Topic 2.6 Membrane Transport: describe the mechanisms that organisms use to transport large and small molecules across the membrane and the energy requirements of passive and active transport.

What this topic is asking

The College Board (Topic 2.6) wants you to describe how cells transport small and large molecules across the membrane and to distinguish passive transport (no energy) from active transport (uses energy), based on movement relative to the concentration gradient.

Passive transport

Passive transport includes:

• Simple diffusion: small nonpolar molecules (oxygen, carbon dioxide) cross the bilayer directly.
• Osmosis: the diffusion of water across a selectively permeable membrane, often through aquaporins.
• Facilitated diffusion: polar molecules and ions cross down their gradient with the help of channel or carrier proteins (covered in Topic 2.7).

The rate of diffusion increases with a steeper gradient, higher temperature, larger surface area and shorter distance.

Active transport

Because the cell must do work to move substances uphill, cells with high active-transport demand (such as kidney and nerve cells) contain many mitochondria to supply ATP.

Why transport keeps cells far from equilibrium

A living cell is never at equilibrium with its surroundings, and transport is what keeps it that way. Left alone, diffusion would erase every concentration difference until inside and outside matched, which for a cell means death. Passive transport lets the cell take advantage of favorable gradients (importing oxygen, exporting carbon dioxide) at no energy cost, while active transport lets it build and hold the unfavorable gradients it needs (high potassium inside, high sodium outside). The College Board frames this through the big idea of energetics: maintaining order and concentration differences requires a constant input of free energy, and a cell that stops spending energy on active transport quickly drifts toward equilibrium and loses its organization. Membrane transport is thus a continuous, energy-dependent balancing act.

Primary and secondary active transport

The College Board distinguishes two kinds of active transport, and the difference is where the energy comes from. Primary active transport uses chemical energy directly: a pump hydrolyzes ATP to change shape and move a substance against its gradient. The sodium-potassium pump is the standard example, hydrolyzing one ATP to export three $Na^+$ and import two $K^+$ per cycle, which builds steep ion gradients across the membrane.

Secondary active transport (cotransport) does not spend ATP at the moment of transport; instead it harnesses the gradient that primary transport already built. As the stored ions flow back down their gradient through a coupled carrier, the energy released drags a second substance along, often uphill against its own gradient. A classic case is the $\text{Na}^+$/glucose symporter in the gut and kidney: sodium flooding back into the cell down the gradient set up by the pump powers the uptake of glucose against its gradient. The transport is still ultimately ATP-dependent (the pump must keep restoring the sodium gradient), but the energy is spent earlier and stored as the gradient, which is why it is called secondary. In a symport both substances move the same way; in an antiport they move in opposite directions.

Bulk transport

Very large molecules and particles are too big for proteins, so they move in membrane-bound vesicles:

• Endocytosis: the membrane folds inward to engulf material, forming a vesicle (phagocytosis for solids, pinocytosis for fluids, receptor-mediated for specific molecules).
• Exocytosis: a vesicle fuses with the plasma membrane to release its contents (for example secreting hormones or neurotransmitters).

Both require energy and link to the endomembrane system.

Try this

Q1. Identify whether osmosis is active or passive and justify. [2 points]

• Cue. Passive; water moves down its own concentration gradient across a selectively permeable membrane without the cell using energy.

Q2. Explain why a cell carrying out a lot of active transport contains many mitochondria. [2 points]

• Cue. Active transport requires ATP, and mitochondria produce ATP by respiration, so cells with high active-transport demand need many mitochondria.