State that the energy for diffusion comes from the kinetic energy of molecules and ions.

3.1 Diffusion

Learning objective

State that the energy for diffusion comes from the kinetic energy of the molecules and ions themselves (derived from temperature) and that diffusion is a **passive** process – no cellular ATP is required.

What is diffusion?

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration **through a semi‑permeable membrane (or directly through a fluid)** until equilibrium is reached.

Source of energy for diffusion

Diffusion does not need an external energy source such as ATP. The required energy is supplied by the particles themselves in the form of kinetic energy.

The kinetic energy (KE) of a particle is given by:

$$KE = \tfrac{1}{2}mv^{2}$$

• m = mass of the particle
• v = velocity of the particle

Because particles are in constant random motion, they possess kinetic energy that enables them to move and spread out. This kinetic energy originates from the temperature of the system – higher temperature → greater kinetic energy → faster diffusion.

Factors influencing the rate of diffusion (required syllabus factors)

Extension (optional for deeper study)

• Particle size: Smaller particles have higher average speeds and therefore diffuse more quickly.
• Medium of diffusion: Gases diffuse fastest, liquids slower, and solids very slowly.

Suggested classroom investigation

Title: Diffusion of a coloured dye through agar gel

1. Place a drop of dye on one side of a thin agar slab.
2. Measure the distance the dye front travels after a fixed time under different conditions:
   • Vary temperature (room temperature vs. warm water bath).
   • Vary concentration gradient by using different dye concentrations.
   • Vary surface area by using agar slabs of different widths.
   • Vary diffusion distance by using slabs of different thicknesses.
3. Record the distance travelled and discuss which factor gave the greatest change in rate.

This investigation addresses the four required factors and can be performed with simple school‑lab equipment.

Links to other syllabus topics

• Gas exchange: Oxygen diffuses from alveolar air into blood capillaries, and carbon dioxide diffuses in the opposite direction.
• Osmosis: Water moves by diffusion (osmosis) across a selectively permeable membrane; the same principles of kinetic energy and concentration gradient apply.

Examples of diffusion in living organisms

• Simple diffusion:
  • Oxygen (O₂) moving from alveolar air into blood.
  • Carbon dioxide (CO₂) moving from blood into the alveoli.
• Facilitated diffusion (carrier‑protein mediated):
  • Glucose entering cells via a glucose carrier protein.
  • Sodium (Na⁺) and potassium (K⁺) ions moving through ion channels in nerve cells.

Summary

Diffusion is a passive process driven solely by the kinetic energy of the molecules and ions involved. This kinetic energy is supplied by temperature; as temperature rises, particles move faster, increasing the rate of diffusion. No cellular energy (ATP) is required.

Self‑check questions

1. Explain why diffusion does not require ATP.
2. How does temperature affect the kinetic energy of particles and consequently the rate of diffusion?
3. Why do gases diffuse more rapidly than liquids?
4. Give two biological examples where diffusion is essential for organism function.
5. Describe how each of the four required factors (temperature, concentration gradient, surface area, distance) influences the rate of diffusion.

Suggested answer length: 1–2 sentences per question, including the key term(s) highlighted in the syllabus.