[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to use the idea of surface area to volume ratio to explain why multicellular organisms require exchange surfaces and a transport system you should then be able to describe how gills increase the rate of transport of gases into and out of fish in the last video we saw that molecules move in and out of cells by diffusion for example oxygen diffuses into cells and carbon dioxide diffuses out now one key fact linked to the idea of diffusion is the surface area to volume ratio and we're going to look at that now this shows a single celled organism called an amoeba now organisms like this have a huge surface area for their volume scientists call this the surface area to volume ratio single celled organisms such as an amoeba can rely on diffusion to transport molecules in and out of their cell so for example all of the oxygen that the amoeba needs simply diffuses in through its membrane in your exam you could be asked to calculate the surface are of volume ratio imagine I've got an organism shaped like a cube such as this one the length of each side is 1 mm the area of each side is 1 mm * 1 mm which is 1 square mm I've got got six sides so the total surface area of this organism is 6 Square mm the volume is 1 mm * 1 mm * 1 mm this gives us a volume of 1 cubic mm so the ratio of the surface area to the volume is 6: 1 here's another organism but this one's got sides with a length of 2 mm I'd like you to calculate the surface area to volume ratio of this organism so pause the video now and try this yourself okay the surface area is 2 * 2 * 6 which gives us a value of 24 Square mm the volume is 2 * 2 * 2 giving us a value of 8 cubic millim the surface are of volume ratio is 24 / 8 giving us a ratio of 3: one here's another organism and this one's got size of 3 mm for this organism the surface area of volume ratio Falls to 2: one so as you can see as organisms get larger the surface volume ratio Falls sharply now this presents a huge problem for multicellular organisms and remember that these are organisms with more than one cell their surface area simply isn't large enough for their volume cells on the surface can get enough oxygen simply by diffusion however not enough oxygen can diffuse into the cells in the center of the organism they're simply too far away from the surface now animals have solved this problem in two ways firstly animals have got special structures for gas exchange with a very high surface area for example lungs and mammals secondly animals have a transport system to carry gases around the body in later videos we're going to look at how gases are exchanged and transported in mammals but in this video we're looking at fish fish get their oxygen from the water the oxygen rich water passes into the mouth it then flows over gills where the Oxygen's transported into the bloodstream I'm showing you a picture of gills here as you can see the gills are covered in a very large number of fine filaments this is where gases pass in and out of the blood de oxygenated blood passes into the filament oxygen diffuses from the water into the blood and then the oxygenated blood returns to the body the filaments have got three adaptations to increase the rate of diffusion firstly the filaments give the gills a massive surface area the filaments also have a thin membrane to provide a short diffusion pathway the filaments have an efficient blood supply to take the oxygenated blood away this ensures that the concentration gradient is always high so all of these adaptations make diffusion as efficient as possible remember you'll find plenty of questions on surface area to volume ratio in my Orion workbook and you can get that by clicking on the link above okay so hopefully now you should be able to use the idea of surface area to volume ratio to explain why multicellular organisms require exchange surfaces and a transport system you should then be able to describe how gills increase the rate of transport of gases into and out of fish [Music]