Explain active mineral-ion uptake by root hairs
Show why useful uptake can continue when the soil concentration is lower.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall active transport, root hairs and water potential.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why is active uptake useful in dilute soil?
What you will learn
- Show why useful uptake can continue when the soil concentration is lower.
- Apply the idea to a supplied biological example or investigation.
Relate uptake to a plant need
Root hairs can actively absorb mineral ions from dilute soil solutions. If the ion concentration is already higher inside the cell, continuing inward uptake is against its concentration gradient and requires respiratory energy. Plants need mineral ions for growth and chemical processes, so this ability permits uptake beyond passive equilibration.
Use the specific ion comparison. A soil concentration lower than the cell concentration supports the against-gradient argument; the root-hair shape alone does not prove every ion transfer is active.
Separate two simultaneous processes
Water can enter the same root hair by osmosis when its water-potential gradient favours entry. The mineral ion and water are different moving substances. Do not describe ion uptake as osmosis or use water potential in place of the ion concentration comparison.
A reduced oxygen supply may reduce aerobic respiration and the energy available for active ion uptake, but a biological observation can have additional causes. Frame a prediction as a possible effect under controlled conditions rather than claiming all roots immediately stop every form of respiration.
Pause and explain
Which evidence most directly supports active inward ion uptake?
Worked example
Nitrate concentration is 0.2 units in soil solution and 1.4 units in the root hair. Explain continuing uptake.
Show the worked solution
- Nitrate enters from lower to higher concentration.
- Net diffusion alone would not explain this accumulation.
- Active uptake uses energy from respiration.
Answer Active transport supports nitrate uptake against its gradient.
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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 ion is 0.4 units outside and 2.0 inside. Explain inward accumulation.
Give me a hint
Follow its concentration gradient.
Compare my reasoning
- Inward is lower to higher.
- Active transport uses respiratory energy.
Active uptake against the ion concentration gradient.
Look for these in your work
- Name energy from respiration.
State how water and mineral ions can enter a root hair by different processes.
Give me a hint
Separate the moving substances.
Compare my reasoning
- Water enters by osmosis under a favourable water-potential gradient.
- Ions can be actively accumulated using respiration.
Osmosis for water; active transport for against-gradient ion uptake.
Look for these in your work
- State the relevant condition for each.
Why could reduced aerobic respiration reduce active ion uptake without proving osmosis has stopped?
Give me a hint
Compare energy requirements.
Compare my reasoning
- Active transport uses respiratory energy.
- Osmosis does not require that direct energy supply.
Less respiratory energy can reduce active uptake while osmosis may continue.
Look for these in your work
- Avoid claiming all membrane exchange stops.
Common mistakes
- Ion uptake is not osmosis.
- Mineral-ion concentration and water potential are different comparisons.
What can you explain now?
Which evidence most directly supports active inward ion uptake?
Compare with the explanation
Lower external than internal ion concentration plus energy-dependent inward transfer.
The direction is against the named ion gradient.
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Make it stick
Explain root-hair uptake without calling ions water or calling osmosis pumping.
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.