The Stages of the Carbon Cycle

Carbon, an Ingredient of Life

Carbon is a chemical element found in all living things. The sugars, fats, proteins, and starch in a tortilla all contain carbon. That is why we say that life is "made of carbon."

Carbon is neither created nor destroyed: it changes place and form. It is found in these places, called reservoirs:

• Atmosphere: as carbon dioxide, $\text{CO}_2$, a gas.

• Living things: in organic matter (sugars, fats, proteins).

• Dead matter and soil: fallen leaves, animal remains, humus.

• Oceans: $\text{CO}_2$ dissolved in the water.

• Fossil fuels: oil, gas, and coal, formed millions of years ago.

From the Air to Living Things: Photosynthesis and Eating

Plants, algae, and some bacteria are producers. Using sunlight, they take $\text{CO}_2$ from the air and water, and make sugars. This process is photosynthesis. In it, carbon passes from the atmosphere into living matter, and oxygen is released.

Next comes eating: a rabbit eats grass, a coyote eats the rabbit. Carbon passes from one living thing to another along the food chain. Animals cannot take carbon from the air: they get it only by eating.

Back to the Air: Respiration

To live, cells get energy from sugars through cellular respiration. In this process, oxygen is used, energy is released, and $\text{CO}_2$ is produced and leaves into the atmosphere.

All living things carry out respiration: plants, animals, fungi, and bacteria. When you exhale, you are returning to the air carbon that was once in your food.

Decomposers Close the Cycle

When a plant or an animal dies, its body still contains carbon. Decomposer bacteria and fungi feed on those remains. As they do so:

1. They break down the organic matter into simpler substances.

2. They respire, and release $\text{CO}_2$ into the atmosphere.

3. They leave nutrients in the soil, which plants take advantage of.

This is what happens in Ximena's compost: microorganisms "eat" the peels, and the carbon leaves as $\text{CO}_2$ or stays in the fertile soil.

Without decomposers, remains would pile up and the carbon would stay trapped in dead matter. Nutrients would not return to the soil, and plants would have less and less to grow on.

Combustion and Deforestation: Carbon That Returns All at Once

Fossil fuels (oil, gas, and coal) are the remains of living things that were buried millions of years ago. Their carbon stayed stored all that time. When gasoline, diesel, or LP gas is burned, that carbon goes into the air as $\text{CO}_2$ within minutes: this is combustion.

Deforestation also returns carbon to the air. Trees store carbon in their wood. If they are cut down and burned, or if they rot, that carbon goes into the atmosphere. In addition, there are fewer plants to carry out photosynthesis and capture $\text{CO}_2$.

One example: in the Mexico City Metropolitan Area, millions of cars, buses, and motorcycles are on the road. All of them burn fuel and emit $\text{CO}_2$. Another case is the clearing of jungles and forests to make way for pastures or farmland.

These practices move carbon faster than natural processes can recapture it. Their effects on the climate are studied in the section "Human Impact and Global Warming."

Reading a Diagram of the Cycle

To identify the processes in a diagram, look at where the arrow starts and where it ends:

• Atmosphere → plant: photosynthesis.

• Plant → animal: eating.

• Plant or animal → atmosphere: respiration.

• Dead matter → atmosphere or soil: decomposition.

Back to the beginning: the peels in Ximena's compost turn into soil because decomposers transform them. Part of their carbon leaves into the air as $\text{CO}_2$ and part stays in the soil, from where plants will recover it through photosynthesis.

Summary