Active Transport

Core Concepts

Concentration gradients and the energy bill

A concentration gradient is a difference in the amount of a substance between two regions. Diffusion moves substances down the gradient (high → low) with no energy cost. Moving a substance up the gradient (low → high) is thermodynamically unfavorable, so it must be coupled to an energy-releasing reaction — usually ATP hydrolysis. For charged particles (ions), the relevant gradient is the electrochemical gradient, which combines the concentration difference with the electrical charge difference across the membrane.

Primary active transport: pumps that use ATP directly

In primary active transport, a transport protein (a pump) hydrolyzes ATP itself and uses that energy to move a substance against its gradient. The classic example is the sodium–potassium pump (Na⁺/K⁺-ATPase), found in nearly all animal cells:

• For every ATP molecule hydrolyzed, it moves 3 Na⁺ out of the cell and 2 K⁺ in.
• The result is a cell interior that is rich in K⁺, low in Na⁺, and negatively charged relative to the outside — the setup that powers nerve signaling.

Other ATP-powered pumps include the Ca²⁺ pumps that refill calcium stores in muscle cells (sarcoplasmic reticulum) and the H⁺/K⁺ pump in stomach lining cells that secretes acid.

Secondary active transport: riding an existing gradient

Secondary active transport does not use ATP directly. Instead, it harnesses the energy of one substance flowing down its gradient to drag another substance against its gradient. The ion that flows downhill is almost always Na⁺ (or H⁺), and the gradient it rides was itself built by primary pumps. Two patterns:

• Symport (cotransport): both substances move in the same direction. Example: the Na⁺/glucose symporter in intestinal cells — Na⁺ flows inward down its gradient while glucose is pulled in against its gradient.
• Antiport (countertransport): the two substances move in opposite directions. Example: the Na⁺/Ca²⁺ exchanger in heart muscle cells, which lets Na⁺ in while pumping Ca²⁺ out.

Bulk transport: endocytosis and exocytosis

Small molecules move through transport proteins, but very large items (cells, bacteria, droplets, big macromolecules) move by bulk transport, in which the membrane itself forms vesicles:

• Endocytosis brings material into the cell by folding the membrane inward. Forms include phagocytosis (engulfing large particles, e.g., immune cells eating bacteria), pinocytosis (sipping in fluid), and receptor-mediated endocytosis (specific molecules bind receptors first — this is how cells take up LDL cholesterol).
• Exocytosis releases material by fusing a vesicle with the membrane — how neurons release neurotransmitters and glands secrete hormones.

Bulk transport is often grouped with active transport because it requires energy, even though it does not move molecules through a pump.

Worked Example: Riding the Sodium Train

Walk through absorption in the small intestine. The gut contents have a moderate amount of glucose; the cell's cytoplasm already has a lot. To keep absorbing, the cell uses a symporter that binds one glucose and two Na⁺. Na⁺ is far more concentrated outside the cell because the Na⁺/K⁺ pump keeps pumping it out. So Na⁺ flows inward — downhill, energetically favorable — and the shared carrier protein uses that flow to drag glucose inward — uphill, against its gradient. The Na⁺/K⁺ pump then removes the extra Na⁺, spending ATP. The glucose stays trapped inside (it has no easy way back out) and is used for energy or passed into the blood.

Why this matters for real life: Oral rehydration solutions contain both salt and sugar for exactly this reason. The glucose drags Na⁺ (and water follows) into the body's cells even during diarrhea, when the gut is dumping water out. The sugar isn't just food — it's the engine that pulls sodium in.

Common Confusions