Diffusion

Diffusion is the net movement of particles (molecules or ions) from a region where they are in a higher concentration to a region where they are in a lower concentration. This movement occurs down a concentration gradient. It's a passive process, meaning it doesn't require any metabolic energy from the cell because the particles move due to their own random kinetic energy. This process continues until the particles are evenly spread out, a state known as equilibrium. At equilibrium, particles still move randomly, but there is no overall (net) movement in any particular direction. Diffusion happens in liquids and gases.

Diffusion is vital for life, as it's the primary way substances move over short distances into, out of, and between cells. For example, in the lungs, oxygen from the air you breathe in is at a high concentration in the alveoli and diffuses into the blood where its concentration is lower. At the same time, carbon dioxide, a waste product, diffuses from the blood (high concentration) into the alveoli (low concentration) to be exhaled. In plants, carbon dioxide for photosynthesis diffuses from the air into the leaves. In the small intestine, digested food molecules like glucose and amino acids diffuse from the gut into the bloodstream.

The speed, or rate, of diffusion is not always the same. It is affected by several factors. A steeper concentration gradient (a bigger difference in concentration between two areas) results in a faster net movement of particles. A shorter diffusion distance means particles have less far to travel, so the rate is faster. Higher temperatures increase the rate because particles have more kinetic energy, causing them to move and collide more frequently. Biological systems are adapted to maximise these factors. For example, the alveoli in the lungs have walls that are only one-cell thick (short distance) and a constant blood supply maintains a steep concentration gradient.

The surface area to volume ratio (SA:V ratio) is a critical concept for understanding the limits of diffusion. As an organism or cell gets larger, its volume increases much faster than its surface area (volume is cubed, e.g., length³, while surface area is squared, e.g., 6 x length²). This means that large organisms have a small SA:V ratio. For a small, single-celled organism like an amoeba, its large SA:V ratio means diffusion across its surface is sufficient to supply all its needs. For a large, multicellular organism like a human, the SA:V ratio is too small. The distance to the central cells is too great for diffusion to be fast enough. This is why large organisms have evolved complex exchange surfaces (like lungs or gills) which are highly folded to maximise surface area, and transport systems (like the circulatory system) to carry substances between the exchange surface and the rest of the body.