Active transport — AQA GCSE Biology
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Active transport explained
Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration, that is, against a concentration gradient.
Read the full explanation
This is the opposite direction to simple diffusion, which is passive and down a concentration gradient. Because active transport moves substances against the gradient, it requires energy and carrier proteins in the membrane. In plants, root hair cells use active transport to absorb mineral ions such as nitrate from soil, where the soil solution is more dilute than the cell cytoplasm. In animals, cells lining the small intestine use active transport to absorb glucose from the gut when gut concentration is lower than blood concentration. The process is selective because specific carrier proteins transport specific substances.
This requires energy from respiration.
Active transport requires energy because substances are moved against a concentration gradient, from a more dilute solution to a more concentrated solution. The energy comes from respiration, which releases energy from glucose in cells. This energy is used by carrier proteins in the cell membrane to change shape and transport specific substances across the membrane. Without respiration, active transport stops. This explains why cells that carry out a lot of active transport, such as root hair cells and cells lining the small intestine, have many mitochondria to release energy by aerobic respiration. It also explains why active transport is affected by factors that affect respiration, such as oxygen availability and temperature, within limits.
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.
Active transport moves mineral ions into root hair cells against a concentration gradient, from a more dilute solution in the soil to a more concentrated solution in the cytoplasm. It requires energy from respiration. Root hair cells have a large surface area and many mitochondria to provide the energy needed, so they absorb ions even when soil concentrations are very low. Plants require these ions for healthy growth; for example, magnesium ions are needed to make chlorophyll, and nitrate ions are needed to make amino acids and proteins. If respiration is blocked, ion uptake slows or stops, demonstrating that the process is active rather than passive diffusion.
It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration.
In the gut, sugar molecules can be absorbed from the lumen into the blood even when the sugar concentration in the gut is lower than in the blood. This is active transport because it moves sugar against a concentration gradient. Energy from respiration and carrier proteins in the cell membrane of intestinal cells are needed. The sugar then enters the blood and is carried to cells. This matters after a meal when most sugar has already been absorbed, or when blood sugar is high, because diffusion alone would not move sugar into the blood. The process helps maintain a supply of glucose for respiration.
Sugar molecules are used for cell respiration.
Sugar molecules absorbed into the blood are transported to cells where they are used in cell respiration. Respiration releases energy from sugar, and this energy is used for life processes such as muscle contraction, active transport, cell division and maintaining body temperature. In cells, glucose is broken down in a series of reactions; aerobic respiration uses oxygen and produces carbon dioxide and water, while anaerobic respiration in animals produces lactic acid. The energy released is transferred to ATP, which powers cell activities. This links the absorption of sugar in the gut to the energy needs of the whole organism.
Students should be able to: • describe how substances are transported into and out of cells by diffusion, osmosis and active transport • explain the differences between the three processes.
Substances cross cell membranes in three ways. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, and it is passive because no energy is transferred from respiration. Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (higher water potential) to a concentrated solution (lower water potential). Active transport moves substances from a lower concentration to a higher concentration, against the gradient, using energy from respiration and carrier proteins in the membrane. Root hair cells absorb mineral ions by active transport; oxygen diffuses into alveoli capillaries; water enters root hair cells by osmosis.
Your focus
- State the direction of movement in active transport relative to a concentration gradient.
- Explain the role of carrier proteins and energy in active transport.
- Apply the concept to named examples such as mineral ion uptake by root hair cells.
Show all 18 objectives
- Explain why active transport requires energy from respiration.
- Describe how mitochondria supply energy for active transport in specialised cells.
- Predict the effect of reduced respiration on active transport.
- Describe active transport of mineral ions into plant root hair cells from dilute soil solutions.
- Explain why active transport requires energy from respiration.
- Relate the uptake of specific mineral ions, such as magnesium and nitrate, to their roles in healthy plant growth.
- Describe active transport of sugar from the gut into the blood when gut concentration is lower.
- Explain why active transport is needed rather than diffusion in this situation.
- Link sugar absorption to its later use in cell respiration.
- Describe how sugar molecules are used in cell respiration to release energy.
- Explain the link between sugar absorption in the gut and energy use in cells.
- Compare aerobic and anaerobic respiration in terms of products and energy released.
- Describe diffusion, osmosis and active transport in terms of the direction of movement of particles or water.
- Explain how energy from respiration and membrane proteins are involved in active transport but not in diffusion or osmosis.
- Apply each transport process to a named example in a plant or animal cell.
Active transport exam tips
Marking Points
- Define active transport as movement of substances from a more dilute solution to a more concentrated solution, against a concentration gradient.
- Contrast active transport with diffusion, which is passive and moves substances down a concentration gradient.
- State that active transport requires energy from respiration and carrier proteins in the cell membrane.
- Give a named example, such as root hair cells absorbing mineral ions from dilute soil solution.
- Explain that carrier proteins are specific, so active transport allows selective uptake of particular substances.
- Describe how the direction of movement relates to concentration: from lower concentration to higher concentration.
- State that active transport requires energy released by respiration.
- Explain that energy is needed because substances are moved against a concentration gradient.
- Link the energy to carrier proteins in the membrane, which use energy to transport specific substances.
- Describe how cells specialised for active transport, such as root hair cells, contain many mitochondria to supply energy.
- Explain that if respiration is inhibited, active transport cannot continue.
- Relate energy demand to factors affecting respiration, such as oxygen supply and temperature.
- Active transport moves mineral ions against a concentration gradient, from a dilute soil solution into root hair cells.
- The process requires energy released by respiration, which is provided by mitochondria in the root hair cells.
- Root hair cells are adapted with a large surface area to maximise the uptake of water and mineral ions.
- Plants require specific ions for healthy growth: magnesium ions are used to make chlorophyll for photosynthesis.
- Nitrate ions are absorbed and used to make amino acids, which are then synthesised into proteins for growth.
- If respiration is inhibited, ion uptake decreases, providing evidence that active transport is occurring.
- Sugar molecules are absorbed from the gut lumen into the blood by active transport.
- The movement is against a concentration gradient because gut concentration is lower than blood concentration.
- Energy from respiration and carrier proteins in the intestinal cell membrane are required.
- The absorbed sugar enters the blood and is transported to body cells.
- Active transport allows absorption to continue even when diffusion would not occur.
- Sugar molecules are transported in the blood to body cells.
- Sugar is broken down in cell respiration to release energy.
- The energy released is used for life processes such as muscle contraction, active transport and cell division.
- Aerobic respiration uses oxygen and produces carbon dioxide and water.
- Anaerobic respiration in animals produces lactic acid and releases less energy per sugar molecule.
- Diffusion is described as net movement of particles from higher to lower concentration, down a concentration gradient, and as a passive process requiring no energy from respiration.
- Osmosis is described as the diffusion of water through a partially permeable membrane from a dilute solution to a concentrated solution, or from higher to lower water potential.
- Active transport is described as movement of substances from lower to higher concentration, against a concentration gradient, requiring energy from respiration and carrier proteins.
- A valid comparison is made: diffusion and osmosis are passive, whereas active transport is active and uses energy released by respiration.
- A correct example is linked to each process, such as oxygen diffusing into blood, water entering root hair cells by osmosis, and mineral ions being absorbed by root hair cells by active transport.
- The role of the partially permeable membrane is identified in osmosis, and the role of carrier proteins is identified in active transport.
Examiner Tips
- 💡Use the phrase against a concentration gradient when defining active transport, as this distinguishes it from diffusion.
- 💡When giving an example, name the substance and the direction of movement, such as mineral ions entering root hair cells from dilute soil solution.
- 💡If asked to compare transport processes, make clear that active transport requires energy and carrier proteins, whereas diffusion does not.
- 💡When explaining why active transport needs energy, always refer to movement against a concentration gradient.
- 💡Link energy to respiration and mitochondria, as this shows understanding of cellular energy supply.
- 💡If asked about adaptations, mention that cells with high active transport have many mitochondria to release more energy.
- 💡Name the specific ion and its use, such as magnesium for chlorophyll or nitrate for proteins, when discussing plant growth.
- 💡Link the need for energy to respiration and mitochondria rather than simply writing that energy is needed.
- 💡Compare the concentrations in the gut and blood explicitly before naming the transport process.
- 💡State that energy comes from respiration and is used by carrier proteins.
- 💡Use the term concentration gradient and say whether movement is with or against it.
- 💡Write a word equation for aerobic respiration to show the reactants and products.
- 💡Link the energy released to a named life process rather than leaving it general.
- 💡Distinguish aerobic from anaerobic respiration by naming the products and relative energy release.
- 💡Use the phrase 'net movement' when defining diffusion, because it shows you understand that particles move both ways but the overall movement is down the gradient.
- 💡When comparing the three processes, structure your answer around direction of movement, membrane involvement and energy requirement.
- 💡Link each process to a named biological example, such as gas exchange in alveoli, water uptake by root hair cells or ion uptake by root hair cells, to show applied understanding.
Common Mistakes
- Saying active transport moves substances from high to low concentration; correct this by stating it moves from more dilute to more concentrated, against the gradient.
- Confusing active transport with osmosis; correct this by noting that osmosis is passive water movement, while active transport uses energy and carrier proteins.
- Forgetting that active transport requires energy; correct this by linking it to respiration and ATP production in the cell.
- Saying active transport uses energy from photosynthesis; correct this by stating that respiration releases the energy used by cells for active transport.
- Thinking energy is used to make substances move faster by diffusion; correct this by explaining that energy is used to move substances against a concentration gradient via carrier proteins.
- Ignoring the role of mitochondria; correct this by linking cells with high active transport demands to a high number of mitochondria.
- Thinking mineral ions enter by diffusion because the soil solution is dilute; correct this by stating that active transport moves ions against the concentration gradient and requires energy.
- Incorrectly stating that nitrate or potassium is used to make chlorophyll; correct this by specifying that magnesium is required for chlorophyll synthesis.
- Believing active transport uses no energy because it happens in plants; correct this by linking the process to cellular respiration.
- Stating that sugar moves by diffusion because it moves from gut to blood; correct this by noting that diffusion would require a higher concentration in the gut than in the blood.
- Forgetting that carrier proteins are specific; correct this by stating that carrier proteins in the membrane transport sugar molecules.
- Writing that the blood has a lower sugar concentration than the gut in this example; correct this by stating the blood has the higher concentration.
- Saying respiration makes energy; correct this by stating that respiration releases energy from sugar.
- Confusing respiration with breathing; correct this by describing respiration as a chemical process in cells.
- Thinking sugar is used only for energy and not as a building material; correct this by noting that sugar can also be converted into other molecules, but its main role here is respiration.
- Saying that diffusion stops when concentrations become equal; the correction is that net movement stops at dynamic equilibrium, but particles continue to move in both directions.
- Describing osmosis as the movement of any dissolved substance rather than water; the correction is that osmosis refers specifically to water moving across a partially permeable membrane.
- Stating that active transport happens because particles naturally spread out; the correction is that active transport moves substances against the concentration gradient and therefore needs energy from respiration.