Explain how water pressure supports a plant
Link water entry with pressure against a supporting cell wall.
Section B3.2 · Osmosis
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 plant cell wall, membrane and vacuole.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What resists outward pressure in a plant cell?
What you will learn
- Link water entry with pressure against a supporting cell wall.
- Apply the idea to a supplied biological example or investigation.
Connect water to firmness
Plants are supported partly by the pressure of water inside cells pressing outwards against their cell walls. Water entry enlarges the fluid-containing interior, while the wall resists excessive expansion. Many firm cells together help support a soft plant stem or leaf.
The cell wall provides structural resistance; it is not an active water pump. The cell membrane controls osmotic exchange. Wall and membrane therefore have different roles in the same explanation.
Explain loss of support
When cells lose water, internal pressure against the wall can fall and tissue may become less firm or wilt. A visible wilt is evidence of reduced support, but it does not alone identify exactly why water was lost or establish the condition of every cell.
Cell walls and other structural tissues also contribute to support. Do not claim all plant support is exclusively water pressure. Extended terminology for turgor, flaccidity and plasmolysis is introduced separately.
Sources for this lesson
Pause and explain
A soft leaf loses water and becomes less firm. What is a suitable explanation?
Worked example
Why can water entry make a soft plant leaf firmer?
Show the worked solution
- Water increases the fluid volume inside cells.
- Internal pressure acts outward against the wall.
- The supporting wall resists expansion.
Answer Greater internal water pressure against walls contributes to firmness.
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.
Name the two roles of membrane and wall in osmotic plant support.
Give me a hint
Separate exchange from resistance.
Compare my reasoning
- Membrane controls water exchange.
- Wall resists outward expansion.
Membrane: osmotic exchange; wall: structural resistance.
Look for these in your work
- Do not swap the roles.
Explain why a water-filled plant cell does not behave exactly like an animal cell.
Give me a hint
Identify the extra supporting boundary.
Compare my reasoning
- Plant cells have a supporting wall.
- It resists excessive expansion under water entry.
The wall limits expansion and supports the internal pressure.
Look for these in your work
- Link wall to pressure.
A student says plant walls are unnecessary because water alone supports plants. Correct the statement.
Give me a hint
Pressure needs a resisting boundary.
Compare my reasoning
- Water presses outward.
- Walls resist and support that pressure.
- Other structural tissues also contribute.
Water pressure and supporting structures work together.
Look for these in your work
- Avoid an exclusive water-only claim.
Common mistakes
- The wall is supporting, not pumping.
- Wilting does not prove every cell is plasmolysed.
What can you explain now?
A soft leaf loses water and becomes less firm. What is a suitable explanation?
Compare with the explanation
Reduced internal pressure against cell walls.
Less water can reduce the pressure contribution to support.
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Make it stick
Explain wilt using water loss, pressure and the wall.
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.