Diffusion, Osmosis & Active Transport

What active transport is

Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy released by respiration

Two things make active transport different from diffusion and osmosis:

• The direction is against the concentration gradient. Particles are being moved "uphill" from a place where they are scarce to a place where they are already concentrated.
• This needs energy from respiration (specifically, energy carried by ATP produced in the mitochondria). Cells that do a lot of active transport (root hair cells, gut lining cells, kidney tubule cells) are packed with mitochondria.

How it works at the molecular level

Active transport relies on protein pumps embedded in the cell membrane. Each pump:

• Binds the specific particle it transports on one side of the membrane
• Uses energy from ATP to change shape
• Releases the particle on the other side of the membrane (the high-concentration side)

A single pump can move thousands of particles per second when supplied with enough ATP.

Important examples of active transport

• Root hair cells in plants absorb mineral ions (nitrate, magnesium, potassium, etc.) from the soil. Soil water typically has very low ion concentrations, while the root cells have much higher concentrations inside. The plant cannot rely on diffusion; it has to pump the ions in against the gradient.
• Cells lining the small intestine absorb glucose and amino acids from digested food. By the end of digestion, the gut contents have a lower concentration of these molecules than the blood, but the body still needs to take in every last molecule. Active transport pumps them in.
• Kidney tubule cells reabsorb glucose from the urine back into the blood by active transport, even though the glucose in the urine is at a lower concentration than in the blood.

Side-by-side comparison