Choose between diffusion osmosis and active transport from evidence
Use substance, boundary, gradient and energy to justify a mechanism.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall Core definitions plus Extended water potential and carriers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Only an inward arrow is given. What is the best response?
What you will learn
- Use substance, boundary, gradient and energy to justify a mechanism.
- Apply the idea to a supplied biological example or investigation.
Ask four questions
First identify the moving substance: water, a gas or a solute. Then identify the boundary and compare the relevant concentrations or water potentials. Finally ask whether respiratory energy is supplied for against-gradient transfer. A mechanism name should follow evidence rather than a memorised arrow direction.
Diffusion gives net movement down a concentration gradient from random kinetic motion. Osmosis concerns water through a partially permeable membrane, from higher to lower water potential. Active transport uses respiratory energy and membrane protein carriers for the stated against-gradient particle transfer.
Use only the evidence provided
Inward arrows can represent all three processes in different conditions. A question that gives only an inward arrow lacks enough information to distinguish them. State what comparison or energy evidence is needed instead of guessing.
The processes can coexist across one membrane. Oxygen can diffuse while ions are actively accumulated and water crosses by osmosis. They do not have to share the same gradient, direction or energy requirement. This is an Extended integration lesson, building on the Core diffusion and active-transport definitions.
Sources for this lesson
Pause and explain
Which pairing is correct?
Worked example
A cell takes in oxygen from higher external oxygen concentration, water from higher external water potential, and an ion against its gradient using a carrier and respiratory energy. Name the three processes.
Show the worked solution
- Oxygen follows its concentration gradient: diffusion.
- Water follows water potential across the membrane: osmosis.
- The carrier transfers the ion against its gradient using energy: active transport.
Answer Oxygen: diffusion; water: osmosis; ion: active transport.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
An ion enters from lower external to higher internal concentration using a carrier and respiration. Justify the mechanism.
Give me a hint
Apply all active criteria.
Compare my reasoning
- Against-gradient ion movement.
- Carrier and respiratory energy supplied.
Active transport.
Look for these in your work
- Use direction, carrier and energy.
Water enters through a partially permeable membrane from higher outside water potential. Justify the mechanism.
Give me a hint
The moving substance is water.
Compare my reasoning
- Water crosses the required membrane.
- Net direction is higher to lower water potential.
Osmosis.
Look for these in your work
- Give substance, membrane and comparison.
A diagram shows only a dissolved substance moving out. Give two pieces of missing evidence needed to separate diffusion from active transport.
Give me a hint
Compare concentrations and energy.
Compare my reasoning
- Ask the substance concentrations on each side.
- Ask whether respiratory energy is supplied for transfer.
Its gradient direction and energy requirement; membrane/carrier information can also help.
Look for these in your work
- Do not infer mechanism from outward direction alone.
Common mistakes
- An arrow alone cannot name the mechanism.
- Different substances can cross the same membrane differently.
What can you explain now?
Which pairing is correct?
Compare with the explanation
Osmosis: water potential; active ion uptake: ion concentration and respiratory energy.
Use the appropriate driving comparison.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Classify three new examples using evidence rather than arrow orientation.
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.