The carbon cycle

Last time

Before we start: last time, in Ecosystems, food chains and food webs, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The same carbon, again

The carbon in your body was once carbon dioxide in the air. It has been in a plant, an animal, perhaps a fossil, and back into the air again. Today: the carbon cycle, the processes that move carbon around, and how burning fossil fuels has upset the balance.

Where carbon is found

The air contains carbon dioxide, only about four parts in ten thousand, but it is essential for photosynthesis. Living things contain carbon in organic compounds: carbohydrates, proteins and fats. Fossil fuels such as coal, oil and gas are carbon stores made from dead organisms millions of years ago.

Taking carbon out of the air

Producers absorb carbon dioxide from the air and use it in photosynthesis to make glucose. The carbon is now in organic compounds in the plant: carbohydrates, proteins and fats. Consumers get the carbon by feeding on the plants, and on each other.

Putting carbon back

All living things respire, and respiration releases carbon dioxide into the air. When organisms die, decomposers break down the remains, and they respire, releasing carbon dioxide. Burning wood or fossil fuels, called combustion, also puts carbon dioxide into the air.

Photosynthesis and respiration

Photosynthesis: six carbon dioxide plus six water makes glucose and six oxygen, using light. Respiration is almost the reverse: glucose and oxygen make carbon dioxide and water, releasing energy. In a balanced ecosystem, the carbon dioxide taken in and put back is about equal.

The carbon cycle

Here is the carbon cycle. Carbon dioxide in the air goes into plants by photosynthesis, along the food chain by feeding, and back to the air by respiration, decay and combustion.

How fossil fuels form

Dead plants and animals can be buried before they decay. Over millions of years, heat and pressure turn them into coal, oil and gas. This takes millions of years, so fossil fuels are non-renewable. We use them far faster than they form. Burning them returns the carbon to the air as carbon dioxide in a very short time.

Restoring the balance

Plant trees and protect forests, so more carbon dioxide is absorbed by photosynthesis. Burn less fossil fuel, by using renewable energy such as wind, solar and hydroelectric power. Together, these reduce the carbon dioxide concentration and slow climate change.