Relate the cell-surface membrane to selective exchange
The cell-surface membrane forms the boundary of a cell and controls exchange with its surroundings.
AS Level · Cambridge International Biology 9700, 2028–2030. Interpret only the structures supported by the provided image or description. Written practice is self-checked; it does not demonstrate practical microscopy competence.
Before you begin
- Recognise that organisms are made of cells and that a light microscope produces a magnified image.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which structure controls selective exchange at a cell boundary?
What you will learn
- Recognise the cell-surface membrane in eukaryotic cells
- Outline its role as a selectively permeable boundary
A thin boundary around the cytoplasm
The cell-surface membrane is a thin boundary around the cell. It separates the cytoplasm from the external environment while permitting selected substances to cross. In an electron micrograph it may appear as a narrow boundary; its appearance depends on image preparation and resolution.
The membrane is not the same as a plant cell wall. A plant cell has a cell-surface membrane just inside its cellulose wall. Animal cells have a cell-surface membrane but no cellulose wall.
Selectivity supports cell function
Water, ions, nutrients and waste products cross by different processes. The membrane’s selective permeability helps maintain conditions needed for cell processes. At this level, connect the structure with controlled exchange; detailed membrane transport pathways are taught in Topic 4.
A membrane should not be described as an impermeable barrier. It separates compartments while enabling regulated movement. A visible boundary in a drawing may be schematic rather than a literal scale representation.
Pause and explain
Where is the cell-surface membrane in a typical plant cell?
Worked example
A plant cell is described as having a cellulose wall and a selectively permeable boundary. Name both structures and explain their positions.
Show the worked solution
- The outer supporting structure is the wall.
- The boundary controlling exchange is the membrane.
Answer The cellulose cell wall is outside. The cell-surface membrane lies just inside the wall and controls selective exchange between the cytoplasm and surroundings.
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.
A student says the cell wall controls everything that enters a plant cell. Correct the statement.
Give me a hint
Separate support from selective exchange.
Compare my reasoning
- The cell wall provides support.
- The cell-surface membrane controls selective exchange.
The cellulose wall supports the plant cell, but the cell-surface membrane just inside it controls selective exchange between the cytoplasm and the surroundings.
Look for these in your work
- Wall role corrected
- Membrane role and position stated
Explain why a selectively permeable membrane is useful to a cell.
Give me a hint
Consider internal conditions and controlled movement.
Compare my reasoning
- The membrane separates the cytoplasm from the environment.
- Selective exchange helps maintain suitable internal conditions.
A selectively permeable membrane controls which substances enter and leave, helping the cell maintain internal conditions while exchanging materials with its surroundings.
Look for these in your work
- Selective exchange stated
- Cell function linked
An image does not clearly show a thin cell boundary. Can you conclude that no membrane is present? Explain.
Give me a hint
Distinguish absence of evidence from evidence of absence.
Compare my reasoning
- Image resolution and preparation affect visibility.
- Cell-surface membranes are a feature of eukaryotic cells.
No. A thin membrane may not be resolved clearly in the image. The image’s limits do not establish that the cell lacks a membrane.
Look for these in your work
- Imaging limit recognised
- Conclusion appropriately cautious
Common mistakes
- Using “cell wall” for the selective boundary
- Calling the membrane completely impermeable
- Assuming the membrane is absent when a low-resolution image cannot show it
What can you explain now?
Which structure controls selective exchange at a cell boundary?
Compare with the explanation
Cell-surface membrane
The membrane regulates movement across the boundary.
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Make it stick
How does the cell-surface membrane differ from the cell wall in position and role?
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.