Diffusion and Facilitated Diffusion

Cells need to move molecules across their membranes constantly: oxygen in, carbon dioxide out, glucose in, waste out. The simplest mechanism for this is diffusion, a passive process requiring no metabolic energy. This lesson covers OCR A-Level Biology A specification point 2.1.5 (e)(i) — the movement of molecules across membranes by simple and facilitated diffusion.

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1. What Is Diffusion?

Key Definition — Diffusion: The net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random thermal motion.

Diffusion is a passive process — it requires no ATP. The random kinetic motion of particles is the only driving force. Equilibrium is reached when particles are evenly distributed, but net movement stops long before every particle comes to rest; it is the net movement that is zero at equilibrium.

Diffusion occurs in gases, liquids and even solids (slowly). In biology the key examples are:

• Oxygen diffusing from alveolar air into blood plasma and across red blood cell membranes.
• Carbon dioxide diffusing from respiring tissues into capillary blood.
• Water molecules diffusing across membranes (the special case of osmosis, covered in Lesson 4).

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2. Fick's Law and Factors Affecting the Rate of Diffusion

The rate at which substances diffuse across a surface can be expressed by Fick's law:

Rate ∝ (Surface Area × Concentration Gradient) / Diffusion Distance

This gives us the three main factors that exam answers must refer to.

2.1 Concentration Gradient

The steeper the gradient, the faster the rate. A large difference in concentration means more particles move from high to low per unit time.

2.2 Surface Area

A bigger surface area allows more particles to cross simultaneously. Exchange surfaces in biology (alveoli, villi, gills, root hairs) are highly folded to maximise surface area.

2.3 Diffusion Distance (Path Length)

The shorter the path, the faster the diffusion. Alveoli and capillaries share a membrane just ~0.5 μm thick; a red blood cell squeezes through a capillary so that haemoglobin is close to the alveolar air.

2.4 Additional Factors

• Temperature — more kinetic energy → faster diffusion.
• Size and mass of particles — small particles diffuse faster.
• Nature of particles — lipid-soluble molecules cross faster than polar ones.

Factor	Effect on rate
Larger concentration gradient	Faster
Larger surface area	Faster
Shorter distance	Faster
Higher temperature	Faster
Smaller molecules	Faster
Lipid-soluble molecules	Faster across bilayer

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3. Simple Diffusion Across the Membrane

Simple diffusion occurs directly through the phospholipid bilayer. Only certain molecules can do this:

• Non-polar, lipid-soluble molecules — O₂, CO₂, N₂, steroid hormones, fatty acids — dissolve in and cross the hydrophobic core easily.
• Small polar molecules — water (slowly), urea — can pass, but more slowly.
• Ions (Na⁺, K⁺, Cl⁻, Ca²⁺) — cannot pass directly because the hydrophobic core repels charged particles.
• Large polar molecules — glucose, amino acids — cannot pass directly because they are too large and too hydrophilic.

Exam Tip: "Lipid-soluble = simple diffusion; charged or large polar = needs a protein."

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4. Facilitated Diffusion

When molecules cannot cross the bilayer directly, they can still move down a concentration gradient passively if a protein helps them. This is facilitated diffusion.

Key Definition — Facilitated Diffusion: The passive movement of molecules or ions across a membrane down a concentration gradient, through specific transmembrane proteins (channel or carrier proteins). Like simple diffusion, it requires no ATP.

Facilitated diffusion is passive — no ATP is used — but it differs from simple diffusion because it depends on specific proteins that determine which molecules cross.

graph TD
  A[Movement across membrane] --> B[Simple diffusion<br/>through bilayer]
  A --> C[Facilitated diffusion<br/>through proteins]
  A --> D[Active transport<br/>with ATP]
  C --> C1[Channel proteins<br/>ions and water]
  C --> C2[Carrier proteins<br/>glucose amino acids]

4.1 Channel Proteins

Channel proteins form hydrophilic pores through the bilayer that allow specific ions or polar molecules to pass. They are highly selective — a K⁺ channel admits K⁺ but not Na⁺, for example, due to the geometry of the pore and the charges lining it.

Channels are often gated: