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Biology · Transport in Cells

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Active Transport

Diffusion and osmosis only move things down a gradient. Sometimes a cell has to move a substance the other way, and that costs energy. This lesson covers active transport, where it happens in plants and in the gut, and how to tell the three transport processes apart.

  • 9 key terms
  • All boards

Learning Objectives

  1. 1Describe active transport and state what it needs.
  2. 2Explain why root hair cells and the cells of the gut use active transport.
  3. 3Explain why cells that carry out active transport contain many mitochondria.
  4. 4Describe how substances are transported into and out of cells by diffusion, osmosis and active transport, and explain the differences between them.

Retrieval practice

  1. 1

    What is diffusion?

    Show answerHide answer

    The net movement of particles from a higher to a lower concentration.

  2. 2

    What is osmosis?

    Show answerHide answer

    The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

  3. 3

    What does a mitochondrion do?

    Show answerHide answer

    It releases energy from respiration.

  4. 4

    How is a root hair cell adapted?

    Show answerHide answer

    It has a long thin extension, which gives a large surface area.

Moving things the hard way

Diffusion and osmosis are like a ball rolling downhill: they happen on their own, and they only go in one direction. But sometimes a cell needs more of a substance than there is outside, so it has to pull it in from a place where the concentration is lower than it already is inside. That is like pushing a ball uphill, and it needs energy. This is active transport, and without it a plant could not feed and a human could not absorb all the sugar from their food.

What is active transport?

Active transport moves substances from a more dilute solution to a more concentrated solution, which is against a concentration gradient. This requires energy from respiration.

  • Against the gradient

    The substance moves from where there is less of it to where there is more, the opposite of diffusion.

  • Needs energy

    The cell has to use energy released by respiration. If respiration stops, active transport stops.

  • Carrier proteins

    In the cell membrane there are carrier proteins that pick up a particular substance and move it across.

  • Many mitochondria

    Cells that do a lot of active transport contain many mitochondria, which release the energy.

Active transport in plants

Plants need mineral ions for healthy growth. The soil solution is very dilute, and the cells of the root hairs hold the ions at a higher concentration.

  • The problem

    The ions are at a lower concentration in the soil than in the root hair cell. Diffusion would move them out of the plant, not in.

  • The solution

    Active transport takes mineral ions into the root hair cells from the very dilute soil solution, against the concentration gradient.

  • Why plants need them

    Nitrate ions are used to make proteins, and magnesium ions are used to make chlorophyll, so a plant that cannot take them up grows poorly.

  • Adaptations

    Root hair cells have a large surface area, and contain many mitochondria to release the energy for active transport.

Active transport in the gut

Active transport also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher sugar concentration.

  • After a meal

    The concentration of sugar in the gut is high, so some sugar can diffuse into the blood.

  • Later

    The concentration in the gut falls below the concentration in the blood, so diffusion cannot take any more in.

  • Active transport

    The cells lining the gut carry on moving sugar against the gradient, so nearly all of it is absorbed.

  • Why it matters

    Sugar molecules are used for cell respiration, so the body cannot afford to lose them.

Diffusion and active transport

Diffusion

  • From a higher to a lower concentration
  • Down the concentration gradient
  • No energy needed from the cell
  • Carries on until the concentrations are equal

Active transport

  • From a lower to a higher concentration
  • Against the concentration gradient
  • Needs energy from respiration
  • Can keep going until the substance is all absorbed

Telling the three processes apart

In an exam question, ask three things about what is moving.

  • Is it water crossing a partially permeable membrane?

    Then it is osmosis, and it goes from the dilute solution to the concentrated one.

  • Is the substance moving from low concentration to high, or does the question mention energy or respiration?

    Then it is active transport.

  • Is it simply moving from high to low concentration, with no energy?

    Then it is diffusion.

  • Check the clues

    Words such as "against the concentration gradient", "very dilute solution" and "mitochondria" point to active transport.

Which process is it?

Nitrate ions are at a concentration of 0.01 mol/dm³ in the soil and 0.10 mol/dm³ inside a root hair cell. The cell takes up nitrate ions, and uptake stops when the cell is given a poison that stops respiration. Name the process, and explain how you know.

Show the solutionHide the solution
  1. 1 Compare the concentrations The ions are at a lower concentration outside (0.01) than inside (0.10).
  2. 2 Work out the direction They move from low concentration to high concentration, which is against the gradient. Diffusion could not do this.
  3. 3 Use the clue about respiration When respiration stops, uptake stops, so the process needs energy from respiration.
  4. 4 Name it The process is active transport.

AnswerActive transport, because the ions move against the concentration gradient and the process needs energy from respiration.

Case study

Why waterlogged soil harms plants

In waterlogged soil, the spaces between soil particles are full of water, and the roots cannot get enough oxygen. Without oxygen the root cells cannot release as much energy by respiration, so active transport slows down. The plant takes up fewer mineral ions, even though they are in the soil, and its growth suffers.

Less oxygen So less respiration in the roots
Less energy So less active transport of ions

The definition to learn

Active transport moves substances from a more dilute solution to a more concentrated solution, and it needs energy from respiration.

Against a concentration gradient, using energy. That is what makes it "active".

Active transport

Active transport

  • Definition

    • from dilute to concentrated
    • against the gradient

  • Needs

    • energy from respiration
    • mitochondria

  • In plants

    • mineral ions into root hair cells from dilute soil solution

  • In the gut

    • sugar into the blood from lower concentration

  • Compare

    • diffusion and osmosis go down the gradient and need no energy

Summary and exam focus

  • Active transport moves substances from a more dilute solution to a more concentrated one, against a concentration gradient.
  • It needs energy from respiration.
  • Mineral ions are taken into root hair cells by active transport from the very dilute soil solution.
  • Sugar is absorbed from the gut into the blood by active transport when the gut concentration is lower.
  • Diffusion and osmosis need no energy, and go from high to low concentration (of particles, or of water).

Exam focus

Compare diffusion, osmosis and active transport. (6 marks) (6 marks)

Write about direction, what moves and energy for each of the three. A clear way is a sentence on each: diffusion is any particles from high to low with no energy, osmosis is water only through a partially permeable membrane, active transport is against the gradient with energy.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Active transport
The movement of substances from a more dilute solution to a more concentrated solution, against a concentration gradient, using energy from respiration.
Against the concentration gradient
From a lower concentration to a higher concentration.
Respiration
The process in cells that releases energy from glucose.
Mineral ions
Dissolved substances, such as nitrate ions, that plants take up from the soil for healthy growth.
Carrier protein
A protein in the cell membrane that picks up a particular substance and moves it across.
Root hair cell
A root cell with a long thin extension, adapted to take up water and mineral ions from the soil.
Mitochondria
The sub-cellular structures where aerobic respiration releases energy.
Absorption
Taking a substance in, for example sugar moving from the gut into the blood.
Passive
Not needing energy from the cell. Diffusion and osmosis are passive.

Questions and answers

20 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question State 1 mark Easier

State what is needed for active transport.

Mark scheme — 1 mark available

  • Energy from respiration — 1 mark

Model answer

Energy (from respiration).

2. Exam question Describe 2 marks Easier

Describe what is meant by active transport.

Mark scheme — 2 marks available

  • Movement from dilute to concentrated / against a concentration gradient — 1 mark
  • Needs energy from respiration — 1 mark

Model answer

The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient), using energy from respiration.

3. Exam question Name 4 marks Easier

Figure 1 shows ions being taken into a cell. A is a carrier protein. Name the structure B, name the process shown, and explain how you know it is that process.

Ions moving through a carrier protein from a low concentration to a high concentration, with energy supplied by a mitochondrion.

Mark scheme — 4 marks available

  • B: mitochondrion — 1 mark
  • Active transport — 1 mark
  • Ions move from lower to higher concentration / against the gradient — 1 mark
  • Needs energy from respiration — 1 mark

Model answer

B is a mitochondrion. The process is active transport. The ions move from a lower to a higher concentration, which is against the concentration gradient, and the mitochondrion supplies energy from respiration, which diffusion would not need.

4. Exam question Give 1 mark Easier

Give one example of active transport in a plant.

Mark scheme — 1 mark available

  • Mineral ions into root hair cells — 1 mark

Model answer

Mineral ions being taken into root hair cells from the soil.

5. Exam question Explain 3 marks Easier

Explain why root hair cells take up mineral ions by active transport and not by diffusion.

Mark scheme — 3 marks available

  • Lower concentration of ions in the soil than in the cell — 1 mark
  • Diffusion would not move ions into the cell / only goes down the gradient — 1 mark
  • Active transport moves them against the gradient using energy — 1 mark

Model answer

The concentration of mineral ions in the soil solution is lower than in the root hair cell. Diffusion only moves substances from a higher to a lower concentration. Active transport can move ions against the concentration gradient, using energy.

6. Exam question Explain 2 marks Easier

Sugar is absorbed from the gut into the blood by active transport. Explain why the sugar must be absorbed.

Mark scheme — 2 marks available

  • Sugar is used for respiration — 1 mark
  • To release energy for the body's cells — 1 mark

Model answer

Sugar molecules are used for cell respiration, so the body needs to absorb them.

7. Exam question Explain 3 marks Easier

Root hair cells contain large numbers of mitochondria. Explain why.

Mark scheme — 3 marks available

  • Mitochondria release energy / respiration — 1 mark
  • Active transport needs energy — 1 mark
  • Root hair cells take in ions by active transport — 1 mark

Model answer

Mitochondria release energy by respiration. Root hair cells take up mineral ions by active transport, which needs energy. So they need many mitochondria to supply it.

