Carbon is one of the many natural elements that can be found on and in the Earth. It is one of the most abundant elements after hydrogen, helium and oxygen, and is an integral part of all human, animal and plant life. Carbon is particularly important in biology because it can easily form multiple bonds in a variety of organic molecules. These bonds are strong, which means they can form rings and long, connected chains, which are the backbone of most living cells.
These carbon bonds also contain a lot of energy, which can be stored or released if the bonds are broken. For this reason, carbon is an important and often used fuel source for living things.
Where is carbon found?
Carbon is found in the four spheres of the Earth. The lithosphere, or the rock and earth aspect of the Earth, contains a large amount of carbon. In fact, it’s in the top 15 for the elements that make up the earth’s crust. Carbon can also be found in the atmosphere, most often in the form of carbon dioxide, an organic compound made up of both carbon and oxygen. The hydrosphere, which includes all bodies of water on Earth, also contains carbon in the form of carbon dioxide, as does the biosphere, which is the term used for all organic life.
The carbon cycle
Carbon on Earth and in the four terrestrial spheres is constantly in motion. This exchange of carbon and carbon compounds is called the carbon cycle. There are different processes, both natural and synthetic, that cause carbon to be exchanged from one sphere to another. Sometimes carbon moves rapidly within Earth systems, and other times it can be held in one place for many years and therefore is exchanged at a much slower rate. For this reason, two distinctions have been made. These are known as the fast cycle and the slow cycle.
The rapid cycle
The rapid cycle, as the name suggests, encompasses those aspects of carbon exchange that occur more quickly. This includes the carbon that passes between living organisms and the exchange of carbon between living things and the atmosphere. In this way, the rapid cycle is measured in lifespan: the lifespan of a given organism, whether plant or animal.
Carbon that exists in the atmosphere, in carbon dioxide (CO2) form, is taken up by plants. Plant life uses carbon dioxide, along with sunlight and water, and turns them into sugars, in a process called photosynthesis. Carbon is extracted from the air through a stoma on the leaves of a plant. This then interacts with sunlight and water to provide the energy a plant needs to thrive. Oxygen and sugar are products of this process. This is why plants are so important for human life because they provide oxygen. Without carbon, this process could not exist.
A plant can also transfer its carbon to animals when it is consumed. When a plant is eaten, its energy stores are then transferred to that creature, along with the plant’s carbon atoms. This is the second part of the rapid cycle. Carbon moves through a food chain in a given ecosystem as small prey are eaten by predators higher in the food chain. Carbon remains in the food chain until it is released by decomposition following the death of a plant or animal, or by respiration.
As mentioned above, plants take in CO2 and release oxygen (O2). Animals, including humans, breathe this O2. The process of respiration in humans and animals involves these O2 molecules that bind to carbon to produce CO2. The process of breathing, which we generally think of simply as breathing, supplies oxygen to our bloodstream and body and creates CO2 by-product, which we then either expire or release into the atmosphere. In this way, carbon easily moves from the atmosphere to a plant, to an animal, and then back to the air. Carbon is constantly in motion in this rapid carbon cycle. About 1,000 to 100,000 million metric tonnes of carbon pass through the rapid carbon cycle each year.
Decomposition occurs after the death of a plant or animal. Once it is dead, bacteria break down the matter of these organisms, breaking the bonds between these strong carbon compounds. As these bonds are broken, the CO2 is released into the atmosphere. In this way, the carbon completes a complete cycle, passing from the air to plants and / or animals, and is returned to the atmosphere.
While a large amount of carbon circulates in the fast cycle, there is another aspect of the carbon cycle known as the slow cycle. In this cycle, carbon is exchanged at a much slower rate, over thousands and millions of years, between the atmosphere and the lithosphere, which primarily includes the earth’s crust. Much of the rocks and minerals in the earth’s crust contain carbon or carbon compounds.
The exchange of carbon from the atmosphere to the earth occurs in different ways. One of these means is rainwater. CO2 collects in the air and saturates the rainwater to form a mild type of acid rain. The acidic nature of this rainwater causes it to eat away at solid rock, dissolving and eroding it over time. As rocks are weathered, minerals are released which in turn create inorganic carbons. Carbon and acid rainwater can also land in the ocean. When high carbon water reaches the ocean, it dissolves and produces bicarbonate ions. This can then bind with calcium to create the compound known as calcium bicarbonate, an essential component in the makeup of shells and many crustaceans. Seashells that sink to the bottom of the seabed (for example, when seashells die) will eventually, over time and pressure, compact and melt into the ocean floor, becoming part of the lithosphere.
Decomposition and fossil fuels
Likewise, organic life can get trapped in mud, sand, or bogs, when it dies and begins to decompose. During normal decomposition, carbon bonds are broken by bacteria and released into the atmosphere. But in other cases, the body does not break down aerobically and the carbon becomes trapped. Over time, strong pressure will compress and trap these decaying carbon-based life forms, creating pockets of carbon-based fuel, known as fossil fuels. These fuels can be released from the lithosphere naturally as tectonic plates move or, more commonly, when they are mined and burned as fuel by humans. This way the carbon goes through a much slower cycle, but still rotates from the atmosphere to the ground, and then eventually comes back.