Active Transport and Co-transport Revision Notes | A Level AQA Biology

1. Why Cells Cannot Always Go With the Flow

Diffusion and osmosis are passive — they move substances down gradients without any energy cost. But cells often need to accumulate substances at concentrations far higher than their surroundings, or expel substances against a gradient. Passive processes cannot do this. That is where active transport comes in.

Picture a cell in your small intestine that needs to absorb glucose from the gut lumen even when glucose is already more concentrated inside the cell than outside. Moving glucose in that direction is like pushing water uphill — it requires a direct energy input. Understanding how cells achieve this, and how they are structurally adapted to do it efficiently, is central to this topic.

2. Active Transport: The Mechanism

Here is how the process works at the molecular level. A carrier protein embedded in the membrane has a specific binding site that recognises the target molecule or ion. When the molecule binds, a molecule of ATP is hydrolysed — broken down into ADP and inorganic phosphate (Pi) — and the energy released causes the carrier protein to change shape. This conformational change physically moves the bound molecule to the other side of the membrane and releases it, even though that side already has a higher concentration of that substance.

The carrier protein then returns to its original conformation, ready to bind another molecule. This cycle repeats continuously, driven entirely by the cell's supply of ATP. Without ATP, active transport stops — which is why metabolic poisons that block respiration also halt active transport within minutes.

A key distinction worth locking in: carrier proteins used in active transport are not the same as channel proteins. Channel proteins form water-filled pores and allow passive movement. Carrier proteins bind their specific substrate and undergo a shape change — that shape change is what makes active transport possible, and it is what requires energy.

Now that the mechanism is clear, consider what determines how fast active transport can proceed. Three factors are critical: the steepness of the concentration gradient being worked against, the number of carrier proteins available in the membrane, and the surface area of the membrane itself. A greater surface area means more membrane space for carrier proteins; more carrier proteins mean more simultaneous transport events; and a steeper gradient does not slow active transport (unlike diffusion) but does determine how much energy is needed to maintain it.

3. Co-transport: Using One Gradient to Drive Another

Active transport becomes even more sophisticated in the mammalian ileum, where the body must absorb glucose and amino acids from digested food. The challenge is that by the time food reaches the ileum, glucose concentrations inside the epithelial cells lining the gut wall are already high. Moving more glucose in would require working against a steep gradient. The cell's solution is elegant: it uses co-transport.

In the ileum, the co-transport carrier protein on the apical (lumen-facing) surface of the epithelial cell has binding sites for both sodium ions ($\text{Na}^+$) and glucose. Sodium ions move into the cell down their concentration gradient — they are at higher concentration in the gut lumen than inside the cell. This downhill movement of $\text{Na}^+$ releases enough energy to simultaneously drag glucose into the cell against its own concentration gradient. One protein, two passengers, one direction.

But here is the critical question: where does the sodium gradient come from in the first place? This is where conventional active transport re-enters the picture. On the basolateral (blood-facing) side of the epithelial cell, sodium-potassium pumps ($\text{Na}^+$/$\text{K}^+$ ATPase) use ATP to actively pump $\text{Na}^+$ out of the cell into the blood. This continuously lowers the intracellular $\text{Na}^+$ concentration, maintaining the steep gradient that drives co-transport on the other side. The whole system depends on ATP — it is just used indirectly to power glucose absorption.

Worked example: Tracing Glucose Absorption in the Ileum: Step by Step

Describe, in sequence, how a glucose molecule moves from the gut lumen into the bloodstream via the epithelial cells of the ileum.

1. $\text{Na}^+$/$\text{K}^+$ ATPase pumps on the basolateral membrane use ATP hydrolysis to actively transport $\text{Na}^+$ out of the epithelial cell into the blood. — This keeps intracellular [Na+] low, creating a steep concentration gradient: high [Na+] in the lumen, low [Na+] inside the cell.
2. The co-transport carrier protein on the apical membrane binds both $\text{Na}^+$ and glucose. $\text{Na}^+$ moves down its gradient into the cell, and glucose is carried in simultaneously against its own gradient. — No direct ATP is used here — the energy comes from the Na+ electrochemical gradient established in Step 1.
3. Glucose concentration inside the epithelial cell rises above that in the blood capillary. — This creates a concentration gradient from cell to blood — glucose can now move passively.
4. Glucose moves out of the epithelial cell across the basolateral membrane via carrier proteins (facilitated diffusion) into the blood capillary. — This step is passive — no ATP required. Glucose moves down its concentration gradient from cell to blood.

Answer: Glucose moves from lumen to blood via: (1) active Na+ expulsion by Na+/K+ ATPase, (2) co-transport of Na+ and glucose into the epithelial cell, (3) facilitated diffusion of glucose into the blood capillary.

4. Factors Affecting the Rate of Transport Across Membranes

Whether a substance moves by diffusion, osmosis, facilitated diffusion, or active transport, the rate of movement is governed by the same three structural and chemical variables: surface area, the number of channel or carrier proteins, and the gradient (concentration gradient for solutes, water potential gradient for water).

Notice that for active transport, increasing the number of carrier proteins has a direct effect on rate — up to the point where ATP supply becomes the limiting factor. This is why cells that carry out intensive active transport are packed with mitochondria: they need a continuous, high-volume ATP supply to keep all their carrier proteins cycling.

5. Cell Adaptations for Rapid Transport

The factors above are not just theoretical — cells exploit them structurally. When a cell needs to transport substances rapidly, natural selection favours adaptations that maximise surface area and protein number. These two strategies are the main routes to faster transport.

• Microvilli (brush border): finger-like projections on the apical surface of ileum epithelial cells that dramatically increase membrane surface area, allowing more co-transport carrier proteins to be embedded per cell.
• Folded internal membranes: mitochondrial cristae increase the surface area for ATP synthesis, supporting the high ATP demand of active transport.
• High mitochondrial density: cells performing intensive active transport contain many mitochondria, ensuring ATP supply matches the demand of carrier proteins.
• High density of carrier or channel proteins: cells can upregulate the expression of specific transport proteins in their membranes, increasing the number of functional transport pathways.
• Thin membranes or short diffusion distances: reduce the path length substances must travel, increasing the rate of passive movement.

The ileum epithelial cell is a textbook example of multiple adaptations working together. Microvilli increase surface area on the apical side; co-transport carrier proteins are densely packed into those microvilli; and abundant mitochondria in the cell body supply the ATP needed to maintain the sodium gradient via Na+/K+ ATPase. Each adaptation targets one of the three rate-determining factors.

6. Required Practical 3: Water Potential of Plant Tissue

To measure the water potential of plant tissue, you cannot insert a probe directly into a cell. Instead, you use osmosis as a tool: place tissue in solutions of known water potential and observe whether the tissue gains or loses mass. The solution in which mass does not change is at the same water potential as the tissue. Required Practical 3 formalises this approach using a dilution series to generate a calibration curve. \n\n**How to produce a dilution series for Required Practical 3:** \nTo create a series of solutions with different, known concentrations of sucrose, start with a stock solution of known concentration (e.g., 1.0 mol dm⁻³ sucrose). Use a volumetric pipette or measuring cylinder to mix specific volumes of stock solution and distilled water in test tubes or beakers, according to a dilution table (for example, 10 cm³ of stock + 0 cm³ water for 1.0 mol dm⁻³; 8 cm³ stock + 2 cm³ water for 0.8 mol dm⁻³; 6 cm³ stock + 4 cm³ water for 0.6 mol dm⁻³, etc.). Mix thoroughly. Repeat as needed for all concentrations required. Plant tissue samples (e.g. potato cylinders) are then placed into each sucrose solution.

Using percentage change in mass rather than absolute mass change is a deliberate methodological choice. If cylinders are not perfectly equal in initial mass, absolute changes would be misleading — a heavier cylinder absorbs more water even at the same concentration. Percentage change normalises for this variation, making comparisons between cylinders valid.

7. Exam Technique and Common Mistakes

Active transport and co-transport questions reward precise language. Examiners award marks for specific mechanistic detail — vague answers about 'energy' or 'proteins' without qualification will not score. The guidance below targets the errors that most frequently cost marks in AQA mark schemes.