Sign up to access the complete lesson and track your progress!
Unlock This CourseAll the materials in the living world are recycled to provide the building blocks for future organisms. Nothing is made new each time.
The materials move between the abiotic parts of an ecosystem (the air, water and soil) and the biotic parts (the living organisms). Many different materials do this, including carbon, water and mineral ions. If they were not recycled, they would run out and life would stop.
Key terms:
Carbon is in every living thing. It is in sugars, proteins, fats and DNA. The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, ready to be used by plants in photosynthesis.
Plants and algae take carbon dioxide from the air and use it in photosynthesis to make glucose. The carbon is now part of the plant. When animals eat the plant, and other animals eat them, the carbon passes along the food chain.
Living things release carbon dioxide by respiration, including plants. Dead organisms and waste are broken down by decay, which also releases carbon dioxide. Burning fuels and wood by combustion does the same.
So carbon dioxide is taken out of the air by photosynthesis and put back by respiration, decay and combustion. In a balanced ecosystem the two sides roughly match, so the amount of carbon dioxide in the air stays steady.
A carbon atom in the air is taken in by an oak leaf during photosynthesis. A caterpillar eats the leaf, and a bird eats the caterpillar. When the bird dies, microorganisms break down its body and the atom is released as carbon dioxide into the air. It is then ready to be taken in by another plant.
Living things cannot survive without fresh water. The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.
Organisms also return water to the air. Plants lose water vapour through their leaves and animals breathe out water vapour and sweat. This adds to the water going back into the atmosphere.
Plants use water in photosynthesis and to keep their cells firm. They take in fresh water through their roots from rain that has soaked into the soil.
Animals drink fresh water or get it from their food. Their cells need it for the chemical reactions of life. Sea water is too salty for most land animals to drink.
Microorganisms such as bacteria and fungi are the recyclers of the living world. When plants and animals die, and when animals produce waste, microorganisms feed on the dead matter. This is what causes decay.
Plants take the mineral ions up through their roots and use them to grow. Without microorganisms, dead material would pile up, carbon would stay locked inside it, and the soil would run out of mineral ions.
What affects the speed of decay is covered in the next lesson, Decomposition.
Exam questions often give you a diagram of the carbon cycle or water cycle. Arrows show the direction of movement. Use this method:
Saying that plants only photosynthesise and never respire. Plants respire all the time, day and night. Another mistake is saying that carbon is used up. It is never destroyed, it only moves from one place to another. A third is writing that water disappears when it evaporates. It becomes water vapour in the air.
(a) Name the process that removes carbon dioxide from the air. [1 mark]
(b) Give two processes that return carbon dioxide to the air. [2 marks]
(c) Explain why microorganisms are important in the carbon cycle and in keeping soil fertile. [3 marks]
(a) Photosynthesis (1).
(b) Any two from: respiration (1), decay (1), combustion (1).
(c) Microorganisms feed on dead organisms and waste, causing decay (1). They release carbon dioxide into the air when they respire, so the carbon can be used again by plants (1). They also release mineral ions into the soil, which plants take up to grow (1).
In part (c), the question asks about both the carbon cycle and the soil, so make sure you give one point for each. Name the material that is returned: carbon dioxide to the air and mineral ions to the soil.