KS3 Chemistry - The Carbon Cycle

In this video we're going to learn about the carbon cycle. So we'll basically be covering all the different processes it involves, including how photosynthesis removes carbon dioxide from the atmosphere, how carbon moves through food chains, and how carbon dioxide can be returned to the atmosphere by respiration, decomposition, or combustion. Before we start though you need to know why carbon is so important and where it's found. Now carbon is the main element used to build molecules in living organisms, and so it's essential for life. Most of the carbon is split between six main stores. The atmosphere, plants, animals, decomposers like fungi and bacteria, dead organisms and waste, and in fossil fuels. Importantly though, carbon doesn't stay in just one place. Instead it's constantly being recycled, so moving between these different stores. And it's this movement of carbon that we refer to as the carbon cycle. So to kick us off let's take a look at how carbon moves from the atmosphere into plants via a process called photosynthesis. To understand photosynthesis you need to know that carbon exists in the atmosphere as CO2, which is short for carbon dioxide, and it can be absorbed by plants and some other organisms like algae. The equation for photosynthesis is carbon dioxide plus water produces glucose plus oxygen. This process also needs light, so we sometimes write light above the arrow in the equation like this. Now the carbon from carbon dioxide is used to produce glucose, which is then used to make carbohydrates, fats and proteins, as well as other substances that are essential for many life processes including growth. So overall photosynthesis removes carbon dioxide from the atmosphere and uses it to build new living cells. Next up, let's cover how carbon dioxide moves from plants to animals during the process of feeding. When plants are eaten by animals they take in some of the carbohydrates, fats and proteins that the plants made and use these carbon-based substances for their own life processes. This carbon is then transferred from one organism to another as animals are consumed by other animals higher up in the food chain, creating a flow of carbon through the ecosystem. At some point though all of these plants and animals are going to turn into dead organisms and waste, which happens via the processes of death and excretion. As we mentioned, at some point all plants animals die and if they're not immediately eaten by another animal some of the carbon in their remains can stick around. How long for ranges massively, sometimes only for a short time but sometimes for thousands of years. In addition, animals also excrete waste though and this adds to the supply of carbon left behind by dead plants and animals. Next, let's take a look at how the carbon in animals and plants can be returned to the atmosphere, which happens in three main ways. Decomposition, respiration and combustion. For decomposition, this is a process carried out by a group of organisms called decomposers and these are things like bacteria and fungi. Decomposers consume the bodies of dead organisms as well as their waste and use it as a food source. In doing so, carbon is transferred from the dead organisms to the decomposers. This leads us on to the next part of the cycle, respiration, which is how carbon from all the different organisms gets transferred back into the atmosphere. During respiration, the carbon in glucose is converted back into carbon dioxide. You could also think of the process as an equation. Glucose and oxygen are transformed into carbon dioxide and water, releasing energy at the same time. This energy lets organisms carry out essential functions such as movement and reproduction. The carbon dioxide that's produced during respiration is usually released back into the atmosphere. There's another process that releases carbon dioxide into the atmosphere though and that's combustion. When dead animals and plants, or their waste, aren't eaten and no decomposition occurs, they can be buried underground, sometimes forming fossils over millions of years. These fossils can accumulate and, under certain specific conditions, can transform into fossil fuels like coal, oil and natural gas and these contain a lot of carbon. Combustion then occurs when humans burn these fossil fuels to release energy and the stored carbon is also released into the atmosphere as carbon dioxide. Now the carbon dioxide released through decomposition, respiration and combustion can then be absorbed by plants and algae during photosynthesis, restarting the carbon cycle again. And remember, this whole process is a cycle, so carbon is constantly moving between all of these different stores. 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