 As you know, life is made of carbon compounds. We will see how the element carbon is taken in as carbon dioxide by plants, used and reused by all living things, before being respired back into the environment as carbon dioxide once again. So let's start with the gas carbon dioxide. Do you know how much there is of it in the air? Well, the air contains nearly 0.04% carbon dioxide. But hundreds of years ago, before we started using fossil fuels, it was only 0.28%. Not much, you might think, but enough to enable plants to build up all the structures they need with a little help from the water and minerals they take up from their roots, and energy from the sun. Plants use the sunlight to pull oxygen away from water and carbon dioxide, allowing the plant to capture the carbon. This happens in the green chloroplasts found in the leaves of plants. What is the name of this reaction? Perhaps the most important chemical reaction on the planet. Pause the video whilst you think of your answer. It is photosynthesis, well done if you got it right. Photo means light, and synthesis means building up. However, in its simplest terms, we are actually breaking carbon dioxide apart, releasing the reactive gas oxygen into the environment, allowing the carbon to be captured by the plants to build up their structures as they grow. Animals, in their turn, get carbon by eating plants or other animals. All this of course requires energy. In order to fuel these living processes, plants and animals take back oxygen from the air, and rejoin it with the carbon releasing carbon dioxide back into the environment. Do you know the name of this energy releasing process? Pause the video whilst you think of your answer. Well, it's called respiration, and here is a simple representation of the two processes. First of all, sunlight breaks the bonds, allowing the carbon dioxide and water to form oxygen and carbohydrate, which is a compound of carbon and hydro or water, hence C-H2O, a simple representation of the carbohydrate molecule. That's photosynthesis. Energy is now stored, and living things can use the carbohydrates to build their bodies and structures. To obtain energy, they simply reverse this reaction, letting the oxygen and the carbohydrate to rejoin, forming carbon dioxide and water all over again, and this is respiration. Let's look at these two processes more closely. They both need catalysts to enable them to work at normal temperatures. These catalysts are called enzymes. Photosynthesis also needs a pigment called chlorophyll. The active site is a magnesium atom, coloured green in this model, which absorbs blue and red light from the sun's spectrum to drive the photosynthesis reactions, leaving the green light to be reflected, making the leaves look well green. The reaction takes place by a number of complex stages. It's a reaction which evolved early on in the lifetime of our planet, and the blue-green algae, also known as cyanobacteria, are still alive today living in the sea. The chloroplasts you find in the green leaves of plants evolved from these simple bacteria. If only we could find a way to use sunlight to split carbon dioxide and water to make fuel and oxygen for ourselves, we could solve our green energy problems. Scientists are trying to find alternative catalysts to chlorophyll, but if there was a better catalyst, maybe life would have evolved it already. Respiration also needs a series of enzymes to enable oxygen to rejoin with the carbohydrate fuel. The series of reactions takes place in so-called mitochondria in the cells of plants and animals. In humans and most other vertebrates, their blood transports the oxygen from the lungs and the digested food from the gut to all the cells in their body. There, the food and oxygen react, and the blood takes the carbon dioxide back to the lungs to be excreted. Plants also respire, but you only notice it in the dark because in the sunlight they release more oxygen from photosynthesis than they use in respiration. But think of the roots and seeds which are hidden from the sun. Pause the video and think how roots and seeds get energy. The answer is that they need a constant supply of oxygen, which means that the soil must be full of air and not waterlogged. The roots get sugars sent down from the leaves, and seeds have enough of their own fuel supply until they put out green shoots and start making more. Respiration of food is similar to combustion. In both cases, oxygen is added to the fuel or the food, and carbon dioxide is built up. In respiration the reaction happens at normal temperatures using enzymes, for example in the cells of our bodies, whereas combustion, having no catalyst, only takes place at high temperatures, but both reactions release energy.