Explain why active transport is needed to absorb mineral ions into root hair cells, even when the concentration of mineral ions is already higher inside the root hair cell than in the surrounding soil water.

Pearson Edexcel International GCSE Biology (4BI1) — 2.4 Movement of substances into and out of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Active transport is required because the mineral ions must move against the concentration gradient – from the dilute soil solution into the more concentrated root‑hair cell. Diffusion can only move substances down a concentration gradient, so it cannot bring ions into the cell when the internal concentration is already higher. Active transport therefore uses energy from cellular respiration to move the ions from the soil into the cell, allowing the cell to absorb the ions it needs.

Examiner tips

  • Show the concentration‑gradient point first, then explain why diffusion cannot do it, and finish with the energy requirement and the direction of transport.
  • Use the exact phrase ‘against the concentration gradient’ and ‘energy from respiration’ – these are key words in the mark scheme.
  • Mention that the cell needs the ions even though its internal concentration is higher – this shows understanding of the need for active transport.

Common mistakes

  • Confusing diffusion with active transport – writing that diffusion can move ions into the cell.
  • Omitting the energy source (respiration) or the direction of movement (soil to cell).

Mark scheme (4 marks)

  1. Mineral ions must move against the concentration gradient (from a more dilute solution to a more concentrated solution)
  2. Diffusion cannot move substances against the concentration gradient / diffusion only moves particles from high to low concentration
  3. Active transport requires energy from respiration
  4. Active transport moves mineral ions from the soil water (more dilute) into the root hair cell (more concentrated), allowing the cell to absorb ions it needs

Key terms in this question

active transport

Related

More Movement of substances into and out of cells questions

▶ Try answering this question with AI marking (free) →