8. Exam question Explain 3 marks Easier

A farmer notices that plants in waterlogged soil take up fewer mineral ions, although the soil contains plenty. Use ideas about active transport to explain why.

Mark scheme — 3 marks available

  • Little oxygen in the waterlogged soil, so less respiration — 1 mark
  • Less energy released — 1 mark
  • Active transport of ions needs energy, so less uptake — 1 mark

Model answer

Waterlogged soil contains little oxygen, so the root cells respire less. They release less energy, and active transport of mineral ions needs energy. So fewer ions are taken up.

9. Exam question Compare 3 marks Easier

Give three differences between diffusion and active transport.

Mark scheme — 3 marks available

  • Direction: high to low (diffusion) compared with low to high (active transport) — 1 mark
  • Diffusion needs no energy, active transport does — 1 mark
  • Down the gradient compared with against the gradient — 1 mark

Model answer

Diffusion moves substances from a higher to a lower concentration, but active transport moves them from a lower to a higher concentration. Diffusion does not need energy, but active transport needs energy from respiration. Diffusion goes down the concentration gradient, but active transport goes against it.

10. Exam question Compare 6 marks Core

Compare diffusion, osmosis and active transport.

What the examiner wants: For each process say what moves, which way it moves and whether it needs energy.

Mark scheme — 6 marks available

  • Diffusion: particles from high to low concentration — 1 mark
  • Diffusion needs no energy — 1 mark
  • Osmosis: water only — 1 mark
  • Osmosis: dilute to concentrated through a partially permeable membrane — 1 mark
  • Active transport: dilute to concentrated (against the gradient) — 1 mark
  • Active transport needs energy from respiration — 1 mark

Model answer

Diffusion is the net movement of particles from a higher to a lower concentration, down the concentration gradient, and needs no energy. Osmosis is the diffusion of water only, from a dilute solution to a concentrated solution through a partially permeable membrane, and needs no energy. Active transport moves substances from a more dilute to a more concentrated solution, against a concentration gradient, and needs energy from respiration.

11. Multiple choice 1 mark Core

What does active transport need?

  1. A A partially permeable membrane only
  2. B Light
  3. C Energy from respiration Correct
  4. D Nothing at all

Why: Active transport needs energy from respiration, because it moves substances against a concentration gradient.

12. Multiple choice 1 mark Core

In which direction does active transport move substances?

  1. A From low concentration to high concentration Correct
  2. B From high concentration to low concentration
  3. C Only out of cells
  4. D Only into plants

Why: Active transport moves substances from a more dilute to a more concentrated solution, against the concentration gradient.

13. Multiple choice 1 mark Core

Which structures supply the energy for active transport?

  1. A Ribosomes
  2. B The nucleus
  3. C The cell wall
  4. D Mitochondria Correct

Why: Mitochondria release energy from respiration, so cells that do a lot of active transport have many of them.

14. Multiple choice 1 mark Core

Which of these is an example of active transport?

  1. A Oxygen moving into a respiring cell
  2. B Mineral ions moving into root hair cells Correct
  3. C Water moving into a potato cell
  4. D Carbon dioxide moving out of a cell

Why: Root hair cells take in mineral ions from the very dilute soil solution by active transport.

15. Multiple choice 1 mark Core

Why do plants need mineral ions?

  1. A For healthy growth Correct
  2. B To make energy by respiration
  3. C To absorb water by osmosis
  4. D To make sugar from the soil

Why: Plants need ions for healthy growth, for example nitrate to make proteins and magnesium to make chlorophyll.

16. Multiple choice 1 mark Core

Sugar concentration is lower in the gut than in the blood. How does sugar move into the blood?

  1. A By diffusion
  2. B By osmosis
  3. C By active transport Correct
  4. D It does not move

Why: It has to move against the concentration gradient, so it is absorbed by active transport.

17. Multiple choice 1 mark Core

Which process needs no energy from the cell?

  1. A Active transport only
  2. B Neither diffusion nor osmosis
  3. C Pumping ions
  4. D Diffusion and osmosis Correct

Why: Diffusion and osmosis are passive processes, which move things down a gradient without energy from the cell.

18. Multiple choice 1 mark Core

What happens to active transport if respiration in a cell stops?

  1. A It speeds up
  2. B It stops or slows down Correct
  3. C It does not change
  4. D It turns into diffusion

Why: Active transport needs energy from respiration, so it stops or slows down.

19. Multiple choice 1 mark Core

Which statement about osmosis is correct?

  1. A It needs no energy and moves only water Correct
  2. B It needs energy from respiration
  3. C It moves any substance
  4. D It moves water from concentrated to dilute

Why: Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. It needs no energy.

20. Multiple choice 1 mark Stretch

A cell takes in a substance from a place where it is at a lower concentration. Which is the most likely process?

  1. A Diffusion
  2. B Osmosis
  3. C Active transport Correct
  4. D Cell division

Why: Moving a substance from a lower to a higher concentration is against the gradient, so it must be active transport.