The Carbon Cycle

Essential Question

How do photosynthesis and cellular respiration move carbon through Earth’s systems?

The carbon atoms in your body came from your food, and the carbon in that food came from carbon dioxide in the air. Over Earth’s history, the same atoms have also spent time in the ocean, in rocks, and in other living things. In this lesson, you’ll follow carbon as photosynthesis pulls it out of the air and cellular respiration puts it back. You’ll also use data to see how people are changing the carbon cycle.

Carbon: The Backbone of Life

• All four types of biomolecules have carbon backbones
• Carbon atoms bond in chains and rings
• Carbon atoms are recycled, never used up

A carbon atom can form four bonds, so carbon atoms link into long chains and rings. These carbon skeletons are the backbone of all four kinds of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. The amount of carbon on Earth stays the same. Living things build their bodies from carbon that has cycled through the air, water, rocks, and other organisms many times.

Photosynthesis and Cellular Respiration

• Photosynthesis: CO₂ + H₂O + light energy → glucose + O₂
• Respiration: glucose + O₂ → CO₂ + H₂O + usable energy
• Each process uses the other’s products

Producers, such as plants and algae, use light energy to combine carbon dioxide and water into glucose, releasing oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This moves carbon from the air into living things. Almost all organisms, including plants, then break down glucose in cellular respiration to release usable energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. This returns carbon to the air. The products of each process are the reactants of the other.

Carbon Reservoirs and Flows

• Reservoirs: air, living things and soil, oceans, rocks
• Photosynthesis and oceans remove CO₂ from the air
• Respiration, decomposition, and burning return it

Carbon is stored in four main reservoirs: the atmosphere, the biosphere (living things and soil), the oceans, and the geosphere (rocks and fossil fuels). The numbers are in gigatons of carbon (GtC); one gigaton is a billion metric tons. The arrows show approximate yearly flows, rounded for this model. The photosynthesis arrow shows photosynthesis on land only, and published estimates of these flows vary. Photosynthesis and the oceans remove carbon from the air, while respiration, decomposition, the oceans, and burning return it. Fossil fuels formed over millions of years from buried organisms, so burning them moves ancient carbon into the air very quickly.

Carbon Dioxide in the Air Is Rising

• CO₂ measured in Hawaii since 1958
• Rising trend: from about 317 ppm to over 420 ppm
• Yearly zigzag: plants growing and resting

Since 1958, scientists at Mauna Loa Observatory in Hawaii have measured the amount of CO₂ in the air, in parts per million (ppm). The graph is a smoothed model of those measurements. The long-term trend rises steadily, mainly because of burning fossil fuels and clearing forests. The small yearly zigzag comes from plants. Most land is in the Northern Hemisphere, so CO₂ drops during the northern spring and summer, when photosynthesis is fastest, and rises in fall and winter, when respiration and decomposition outpace photosynthesis.

People and the Carbon Cycle

• Burning fossil fuels and forests adds about 11 GtC per year
• Plants and oceans absorb only part of it
• The rest builds up in the air

The bar graph shows approximate yearly flows of carbon into and out of the atmosphere. Without people, the flows in and out would nearly balance. Burning fossil fuels and clearing forests add about 11 GtC each year. Land plants and the oceans absorb some of the extra, but almost half stays in the air. Extra CO₂ traps heat and warms the climate, and CO₂ absorbed by the oceans makes seawater more acidic, which harms corals and shellfish.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Tracing a Carbon Atom

A carbon atom moves from the air into a blade of grass, then into a rabbit that eats the grass, then back to the air. Name the process at each step.

Show the solution

1. Step 1: photosynthesis. The grass uses light energy to build CO₂ into glucose.
2. Step 2: eating and digestion. The rabbit eats the grass and uses carbon from the glucose to build its own molecules, such as proteins.
3. Step 3: cellular respiration. The rabbit breaks down carbon compounds for energy and breathes out CO₂. (If the rabbit dies first, decomposers release the carbon instead.)
4. Answer: Photosynthesis, then eating, then cellular respiration.

Example 2: Is Carbon Building Up in the Air?

Use the yearly flows below, which are rounded values for a simplified model. Find the total carbon added to the air and the total removed. What is the net change each year? About 2.1 GtC of carbon raises CO₂ in the air by 1 ppm. How fast is CO₂ rising?

Flow	Direction	GtC per year
Cellular respiration and decomposition	into the air	117
Released by the ocean	into the air	77
Burning fossil fuels and clearing forests	into the air	11
Photosynthesis	out of the air	120
Absorbed by the ocean	out of the air	80

Show the solution

1. Into the air: 117 + 77 + 11 = 205 GtC
2. Out of the air: 120 + 80 = 200 GtC
3. Net change: 205 GtC − 200 GtC = +5 GtC per year
4. Rate of rise: 5 GtC ÷ 2.1 GtC per ppm ≈ 2.4 ppm per year
5. Answer: The air gains about 5 GtC each year, raising CO₂ by about 2.4 ppm per year.

Example 3: The Yearly Zigzag

The graph shows monthly CO₂ values at Mauna Loa for two years (sample values). In which months does CO₂ fall? Explain why. Then compare May 2022 with May 2023 to find the yearly increase.

Show the solution

1. Pattern: CO₂ falls from about May to September or October, then rises again.
2. Reasoning: during the Northern Hemisphere summer, plants grow fast, and photosynthesis takes in more CO₂ than respiration and decomposition release.
3. Yearly increase: 423.4 ppm − 420.8 ppm = 2.6 ppm
4. Answer: CO₂ falls from May to about September and rose 2.6 ppm from May 2022 to May 2023.

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Practice

Compare photosynthesis and respiration, name the flows in a carbon cycle model, calculate the net change in atmospheric carbon, and explain experiment and CO₂ data. Try it on your own first, then check your work with the answer key.

• The Carbon Cycle Practice Worksheet (PDF)
• The Carbon Cycle Practice Answer Key (PDF)

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Lesson Slides

• 1.3 The Carbon Cycle Slides (PDF)

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Big Ideas

• Carbon is the backbone of all biomolecules
• Photosynthesis removes CO₂; respiration returns it
• Carbon cycles among air, life, oceans, and rocks
• Burning fossil fuels adds carbon faster than it is removed

Back to the essential question: photosynthesis moves carbon from the air into the sugars of producers, and cellular respiration in nearly all organisms returns it to the air as CO₂. Carbon also cycles through the oceans, soil, and rocks. Today, burning fossil fuels adds carbon faster than plants and oceans remove it, so CO₂ in the air keeps rising. Next, you’ll follow nitrogen, an element living things need but cannot take straight from the air.

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