Principles and Factors of Diffusion

Particles spread because they are always moving

Particles in liquids and gases move randomly. Imagine adding a drop of coloured dye to water. At first, many dye particles occupy a small region, so their concentration is high there. Their random movement gradually spreads them through regions containing fewer dye particles.

This process is diffusion: the spreading out of the particles of any substance in a solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

“Net” is important. Individual particles can move in either direction. However, when one region contains more particles than another, more particles are likely to leave the higher-concentration region than return to it. The overall result is movement down the concentration gradient.

As diffusion continues, the difference in concentration becomes smaller. If the particles eventually become evenly distributed, the system has reached equilibrium. Particles still move randomly, but equal movements in opposite directions mean that there is no net movement.

Diffusion across cell membranes

A cell membrane separates the cell’s contents from its surroundings. Substances that can cross the membrane may diffuse into or out of the cell, provided that there is a concentration difference. The direction is determined separately for each substance: it moves overall from where its concentration is higher to where its concentration is lower.

Important biological examples include:

• oxygen, which diffuses during gas exchange and can then enter cells where its concentration is lower;
• carbon dioxide, which can diffuse out of respiring cells and is exchanged at gas-exchange surfaces;
• urea, a waste product that diffuses from cells into the blood plasma before being excreted by the kidneys.

Diffusion therefore helps cells obtain substances they need and remove some waste products.

A steeper concentration gradient increases the rate

A concentration gradient is the difference in concentration between two regions. The greater this difference, the steeper the gradient and the faster the net rate of diffusion.

Suppose there are many oxygen particles on one side of a membrane and few on the other. Random movement occurs both ways, but the imbalance is large, so there is a large net movement towards the lower-concentration side. As diffusion reduces the difference, the gradient becomes less steep and the net rate slows.

Higher temperature increases the rate

Increasing temperature gives particles more kinetic energy. They move faster, so they spread through a liquid or gas more rapidly and cross a suitable membrane more frequently. Diffusion is therefore faster at a higher temperature, provided the other factors are unchanged.

A larger membrane surface area increases the rate

Diffusion can occur at every suitable part of a membrane. A larger membrane surface area provides more space across which particles can pass at the same time. Consequently, more particles can cross per second and the rate of diffusion is greater.

This comparison must be fair: when considering surface area, factors such as temperature and concentration gradient should be kept the same. Concentration gradient, temperature and membrane surface area can all act together, so changing more than one makes it difficult to identify the cause of a change in rate.

Definition and direction

Diffusion is the spreading of particles in a solution or gas, producing a net movement from higher to lower concentration. Particles move randomly in both directions; “net” describes the overall result.

Across cell membranes:

• oxygen and carbon dioxide diffuse in gas exchange;
• urea diffuses from cells into blood plasma for excretion by the kidneys.

Rate of diffusion

Diffusion is faster when: