Describe the importance of diffusion of gases and solutes in living organisms.

3.1 Diffusion

Objective

Describe the importance of diffusion of gases and solutes in living organisms.

Definition (IGCSE wording)

Diffusion is the net movement of particles (atoms, ions or molecules) from a region of higher concentration to a region of lower concentration until equilibrium is reached.

Source of Energy

The energy for diffusion comes from the kinetic energy of the particles themselves; this kinetic energy increases as temperature rises.

Substances that Diffuse Through the Cell Membrane

Some substances move into and out of cells by simple or facilitated diffusion through the cell membrane, for example:

• Gases – O₂, CO₂
• Small, non‑polar molecules – glucose, urea
• Ions – Na⁺, K⁺, Cl⁻

Why Diffusion Is Important for Life (ordered to match the syllabus)

1. Gas exchange – provides a passive, energy‑free way for O₂ to enter the body and CO₂ to be removed.
2. Solute (nutrient) uptake – essential nutrients such as glucose and amino acids move from blood or extracellular fluid into cells.
3. Waste removal – metabolic wastes (e.g., urea, ammonia, CO₂) diffuse out of cells into blood or excretory fluids.
4. Water balance – diffusion of water (osmosis) helps maintain cell turgor and overall fluid balance.

Key Biological Examples

1. Respiratory surfaces
   • Human alveoli – O₂ diffuses from alveolar air into capillary blood; CO₂ diffuses in the opposite direction.
   • Plant leaves – CO₂ diffuses into stomata and O₂ diffuses out of the mesophyll.
   • Fish gills – dissolved O₂ diffuses from water into the blood.
2. Cellular level
   • Glucose entering a cell via facilitated diffusion through carrier proteins.
   • Na⁺ and K⁺ moving down their concentration gradients through ion channels.
3. Excretion
   • Urea (or ammonia) diffusing from blood into the renal tubules for elimination.

Core Factors that Influence the Rate of Diffusion

Qualitative Relationship

The diffusion rate increases when the concentration difference is larger, the surface area is greater, the distance to be crossed is shorter, and the temperature is higher.

Consequences of Impaired Diffusion

• Reduced oxygen supply → fatigue, organ dysfunction.
• Accumulation of carbon dioxide → respiratory acidosis.
• Insufficient nutrient delivery → poor growth and impaired repair.
• Failure to remove metabolic wastes → toxicity.

Adaptations that Enhance Diffusion

• Very thin respiratory membranes (e.g., alveolar walls, gill lamellae).
• Extremely large surface areas (millions of alveoli, leaf mesophyll).
• Moist surfaces to dissolve gases.
• Ventilation or water flow that continually refreshes the surrounding medium.

Investigation Ideas (Practical Work)

• Measure the effect of temperature on the rate at which a coloured dye diffuses through agar.
• Compare diffusion rates across membranes of different thicknesses using gelatin sheets.
• Investigate how surface area influences diffusion by using dialysis tubing of varying sizes.
• Explore the impact of concentration gradient by placing a drop of starch solution in iodine solutions of different concentrations.

Summary

Diffusion is a passive, energy‑free process that underpins gas exchange, solute uptake, waste removal and water balance in all living organisms. Its efficiency depends on four core factors—concentration gradient, surface area, distance and temperature—and is maximised by structural adaptations such as thin membranes and large surface areas. Understanding diffusion explains the design of respiratory organs and the problems that arise when diffusion is compromised.