Why can you smell someone's stinky PE socks from all the way across the classroom? Yuk! Normally, it's because sweat and other molecules are moving away from the socks and spreading out in the air. This is called diffusion.
Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in movement from an area of higher concentration (where there's more particles in a certain solution or area) to an area of lower concentration (where there are fewer particles).
Living organisms need different substances to be able to survive and function. These substances need to be transported in and out of their cells through diffusion (as well as osmosis and active transport - you will learn more about these later!). During diffusion, molecules move from an area of high concentration to an area of low concentration. They are said to move down a concentration gradient. Particles diffuse until they are evenly spaced apart. Diffusion is a passive process, which means that no energy is needed and it happens naturally.
Diffusion in organisms
The cell membrane has an important job to do. It is selective. This means it won't just let any random particle through - it can monitor which particle is allowed through it and also how many. Pretty clever right?!
In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. Diffusion is the main way that substances move over short distances in organisms.
Breathing involves exchanging gases in the lungs - this requires diffusion. When you breathe in, oxygen in the inhaled air diffuses through the tiny air sacs (alveoli) in your lungs into your bloodstream and is transported around your body. Carbon dioxide is the waste gas produced by respiration - it diffuses from cells into the bloodstream and is exhaled by the lungs.
Another example of diffusion is in the small intestine. Digested food is broken down into small molecules, such as glucose and amino acids. These important molecules need to be transported around the body in the blood. The small intestine is lined with many finger-like projections called villi. The molecules diffuse through the villi of the small intestine into the blood to be transported around the body.
In the science lab, something called a Visking tube can be used to model a cell membrane. You can see a diagram of a Visking tube below:
The Visking tube can mimic the action of the gut. In the gut, small food molecules can diffuse out while the larger molecules can't. The Visking tube does something similiar and only lets certain molecules through, like glucose for example.
Diffusion also occurs in plants. Plants take in carbon dioxide for photosynthesis and produce oxygen. These enter and leave the plant through the process of diffusion.
Factors affecting diffusion
Different factors can affect diffusion and how quickly it happens. Some of these factors are:
The difference in concentrations (concentration gradient) - having a large difference in concentrations means that diffusion can occur at a quicker rate, as particles will naturally move from a high to low concentration.
The temperature - the higher the temperature, the more energy the particles will have to move and spread out.
The surface area of the membrane - the larger the surface area, the faster the rate of diffusion. This is because more particles can pass through the membrane since there is more space, as in the alveoli in the lungs.
In the following activity, you will define and describe diffusion.