Diffusion

This lesson covers diffusion — one of the key processes by which substances move in and out of cells — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definition, factors affecting the rate of diffusion and examples of diffusion in living organisms.

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What Is Diffusion?

Diffusion is the 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 happens because particles are in constant random motion (they move in all directions).
• Net movement occurs from high to low concentration until the particles are evenly distributed (equilibrium).
• Diffusion occurs in both liquids and gases (not in solids, as particles are fixed in position).

Exam Tip: Always refer to the "net movement" of particles. Individual particles move randomly in all directions, but the overall (net) movement is from high to low concentration. The word "net" is important for full marks.

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The Concentration Gradient

The concentration gradient is the difference in concentration between two areas. Diffusion always occurs down the concentration gradient (from high to low).

graph LR
    A["HIGH concentration<br/>Many particles"] -->|"Net movement<br/>(diffusion)"| B["LOW concentration<br/>Few particles"]

When the particles are evenly spread (equilibrium), there is no longer a net movement in either direction — particles still move randomly, but equal numbers move in each direction.

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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 gradient (bigger difference in concentration) increases the rate. More particles move from the area of higher concentration.
Temperature	Higher temperature increases the rate. Particles have more kinetic energy and move faster, so they spread out more quickly.
Surface area	A larger surface area increases the rate. More area is available for particles to diffuse across.
Distance	A shorter distance (thinner membrane or barrier) increases the rate. Particles have less far to travel.

Exam Tip: You can remember the factors using the mnemonic: Concentration, Temperature, Surface area, Distance — or simply think "Can This Speed up Diffusion?"

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Diffusion Across Cell Membranes

Cell membranes are selectively permeable — they allow some substances to pass through but not others. Substances that can diffuse across cell membranes include:

Substance	Type	Direction
Oxygen	Small, non-polar molecule	Into cells (for respiration)
Carbon dioxide	Small, non-polar molecule	Out of cells (waste product of respiration)
Water	Small molecule	In both directions (osmosis — a special case of diffusion)
Urea	Small molecule	Out of cells (waste product)
Glucose	Larger molecule	Cannot diffuse freely — requires facilitated diffusion or active transport
Amino acids	Larger molecule	Cannot diffuse freely — requires facilitated diffusion or active transport

Exam Tip: Only small, non-polar molecules can diffuse directly through the phospholipid bilayer of the cell membrane. Larger molecules and ions require transport proteins.

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Examples of Diffusion in the Body

1. Gas Exchange in the Lungs

• Oxygen diffuses from the air in the alveoli (high concentration) into the blood in the capillaries (low concentration).
• Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration), to be breathed out.

The alveoli are adapted for efficient diffusion:

Adaptation	How It Helps
Very thin walls (one cell thick)	Short diffusion distance
Large surface area (millions of alveoli)	More area for gas exchange
Rich blood supply	Maintains a steep concentration gradient by constantly carrying gases away
Moist lining	Gases dissolve before diffusing

2. Gas Exchange in Plants

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

3. Absorption in the Small Intestine

• Products of digestion (e.g. glucose, amino acids) are absorbed from the gut lumen into the blood by a combination of diffusion and active transport.
• The small intestine has villi — finger-like projections that increase the surface area for absorption.

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Worked Example

Question: Explain why the rate of diffusion of oxygen into a muscle cell increases during exercise.

Solution:

During exercise, the muscle cell uses oxygen faster for aerobic respiration to release energy for contraction. This decreases the concentration of oxygen inside the cell. Meanwhile, oxygenated blood continues to supply oxygen to the area around the cell. This creates a steeper concentration gradient between the blood (high oxygen) and the inside of the cell (low oxygen). A steeper concentration gradient increases the rate of diffusion of oxygen into the cell.

Additionally, during exercise, body temperature increases, giving particles more kinetic energy, which also increases the rate of diffusion.

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Surface Area to Volume Ratio

The efficiency of diffusion depends on the surface area to volume ratio (SA:V) of an organism or cell.

• Smaller organisms have a larger SA:V ratio, meaning they can exchange substances efficiently by simple diffusion alone.
• Larger organisms have a smaller SA:V ratio, so they need specialised exchange surfaces (e.g. lungs, villi, gills) with adaptations to increase the efficiency of diffusion.

Common Adaptations of Exchange Surfaces

Adaptation	Effect
Large surface area (e.g. folds, villi, alveoli)	More area for diffusion
Thin walls (often one cell thick)	Short diffusion distance
Good blood supply	Maintains steep concentration gradient
Ventilation (e.g. breathing)	Maintains steep concentration gradient for gases

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Worked Example — SA:V Ratio

Question: A cube has sides of 2 cm. Calculate its surface area to volume ratio.

Solution:

$\text{Surface area} = 6 \times 2^2 = 6 \times 4 = 24 \text{ cm}^2$Surface area=6×22=6×4=24 cm2