Diffusion is the spreading out of particles, either gas particles or particles dissolved in a solution. The particles end up with a net movement from where they are at a higher concentration to where they are at a lower concentration. Cells rely on diffusion at their membranes. Oxygen and carbon dioxide move by diffusion during gas exchange, and the waste product urea diffuses from cells into the blood plasma, which carries it to the kidney for excretion.
Three factors affect the rate of diffusion. A bigger difference in concentration, called a steeper concentration gradient, makes diffusion faster. A higher temperature makes it faster because the particles have more energy and move more quickly. A larger surface area of membrane makes it faster because more particles can cross at the same time.
A single-celled organism has a large surface area to volume ratio, so diffusion across its surface supplies enough molecules for its needs. To find the ratio, divide the surface area by the volume. A cube with 2 cm sides has a surface area of 6 × 2 × 2 = 24 cm2 and a volume of 2 × 2 × 2 = 8 cm3, so the ratio is 24 : 8, which simplifies to 3 : 1. As an organism gets bigger its ratio gets smaller, so a multicellular organism needs exchange surfaces and a transport system.
A good exchange surface has a large surface area and a thin membrane, which gives a short diffusion path. In animals it also has an efficient blood supply, and for gas exchange it is ventilated. The small intestine and lungs in mammals, the gills in fish, and the roots and leaves in plants are all adapted for exchanging materials in these ways.
