Diffusion

This lesson covers diffusion as required by the Edexcel GCSE Combined Science specification (1SC0). You need to define diffusion, explain the factors that affect the rate of diffusion, give biological examples and understand the importance of surface area to volume ratio.

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What Is Diffusion?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Key points:

• Diffusion is a passive process — it does not require energy from respiration.
• It occurs because particles are in constant random motion (kinetic energy).
• Diffusion continues until particles are evenly distributed (equilibrium is reached), although random movement still occurs.
• Diffusion can occur in gases and liquids (solutions), and across partially permeable membranes.

Exam Tip: The specification requires you to say "net movement". Individual particles move randomly in all directions, but overall the net movement is from high concentration to low concentration.

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Factors Affecting the Rate of Diffusion

Several factors affect how quickly diffusion occurs:

Factor	Effect on Rate of Diffusion
Concentration gradient	A steeper concentration gradient (bigger difference between regions) increases the rate
Temperature	Higher temperature gives particles more kinetic energy, so they move faster and diffuse more quickly
Surface area	A larger surface area allows more particles to cross at the same time, increasing the rate
Distance (thickness of membrane)	A shorter distance (thinner membrane) means particles cross more quickly
Size of particle	Smaller particles diffuse faster than larger ones

The Concentration Gradient

The concentration gradient is the difference in concentration between two areas. The greater this difference, the faster the rate of diffusion. As diffusion occurs and particles spread out, the gradient decreases and the rate slows down until equilibrium is reached.

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Examples of Diffusion in Living Organisms

Gas Exchange in the Lungs

• Oxygen diffuses from the air in the alveoli (high O₂ concentration) into the blood in the capillaries (low O₂ concentration).
• Carbon dioxide diffuses from the blood (high CO₂) into the alveoli (low CO₂) to be exhaled.
• The alveoli are adapted for efficient diffusion: large surface area, thin walls (one cell thick), good blood supply to maintain the concentration gradient.

Gas Exchange in Leaves

• Carbon dioxide diffuses into the leaf through stomata for photosynthesis.
• Oxygen (a product of photosynthesis) diffuses out through the stomata.
• Water vapour also diffuses out through stomata (transpiration).

Absorption in the Small Intestine

• Digested food molecules (e.g. glucose, amino acids) diffuse from the small intestine (high concentration) into the blood (low concentration).
• Villi increase the surface area for absorption.

Exam Tip: When explaining any diffusion example, always state: (1) what is diffusing, (2) from where to where, (3) from high to low concentration, (4) down the concentration gradient.

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Surface Area to Volume Ratio

The surface area to volume ratio (SA:V) is important in biology because it affects the rate of exchange of substances.

Calculating SA:V Ratio

For a cube with side length s:

• Surface area = 6s²
• Volume = s³
• SA:V ratio = 6s² ÷ s³ = 6/s

Cube Side Length	Surface Area	Volume	SA:V Ratio
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1
3 cm	54 cm²	27 cm³	2 : 1
4 cm	96 cm²	64 cm³	1.5 : 1

As the size of an organism increases, its SA:V ratio decreases. This means larger organisms have relatively less surface area for the exchange of substances compared to their volume.

Why SA:V Ratio Matters

• Single-celled organisms (e.g. amoeba) have a large SA:V ratio, so diffusion across their surface is sufficient for all their needs.
• Multicellular organisms have a smaller SA:V ratio, so they need specialised exchange surfaces (e.g. lungs, villi, gills) and transport systems (e.g. blood circulatory system) to move substances efficiently.

Exam Tip: If asked why multicellular organisms need specialised exchange surfaces, explain that as organisms get larger, the SA:V ratio decreases, so diffusion alone is too slow to meet the organism's needs.

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Worked Example

Question: A cube has sides of 3 cm. Calculate its surface area to volume ratio.

Surface area = 6 × 3² = 6 × 9 = 54 cm²

Volume = 3³ = 27 cm³

SA:V ratio = 54 ÷ 27 = 2 : 1

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Summary

• Diffusion is the net movement of particles from high concentration to low concentration, down the concentration gradient. It is a passive process.
• Factors affecting the rate: concentration gradient, temperature, surface area, distance and particle size.
• Biological examples include gas exchange in the lungs and leaves, and absorption of nutrients in the small intestine.
• Surface area to volume ratio decreases as organisms get larger, which is why multicellular organisms need specialised exchange surfaces and transport systems.
• Always state the direction of movement (from high to low concentration) when describing diffusion in the exam.

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Deeper Look: Why Diffusion Matters for Life

Diffusion is the primary way substances move in and out of small cells, and across exchange surfaces in larger organisms. Oxygen reaches your cells by diffusion; carbon dioxide leaves your cells by diffusion. Nutrients pass into cells by diffusion (or active transport), and waste products exit the same way. Without diffusion, life at the cellular level would not be possible.

Fick's Law in Simple Terms

Although you do not need to memorise the formula in detail, it is useful to know that the rate of diffusion is proportional to:

$\text{Rate} \propto \frac{\text{Surface area} \times \text{Concentration difference}}{\text{Distance (membrane thickness)}}$Rate∝Distance (membrane thickness)Surface area×Concentration difference​

This formula encapsulates three of the factors from the table: greater surface area, steeper concentration gradient and shorter diffusion distance all increase the rate. Temperature affects rate separately by changing kinetic energy.

Worked Example: Comparing SA:V Ratios

Question: Compare the surface area to volume ratio of a cube of side length 1 cm, a cube of side length 5 cm, and a cube of side length 10 cm.

Working: