[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe what's meant by the respiratory quotient or RQ you should then be able to describe why the respiratory quotient is different for different respiratory substrates and this is for the OCR spec now I'm going to explain the respiratory quotient based on the molecules involved in respiration this may seem a bit tricky but stick with it and you will get it now when organisms respire they can use a carbohydrate such as glucose however respiration can use other respiratory substrates such as lipids or proteins now if we know the amount of carbon dioxide produced during respiration and the amount of oxygen consumed then we can work out which molecule or substrate as being respired and to do that we calculate the respiratory quotient or RQ I'm going to show you how to work out the RQ for different respiratory substrates we're going to start with carbohydrates such as glucose the air aerobic restoration of glucose requires oxygen and produces carbon dioxide and water now this chemical equation is not balanced so I'm going to balance it now one molecule of glucose contains six carbon atoms each carbon atom makes one molecule of carbon dioxide so we make six molecules of carbon dioxide one molecule of glucose also contains 12 hydrogen atoms we need two hydrogen atoms to make each molecule of water so we make six molecules of water now we need to calculate the number of oxygen molecules required for this reaction we have 12 atoms of oxygen in the carbon dioxide and we have six atoms of oxygen in the water So based on that this reaction needs 18 atoms of oxygen however each glucose molecule contains six oxygen atoms so that means that the reaction needs 12 additional oxygen atoms each molecule of oxygen has two oxygen atoms so for 12 atoms of oxygen we need six oxygen molecules so here's the balanced chemical equation for the aerobic respiration of glucose now the respiratory quotient or RQ is the amount of carbon dioxide produced divided by the amount of oxygen consumed in the case of glucose we produce six carbon dioxide molecules and we consume six oxygen molecules so the RQ for glucose is six divided by six which is one so if we get an RQ of one this tells us that the organism is respiring a carbohydrate such as glucose okay I'm showing you here the equation for the aerobic respiration of a fatty acid which we find in lipids different fatty acids have different formulas but they all produce the same RQ value I'd like you to balance this equation so pause the video now and try this yourself okay from one molecule of this fatty acid we can make 16 molecules of carbon dioxide and 16 molecules of water to make these we need 48 atoms of oxygen the fatty acid already contains two oxygen atoms so we need 46 additional oxygen atoms each molecule of oxygen has two oxygen atoms so for 46 atoms of oxygen we need 23 oxygen molecules now if we calculate the RQ for this reaction we can see that it comes to 0.7 so if we get an RQ of 0.7 this tells us that the organism is respiring a lipid now with proteins the RQ is difficult to calculate that's because different proteins contain different amino acids and each amino acid has a different formula however generally speaking If an organism is respiring protein then the RQ will be around 0.9 okay so hopefully now you can describe what's meant by the respiratory quotient [Music] thank you