Channel proteins provide selective hydrophilic pathways for facilitated diffusion of ions and polar molecules down concentration gradients without ATP; Selectivity comes from channel structure and the chemical nature of the transported particle.
Coverage 2013–2023 · Updated 15 Jul 2026
Channel proteins form selective hydrophilic pores through the bilayer, allowing particular ions or polar molecules to cross down an electrochemical gradient.
The pore exposes a compatible interior while the surrounding lipid tails remain excluded. Size, charge and binding sites determine which solutes pass; channels do not normally supply the energy for uphill movement.
For a channel claim, name the solute, pore selectivity and gradient direction.
An ion channel can let potassium move rapidly down its gradient while excluding a larger or differently charged ion.
A channel is not automatically an active pump. If movement is against a gradient, another energy-coupled mechanism is required.
This objective is assessed through structured response, commonly using Identify / Explain / Describe.
Identify / Explain / Describe / Outline
Build the answer around this relationship: Channel proteins form hydrophilic pathways through the membrane.
Adding ATP to facilitated diffusion even when the movement is passive.
Representative question
Explain facilitated diffusion.
The SL membrane model is a decision system. The bilayer forms because phospholipids are amphipathic, and the hydrophobic core creates selective permeability. Small non-polar molecules diffuse directly; water moves by osmosis and often through aquaporins; ions and polar molecules use channels or transporters; pumps use ATP for movement against gradients. Proteins and glycocalyx components add transport, recognition, and model evidence.
