Connect membrane diffusion with gas and solute exchange
Use separate gradients to explain useful movement into and out of cells.
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 membrane function and diffusion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which boundary controls exchange into a typical animal cell?
What you will learn
- Use separate gradients to explain useful movement into and out of cells.
- Apply the idea to a supplied biological example or investigation.
A membrane can permit diffusion
Some substances move into and out of cells by diffusion through the cell membrane. Whether a particular substance crosses depends on the membrane and substance. Do not assume every molecule crosses equally easily or that a supporting cell wall controls the same exchange as the membrane.
For each substance, compare its own concentration on the two sides. Oxygen and carbon dioxide can have opposite net directions across the same boundary because their individual concentration gradients differ.
Relate exchange to living processes
A respiring cell uses oxygen, helping maintain lower oxygen concentration inside than outside; oxygen can diffuse inward. Production of carbon dioxide can maintain a higher concentration inside, allowing it to diffuse outward. A photosynthesising leaf can take in carbon dioxide down a gradient maintained by its use.
Solutes are substances dissolved in a solvent. In a stated permeable-membrane model, a dissolved nutrient can diffuse toward a region of lower concentration. Not all nutrient uptake is diffusion; when evidence shows transfer against a gradient, active transport is needed to explain it.
Sources for this lesson
Pause and explain
Why can carbon dioxide diffuse out while oxygen diffuses in?
Worked example
A cell has lower oxygen concentration but higher carbon dioxide concentration than its surroundings. Predict the two net directions.
Show the worked solution
- Compare oxygen on each side: higher outside.
- Compare carbon dioxide on each side: higher inside.
- Treat each gas independently.
Answer Oxygen inward; carbon dioxide outward.
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.
Oxygen is more concentrated outside a respiring cell. State its net diffusion direction.
Give me a hint
Follow the oxygen gradient.
Compare my reasoning
- Higher concentration is outside.
- Lower concentration is inside.
Into the cell.
Look for these in your work
- Use the named substance.
A membrane permits a dissolved nutrient to pass. Its concentration is 8 units outside and 3 inside. Predict net diffusion.
Give me a hint
Compare the supplied concentrations.
Compare my reasoning
- 8 is higher than 3.
- The permitted nutrient crosses down its gradient.
Outside to inside.
Look for these in your work
- State the permeability assumption.
Why does oxygen consumption inside a cell help maintain oxygen entry?
Give me a hint
Link use to the concentration difference.
Compare my reasoning
- Respiration uses oxygen.
- Internal oxygen concentration can remain lower.
- The inward gradient persists while supply is maintained.
Consumption helps maintain an inward oxygen concentration gradient.
Look for these in your work
- Link consumption to concentration and direction.
Common mistakes
- Different substances can have opposite gradients.
- A permeable pathway must be present.
What can you explain now?
Why can carbon dioxide diffuse out while oxygen diffuses in?
Compare with the explanation
Their concentration gradients differ.
Net direction is considered separately for each gas.
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Make it stick
Predict two gas directions from separate gradients.
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.