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2.4.5Cell Structure

Active Transport and Co-transport — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 27 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.4.5 Active Transport and Co-transport — AQA A Level Biology 7402 notes: Explain how active transport and co-transport move substances against concentration gradients, and how co-transport absorbs glucose and amino acids in the ileum.Active transport is the movement of molecules…

  • against a concentration gradient
  • carrier protein
  • directional
  • sodium ion gradient
  • co-transporter protein
  • oral rehydration therapy

Section 1 of 9Active transport

Active transport is the movement of molecules or ions into or out of a cell against a concentration gradient (from lower to higher concentration), using ATP and carrier proteins. Unlike diffusion, it needs energy from respiration.

Carrier protein
A specific carrier protein in the membrane binds the molecule or ion on one side.
ATP binds to the carrier and is hydrolysed to ADP and inorganic phosphate (Pi) by the carrier protein, which acts as an ATP hydrolase.
The phosphate released causes the carrier to change shape, releasing the molecule on the other side of the membrane.
The carrier then returns to its original shape.

Featuresit is specific (each carrier handles one type of molecule), it is directional, it can be stopped by respiratory inhibitors, and the rate depends on the number of carriers and the rate of ATP supply.

Section 2 of 9Comparing transport mechanisms

FeatureSimple diffusionFacilitated diffusionActive transport
DirectionDown gradientDown gradientAgainst gradient
ATP neededNoNoYes
ProteinsNoChannel or carrierCarrier only
SubstancesSmall, non-polarPolar molecules, ionsIons, glucose, amino acids
Rate limited byGradient, area, distanceNumber of proteins, gradientNumber of carriers, ATP supply

Section 3 of 9Co-transport

Co-transport is the coupled movement of two different substances by the same carrier protein, where the movement of one substance down its gradient provides the energy to move another against its gradient. The classic example is absorption of glucose and amino acids from the ileum into epithelial cells, and the key is the sodium ion gradient.

Sodium ions are actively transported out of the epithelial cell into the blood by the sodium-potassium pump, lowering the sodium ion concentration inside the cell.
Co-transporter protein
Sodium ions then diffuse down this concentration gradient from the lumen into the cell via a co-transporter protein, bringing glucose or an amino acid with them.
As a result, glucose builds up inside the epithelial cell to a concentration higher than in the lumen.
Glucose then moves out of the cell into the blood by facilitated diffusion through a different carrier protein in the basal membrane.

The co-transporter itself does not use ATP; ATP is used indirectly by the sodium-potassium pump to maintain the sodium gradient.

Section 4 of 9Applications

Co-transport is important in absorbing all of the glucose from digested food even when its concentration in the lumen is lower than in the blood.

Oral rehydration therapy
It is also the basis of oral rehydration therapy: in diarrhoea, patients lose water and ions.
Giving a solution containing glucose and sodium ions stimulates co-transport of sodium and glucose into the cells,
  • which lowers the water potential in the cells and so water follows by osmosis, rehydrating the patient.

Active transport also occurs in root hair cells (absorbing mineral ions such as nitrate), in kidney tubules (reabsorbing glucose), and in loading of sucrose into phloem companion cells. Cells with a large amount of active transport have many mitochondria and a large surface area, often provided by folded membranes or microvilli.

Section 5 of 9Interpreting data

If the rate of uptake of an ion is plotted against oxygen concentration, active transport shows an increase as oxygen rises (more aerobic respiration, more ATP) until another factor (number of carrier proteins) limits it.

Interpreting data
If a respiratory inhibitor such as cyanide is added, uptake falls sharply.
In contrast, uptake by facilitated diffusion would be unaffected by the inhibitor.
Uptake against a concentration gradient (e.g. a cell concentration higher than the surroundings) is evidence for active transport.

Section 6 of 9Rates and limiting factors in active transport

The rate of active transport depends on the number and activity of carrier proteins, the concentration of the substance, the supply of ATP and temperature.

Rates and limiting factors in active transport
Oxygen concentration and respiratory substrate affect ATP supply.
A graph of uptake against external concentration for a diffusing substance is a straight line,
  • whereas for active transport it shows saturation because carriers become fully occupied.
Low temperature slows the process by reducing kinetic energy and the rate of respiration, while very high temperature denatures the carrier proteins.

Exam questions often provide data showing uptake with and without an inhibitor.

Rates and limiting factors in active transport
A metabolic inhibitor lowers uptake of an actively transported ion but not of a substance that moves by diffusion.
If an inhibitor binds to the carrier protein, uptake falls even if ATP is present.
Be careful to give the evidence, for example 'uptake continues against the concentration gradient' or 'uptake decreases with cyanide' when you are asked what shows that transport is active.

Section 7 of 9Exam-style questions

  1. Explain how glucose is absorbed from the ileum into the blood against its concentration gradient. [5]5 marks

  2. Cells in a solution took up ion X faster than expected from diffusion alone, and uptake fell when cyanide was added. Explain what this shows. [3]3 marks

  3. Explain why epithelial cells of the ileum contain many mitochondria. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Active transportMovement of substances against a concentration gradient using ATP and carrier proteins
Carrier proteinMembrane protein that changes shape to move a specific molecule across
Co-transportMovement of two substances at once by one protein, one down its gradient driving the other
Sodium-potassium pumpCarrier that uses ATP to move sodium ions out and potassium ions in
ATP hydrolaseEnzyme activity of the carrier hydrolysing ATP to ADP and Pi
Facilitated diffusionPassive movement through channel or carrier proteins
Oral rehydration therapyTreatment with glucose and salts solution using co-transport to restore water

Section 9 of 9Exam technique

1
Saying co-transport uses ATP directly at the co-transporter - ATP is used by the sodium pump, not by the co-transporter.
2
Stating that glucose enters the epithelial cell by active transport using ATP - it enters by co-transport driven by the sodium gradient.
3
Writing that active transport moves substances down a concentration gradient.
4
Forgetting that glucose leaves the cell by facilitated diffusion.
5
Confusing channel and carrier proteins - active transport always uses carriers.

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