Active transport — AQA GCSE Combined Science
Test yourself on Active transport with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Active transport explained
Active transport is the carrier-mediated movement of a substance across a membrane from a region where it is more dilute to a region where it is more concentrated, so it runs against the concentration gradient.
Read the full explanation
Because the substance is being moved away from equilibrium, the process cannot happen by diffusion and the cell must supply energy. That energy comes from respiration, which releases energy from glucose; the energy is used by carrier proteins in the membrane to change shape and transfer the substance. A concrete example is a root hair cell absorbing nitrate ions from soil solution that is more dilute than the cell contents. The rate of active transport depends on the number of carrier proteins and on the rate of respiration, so it stops if respiration is inhibited or oxygen is unavailable.
Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.
Plant root hair cells absorb mineral ions such as nitrate, phosphate and potassium from soil solution. These ions are often present in the soil at a lower concentration than inside the root hair cell, so diffusion would not move them into the cell. Active transport uses energy from respiration and carrier proteins in the cell membrane to move the ions from the more dilute soil solution into the more concentrated cell contents, against the concentration gradient. The ions are then used to make compounds needed for healthy growth, including proteins, DNA and chlorophyll. A root hair cell has a long, thin extension that increases its surface area for absorption, and it has many mitochondria to release the energy needed for active transport.
It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.
Active transport moves substances against a concentration gradient, from a lower concentration to a higher concentration, using energy from respiration. In the gut, sugar molecules may be at a lower concentration in the gut lumen than in the blood, so diffusion cannot supply them. Carrier proteins in the cell membrane of gut lining cells bind the sugar and use energy from ATP to change shape and transfer it into the blood. This allows sugar to be absorbed even when gut concentration is low, so respiration can continue. The sugar is then delivered to cells, where it is broken down in respiration to release energy for processes such as muscle contraction and active transport itself.
describe how substances are transported into and out of cells by diffusion, osmosis and active transport
Substances cross cell membranes by three processes. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, and is passive. Osmosis is the diffusion of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution, also passive. Active transport moves substances from a lower concentration to a higher concentration, against the concentration gradient, using energy from respiration. For example, root hair cells use active transport to absorb mineral ions from soil, while oxygen enters cells by diffusion and water enters by osmosis.
explain the differences between the three processes.
Diffusion, osmosis and active transport differ in what moves, which direction relative to the concentration gradient, and whether energy is needed. Diffusion moves any dissolved or gaseous particles down a concentration gradient and is passive. Osmosis moves only water molecules across a partially permeable membrane, from a dilute solution to a concentrated solution, and is also passive. Active transport moves substances against a concentration gradient, from lower to higher concentration, using energy from respiration and carrier proteins. For example, oxygen enters cells by diffusion, water enters root hair cells by osmosis, and mineral ions enter root hair cells by active transport.
Your focus
- State the direction of movement in active transport and identify it as movement against a concentration gradient.
- Explain why active transport requires energy and identify respiration as the source of that energy.
- Apply the concept of active transport to a named example, such as absorption of mineral ions by a root hair cell.
Show all 15 objectives
- Describe how root hair cells absorb mineral ions from dilute soil solution by active transport.
- Explain why active transport is needed for mineral ion uptake and how the structure of a root hair cell supports this process.
- State why plants require mineral ions for healthy growth and give examples of substances made using them.
- Describe how sugar molecules are absorbed from the gut into the blood when the gut concentration is lower than the blood concentration.
- Explain why active transport, rather than diffusion, is required for this absorption.
- Relate the absorbed sugar molecules to their use in cell respiration.
- Describe diffusion as the net movement of particles from higher to lower concentration, down a concentration gradient, without requiring energy.
- Describe osmosis as the net movement of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution.
- Describe active transport as the movement of substances against a concentration gradient, requiring energy from respiration.
- Explain that diffusion and osmosis are passive, whereas active transport requires energy from respiration.
- Explain that diffusion moves any particles down a concentration gradient, osmosis moves water across a partially permeable membrane, and active transport moves substances against a concentration gradient.
- Compare the three processes using correct terminology, including concentration gradients, partially permeable membranes and carrier proteins.
Active transport exam tips
Marking Points
- Active transport moves a substance from a more dilute solution to a more concentrated solution, which is against the concentration gradient.
- Movement against a concentration gradient cannot occur by diffusion alone and requires an input of energy.
- The energy for active transport is released by respiration, so the process depends on a supply of glucose and, in aerobic cells, oxygen.
- Carrier proteins in the cell membrane bind the substance and use energy to change shape, transferring the substance across the membrane.
- The rate of active transport is limited by the number of carrier proteins and by the rate of respiration, so it falls when respiration is inhibited.
- Active transport differs from diffusion because diffusion is passive and down a concentration gradient, whereas active transport is energy-requiring and up a concentration gradient.
- Root hair cells absorb mineral ions such as nitrate, phosphate and potassium from soil solution.
- The soil solution can be more dilute than the cell contents, so the ions are moved against a concentration gradient by active transport.
- Active transport in root hair cells requires energy released by respiration, which is why root hair cells contain many mitochondria.
- The absorbed mineral ions are used by the plant to make substances needed for healthy growth, including proteins, DNA and chlorophyll.
- A root hair cell has a large surface area because of its long, thin extension, which increases the rate of absorption of ions and water.
- If active transport is prevented, for example by a lack of oxygen, the plant cannot absorb enough mineral ions and growth is affected.
- Active transport moves sugar from a lower concentration in the gut to a higher concentration in the blood, against the concentration gradient.
- Energy is required because the movement is against the gradient; this energy comes from respiration, often as ATP.
- Carrier proteins in the membrane of gut lining cells bind the sugar and change shape to transfer it across the membrane.
- The sugar absorbed into the blood is transported to cells and used in cell respiration to release energy.
- Without active transport, sugar would not be absorbed when gut concentration is lower than blood concentration, so respiration could be limited.
- Diffusion: net movement of particles from higher to lower concentration, down a concentration gradient, passive (no energy from respiration required).
- Osmosis: net movement of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution, passive.
- Active transport: net movement of substances from lower to higher concentration, against a concentration gradient, requiring energy from respiration.
- Examples: oxygen and carbon dioxide exchange by diffusion; water uptake by root hair cells by osmosis; mineral ion uptake by root hair cells by active transport.
- Reference to partially permeable membranes is expected for osmosis, while concentration gradients should be mentioned for all three processes.
- Diffusion and osmosis are passive and do not require energy from respiration; active transport requires energy from respiration.
- Diffusion moves any particles down a concentration gradient; osmosis moves only water molecules down a water potential gradient across a partially permeable membrane.
- Active transport moves substances against a concentration gradient, from lower to higher concentration, using carrier proteins.
- Osmosis involves a partially permeable membrane; diffusion may occur across a membrane or in a fluid without one; active transport always involves carrier proteins in a membrane.
- Comparisons should be made explicitly, for example using 'whereas' or 'unlike', to show the differences between the processes.
Examiner Tips
- 💡Define the direction of movement precisely using the words more dilute and more concentrated, then state that this is against the concentration gradient.
- 💡When asked why a cell needs energy, link the answer to respiration releasing energy from glucose rather than simply writing that energy is needed.
- 💡Use a named example, such as a root hair cell absorbing mineral ions, to show that you can apply the definition rather than only recall it.
- 💡Name at least one mineral ion, such as nitrate or phosphate, and state that it is absorbed from a very dilute soil solution.
- 💡Explain the advantage of active transport by referring to the concentration gradient between the soil solution and the cell contents.
- 💡Link the structure of the root hair cell, including its large surface area and many mitochondria, to the process of absorbing ions.
- 💡Always name the direction of movement: from a lower concentration in the gut to a higher concentration in the blood, and state that this is against the concentration gradient.
- 💡Link the need for energy to respiration explicitly, for example 'energy from respiration is used by carrier proteins to change shape'.
- 💡When asked why sugar is absorbed, connect it to cell respiration and the release of energy for the organism, not just to 'making energy'.
- 💡Use the phrase 'net movement' when defining diffusion and osmosis to show you understand it is an overall direction, not individual particle motion.
- 💡When describing osmosis, always mention the partially permeable membrane and that water moves from a dilute to a concentrated solution.
- 💡Structure comparison answers by addressing one difference at a time, such as direction of movement, then energy requirement, then substances involved.
- 💡Use comparative connectives like 'whereas' and 'unlike' to make the differences explicit and avoid simply describing each process separately.
- 💡Refer to specific examples, such as oxygen by diffusion, water by osmosis and mineral ions by active transport, to support each comparison.
Common Mistakes
- Saying that active transport moves substances from a concentrated solution to a dilute solution: the correct direction is from a more dilute solution to a more concentrated solution, against the concentration gradient.
- Stating that active transport needs energy but not naming respiration as the source: the energy is released by respiration, so always link the process to respiration.
- Confusing active transport with osmosis: osmosis is the passive movement of water across a partially permeable membrane down a water potential gradient, whereas active transport uses energy and carrier proteins to move solutes against a concentration gradient.
- Writing that mineral ions enter root hair cells by diffusion: because the soil solution is more dilute than the cell contents, the ions must be taken in by active transport.
- Saying that plants need mineral ions only for energy: mineral ions are used to make proteins, DNA and chlorophyll, while energy comes from respiration of glucose.
- Forgetting to link the structure of the root hair cell to its function: the long extension increases surface area, and the many mitochondria supply energy for active transport.
- Thinking that diffusion can absorb sugar from a lower concentration in the gut into a higher concentration in the blood; correction: diffusion only moves substances down a concentration gradient, so active transport is needed.
- Stating that active transport does not need energy; correction: active transport requires energy from respiration because it moves substances against a concentration gradient.
- Confusing the role of sugar with the role of oxygen in respiration; correction: sugar is a reactant broken down in respiration, while oxygen is also a reactant, and both are needed for aerobic respiration.
- Stating that diffusion requires energy from respiration; correction: diffusion is passive and relies on the random motion of particles; no metabolic energy is needed.
- Describing osmosis as the movement of any dissolved substance rather than water; correction: osmosis specifically refers to the net movement of water molecules across a partially permeable membrane.
- Saying active transport moves substances down a concentration gradient; correction: active transport moves substances against a concentration gradient, from lower to higher concentration, and requires energy.
- Error: saying diffusion and osmosis both require energy. Correction: both are passive processes; only active transport requires energy from respiration.
- Error: stating that osmosis moves solutes rather than water. Correction: osmosis is the movement of water molecules only, across a partially permeable membrane.
- Error: describing active transport as moving substances down a concentration gradient. Correction: active transport moves substances against a concentration gradient, from lower to higher concentration.