Separate water entry from mineral-ion uptake
Apply osmosis and active transport to the root without confusing the two processes.
Section B8.2 · Water uptake
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Recall osmosis and active transport lessons.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which substance moves in osmosis?
What you will learn
- Explain water entry using water potential
- Explain a stated against-gradient mineral uptake
Water and ions have different mechanisms
Water enters a root hair cell by osmosis when the soil solution has higher water potential than the cell contents. Osmosis is net water movement through a partially permeable membrane down a water-potential gradient. It does not mean that every dissolved substance crosses with water.
A concentrated external solution can lower soil water potential enough to reduce entry or cause water loss. The direction depends on the gradient; roots do not gain water by osmosis under every condition.
Ions require their own explanation
If the stated ion concentration is lower outside than inside the cell, continued net uptake against that concentration gradient requires active transport. Transport proteins move ions using energy supplied by respiration. Water does not need the same active-transport explanation.
Not every possible ion movement must be against a gradient. Use the supplied concentrations before naming the mechanism. Root hair surface increases available contact, while membrane transport and energy explain a particular uptake process.
Sources for this lesson
Pause and explain
Which observation supports against-gradient nitrate uptake?
Worked example
Soil has higher water potential than a root hair, but lower nitrate concentration than inside. Explain net entry of each.
Show the worked solution
- Water enters down its water-potential gradient by osmosis.
- Nitrate uptake against its concentration gradient uses active transport and respiratory energy.
Answer Water: osmosis into the cell. Nitrate: active transport in the stated against-gradient case.
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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.
Soil water potential is higher than root-cell water potential. Predict net water movement and name the process.
Give me a hint
Water moves down this stated gradient across the membrane.
Compare my reasoning
- Compare the two water potentials.
- Name net entry as osmosis.
Water enters the root hair by osmosis.
Look for these in your work
- Give the correct direction
- Name osmosis
Nitrate concentrations are 2 units outside and 7 inside. What explains continued net uptake?
Give me a hint
Entry is from lower to higher concentration here.
Compare my reasoning
- 2 to 7 is against the concentration gradient.
- Active transport uses proteins and energy from respiration.
Active transport against the nitrate concentration gradient, using respiratory energy.
Look for these in your work
- Identify the gradient direction
- Include the energy source
A fertiliser solution makes external water potential lower than cell water potential. Explain why adding more fertiliser need not improve water uptake.
Give me a hint
Use water potential rather than amount of mineral alone.
Compare my reasoning
- The changed external solution reverses the water gradient.
- Water can leave the root hair by osmosis instead of entering.
Water uptake can decrease or reverse; more ions outside do not guarantee greater water entry.
Look for these in your work
- Use the reversed gradient
- Separate mineral supply from water entry
Common mistakes
- Calling ion uptake osmosis.
- Saying roots always gain water regardless of the external solution.
What can you explain now?
Which observation supports against-gradient nitrate uptake?
Compare with the explanation
Net entry when nitrate concentration is lower outside
Active transport explains this stated gradient.
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Make it stick
State one difference between water uptake by osmosis and the stated nitrate uptake by active transport.
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.