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B1.9 Active Transport

GCSE Combined Science · AQA · Chapter B1: Cell Structure and Transport

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: moving substances against a concentration gradient using energy from respiration. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Define active transport and explain why it needs energy
  • Describe how root hair cells absorb mineral ions by active transport
  • Describe how sugar molecules are absorbed from the gut by active transport
  • Compare diffusion, osmosis and active transport

Key words

  • Active transport: The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient). It needs energy from respiration.
  • Carrier protein: A protein in the cell membrane that picks up a particle on one side and releases it on the other.
  • Mineral ions: Charged particles, such as nitrate and magnesium ions, that plants need for healthy growth.
  • Respiration: The reactions in cells (mostly in mitochondria) that release energy from glucose.

Moving against the gradient

Diffusion and osmosis only move substances down a concentration gradient. Sometimes a cell needs to take in a substance that is already more concentrated inside the cell than outside. To do this it uses active transport.

Key fact: Active transport needs energy from respiration. Cells that do a lot of active transport have many mitochondria. If respiration stops (for example, with no oxygen or a poison such as cyanide), active transport stops too.

Active transport in plants and animals

  • Root hair cells; What is moved mineral ions (e.g. nitrate, magnesium) from the soil; Why active transport is needed the soil water is a very dilute solution of ions, more dilute than the cell sap, so diffusion would move ions out of the root
  • Small intestine (gut lining); What is moved glucose and other sugar molecules; Why active transport is needed lets sugar be absorbed into the blood even when its concentration in the gut is lower than in the blood, so none is wasted

Plants need nitrate ions to make proteins for growth and magnesium ions to make chlorophyll. Without active transport they could not collect enough from the soil.

Worked example: how much more concentrated?

The concentration of nitrate ions in the cell sap of a root hair cell is 36 mmol/dm³. In the soil water it is 0.2 mmol/dm³.
36 ÷ 0.2 = 180

Answer: the cell sap is 180 times more concentrated, so the ions must be moved in by active transport

Evidence: oxygen and ion uptake

Scientists measured how fast wheat roots took up nitrate ions in solutions with different amounts of oxygen.

  • With almost no oxygen, uptake is very slow: little aerobic respiration means little energy for active transport.
  • As oxygen rises, uptake increases, because more respiration releases more energy.
  • Above about 20% oxygen, the rate levels off. Something else now limits the rate, such as the number of carrier proteins.

Comparing the three transport processes

  • What moves; Diffusion any particles in a gas or solution; Osmosis water only; Active transport particles such as ions and sugars
  • Direction; Diffusion higher → lower concentration; Osmosis dilute → more concentrated solution; Active transport lower → higher concentration (against the gradient)
  • Energy from respiration?; Diffusion no (passive); Osmosis no (passive); Active transport yes
  • Membrane needed?; Diffusion no; Osmosis yes, partially permeable; Active transport yes, with carrier proteins

Common mistakes

  • Saying active transport moves substances “down” or “along” a gradient. It moves them against the gradient.
  • Saying active transport “uses energy” without saying where it comes from. Say “energy from respiration”.
  • Saying root hair cells take in water by active transport. Water enters by osmosis; mineral ions by active transport.
  • Thinking a levelled-off graph means energy has run out. It means another factor, such as the number of carrier proteins, is now limiting.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the best description of active transport? (Choose from: Movement of water through a membrane, Movement of substances from a more dilute to a more concentrated solution, using energy from respiration, Net movement of particles down a concentration gradient, Movement of gases into the lungs.)

Answer: Movement of substances from a more dilute to a more concentrated solution, using energy from respiration

Working: Active transport moves substances against a concentration gradient (dilute → more concentrated) using energy from respiration.

Question 2. True or false? Active transport needs energy from respiration.

Answer: True

Working: True. Moving substances against a concentration gradient needs energy released by respiration.

Question 3. The graph shows how the rate of potassium ion uptake by roots changes with the oxygen concentration. Why does the rate increase between 0% and 15% oxygen? (Choose from: More oxygen diffuses into the roots carrying potassium ions with it, More aerobic respiration releases more energy for active transport, The ions move by osmosis faster, The roots have more chloroplasts.)

Answer: More aerobic respiration releases more energy for active transport

Working: Oxygen is needed for aerobic respiration. More respiration releases more energy, so more ions are taken in by active transport. Above about 20% another factor limits the rate.

Related lessons

Continue in the full course

This lesson is section B1.9 (AGCSC0108) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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