Diffusion and Facilitated Diffusion

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.5 — Biological membranes (refer to the official OCR H420 specification document for exact wording). It develops the movement of molecules across membranes by simple and facilitated diffusion, the role of channel and carrier proteins, and the factors that govern the rate of passive transport.

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 develops the OCR H420 Module 2.1.5 content on 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:

• Voltage-gated channels open or close in response to membrane potential changes (e.g. the Na⁺ and K⁺ channels in neurones).
• Ligand-gated channels open when a specific chemical (a neurotransmitter or hormone) binds (e.g. acetylcholine-gated channels at the neuromuscular junction).
• Mechanically gated channels respond to pressure or stretch.

A famous example of a channel is the aquaporin, which allows water molecules to cross the bilayer about 10× faster than they would by simple diffusion.

4.2 Carrier Proteins

Carrier proteins bind a specific substrate on one side of the membrane, undergo a conformational change (shape change), and release it on the other side. Unlike channels they do not form a continuous open pore.

• Carriers are used for larger polar molecules such as glucose (the GLUT family of glucose transporters) and amino acids.
• Each shape change moves one substrate molecule at a time, so the rate is lower than for channels but highly selective.
• Carriers used for facilitated diffusion do not require ATP; those used for active transport do.

Feature	Channel proteins	Carrier proteins
Mechanism	Open pore	Shape change
Selectivity	High (by size and charge)	High (by shape)
Rate of transport	Very high	Lower than channels
Typical substrates	Ions, water	Glucose, amino acids
Energy required	None (for facilitated diffusion)	None (for facilitated diffusion)

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5. Rates of Simple vs Facilitated Diffusion

If you plot rate of transport against substrate concentration:

• Simple diffusion gives a straight line — the rate is proportional to the concentration gradient and never saturates.
• Facilitated diffusion gives a curve that levels off — the rate rises, then reaches a plateau (V_max) when all the carriers/channels are saturated (working as fast as they can).

graph LR
  A[Low conc gradient] --> B[Low rate - both mechanisms]
  B --> C[Rising conc gradient]
  C --> D[Simple diffusion: linear rise]
  C --> E[Facilitated diffusion: plateaus at Vmax]

This saturation is an excellent diagnostic for facilitated diffusion in exam data questions.

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6. Biological Examples

• Oxygen diffusing from alveoli into capillaries — simple diffusion.
• Carbon dioxide leaving tissues — simple diffusion.
• Glucose entering red blood cells via GLUT1 — facilitated diffusion.
• Glucose entering liver cells via GLUT2 — facilitated diffusion.
• Chloride ions leaving red blood cells via the band 3 transporter — facilitated diffusion.
• K⁺ and Na⁺ moving through voltage-gated channels during an action potential — facilitated diffusion (driven by the gradient set up by the sodium-potassium pump).

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7. Common Exam Mistakes

• Saying facilitated diffusion "uses ATP". It does not — only active transport does.
• Saying diffusion "stops" at equilibrium. The particles keep moving; the net movement is zero.
• Writing that simple diffusion "uses channels". Channels are facilitated diffusion; simple diffusion is through the bilayer.
• Forgetting to state that facilitated diffusion is down a concentration gradient.
• Claiming all transmembrane proteins are channels — carriers work by shape change, not pores.
• Using "membrane" when you mean "phospholipid bilayer" or vice versa.
• Not explaining why ions cannot cross the bilayer (hydrophobic core repels charged particles).

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8. Exam-Style Questions

1. State three factors that affect the rate of diffusion. Use Fick's law to justify each. (3)
2. Explain why glucose cannot cross a phospholipid bilayer by simple diffusion. (2)
3. Compare and contrast simple and facilitated diffusion. (4)
4. Describe how a graph of rate against substrate concentration can show whether a substance crosses by simple or facilitated diffusion. (3)

Model answer for (3): "Both simple and facilitated diffusion move substances down a concentration gradient, do not require ATP and are passive. In simple diffusion, molecules pass directly through the phospholipid bilayer, whereas in facilitated diffusion they pass through specific channel or carrier proteins. Simple diffusion is for small, non-polar molecules such as O₂ and CO₂, while facilitated diffusion is for ions and large polar molecules such as glucose. Facilitated diffusion can become saturated (Vmax) at high concentrations because there are only a limited number of transporters; simple diffusion does not saturate."

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Summary

• Diffusion is the passive net movement of particles down a concentration gradient.
• Fick's law — rate ∝ (area × concentration gradient) / distance — captures the main factors affecting rate.
• Simple diffusion is through the phospholipid bilayer for lipid-soluble and very small molecules.
• Facilitated diffusion is through channel (ions, water) or carrier (glucose, amino acids) proteins and is also passive.
• A saturation curve (plateau at V_max) is diagnostic of facilitated diffusion.
• Neither mechanism requires ATP — that is the defining feature of active transport.

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9. Fick's Law — Quantitative Treatment

The qualitative statement rate $\propto$∝ (area $\times$× gradient) / distance is a teaching simplification of Fick's first law of diffusion, which is a quantitative differential equation:

$J = -D \frac{dc}{dx}$J=−Ddxdc​

where:

• $J$J is the flux — the number of moles of solute crossing unit area per unit time, in mol m$^{-2}$−2 s$^{-1}$−1.
• $D$D is the diffusion coefficient — a property of the solute and the medium, in m$^2$2 s$^{-1}$−1. Smaller, less-polar molecules have higher $D$D values in water and bilayers.
• $\frac{dc}{dx}$dxdc​ is the concentration gradient in mol m$^{-3}$−3 per m. The negative sign reflects that net flux is down the gradient — from high to low concentration.

Integrating across a membrane of thickness $d$d with concentration $c_1$c1​ on one side and $c_2$c2​ on the other:

$J = \frac{D (c_1 - c_2)}{d}$J=dD(c1​−c2​)​

Total transport rate (mol s$^{-1}$−1) is then $J \times A$J×A, where $A$A is the cross-sectional area of the membrane. This is the quantitative form of the schoolbook proportionality.

9.1 The permeability coefficient $P$P

For a particular solute crossing a particular membrane, the diffusion coefficient $D$D and the partition coefficient $K$K (how well the solute dissolves in the bilayer vs water) are usually combined into a single permeability coefficient $P$P:

$P = \frac{KD}{d}$P=dKD​

$J = P \cdot (c_1 - c_2)$J=P⋅(c1​−c2​)

Lipid-soluble molecules (high $K$K, high $D$D) have large $P$P; ions and large polar molecules have $P \approx 0$P≈0 across a pure bilayer and rely on channels or carriers. This is the quantitative basis of the Overton rule (1899).

9.2 Worked example

A gas of partial pressure $p$p dissolves in plasma at concentration $c = \alpha p$c=αp (Henry's law). For oxygen across the alveolar membrane:

• Alveolar partial pressure $p_{O_2} \approx 13.3$pO2​​≈13.3 kPa; capillary $p_{O_2} \approx 5.3$pO2​​≈5.3 kPa — gradient is ~8 kPa.
• Effective alveolar-capillary membrane thickness $d \approx 0.5 \mu m$d≈0.5μm — minimised by squamous epithelium.
• Total alveolar surface area $A \approx 70$A≈70 m$^2$2 in an adult human — maximised by ~300 million alveoli.

Each of these three numbers multiplies the diffusion rate: doubling area or gradient doubles flux; doubling thickness halves it. Exam answers should refer to these three factors quantitatively where possible.

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10. SVG — Channel vs Carrier Protein