Active Transport

Cambridge definition (Extended):

"Movement of particles through a cell membrane from a region of LOWER concentration to a region of HIGHER concentration (AGAINST the concentration gradient), using ENERGY from respiration and PROTEIN CARRIER molecules."

Three keywords Cambridge marks:

1. Against the concentration gradient (low → high).
2. Energy from respiration (ATP).
3. Protein carriers (specific transporters).

Where the energy comes from. ATP is generated by respiration in mitochondria. Cells with lots of active transport (e.g. root hair cells, kidney tubule cells, gut lining cells) have MANY MITOCHONDRIA to supply ATP.

How it works (simplified).

1. The substance binds to a SPECIFIC protein carrier on one side of the membrane.
2. ATP provides energy that changes the carrier's shape.
3. The substance is released on the other side (often at higher concentration).
4. The carrier returns to its original shape, ready for another transport cycle.

Specificity. Each carrier protein only handles ONE type of substance (like a key fits one lock). Different ions need different carriers.

Feature	Diffusion	Active transport
Direction	Down concentration gradient (high → low)	Against (low → high)
Energy from cell?	NO (passive)	YES (ATP)
Mechanism	Random molecular motion	Specific protein carriers
Speed	Limited by gradient	Limited by carriers + ATP supply
Stopping condition	Equilibrium	When ATP runs out or carrier saturated
Examples	Gas exchange in alveoli; CO₂ to chloroplasts	Mineral uptake by roots; glucose reclamation in kidneys

Worked qualitative. A respiring root cell suddenly has its O₂ supply cut. What happens to mineral uptake?

• Without O₂, no aerobic respiration.
• ATP production drops.
• Active transport STOPS.
• Mineral uptake fails (root cell can no longer accumulate ions against the gradient).

Worked qualitative. Why does diffusion not get a cell as much glucose as it needs sometimes?

• If outside glucose is LOWER than inside, diffusion would lose glucose, not gain it.
• Active transport lets the cell pull glucose IN even when outside concentration is lower.

Cambridge tip. When asked "why is active transport important?", give an example: "Root cells use active transport to take up mineral ions like nitrates from soil — the soil concentration is usually lower than inside the cell, so diffusion would not work."

1. Root hair cells absorbing mineral ions from soil.

• Soil concentration of nitrates, phosphates etc. is usually LOWER than in the root cell.
• Diffusion alone would deplete the cell.
• Active transport pulls minerals IN against the gradient.
• Adaptation: root hair cells have MANY MITOCHONDRIA to supply ATP.

2. Kidney tubules reclaiming glucose.

• Filtered blood enters the kidney's tubules.
• Glucose initially passes from blood into the filtrate (filtration is non-selective).
• Tubule cells then pump glucose BACK into the blood by active transport — even after most has been reclaimed (low filtrate concentration).
• Healthy people have NO glucose in urine.

3. Glucose absorption in the small intestine.

• After a meal, gut glucose is high — diffuses into blood.
• Eventually gut glucose drops below blood glucose.
• Active transport then ensures ALL the glucose is absorbed (against the gradient).

4. Sodium-potassium pump in nerve cells.

• Maintains the resting potential by pumping Na⁺ OUT and K⁺ IN against their gradients.
• Continuously uses ATP.
• Essential for nerve impulses (covered in Coordination and Response).

Worked qualitative. Why are root hair cells so rich in mitochondria? They do a lot of active transport (mineral ion uptake), which needs continuous ATP. Many mitochondria → high ATP supply → high transport rate → cells absorb minerals efficiently.