 We're looking at how organisms use their energy now. We know we can classify organisms by how they get their energy. There's autotrophs, heterotrophs. There's also radio tropes, some fungi that seem to get their energy from radiation, and photo heterotrophs. If after this you go look up photosynthesizing animals, you'll find a really cute looking sea slug. Okay, so autotrophs. They make their own energy from the sun using photosynthesis. The equation for photosynthesis is carbon dioxide plus water. With sunlight, we'll go to glucose and oxygen. We should balance that out like chemists. We start with carbons. There's six in the glucose molecule, so we'll make six carbon dioxide. There's 12 hydrogens, so we'll need to put a six there. Six times two is 12. And there's six oxygens in glucose, but there's another two in oxygen, which would make eight. Having a look at the other side, there's six times two is 12 plus six would make 18. So we need to balance the other side of the equation, and if we put a six in front of oxygen, we'll have 12 plus six is 18. There we go. It's important to remember that for photosynthesis, we need the energy going in, and that comes from the sunlight. In chemistry, we call this an endothermic reaction. This is a type of reaction where the newly made molecule has more energy in its bonds than the molecules from which it was created. So that's how energy from the sun is turned into a chemical energy. When an organism needs some energy, it takes that same reaction and does it backwards. So some glucose plus some oxygen can go to carbon dioxide and water, and we'll balance that out again. And in doing this, they can access some of that energy that was stored in the bonds of the glucose molecule. So cells use this chemistry whenever they need energy to grow, to create new molecules or break down molecules, if they need to transport things, and if they need to replicate. Some of the energy is also lost as heat, but that also keeps the organism warm. This process, this back to front photosynthesis, is called respiration, or cellular respiration to be clear, so it's not confused with the respiratory system. To help you remember, you might like to think of it as respiration, because it's also oxygen in and carbon dioxide out. All living things need energy to do those things we listed and more, and most organisms use cellular respiration as their way of getting that energy. That includes plants, because doing photosynthesis just stores their energy for them. They then need to break down the sugars to be able to do anything with it. And because respiration is the reverse of photosynthesis, it is an exothermic reaction, which means that energy is released when the bonds in one of the reactant molecules, here the glucose, are broken down. Cellular respiration looks quite simple here, but it's a very involved process with many steps, and organisms can actually use molecules other than glucose for their energy. And there's a similar process which cells can use when they don't have access to oxygen, and we're going to look at all of this next.