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2.6 Facilitated Diffusion

Syllabus
2025
Topic
2.6
Level
—

Learning objectives

2.6A—Explain how the structure of a molecule affects its ability to pass through the plasma membraneExplain how the structure of a molecule affects its ability to pass through the plasma membrane.• Facilitated diffusion requires transport or channel proteins to enable the movement of charged ions across the membrane.- i. Membranes may become polarized by the movement of ions across the membrane.- ii. Charged ions, including Na+ (sodium) and K+ (potassium), require channel proteins to move through the membrane.• Facilitated diffusion enables the movement of large polar molecules through membranes with no energy input. In this type of diffusion, substances move down the concentration gradient.• Aquaporins transport large quantities of water across membranes.

Facilitated Diffusion Provides a Protein Pathway

Facilitated diffusion is passive transport through a membrane protein. A charged or strongly polar substance is poorly matched to the bilayer's hydrophobic interior, so a channel or transport protein provides a compatible route across the membrane.

Substance property or example Why direct bilayer passage is limited Facilitated route
Charged ions such as Na+Na^+ and K+K^+ Charge is incompatible with the hydrophobic interior Ion channel
Large polar molecules Size and polarity make direct passage unfavorable Specific transport protein
Water in large quantities Direct passage does not provide the required high flow Aquaporin channel

The substance moves down its concentration gradient through the protein, so no direct metabolic-energy input is required. Selective opening or availability of channels changes which ions can cross. Unequal movement of charged ions can separate charge across the membrane, causing the membrane to become polarized.

Using a protein does not automatically make transport active. Facilitated diffusion uses a protein but remains passive because net movement is down the concentration gradient and does not require direct energy input.