Identify the kinetic energy that drives diffusion
Explain why diffusion does not require energy supplied by respiration.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall net diffusion down a concentration gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What supplies the energy for diffusion?
What you will learn
- Explain why diffusion does not require energy supplied by respiration.
- Apply the idea to a supplied biological example or investigation.
Particles already have kinetic energy
Molecules and ions in liquids and gases are in continual random motion. The energy associated with their movement is kinetic energy. This motion provides the energy for diffusion; a cell does not have to supply respiratory energy to make net diffusion down a gradient occur.
Diffusion can take place in a non-living solution. A coloured dissolved substance spreads through water even though the water has no respiration machinery. Random movement plus a concentration difference explains the net spreading.
Do not confuse sources of energy
Heating usually increases average particle kinetic energy and speeds diffusion when the other conditions are comparable. This does not turn diffusion into active transport. Active transport requires energy from respiration for a different task: movement against a gradient across a cell membrane.
A temperature experiment in living tissue may also change membrane properties or damage cells. State the conditions of the model rather than assuming every biological process increases indefinitely with temperature.
Sources for this lesson
Pause and explain
Dye diffuses through water in a beaker. Which claim is justified?
Worked example
A dye spreads through non-living water without stirring. Does this require respiration?
Show the worked solution
- Identify the moving dye and water particles.
- They have kinetic energy and move randomly.
- No living respiratory supply is required for diffusion.
Answer No; random particle motion supplies kinetic energy.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the source of energy for oxygen molecules diffusing into a cell.
Give me a hint
Name the type of particle energy.
Compare my reasoning
- Oxygen molecules move randomly.
- Their movement has kinetic energy.
Kinetic energy of random molecular movement.
Look for these in your work
- Do not substitute respiratory energy.
Why does diffusion in water not stop just because no living cells are present?
Give me a hint
Separate motion from metabolism.
Compare my reasoning
- Molecules continue random motion.
- A gradient gives net transfer.
Random kinetic motion and a gradient do not depend on living cells.
Look for these in your work
- Include random movement.
A student says faster dye spreading in warm water is active transport. Correct the claim.
Give me a hint
Ask whether a membrane and respiratory energy drive movement against a gradient.
Compare my reasoning
- Heating increases average kinetic energy.
- The dye still spreads down its gradient.
- No cellular pumping is established.
It remains diffusion; warmth can increase random motion.
Look for these in your work
- Distinguish kinetic energy from respiratory supply.
Common mistakes
- Diffusion is passive, not energy-free particle motion.
- Higher temperature does not prove active transport.
What can you explain now?
Dye diffuses through water in a beaker. Which claim is justified?
Compare with the explanation
Diffusion can occur in non-living material.
Living cells are not required.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare the energy sources of diffusion and active transport.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.