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B2.1.6—Channel proteins

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.

Syllabus
First assessment 2025
Objective
B2.1.6
Level
SL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–2

Common command terms

  • Identify
  • Explain
  • Describe
  • Outline

Scoring notes

Common mistake
Adding ATP to facilitated diffusion even when the movement is passive.

Recent exam appearances

May 2025Paper2 ["SL"] · TZ14(c)[ 2 ]B2.1.6—Channel proteins
November 2024Paper2 ["SL"] · TZ16(b)[ 4 ]B2.1.6—Channel proteins
May 2024Paper1 ["SL"] · TZ24[ 1 ]B2.1.6—Channel proteins
May 2022Paper1 ["SL"] · TZ13[ 1 ]B2.1.6—Channel proteins
May 2021Paper1 ["SL"] · TZ24[ 1 ]B2.1.6—Channel proteins

Coverage 2010–2025 · Updated 15 Jul 2026

Channels Provide Selective Downhill Routes

A channel is a hydrophilic pore that permits selected ions or molecules to diffuse down an electrochemical gradient without ATP.

Show hydrophilic pore selectivity so the learner can choose The Right Channel.

Pore diameter + charge + chemical lining determine selectivity. Some channels remain open; gated channels change between closed and open states.

SL Summary: Choose a Membrane Route

Particle property → gradient → route → energy.
Non-polar: bilayer. Water: osmosis/aquaporin. Ion or polar solute downhill: channel/carrier. Uphill: ATP-coupled pump.

The bilayer creates the barrier; proteins make it selective; carbohydrates create an outward interface; component mobility makes the membrane adaptable.

Channel proteins

[ 3 ]

Explain facilitated diffusion.

Concept essentials

  • Channel proteins form hydrophilic pathways through the membrane.
  • Facilitated diffusion moves substances down gradients without ATP.
  • Specific channels are needed for many ions and polar solutes.
  • The rate of facilitated diffusion can depend on the number of transport proteins.