the carbon cycle

the carbon cycle

Carbon is one of the most common elements found in living organisms. Chains of carbon molecules form the backbones of many organic molecules, such as carbohydrates, proteins, and lipids. Carbon is constantly cycling between living organisms and the atmosphere ( Figure 1.1). The cycling of carbon occurs through the carbon cycle. Living organisms cannot make their own carbon, so how is carbon incorporated into living organisms? In the atmosphere, carbon is in the form of carbon dioxide gas (CO2 ). Recall that plants and other producers capture the carbon dioxide and convert it to glucose (C6 H12 O6 ) through the process of photosynthesis. Then as animals eat plants or other animals, they gain the carbon from those organisms. The chemical equation of photosynthesis is 6CO2 + 6H2 O C6 H12 O6 + 6O2 . How does this carbon in living things end up back in the atmosphere? Remember that we breathe out carbon dioxide. This carbon dioxide is generated through the process of cellular respiration, which has the reverse chemical reaction as photosynthesis. That means when our cells burn food (glucose) for energy, carbon dioxide is released. We, like all animals, exhale this carbon dioxide and return it back to the atmosphere. Also, carbon is released to the atmosphere as an organism dies and decomposes. Cellular respiration and photosynthesis can be described as a cycle, as one uses carbon dioxide (and water) and makes oxygen (and glucose), and the other uses oxygen (and glucose) and makes carbon dioxide (and water). The carbon cycle. The cycling of car- bon dioxide in photosynthesis and cellular respiration are main components of the carbon cycle. Carbon is also returned to the atmosphere by the burning of organic matter (combustion) and fossil fuels and decomposition of organic matter.


formation of fossil fuels

Millions of years ago, there were so many dead plants and animals that they could not completely decompose before they were buried. They were covered over by soil or sand, tar or ice. These dead plants and animals are organic matter made out of cells full of carbon-containing organic compounds (carbohydrates, lipids, proteins and nucleic acids). What happened to all this carbon? When organic matter is under pressure for millions of years, it forms fossil fuels. Fossil fuels are coal, oil, and natural gas. When humans dig up and use fossil fuels, we have an impact on the carbon cycle ( Figure 1.2). This carbon is not recycled until it is used by humans. The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis. So, there is more carbon dioxide entering the atmosphere than is coming out of it. Carbon dioxide is known as a greenhouse gas, since it lets in light energy but does not let heat escape, much like the panes of a greenhouse. The increase of greenhouse gasses in the atmosphere is contributing to a global rise in Earths temperature, known as global warming or global climate change.

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carbon is necessary for living organisms to make organic compounds.

-->  a. true

b. false

people are able to make the carbon they need for their cells.

a. true

-->  b. false

where does the carbon dioxide we exhale come from?

-->  a) carbon dioxide is a product of cellular respiration.

b) carbon dioxide is obtained from the atmosphere.

c) carbon dioxide is obtained from the food we eat.

d) all of the above

burning of organic matter releases carbon dioxide into the atmosphere. this process is known as

a) respiration.

b) consumption.

-->  c) combustion.

d) decomposition.

which depicts cellular respiration?

a) uses glucose and carbon dioxide and makes oxygen and water

-->  b) uses glucose and oxygen and makes carbon dioxide and water

c) uses oxygen and carbon dioxide and makes glucose and water

d) uses water and carbon dioxide and makes glucose and oxygen

when organisms die, what process returns their carbon back into the atmosphere?

a) respiration

b) erosion

c) combustion

-->  d) decomposition

which of the following are fossil fuels?

-->  a) coal, oil, and natural gas

b) coal, tar and gasoline

c) tar, oil and gas

d) coal, oil and natural wood

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