Active transport

Define active transport.

Active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient), using energy from the hydrolysis of ATP and carrier proteins.

• It is an active process, meaning it requires energy.

Why does active transport require energy?

• Molecules are moved against their concentration gradient, which is energetically unfavourable.

• ATP (adenosine triphosphate) is hydrolysed to ADP + Pi, releasing energy.

• This energy is used to change the shape (conformation) of the carrier protein, allowing the molecule or ion to be transported across the membrane.

What is the role of carrier proteins in active transport?

• Carrier proteins are embedded in the cell membrane.

• Each is specific to a particular molecule or ion.

• They bind to the molecule on one side of the membrane, change shape using ATP, and release it on the other side.

• After transport, the carrier protein returns to its original shape ready to transport another molecule.

How does active transport differ from facilitated diffusion?

Feature	Active Transport	Facilitated Diffusion
Direction	Against concentration gradient	Down concentration gradient
Energy required	✅ Yes (ATP)	❌ No
Type of protein	Carrier proteins	Channel and carrier proteins
Specificity	Highly specific	Also specific
Example	Ion uptake in roots	Glucose uptake in red blood cells

What is the energy source for active transport and how is it used?

• ATP is the energy source.

• It is produced during respiration (mainly in mitochondria).

• ATP → ADP + Pi, releasing energy.

• This energy causes the carrier protein to change shape, moving molecules across the membrane.

List and explain four biological examples where active transport is essential.

• Kidney reabsorption:

  • Useful molecules like glucose and ions are reabsorbed into the blood after filtration in the kidney tubules.

• Absorption in the digestive system:

  • Active transport moves amino acids and glucose into epithelial cells of the small intestine after diffusion has reached equilibrium.

• Loading of sucrose in plants:

  • Sucrose is actively loaded into phloem companion cells for transport via translocation.

• Ion uptake by roots:

  • Root hair cells actively absorb inorganic ions (e.g. nitrate, phosphate) from the soil, even when ion concentration in the soil is lower than in the cell.

Describe what happens to ATP during active transport.

• ATP is hydrolysed by the enzyme ATP hydrolase, splitting it into ADP + Pi (inorganic phosphate).

• This releases energy which is used to change the tertiary structure of the carrier protein.

• This conformational change moves the molecule through the membrane.

why doesn’t facilitated diffusion need energy even though the carrier protein changes shape

In facilitated diffusion, molecules move down their concentration gradient (from high → low).

• This movement is passive — it happens because of the natural kinetic energy of the particles, not from ATP.

• When the carrier protein changes shape, it does so spontaneously when the molecule binds. The binding itself triggers a conformational change — no external energy is needed because the process is energetically favourable. 

• while in active transport, ATP energy forces the shape change to push molecules against the gradient because it goes against the natural direction of diffusion