 So, in a way, cell respiration is kind of a process of cashing in or swapping for small change. So, imagine if you had a $50 note, you had heaps of money, but to go to buy some lollies you needed small change. So you need to go to a shop somewhere and ask for some coins perhaps, some smaller change so that then you could buy the lollies at the lolly shop or a machine. So to the cell, one whole glucose molecule is like a big shiny yellow $50 note. It's worth a lot, but in its current state it doesn't do much for the cell. It needs to do respiration to break it into little usable parts. We'll take a quick look at the energy efficiency of cell respiration. To be able to calculate the energy efficiency, we need to look at some values that are taken from chemical experiments. It would be very hard to do an experiment with one single glucose molecule on the bench. So in chemistry, we use the unit of moles to talk about a bunch of things that are the same. Moles is kind of a shorthand for talking about really, really big numbers. The mole is a standard defined by a number called Avogadro's number or Avogadro's constant. And it's an exact number that is 6.02214076 times 10 to the power 23. Or usually we can round it to 6.022 by 10 to the power 23. So for whatever we're using, whether it's X or glucose, one mole of it is equal to 6.022 times 10 to the 23 molecules. Which would also be equal to 6.022000000000 blah, blah, blah, blah, blah, molecules or particles of X. We know from experiments that one mole of glucose releases 2,870 kilojoules of energy, and one mole of ATP, denazine triphosphate, releases 30.5 kilojoules of energy. We also know that one glucose molecule converts to 38 molecules of ATP. So one mole of glucose molecules will convert to 38 moles of ATP. We can calculate the efficiency by comparing the energy of 38 moles of ATP with one mole of glucose. And if we divide ATP by glucose, we end up with around 0.4. Which means cell respiration is about 40% efficient. 40% of the energy from that glucose is converted directly into ATP that the cell can use. 40% might not sound great. It's not going to pass a test. If you think about cars and how much we rely on them and how much fuel they use, most cars are around about 25% efficient. Which means your cells are doing a really good job. Where does the rest of the energy go? Mostly as heat. And that's not a bad thing because it keeps us warm.