 the carbon cycle. All elements and some compounds have their natural cycle in nature, for example the water cycle. Matter is cycled using energy usually from the Sun and in this video we'll be considering the carbon cycle. We will see how photosynthesis and respiration, see the linked video, help carbon to be cycled in nature, again using energy from the Sun and we will also see the part played by mankind when we use fossil fuels. First of all the natural carbon cycle. Can you remember what the process is called where plants capture carbon and give off oxygen? Pause the video whilst you think. Did you remember that this process is called photosynthesis? The carbon is used to build up new plant material. Animals eat the plants or other animals to build up their bodies. To obtain energy for all these living processes plants and animals take in oxygen and rejoin it with the carbohydrates so returning the carbon dioxide to the atmosphere. Do you remember what this process is called? Pause the video. This process is called respiration. Now let's put some details into this. As they grow all living things have to build up large polymer molecules from small molecules. Protein for example to make your skin and muscles and other organs comes from joining amino acids together. Cellulose and starch to make the leaves, flowers, trunks, roots and all parts of plants from joining sugars and DNA to make the genes which control living things from the bases and the sugar and phosphate. Plants can make these simple molecules from the carbon they capture from photosynthesis with added elements from the minerals they get from the soil. Animals on the other hand get their molecules ready-made when they eat plants or other animals but first they have to break the food polymers back into the small molecules. Do you know what this process is called? Pause whilst you think of an answer. This is called digestion, breaking the food into little bits so it can get into our blood. All of this of course needs lots of energy and living things get their energy from respiration. Some of the monomers, things like sugar, have to be rejoined with oxygen and for us that is the principal use of the carbohydrates in our diet. The carbon dioxide gets back into the food web through photosynthesis.