The rate of diffusion, osmosis and active transport is higher in cells with a larger surface area to volume ratio. The higher the surface area compared to the volume, the faster particles can move in or out
Surface area to volume ratio = Surface Area / Volume
Substances move in and out of a cell faster if there is a big difference in concentration
Surface area is inversely proportional to volume
Exchanging substances is harder in multicellular organisms as it needs to supply cells with substances e.g. glucose, water, oxygen. It also needs to remove waste products such as urea
Urea is produced as a waste product when proteins cant be stored by the body, so excess amino acids are converted in the liver into fats and carbohydrates which can be stored. Urea is then excreted in the urine
As single celled substances are only one cell big, particles can diffuse straight in and out. This is quick because:
Particles only have to travel a short distance
Large surface area to volume ratio
In multicellular organisms, exchange is more difficult and diffusion is slow because:
Cells are deep inside the organism, long way from them to outside environment
Lower surface area to volume ratio
Multicellular organisms have specialized exchange organisms, each with a specialized exchange surface, as they cannot diffusion through a single membrane
Multicellular organisms also need transport systems to carry substances from exchange organs to body cells and to remove waste products
