Diffusion

This lesson covers diffusion — one of the three transport mechanisms you need to know for the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You must be able to define diffusion, explain the factors that affect its rate, and give examples of diffusion in living organisms.

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What Is Diffusion?

Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Key characteristics:

• Diffusion is a passive process — it does not require energy from the cell (no ATP needed).
• It occurs because particles are in constant random motion (they move in all directions).
• Although individual particles move randomly, the net (overall) movement is from high to low concentration.
• Diffusion continues until the particles are evenly spread (equilibrium is reached), although particles continue to move in both directions at equal rates.

Exam Tip: The definition of diffusion is worth memorising word for word: "The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient." This comes up regularly on Edexcel papers.

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Understanding Concentration Gradients

A concentration gradient exists when there is a difference in concentration between two regions.

• Particles naturally move down the concentration gradient (from high to low).
• The steeper the concentration gradient (the bigger the difference), the faster the rate of diffusion.
• Diffusion continues until the concentration is equal on both sides (dynamic equilibrium).

Visualising Diffusion

Imagine dropping a tea bag into a cup of hot water:

• The tea molecules are highly concentrated inside the tea bag.
• They diffuse outwards into the water (lower concentration).
• Eventually, the tea colour spreads evenly throughout the cup.

This happens because the tea molecules are in constant random motion, and the net movement is from the tea bag (high concentration) to the surrounding water (low concentration).

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Factors Affecting the Rate of Diffusion

Four main factors affect how quickly diffusion occurs:

1. Concentration Gradient

• The greater the difference in concentration between two regions, the faster the rate of diffusion.
• As particles diffuse, the gradient decreases and the rate slows down.
• Organisms maintain steep concentration gradients to ensure rapid diffusion (e.g. blood flow in the lungs constantly removes oxygen, keeping the gradient steep).

2. Temperature

• Higher temperature gives particles more kinetic energy.
• Particles move faster and collide more frequently.
• Therefore, diffusion occurs more quickly at higher temperatures.

3. Surface Area

• A larger surface area provides more space for particles to diffuse across.
• This increases the rate of diffusion.
• Many biological exchange surfaces are adapted to have a very large surface area (e.g. alveoli in the lungs, villi in the small intestine).

4. Distance (Membrane Thickness / Diffusion Path Length)

• A shorter distance (thinner membrane or barrier) means particles have less distance to travel.
• This increases the rate of diffusion.
• Exchange surfaces in the body are adapted to be very thin (e.g. alveolar walls are only one cell thick).

Summary Table: Factors Affecting Diffusion Rate

Factor	Effect on Rate
Steeper concentration gradient	Increases rate
Higher temperature	Increases rate
Larger surface area	Increases rate
Shorter diffusion distance (thinner membrane)	Increases rate

Exam Tip: Use the mnemonic CATS — Concentration gradient, surfAce area, Temperature, Short distance — to remember the four factors affecting diffusion rate.

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Diffusion Across Cell Membranes

Diffusion is one of the main ways substances move into and out of cells across the cell membrane.

The cell membrane is partially permeable (selectively permeable) — it allows some molecules to pass through but not others.

What Can Diffuse Across the Cell Membrane?

Substance	Diffuses?	Direction	Notes
Oxygen (O₂)	Yes	Into cells (for respiration)	Small, non-polar molecule
Carbon dioxide (CO₂)	Yes	Out of cells (waste from respiration)	Small, non-polar molecule
Water	Yes (osmosis — special case)	Both directions depending on concentration	Covered in the osmosis lesson
Glucose	Yes (but slowly)	Into cells (for respiration)	Small enough to pass through
Amino acids	Limited by size	Variable	May require facilitated diffusion
Large molecules (e.g. proteins, starch)	No	Cannot cross by simple diffusion	Too large to pass through the membrane

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Examples of Diffusion in the Body

1. Gas Exchange in the Lungs (Alveoli)

• Oxygen diffuses from the alveoli into the blood (capillaries surrounding the alveoli).
• Carbon dioxide diffuses from the blood into the alveoli (to be exhaled).
• Both gases move down their concentration gradients.

Adaptations of alveoli for efficient diffusion:

Adaptation	How it increases diffusion rate
Large surface area	Millions of alveoli provide an enormous total surface area (~70 m² in humans)
Thin walls	Alveolar walls are only one cell thick (~0.2 μm), so the diffusion distance is very short
Good blood supply	A dense network of capillaries constantly carries oxygenated blood away and brings deoxygenated blood, maintaining a steep concentration gradient
Moist lining	Gases dissolve in the thin film of moisture, aiding diffusion
Ventilation (breathing)	Constantly refreshes the air in the alveoli, maintaining a steep concentration gradient

2. Gas Exchange in Cells

• Respiring cells use oxygen and produce carbon dioxide.
• Oxygen diffuses from the blood (high O₂) into the cells (low O₂) for use in aerobic respiration.
• Carbon dioxide (a waste product of respiration) diffuses from the cells (high CO₂) into the blood (low CO₂) to be transported to the lungs.

3. Gas Exchange in Leaves

• Carbon dioxide diffuses into the leaf through stomata (small pores on the underside of the leaf) for photosynthesis.
• Oxygen (a product of photosynthesis) diffuses out of the leaf through the stomata.
• Water vapour also diffuses out through the stomata (transpiration).
• Inside the leaf, gases diffuse through air spaces between the spongy mesophyll cells.

4. Absorption in the Small Intestine (Villi)

• Digested food molecules (e.g. glucose, amino acids) diffuse from the gut lumen (high concentration after digestion) into the blood (lower concentration) across the wall of the villi.

Adaptations of villi for efficient absorption:

Adaptation	How it increases diffusion/absorption
Large surface area	Millions of finger-like villi (and microvilli on their surface) greatly increase the surface area
Thin walls	Villi walls are only one cell thick — short diffusion distance
Good blood supply	Dense capillary network maintains a steep concentration gradient by carrying absorbed molecules away

Exam Tip: When asked about adaptations of exchange surfaces, always link each adaptation to how it increases one of the four factors: surface area, concentration gradient, thin barrier, or blood supply. Use the specific terminology.

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Diffusion in Plants