Cells and organisation · GCSE Biology
Active transport
Explain active transport for GCSE Biology: movement against a concentration gradient using energy from respiration, root-hair mineral ions, and why mitochondria pack into those cells.
Active transport: low to high, against the gradient, energy from respiration, carrier proteins in the membrane. Root hairs and gut lining are the textbook examples.
The important bits
What you need to know
- 1
Active transport moves substances from a lower concentration to a higher concentration (against the concentration gradient).
- 2
It requires energy from respiration. That is why cyanide, which stops respiration, also stops active uptake of ions.
- 3
Carrier proteins in the cell membrane change shape to pump the particle across. This is not the same as diffusion through the phospholipid bilayer.
- 4
Root hair cells take up mineral ions such as nitrate from dilute soil solution into more concentrated cell sap by active transport, then use nitrate to make amino acids.
- 5
Root hair adaptations: large surface area, thin wall, many mitochondria to transfer energy for the pumps.
- 6
In the small intestine, glucose and amino acids can be absorbed by active transport when their concentration in the gut is lower than in the blood, so all the useful product is taken up.
- 7
Plants still take ions at night, when photosynthesis has stopped, because respiration continues in mitochondria and fuels the pumps.
- 8
Comparison: diffusion and osmosis are down the gradient and passive; active transport is against the gradient and active. Osmosis is water only.
Quotations worth analysing
Short evidence. Real method.
“Active transport moves substances against a concentration gradient using energy from respiration.”
Both halves are required: against the gradient, and energy from respiration. “Needs energy” without the direction is incomplete.
“Root hair cells absorb mineral ions by active transport.”
Name nitrate or mineral ions, not water. Water into roots is osmosis. Ions against the gradient are active transport.
“Mitochondria transfer energy for active transport.”
Link organelle to process. “Lots of mitochondria so it can absorb stuff” is too vague.
Go deeper
Why can diffusion not finish the job in a root hair?
Soil water is often more dilute in nitrate than the cytoplasm. Diffusion would move nitrate out of the cell, not in. The plant still needs those ions to make amino acids and proteins (glucose from photosynthesis plus nitrate). Carrier proteins use energy transferred from aerobic respiration in mitochondria to pump ions in against the gradient. That is why well-aerated soil matters: oxygen for respiration. Waterlogged soil reduces oxygen, respiration slows, ion uptake falls, and the plant may show mineral-deficiency symptoms even if ions are present in the water. In the exam, do not write that minerals “diffuse into the root because the root needs them”. Need is not a mechanism. Write against the gradient, energy, mitochondria, nitrate.
Go deeper
How do I compare the three transport processes in a table answer?
Rows: direction, energy, example. Diffusion: high to low, no energy, oxygen into blood. Osmosis: water from dilute to concentrated solution through a partially permeable membrane, no energy, water into a root hair. Active transport: low to high, energy from respiration, nitrate into a root hair or glucose from ileum into blood. Substances: diffusion can be gases or solutes; osmosis is water; active transport is often ions or glucose. Membrane proteins: required for active transport; not required for simple diffusion of O₂ and CO₂. If a six-mark asks you to explain how a root hair is adapted, split water (osmosis, surface area) from ions (active transport, mitochondria). Mixing the two processes in one sentence loses both marks.
Go deeper
Where else does active transport appear in the course?
Gut lining cells load glucose when the meal has been mostly absorbed and the gut concentration has fallen below the blood. Kidney tubules reabsorb glucose so it is not lost in urine (selective reabsorption). Guard cells pump ions to change water potential and open stomata. Nerve cells pump sodium and potassium to restore resting potential after an impulse (Triple). Phloem companion cells actively load sucrose. The common thread is a membrane protein plus respiration. If a graph shows ion uptake falling when oxygen is removed, the process is active transport. If uptake continues in boiled cells, the proteins are denatured and the answer is that living, respiring cells are required.
See the idea in action
Nitrate concentration is 0.1 mmol/dm³ in soil water and 5 mmol/dm³ inside a root hair cell. Nitrate cannot enter by diffusion because that would be against the gradient. The cell uses energy from aerobic respiration in mitochondria so carrier proteins pump nitrate in. Water still enters by osmosis because cell sap is more concentrated than soil water. A student who writes “minerals enter by osmosis” scores zero for that line.
Exam technique
Turn knowledge into marks
If ions move into a root, write active transport, against the gradient, energy from respiration, mitochondria. If water moves into a root, write osmosis and a partially permeable membrane. Never swap those two sentences.
Common mistakes
Do not give these marks away
- 01
Saying active transport moves down a concentration gradient, or that it does not need energy.
- 02
Claiming water is absorbed by active transport, or that mineral ions enter by osmosis.
- 03
Forgetting to mention mitochondria or respiration when explaining root-hair adaptations.
Which process moves mineral ions from dilute soil solution into a more concentrated root hair cell?
AOsmosis
BDiffusion
CActive transport
DTranspiration
Show the answer
Active transport. The ions move against the concentration gradient, so the cell must use energy from respiration. Diffusion and osmosis only move substances down a gradient. Osmosis is water only.
Quick questions
If this is the bit you searched
What is active transport in GCSE Biology?
The movement of a substance from a lower to a higher concentration, against the gradient, using energy from respiration and carrier proteins in the membrane.
How is active transport different from diffusion?
Diffusion is down a concentration gradient and does not need energy. Active transport is against the gradient and uses energy from respiration.
Why do root hair cells have many mitochondria?
To transfer energy from respiration so mineral ions can be pumped in by active transport.
Does active transport happen in the human gut?
Yes. Glucose and amino acids can be absorbed by active transport so uptake continues even when their concentration in the intestine is lower than in the blood.