DiffusionCambridge IGCSE Biology: Revision notes

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient. This movement is due to the random kinetic movement of molecules and ions.

The key point is that particles are always moving randomly in all directions. However, there are more particles in the high concentration area, so statistically more particles move from high to low concentration than vice versa. This creates a net movement down the concentration gradient.

Energy source: The energy for diffusion comes from the kinetic energy of random molecular movement. Particles diffuse spontaneously without requiring cellular energy (ATP), which makes diffusion a passive process.

Diffusion continues until concentrations are equal (equilibrium is reached), after which there is no net movement, although random movement continues.

Diffusion through the cell membrane occurs when small molecules and ions pass directly through the partially permeable (selectively permeable) membrane without the cell using energy.

Substances that diffuse through cell membranes include:

• Oxygen (O₂) — small molecule, dissolves in lipid component
• Carbon dioxide (CO₂) — small molecule, dissolves in lipid component
• Glucose — can diffuse slowly; moves faster through facilitated diffusion
• Ions — move through ion channels in the membrane
• Water (H₂O) — small molecule, moves by osmosis (a special case of diffusion)

The cell membrane is partially permeable because:

• Small molecules can dissolve in or pass through the lipid bilayer
• Large molecules (proteins, starch) cannot pass through
• Ions require specific channel proteins to pass (though some diffusion of ions occurs)

The rate at which a substance diffuses depends on how easily it can cross the membrane — small, non-polar molecules diffuse faster than large or polar molecules.

Diffusion is essential for survival because it allows the exchange of vital substances across cell membranes without cellular energy expenditure.

Importance of diffusion of gases:

• Oxygen diffuses into cells from areas of high concentration (blood/air) to low concentration (respiring cells). Cells need oxygen for aerobic respiration to produce ATP
• Carbon dioxide produced in respiring cells diffuses out to areas of lower concentration (blood/lungs). CO₂ is toxic if it accumulates
• Ventilation systems (in lungs and gills) maintain steep concentration gradients for efficient gas exchange

Importance of diffusion of solutes:

• Glucose and other nutrients diffuse from the intestine into blood and from blood into cells that need them for respiration and growth
• Ions diffuse to maintain correct concentrations inside and outside cells, essential for nerve impulses and muscle contraction
• Urea diffuses from cells into blood for excretion
• Waste products diffuse out of cells to prevent toxic accumulation

Why diffusion is efficient in small organisms:

• Single-celled organisms rely entirely on diffusion through their cell membrane
• Large organisms have specialised exchange surfaces (lungs, gills, small intestine) with high surface area to increase diffusion rate

The rate of diffusion is determined by four main factors:

1. Concentration gradient (difference in concentration)

• A larger concentration gradient = faster diffusion
• A steep gradient means more particles moving down it per unit time
• Example: A cell with 1% glucose will absorb glucose faster from 10% solution than from 2% solution

2. Surface area

• A larger surface area = faster diffusion
• More membrane area available for particles to cross
• Example: Intestinal villi increase surface area for nutrient diffusion; alveoli in lungs maximise gas exchange
• This is why cells and organisms have adaptations that increase surface area

3. Temperature

• Higher temperature = faster diffusion
• Particles have more kinetic energy and move faster and more frequently
• Doubling temperature roughly doubles the rate of diffusion
• Example: Glucose diffuses faster into cells at 37°C than at 20°C

4. Distance (thickness of exchange surface)

• A smaller distance = faster diffusion
• Particles have less distance to travel
• Membranes must be thin for efficient diffusion
• Example: Capillary walls are one cell thick for rapid diffusion of substances

Combined effect: $\text{Diffusion rate} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Distance}}$

This relationship shows that diffusion is most efficient across large surface areas with steep gradients and minimal distance.

Large multicellular organisms have evolved specialised exchange surfaces to maximise diffusion rates because simple diffusion across the body surface is too slow to meet metabolic demands.

Key adaptations follow the pattern of maximising the factors affecting diffusion:

How these adaptations work together: In the lungs: alveoli have massive surface area, thin walls, are surrounded by capillaries (fresh blood), and breathing brings new oxygen-rich air. All factors combine to maximise oxygen diffusion rate.

In the small intestine: villi increase surface area, are one cell thick, have a blood supply, and muscular contractions mix contents. Nutrients diffuse into blood rapidly.

Why single-celled organisms don't need these adaptations:

• Their high surface area to volume ratio means the entire cell surface can exchange with the environment
• Diffusion directly into the cytoplasm is fast enough for metabolic needs
• No specialised tissues required