Answer:
Key points
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—\text {CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript—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 \text {CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript 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 \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript and affect Earth's climate and oceans.
Carbon: building block and fuel source
About 18% of your body consists of carbon atoms, by mass, and those carbon atoms are pretty key to your existence!^1
1
start superscript, 1, end superscript Without carbon, you wouldn't have the plasma membranes of your cells, the sugar molecules you use for fuel, or even the \text{DNA}DNAstart text, D, N, A, end text 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—\text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript—is released into the air, leading to higher and higher levels of atmospheric \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript. This increase in \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript 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 \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript and carbon use by living organisms fit into the bigger picture of carbon cycling.
The carbon cycle
The carbon cycle is most easily studied as two interconnected subcycles:
One dealing with rapid carbon exchange among living organisms
One dealing with long-term cycling of carbon through geologic processes
Although we will look at them separately, it's important to realize these cycles are linked. For instance, the same pools of atmospheric and oceanic \text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript 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—\text{CO}_2CO
2
start text, C, O, end text, start subscript, 2, end subscript—gas, which dissolves in water and reacts with water molecules to produce bicarbonate—\text{HCO}_3^-HCO
3
−
start text, H, C, O, end text, start subscript, 3, end subscript, start superscript, minus, end superscript. 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.
