Explain water movement and responses in animal cells
Animal cells lack a cell wall, so water uptake can cause swelling or lysis and water loss can cause crenation.
Section 4.2 · Movement into and out of cells
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What may happen to an animal cell in a solution with higher water potential than the cell?
What you will learn
- Predict water movement between animal cells and solutions using water potential
- Compare the effects of water gain and loss in cells without a cell wall
Membrane movement follows water potential
Net water movement across an animal-cell surface membrane is from higher water potential to lower water potential. Compare the cell’s and solution’s overall water potentials to predict direction.
Water crosses a partially permeable membrane; solutes do not necessarily move in the same direction. At equal water potentials there is no net water movement, though molecules continue moving both ways.
No wall resists volume change
In a solution with higher water potential than the cell, water enters and the animal cell swells. Because there is no rigid cell wall, excessive water entry may cause lysis.
In a solution with lower water potential than the cell, water leaves and the cell shrinks; the irregular surface is described as crenated. These terms apply to animal cells, not walled plant cells.
Pause and explain
What is the term for an animal cell that has shrunk after water loss?
Worked example
An animal cell at ψ = −250 kPa is placed in a solution at ψ = −40 kPa. Predict net water movement and a possible outcome.
Show the worked solution
- The solution has higher water potential.
- Water enters the cell; without a wall it can swell and may lyse.
Answer Water moves from the solution (−40 kPa) into the cell (−250 kPa). The animal cell swells because it has no cell wall; excessive water entry may cause lysis.
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.
An animal cell has lower water potential than its surrounding solution. Predict net movement and the first volume change.
Give me a hint
The less negative value is the higher water potential.
Compare my reasoning
- The solution has higher water potential.
- Water moves into the cell and it swells.
Net water moves from the surrounding solution into the animal cell. Its volume increases and it swells.
Look for these in your work
- Values compared correctly
- Direction and effect stated
An animal cell is transferred to a solution with lower water potential. Explain the likely change.
Give me a hint
Water moves toward the lower water potential.
Compare my reasoning
- Water leaves the cell.
- The cell shrinks and becomes crenated.
Water moves out of the cell toward the lower-water-potential solution. The cell loses volume and becomes crenated.
Look for these in your work
- Direction correct
- Animal-cell term used
A student says an animal cell will become turgid like a plant cell in dilute solution. Correct the comparison.
Give me a hint
Identify the missing structural feature.
Compare my reasoning
- Animal cells lack a rigid cell wall.
- They swell and may lyse rather than becoming turgid.
Animal cells lack a rigid cell wall, so water uptake makes them swell and may cause lysis. Turgidity describes a plant cell whose contents press against a resisting cell wall.
Look for these in your work
- Wall distinction identified
- Outcomes correctly contrasted
Common mistakes
- Saying animal cells become turgid because they have no wall
- Using plasmolysis for an animal cell
- Predicting water movement from solute concentration without comparing water potential
What can you explain now?
What may happen to an animal cell in a solution with higher water potential than the cell?
Compare with the explanation
Water enters and the cell swells
Net water moves toward lower water potential inside the cell.
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Make it stick
How do animal cells respond to water uptake and loss, and how does the lack of a cell wall matter?
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.