Lesson Video: The Carbon Cycle Chemistry • 7th Grade

In this video, we will learn how to describe the key steps of the carbon cycle, including respiration, photosynthesis, combustion, and decomposition.


Video Transcript

In this video, we will explore the key steps of the carbon cycle, how carbon is used in organisms, and we will have a look at some chemical reactions occurring at the different stages of the carbon cycle.

Carbon is the key element for life for all organisms, both on land and in bodies of water such as oceans and rivers. Most molecules found in living organisms have carbon as the backbone element. Carbon in the backbone is bonded to other elements like oxygen, hydrogen, sulfur, and nitrogen. These carbon-based molecules in all living organisms include proteins, carbohydrates, lipids, DNA, and much more. Carbon is also an important component of the atmosphere, found predominantly as carbon dioxide and also as some other gases, for example, methane, CH4.

In the ground, carbon is found in the form of limestone and fossils, as well as fossil fuels like coal, natural gas, and oil. And in the bodies of water of the earth, including oceans and rivers, carbon is found in the form of dissolved carbon dioxide, carbonic acid, which is H2CO3, and bicarbonate ions, HCO3−. These three substances have a close relationship. Carbon constantly cycles and recycles through these living and nonliving factors in a cycle known as the carbon cycle.

Let’s start the journey through the carbon cycle with atmospheric carbon, which exists mostly in the form of carbon dioxide gas. Carbon dioxide is absorbed by plants and some bacteria on land and by plankton and algae in the oceans and rivers. These organisms use carbon dioxide along with water to produce organic, carbon-based compounds like simple carbohydrates such as glucose during a process called photosynthesis. These simple compounds are then converted to more complex compounds like proteins, lipids, et cetera. Energy from sunlight is used to drive the photosynthesis reaction, and oxygen gas is also produced. The general photosynthesis equation is 6CO2 plus 6H2O reacting together in the presence of sunlight to give glucose C6H12O6 plus 6O2. Carbon in its various complex forms in photosynthesizing plants and other producers can then enter food chains and food webs when animals consume or eat plant matter. And so, carbon in its various forms can move from animal to animal in this way.

All living organisms in the food chains and food webs, including producers like plants themselves, undergo cellular respiration and release carbon back to the atmosphere as carbon dioxide. The general equation for cellular respiration in plants and animals is C6H12O6 plus 6O2 reacting to give 6CO2 plus 6H2O plus ATP. Respiration produces ATP, adenosine triphosphate, a molecule used to transfer or deliver energy in living cells to drive many cellular processes. So far, we have seen that plants absorb carbon during photosynthesis and all organisms release carbon during respiration.

Carbon can move from some animals to the atmosphere as methane during food digestion or to the soil as waste products such as manure and urine. Wastes in the soil from animals as well as dead plant and animal organisms are broken down by decay organisms in the soil in a process known as decomposition. Decomposition could be considered a type of respiration because organic compounds and oxygen in the cells of the decomposer organisms produce similar products to respiration. The general equation for decomposition is organic compounds plus O2 reacting to give H2O plus CO2 plus inorganic nutrients and energy.

The carbon dioxide gas produced from the decay of rotting matter goes back into the atmosphere. And the inorganic nutrients produced from decomposition include nitrates and phosphates, which go back into the soil, and are used again by growing plants. Not all the carbon in dead decaying organic matter is converted to carbon dioxide gas; some of it over long periods of time and high pressure is converted to underground limestone deposits as well as fossils and deposits of carbonaceous fossil fuels. Some of these fossil fuels include coal, natural gas, and oil.

The fourth main key step in the carbon cycle is a process called combustion. This part of the carbon cycle is largely driven by human activity and the burning of fossil fuels. The emissions produced from the burning of fossil fuels and factories and in vehicles releases carbon dioxide back to the atmosphere. The equation for combustion is similar to that for respiration and decomposition. The general equation is a fuel, usually a fuel is made of carbon and hydrogen, plus O2 reacting to give CO2 plus H2O and releases energy. Carbon dioxide and water are the products of combustion when sufficient oxygen is present. However, CO or carbon monoxide is also a possible carbon product produced from combustion when there is not sufficient oxygen. Carbon monoxide is another form of carbon in the carbon cycle.

Besides human-related combustion, some natural processes release underground carbon to the atmosphere to land and to oceans. These natural processes include volcanic eruptions and emissions. When large quantities of land are deforested and are not sustainably replanted, the balance of CO2 removal from the atmosphere by photosynthesis may be influenced. This could lead to a variety of environmental problems. These potential problems, such as the greenhouse effect, are a topic for another video. Now, it’s time to practice some problems and then summarize everything we have learnt.

