3.1 Diffusion
- Syllabus
- 0610–2026–2027
- Topic
- 3.1
- Level
- —
Learning objectives
3.1.1Diffusion• Describe diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement3.1.2Energy for diffusion comes• State that the energy for diffusion comes from the kinetic energy of random movement of molecules and ions3.1.3Diffusion through cell membranes• State that some substances move into and out of cells by diffusion through the cell membrane3.1.4Importance of diffusion• Describe the importance of diffusion of gases and solutes in living organisms3.1.5Factors that influence diffusion• Investigate the factors that influence diffusion, limited to: surface area, temperature, concentration gradient and distance
Define diffusion precisely
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
Particles move randomly in every direction. Where concentration is higher, more particles leave that region per unit time than return, so the overall—or net—movement is towards the lower concentration.
| Cause | Consequence |
|---|---|
| particles move randomly | they spread through the available space |
| the two regions have different concentrations | there is a concentration gradient |
| more particles cross from high to low than low to high | net movement is down the gradient |
As the difference in concentration becomes smaller, net diffusion slows. At equal concentration, particles still move randomly in both directions, but there is no net movement.
Diffusion is not the one-way movement of every particle. It describes the difference between two opposing random flows, and that net flow is down—not against—the concentration gradient.
Identify the energy source for diffusion
The energy for diffusion comes from the kinetic energy of the random movement of molecules and ions.
Kinetic energy is the energy of movement. Molecules and ions already possess it, so their continuous random motion produces diffusion when a concentration gradient exists.
| Process feature | Diffusion |
|---|---|
| immediate energy source | kinetic energy of randomly moving particles |
| energy from respiration required to drive the movement | no |
| direction of net movement | down the concentration gradient |
At a higher temperature, particles have more kinetic energy and move faster. This can increase the rate of diffusion; it does not change the energy source into respiration or ATP.
Do not say that cells spend energy to make diffusion happen. That describes an energy-requiring process such as active transport, not diffusion.
Trace diffusion through a cell membrane
Some substances move into and out of cells by diffusion through the cell membrane.
For each substance, compare its concentration on the two sides of the membrane. If the membrane allows that substance through, its net movement is from the side of higher concentration to the side of lower concentration.
| Situation | Net movement |
|---|---|
| oxygen concentration is higher outside a respiring cell | oxygen diffuses into the cell |
| carbon dioxide concentration is higher inside a respiring cell | carbon dioxide diffuses out of the cell |
| a small solute is higher inside dialysis tubing and can cross its membrane | the solute diffuses out |
A membrane can allow one substance through while preventing another. Therefore, both the concentration gradient and the membrane's permeability to that substance must be considered.
The membrane does not pull particles in or push them out. Diffusion can occur in either direction, and its net direction depends on the concentration gradient for that substance.
Explain why diffusion matters to organisms
Diffusion transfers gases and dissolved substances between an organism and its surroundings, and between exchange surfaces, body fluids and cells.
| Context | Higher concentration → lower concentration | Importance |
|---|---|---|
| lungs | oxygen: alveoli → blood; carbon dioxide: blood → alveoli | supplies oxygen and removes a respiratory waste gas |
| body tissues | oxygen: blood → cells; carbon dioxide: cells → blood | supports aerobic respiration and waste removal |
| leaf in daylight | carbon dioxide: air spaces → mesophyll cells | supplies carbon dioxide for photosynthesis |
| small soluble products after digestion | gut contents → blood, when their concentration is higher in the gut | helps absorb useful solutes |
A complete explanation names the substance, identifies the two regions, states which has the higher concentration and gives the biological benefit of the transfer.
Blood flow, ventilation and the use or production of substances by cells can maintain concentration differences, allowing diffusion to continue rapidly.
Do not describe only 'gas exchange' or 'absorption'. State the substance and its direction down a concentration gradient; diffusion itself does not transport substances against that gradient.
Investigate factors that change diffusion rate
Diffusion is faster with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter diffusion distance.
| Change | Effect on diffusion rate | Reason |
|---|---|---|
| increase surface area | increases | more particles can cross at the same time |
| increase temperature | increases | particles have more kinetic energy and move faster |
| increase concentration gradient | increases | the difference between opposing particle flows is greater |
| increase diffusion distance | decreases | particles take longer to cross the pathway |
Change one factor across a suitable range, measure a diffusion outcome over a fixed time—or the time to reach a fixed endpoint—and keep the other three factors constant. Repeat each condition and compare mean rates.
For coloured dye diffusing through agar jelly, change temperature while keeping the dye concentration, agar dimensions and observation time constant. Measure diffusion distance, repeat, calculate a mean, and plot mean distance or rate against temperature.
Choose an outcome that measures diffusion, identify the independent and dependent variables, control relevant variables, and use the trend to state how the tested factor affects rate.
A large final distance is not automatically a faster rate unless time is controlled. Rate compares change per unit time, and only one named factor should be deliberately changed in a fair test.