Cells rely on membrane proteins to move substances that cannot pass through the hydrophobic lipid bilayer. Carrier proteins and aquaporins are central to selective, efficient transport without ATP when molecules move down existing gradients.

Core idea: protein-mediated movement down gradients

This animation visualizes facilitated diffusion across a membrane when the solute concentration is higher outside the cell than inside. Molecules move through a membrane protein pathway, with net flux proceeding down the concentration gradient until the gradient diminishes. The motion highlights how proteins provide a selective route for substances that cannot cross the hydrophobic bilayer efficiently. Source

Because the phospholipid bilayer blocks most large polar molecules, cells use embedded proteins to provide an alternative pathway that preserves membrane selectivity.

What “down a gradient” implies

• Net movement occurs from higher to lower concentration until equilibrium is approached.

• The rate depends on:

  • Size of the gradient

  • Number and activity of transport proteins

  • Whether proteins become saturated (all binding sites occupied)

Carrier proteins: moving large polar molecules without ATP

This diagram illustrates carrier-mediated facilitated diffusion: a polar solute binds a specific carrier site, then the protein shifts conformation to expose the binding site to the opposite side of the membrane. The solute is released where its concentration is lower, and the carrier returns to its original state, enabling repeated down-gradient transport. Source

How carrier proteins work

• Specific binding: the solute fits a binding site based on shape and chemical properties.

• Conformational change: binding triggers the protein to shift from outward-facing to inward-facing (or vice versa).

• Release: the solute is released on the side with lower concentration.

Key features AP Biology students should know

• No ATP is used directly: transport is powered by the solute’s own concentration gradient.

• Selectivity: different carriers transport different molecules; similar molecules may compete for the same carrier.

• Saturation: there is a maximum rate once all carriers are busy, unlike simple diffusion through a membrane.

• Directionality depends on the gradient: if the gradient reverses, net movement can reverse.

Aquaporins: rapid water transport

Water can diffuse across membranes slowly, but many cells require much faster movement to maintain appropriate internal conditions. Aquaporins provide a high-throughput route for water while keeping the membrane selectively permeable.

This schematic of aquaporin-1 (AQP1) shows water moving through the channel in single file between extracellular and intracellular vestibules. It also labels structural features that impede proton ($H^+$) transport, explaining how aquaporins can greatly increase water permeability without allowing ions to leak across the membrane. The figure reinforces the idea of rapid, passive water movement driven by a water concentration gradient. Source

Why aquaporins are necessary

• They enable rapid water transport without changing the membrane’s barrier to ions and many solutes.

• They support fast volume adjustments in cells exposed to changing external water availability.

Selectivity and efficiency

• Aquaporins are shaped to favour water molecules while excluding most other substances.

• Many aquaporins limit passage of ions (such as H$^+$), helping maintain electrical and chemical stability while still allowing high water flow.

What aquaporins and carriers have in common

• Both are forms of facilitated diffusion.

• Both are energy-independent for movement down gradients.

• Both increase transport rate compared with direct diffusion through the lipid bilayer (which is very limited for large polar molecules, and relatively slow for water without aquaporins).