All organisms need to take in oxygen for aerobic respiration and get rid of carbon dioxide. This exchange of gases between an organism and its environment is called gas exchange.In very small organisms, such as single-celled organisms like Amoeba, gas exchange happens by diffusion across the cell surface membrane. Oxygen diffuses in because it is used up in respiration, keeping its concentration inside low. Carbon dioxide diffuses out because respiration keeps producing it, so its concentration inside stays high.Diffusion alone is enough for small organisms because:
• they have a large surface area to volume ratio, so there is plenty of surface to supply each unit of volume;
• the diffusion distance from the surface to the centre of the organism is very short, so gases reach every part quickly;
• many have a low metabolic rate, so they need relatively little oxygen.Some multicellular organisms are also small or thin enough to rely on diffusion across their body surface. Flatworms are flat, so no cell is far from the surface. Their thin shape keeps the surface area to volume ratio high even though they are longer than single cells.As a result, these organisms do not need a specialised gas exchange surface or a mass transport system such as blood to carry oxygen.Size is critical, though. If an Amoeba doubled its length while keeping the same shape, its surface area would increase 4 times but its volume 8 times, so its surface area to volume ratio would halve. Beyond a certain size, diffusion across the surface cannot supply the inner cells fast enough.
• they have a large surface area to volume ratio, so there is plenty of surface to supply each unit of volume;
• the diffusion distance from the surface to the centre of the organism is very short, so gases reach every part quickly;
• many have a low metabolic rate, so they need relatively little oxygen.Some multicellular organisms are also small or thin enough to rely on diffusion across their body surface. Flatworms are flat, so no cell is far from the surface. Their thin shape keeps the surface area to volume ratio high even though they are longer than single cells.As a result, these organisms do not need a specialised gas exchange surface or a mass transport system such as blood to carry oxygen.Size is critical, though. If an Amoeba doubled its length while keeping the same shape, its surface area would increase 4 times but its volume 8 times, so its surface area to volume ratio would halve. Beyond a certain size, diffusion across the surface cannot supply the inner cells fast enough.