Many different materials cycle through the abiotic (non-living) and biotic (living) parts of an ecosystem. All materials in the living world are recycled to provide the building blocks for future organisms, so nothing is used up for good.
In the carbon cycle, plants and algae take carbon dioxide from the atmosphere and use it in photosynthesis to make glucose and other molecules. Carbon then passes along food chains as animals eat the plants. Respiration by plants, animals and microorganisms returns carbon dioxide to the atmosphere. Burning fuels also releases carbon dioxide. The carbon cycle is important because it returns carbon to the atmosphere as carbon dioxide so that plants can use it again.
In the water cycle, energy from the Sun makes water evaporate from the land and the sea. The water vapour rises, cools and condenses, then falls as precipitation such as rain. This provides fresh water for plants and animals on land before it drains into the seas, and the cycle repeats continuously.
Microorganisms called decomposers break down dead plants and animals and waste. They return carbon to the atmosphere as carbon dioxide, through respiration, and they return mineral ions to the soil, where plants take them up again. You are not expected to study the nitrogen cycle.
