VMBO Year 3 · Ecology · Section 6.3

The carbon and nitrogen cycles

The carbon cycle

The carbon cycle describes how carbon moves through organisms and the environment. Plants take up carbon dioxide and use photosynthesis to make glucose. They use glucose as a fuel and to build other organic substances, including carbohydrates, fats and proteins.

Carbon passes through food chains:

• Herbivores obtain carbon by eating plants.
• Carnivores obtain carbon by eating other animals.
• Cellular respiration releases carbon dioxide back to air or water.

Dead organisms and faeces also contain carbon. Decomposers, including bacteria and fungi, break down this material. Their respiration releases carbon dioxide. Carbon can also be stored in soil, sediments and fossil fuels, so not all carbon returns immediately to the air.

The nitrogen cycle

Nitrogen is needed to build proteins and DNA. Plants take up nitrogen mainly as nitrate or ammonium from the soil. They combine it with carbon-containing substances produced from glucose to make amino acids and other compounds.

Animals obtain nitrogen by eating plants or other animals. They digest proteins and use the building blocks to make their own proteins and other nitrogen-containing substances.

The role of microorganisms

• Decomposing microorganisms break down nitrogen-containing remains and wastes, releasing ammonia or ammonium.
• Nitrifying bacteria convert ammonium into nitrite and then nitrate, which plants can take up.
• Nitrogen-fixing bacteria convert nitrogen gas from the air into compounds organisms can use.
• Denitrifying bacteria return nitrogen to the atmosphere as nitrogen gas under suitable conditions.

Root nodules

Some nitrogen-fixing bacteria live in root nodules on legumes, such as clover and lupins. The plant supplies sugars and the bacteria provide usable nitrogen compounds. This partnership helps the plant grow in soil with limited available nitrogen.

Like carbon, nitrogen is repeatedly transferred between organisms and their environment.