The Carbon Cycle

Carbon is an essential element in the bodies of living organisms. It is also economically important to modern humans, in the form of fossil fuels.Carbon dioxide from the atmosphere is taken up by photosynthetic organisms and used to make organic molecules, which travel through food chains. In the end, the carbon atoms are released as in respiration.Slow geological processes, including the formation of sedimentary rock and fossil fuels, contribute to the carbon cycle over long timescales.Some human activities, such as burning of fossil fuels and deforestation, increase atmospheric CO2 and affect Earth's climate and oceans.

Carbon: building block and fuel sourceAbout 18% of your body consists of carbon atoms, by mass, and those carbon atoms are prettykey to your existence! Without carbon, you wouldn't have the plasma membranes of your cells,the carbohydrate molecules you use for fuel, or even the DNA that carries instructions to build and run your body.Carbon is part of our bodies, but it's also part of our modern-day industries. Carbon compounds from long-ago plants and algae make up the fossil fuels, such as coal and natural gas, that we use today as energy sources. When these fossil fuels are burned, carbon dioxide is released into the air, leading to higher and higher levels of atmospheric CO2. This increase CO2 levels affects Earth's climate and is a major environmental concern worldwide.Let's take a look at the carbon cycle and see how atmospheric CO2 and carbon use by living organisms fit into the bigger picture of carbon cycling.

The carbon cycleThe carbon cycle is most easily studied as two interconnected subcycles:One dealing with rapid carbon exchange among living organismsOne dealing with long-term cycling of carbon through geologic processesAlthough we will look at them separately, it's important to realize these cycles are linked. For instance, the same pools of atmospheric and oceanic CO2 that are utilized by organisms are also fed and depleted by geological processes.As a brief overview, carbon exists in the air largely as carbon dioxide gas, which dissolves in water and reacts with water molecules to produce bicarbonate. Photosynthesis by land plants, bacteria, and algae converts carbon dioxide or bicarbonate into organic molecules. Organic molecules made by photosynthesizers are passed through food chains, and cellular respiration converts the organic carbon back into carbon dioxide gas.

Image credit: Biogeochemical cycles: Figure 3 by OpenStax College, Biology, CC BY 4.0; modification of work by John M. Evans and Howard Perlman, USGSLongterm storage of organic carbon occurs when matter from living organisms is buried deep underground or sinks to the bottom of the ocean and forms sedimentary rock. Volcanic activity and, more recently, human burning of fossil fuels bring this stored carbon back into the carbon cycle. Although the formation of fossil fuels happens on a slow, geologic timescale, human release of the carbon they contain CO2 is on a very fast timescale.

The biological carbon cycleCarbon enters all food webs, both terrestrial and aquatic, through autotrophs, or self-feeders. Almost all of these autotrophs are photsynthesizers, such as plants or algae.Autotrophs capture carbon dioxide from the air or bicarbonate ions from the water and use them to make organic compounds such as glucose. Heterotrophs, or other-feeders, such as humans, consume the organic molecules, and the organic carbon is passed through food chains and webs.How does carbon cycle back to the atmosphere or ocean? To release the energy stored in carbon-containing molecules, such as carbohydrates, autotrophs and heterotrophs break thesemolecules down in a process called cellular respiration. In this process, the carbons of the molecule are released as carbon dioxide. Decomposers also release organic compounds and carbon dioxide when they break down dead organisms and waste products.Carbon can cycle quickly through this biological pathway, especially in aquatic ecosystems. Overall, an estimated 1,000 to 100,000 million metric tons of carbon move through the biological pathway each year. For context, a metric ton is about the weight of an elephant or asmall car!Respiration releases energy for cells from glucose. This can be aerobic respiration, which needs oxygen, or anaerobic respiration, which does not. During exercise, the breathing rate and heart rate increase. During hard exercise an oxygen debt may build up.

What is aerobic respiration?

Respiration is a series of reactions in which energy is released from carbohydrates. Aerobic respiration is the form of respiration which uses oxygen. It can be summarized by this equation:glucose + oxygen → carbon dioxide + water (+ energy)Energy is shown in brackets because it is not a substance. Notice that:Glucose and oxygen are used upCarbon dioxide and water are produced as waste productsAerobic respiration happens all the time in the cells of animals and plants. Most of the reactions involved happen inside mitochondria, tiny objects inside the cytoplasm of the cell. The reactions are controlled by enzymes.

Using energy

Energy released during respiration is used by the organism in several ways. It may be used to build up larger molecules from smaller ones. For example:Plants make amino acids from sugars, nitrates and other nutrientsThese amino acids are then built up into larger molecules - proteinsEnergy is used by animals to enable the muscles to contract so that the animals can move. Mammals and birds keep their body temperature steady. Energy from respiration is used to dothis when their surroundings are colder than they are.

Exercise

During exercise, the muscle cells respire more than they do at rest. This means that:Oxygen and glucose must be delivered to them more quicklyWaste carbon dioxide must be removed more quicklyThis is achieved by increasing the heart rate, rate of breathing and the depth of breathing.The increased heart rate increases the rate of blood flow around the body. The increased rate and depth of breathing increases the rate of gaseous exchange in the lungs.The muscles store glucose as glycogen. This can then be converted back to glucose for use during exercise.Take care not to get confused: plants store glucose as starch and animals store it as glycogen. In addition, respiration and breathing are not the same thing: respiration releases energy, while breathing lets air into and out of our lungs.