose and urea in solution.

Two boxes divided by a dashed line. At the start, most particles are on the left, high concentration, and few on the right, low concentration; an arrow shows net movement from left to right down the concentration gradient. Later the particles are evenly spread; they still move at random but there is no net movement.
Particles keep moving in both directions; “net” means more move from high to low than the other way.

The rate of diffusion increases with a larger surface area, a higher temperature (more kinetic energy), a steeper concentration gradient, and a shorter distance. This is why exchange surfaces such as alveoli and villi are large, thin and have a good blood supply.

Water and osmosis

Water is the solvent in which substances are carried: digested food molecules dissolve in it for absorption (digestion), urea and excess ions leave in solution in urine (excretion), and blood plasma and xylem sap carry dissolved substances around organisms (transport).

Water moves into and out of cells by osmosis, the diffusion of water through a partially permeable membrane such as the cell membrane. Visking (dialysis) tubing is partially permeable: water molecules pass through it, but sucrose molecules are too large.

A bag of Visking tubing full of sucrose solution stands in a beaker of water, with a capillary tube coming out of the top. At the start the liquid level in the tube is low. After 30 minutes, arrows show water entering the tubing from the beaker, and the level in the capillary tube has risen.
Water enters the tubing by osmosis because the solution inside is more concentrated; the extra volume pushes liquid up the capillary tube.

Plants are supported by the pressure of water inside their cells pressing outwards on the cell wall. If they lose too much water, the cells become soft and the plant wilts.

EXTENDED Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

Three plant cells. In pure water, water enters and the cell is turgid, with the cytoplasm pressed against the wall and a large vacuole. In a dilute solution, some water leaves and the cell is flaccid, the cytoplasm pulling slightly away from the wall. In a concentrated solution, much water leaves; the cell is plasmolysed, with the cytoplasm shrunk away from the wall and the gap filled with the external solution.
EXTENDED Turgid: water pushes out on the wall (turgor pressure). Flaccid: soft, no pressure. Plasmolysed: the membrane pulls away from the wall.
  • EXTENDED Turgid: a cell full of water. The vacuole swells and the cytoplasm presses on the wall — this outward pressure is turgor pressure. The wall stops the cell bursting.
  • Flaccid: the cell has lost water and turgor pressure; it is soft.
  • Plasmolysis: so much water is lost that the cell membrane and cytoplasm pull away from the cell wall.
  • Animal cells have no wall: in pure water a red blood cell swells and bursts; in a concentrated solution it shrinks.

EXTENDED Water potential explains how organisms take up and lose water: root hair cells have a lower water potential than the soil water, so water enters them; cells in a leaf lose water to the air spaces.

Active transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration.

A cell membrane with a protein carrier in it. On the left, soil water with a low concentration of ions; on the right, a root hair cell with a high concentration of ions. An arrow shows ions moving through the carrier from low to high concentration, with energy from respiration.
EXTENDED Protein carriers in the membrane move ions such as nitrate into root hair cells, against the gradient.

EXTENDED Active transport lets cells take in molecules or ions that are scarcer outside than inside: root hairs absorb mineral ions from dilute soil water this way. Protein carriers in the membrane move the molecules or ions across. Anything that stops respiration (lack of oxygen, a poison) stops active transport.

✏️Worked example

Pieces of carrot are weighed, left in sucrose solutions for an hour, then reweighed. The percentage change in mass is plotted below. (a) Explain why the carrot gained mass in 0.0 mol per dm³ (pure water). [2] (b) EXTENDED Estimate the concentration with the same water potential as the carrot cells, and explain your answer. [2] (c) Why is percentage change in mass calculated, rather than the change in grams? [1]
A graph of percentage change in mass against sucrose concentration from 0 to 1.0 mol per cubic decimetre. The points are +14.0 at 0, +4.4 at 0.2, -3.6 at 0.4, -10.0 at 0.6, -14.8 at 0.8 and -18.0 at 1.0. A smooth curve through them crosses zero at about 0.30.

(a) Pure water is more dilute than the cell contents, so water entered the carrot cells by osmosis through the partially permeable cell membranes.

(b) About 0.30 mol per dm³, where the curve crosses 0% change: there is no net movement of water, so the water potential of the solution equals that of the cells.

(c) The pieces started with different masses; a percentage lets them be compared fairly.

Check it. Above the crossing point every piece loses mass, below it every piece gains — the sign change is the evidence, so read the crossing from the line of best fit, not from one point.
“Sucrose moved into the carrot.” Sucrose does not cross; the mass change is water moving by osmosis. And say water moves from dilute to concentrated — not “solution moves”.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Where does the energy for diffusion come from? A: respiration in the cell. B: ATP from mitochondria. C: the kinetic energy of the random movement of particles. D: active transport.
C. Diffusion is passive; it needs no energy from the cell.
2. (Theory.) Explain why oxygen diffuses from the alveoli into the blood. [2]
The concentration of oxygen is higher in the alveolar air than in the blood arriving at the lungs, so there is a net movement down the concentration gradient into the blood.
3. (Practical.) Cubes of pink agar with sides 1 cm and 2 cm are placed in acid, which turns the agar colourless as it diffuses in. (a) Calculate the surface area to volume ratio of each cube. (b) Predict which becomes colourless first, and explain. [4]
(a) 1 cm: 6 cm² : 1 cm³ = 6 : 1. 2 cm: 24 : 8 = 3 : 1. (b) The 1 cm cube: its larger surface area to volume ratio means more surface for the acid to diffuse through for each unit of volume, and the distance to the centre is shorter.
4. (Theory.) Give one example each of water acting as a solvent in digestion, excretion and transport. [3]
Digestion: small soluble products such as glucose dissolve so they can be absorbed. Excretion: urea is removed dissolved in urine. Transport: plasma carries dissolved glucose, ions and hormones (or xylem carries dissolved mineral ions).
5. (Theory.) A plant that has not been watered wilts. Explain how water normally supports the plant. [2]
Water in the cells presses outwards on the cell walls, making the cells firm; without enough water the pressure falls, the cells become soft and the plant wilts.
6. (Theory.) EXTENDED A plant cell is placed in concentrated salt solution. Use the term water potential to explain what happens, and name the state of the cell. [3]
The solution has a lower water potential than the cell contents, so water leaves the cell by osmosis through the partially permeable membrane. The cytoplasm shrinks away from the wall: the cell is plasmolysed.
7. (Theory.) EXTENDED Root hair cells are treated with a chemical that stops respiration. Explain why their uptake of nitrate ions falls, but their uptake of water does not stop. [3]
Nitrate is absorbed by active transport against a concentration gradient, which needs energy from respiration (and protein carriers). Water enters by osmosis, which is passive and needs no energy from the cell.
8. (Theory.) State the difference between diffusion and active transport in terms of concentration gradient and energy. [2]
Diffusion: down a concentration gradient (high to low), no energy from respiration. Active transport: against the gradient (low to high), uses energy from respiration.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Diffusion” — change temperature and particle mass and watch net movement
  • Royal Society of Biology — osmosis in plant tissue, a class practical