What is the name given to an organism that releases carbon dioxide into the atmosphere by breaking down dead organic material? (A) Scavenger, (B) decomposer, (C) composter, (D) carbon dioxider, or (E) decayer.

An organism that breaks down dead organic material and releases carbon dioxide is part of one of the key steps in the carbon cycle. The four main processes in the carbon cycle are photosynthesis, respiration, decomposition, and combustion. In photosynthesis, carbon dioxide is removed from the air. This happens in green plants, algae, some bacteria, and phytoplankton. 6CO2 molecules react with six water molecules, and sunlight energy is used to convert these into glucose, C6H12O6, and six oxygen molecules.

The carbon from carbon dioxide in the atmosphere is incorporated into organic molecules inside the plants. In the other three main processes of the carbon cycle, carbon dioxide gas is released to the air. Their equations are very similar. They all involve a carbon compound reacting with oxygen to produce CO2 and water, as well as energy or molecules which transfer energy. The question asks about the release of carbon dioxide. So, we know the question is not talking about photosynthesis, and we can eliminate it. The question asks about the breaking down of dead organic material.

Let’s look at the remaining three processes to see which one involves the breaking down of dead organic matter. Respiration is a process which occurs in every living cell with a plant or animal. The purpose is to produce ATP molecules from food, and ATP delivers energy to drive cellular processes. Decomposition occurs in decomposer organisms. Decomposition is a type of respiration where dead organic material is the food for the decomposer organism. Combustion, though it has a similar equation to respiration and decomposition, is a human activity. Combustion is the burning of fuel, such as fossil fuels, for example, coal, oil, and natural gas. So, we can see that an organism that releases carbon dioxide by breaking down dead organic material is a decomposer.

Match the equations to the processes. A: CH4 plus 2O2 giving CO2 plus 2H2O, B: 6CO2 plus 6H2O giving C6H12O6 plus 6O2, C: C6H12O6 plus 6O2 giving 6CO2 plus 6H2O. And the possible processes are 1: photosynthesis, 2: combustion, 3: respiration. The answer options are (A) A:3, B:2, C:1; (B) A:2, B:3, C:1; (C) A:1, B:3, C:2; (D) A:1, B:2, C:3 ;or (E) A:2, B:1, C:3.

In the four main processes of the carbon cycle, photosynthesis is the only one which removes carbon dioxide from the atmosphere. In producers such as green plants, phytoplankton, some bacteria, and algae, carbon dioxide and water react together in the presence of energy from sunlight to produce glucose and oxygen. Equation B and process 1 belong together. The other three processes produce carbon dioxide by very similar equations. The general equation for these three processes is carbon compound plus oxygen forming carbon dioxide and water. In respiration, glucose reacts with oxygen to produce carbon dioxide, water, and ATP. And this happens in every living cell, both plant and animal. ATP is the molecule which transfers or delivers energy to carry out cellular processes. We can see that equation C and the process respiration belong together.

Combustion is the process of burning of fuel in the presence of oxygen to produce carbon dioxide, water, and energy. The equation in purple is a specific example of a fuel burning and oxygen. Here, the fuel is methane gas, CH4. This equation is the same as equation A and matches or belongs to process 2, combustion. So, equation A belongs to process 2; equation B, process 1; and equation C, process 3. The last process in the carbon cycle, decomposition, also produces carbon dioxide, water, and energy, as well as inorganic nutrients which go back into the soil, for example, nitrates and phosphates.

In this video, we learned that carbon is the key or backbone element to molecules in all living organisms. We learned that carbon is found in different forms or substances in organisms, in the atmosphere, in the soil and deep underground, and in bodies of water such as oceans and rivers and that carbon in its various forms cycles through these living and nonliving factors continuously. We learned that carbon as carbon dioxide moves from the atmosphere into plants, phytoplankton, and algae during photosynthesis and is converted into many other carbon products. And these organic compounds then are transferred into animals in food webs when consumed or eaten.

We learned that carbon as carbon dioxide moves from plants and animals and all living organisms back to the atmosphere during respiration. We saw that carbon from dead organisms moves back into the air as carbon dioxide and back into the soil through waste products during decomposition. And we learned that when fossil fuels are mined from the ground and burnt or combusted, that carbon returns back to the atmosphere once again as carbon dioxide.

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