Transport in cells (diffusion, osmosis, active transport)

Teach (quick)

Diffusion

• Net movement of particles from higher concentration → lower concentration.
• Happens because particles move randomly.
• Faster when: bigger concentration difference, higher temperature, larger surface area, thinner membrane.

Osmosis

• Diffusion of water through a partially permeable membrane.
• Water moves from a more dilute solution to a more concentrated solution.

Active transport

• Movement of substances against a concentration gradient.
• Requires energy (from respiration).

Worked example

Question: Why do root hair cells use active transport to take up mineral ions?

Answer (mark-scheme style): Because mineral ion concentration is often lower in the soil than inside the root hair cell, so ions must move against the concentration gradient and this requires energy.

Practice

MCQ (choose one)

1. Which process describes the movement of water through a partially permeable membrane?

• A Diffusion
• B Osmosis
• C Active transport
• D Respiration

2. Which process requires energy?

• A Diffusion
• B Osmosis
• C Active transport
• D Evaporation

3. In which direction do particles move in diffusion?

• A Low → high concentration
• B High → low concentration
• C Only into cells
• D Only out of cells

4. Why does diffusion happen?

• A Cells pull particles in
• B Particles move randomly
• C It requires energy
• D It only happens in water

5. Which change would increase the rate of diffusion across a membrane?

• A Lower temperature
• B Smaller surface area
• C Bigger concentration difference
• D Thicker membrane

6. A plant cell is placed in a very concentrated sugar solution. What happens?

• A It becomes turgid
• B It becomes plasmolysed
• C It bursts
• D Nothing changes

7. Which statement about active transport is correct?

• A It moves substances down the gradient
• B It does not need energy
• C It can move substances against the gradient
• D It only happens in the lungs

8. Why are villi in the small intestine helpful for absorption?

• A They decrease surface area
• B They increase surface area
• C They stop diffusion
• D They prevent osmosis

Short answer (1–4 marks)

1. Define diffusion. (2)

2. Define osmosis. (2)

3. A student says: “Osmosis is when water moves from high concentration to low concentration.” Explain what’s wrong and write a correct statement. (3)

4. Explain why active transport is needed to absorb glucose in the small intestine. (3)

5. A root hair cell absorbs mineral ions from soil. Describe how the cell is adapted for this, and name the transport process used. (4)

Required practical: osmosis in potato cylinders (short)

1. State the independent variable and dependent variable. (2)
2. Give two control variables. (2)
3. Why should you blot the potato cylinders before weighing? (1)

Mark scheme (condensed)

• Diffusion: net movement of particles from high → low concentration.
• Osmosis: movement of water through a partially permeable membrane from dilute → concentrated.
• Active transport: moves substances against gradient, uses energy (respiration).
• Plant cells: dilute → turgid; concentrated → flaccid/plasmolysed.
• Villi/root hair cells: large surface area; (root hair) many mitochondria for energy.
• Practical: IV = concentration of solution; DV = change in mass (or % change); controls = time, temperature, size of cylinders; blot dry to avoid extra surface solution mass.

Exam-style question (6 marks)

A student investigates osmosis using potato cylinders placed in different sugar solutions.

Explain how the student should carry out the investigation and how they could use the results to estimate the concentration of the potato cells. (6)

Indicative points (what gets marks)

• range of sugar concentrations
• same size cylinders
• measure mass before + after same time period
• blot dry before weighing
• repeats + mean
• calculate % change in mass
• plot graph: concentration vs % change
• find where % change = 0 (isotonic point)