Nutrient cyclesIB MYP Biology: Revision notes
Section 1
Why do nutrients cycle?
Atoms such as carbon and nitrogen are not used up. They are passed round between living things and the environment in cycles, so the same atoms are used again and again. Producers take nutrients in, consumers pass them on by feeding, and decomposers return them to the soil and air.
Section 2
The carbon cycle
Carbon is in carbon dioxide in the air and in the compounds that make up living things (carbohydrates, proteins, fats).
- Photosynthesis: plants take carbon dioxide from the air and make glucose. This removes carbon dioxide.
- Feeding: carbon compounds pass from plants to animals along food chains.
- Respiration: all living things, including plants and decomposers, release carbon dioxide.
- Decomposition: fungi and bacteria break down dead matter and release carbon dioxide by respiration.
- Combustion: burning wood or fossil fuels (coal, oil, gas) releases carbon dioxide.
- Fossil fuels formed over millions of years from the remains of dead organisms. Burning them returns carbon that has been locked away to the atmosphere.
Respiration and burning put carbon dioxide IN to the air. Only photosynthesis takes it OUT.
Section 3
The nitrogen cycle
Plants and animals need nitrogen to make proteins. About 78% of air is nitrogen gas, but plants cannot use it directly. The cycle changes nitrogen into forms they can use:
- Nitrogen fixation: bacteria in the soil and in root nodules of legumes (beans, peas, clover) convert nitrogen gas into ammonium compounds. Lightning also fixes some nitrogen.
- Nitrification: nitrifying bacteria change ammonium compounds into nitrates.
- Uptake: plants absorb nitrates through their roots and use them to make proteins. Animals get nitrogen by eating plants.
- Decay: decomposers break down dead matter and waste (urea) and release ammonium compounds.
- Denitrification: in waterlogged soil, denitrifying bacteria convert nitrates into nitrogen gas, returning it to the air.
Farmers rotate legumes with other crops so the soil gains nitrates, which means less fertiliser is needed.
Section 4
The water cycle
Water moves between the sea, land, air and living things:
- Evaporation: the Sun heats water in seas, lakes and soil, which changes into water vapour.
- Transpiration: plants take up water through their roots and lose water vapour through their leaves.
- Condensation: rising vapour cools and turns back into liquid droplets, forming clouds.
- Precipitation: water falls as rain, snow or hail.
The water then runs into rivers and seas or soaks into the ground, and the cycle starts again. Cutting down forests reduces transpiration, which can reduce rainfall.
Must know
- Carbon cycle: photosynthesis removes carbon dioxide; respiration, decomposition and combustion release it.
- Nitrogen cycle: fixation, nitrification, uptake, decay and denitrification.
- Water cycle: evaporation, transpiration, condensation and precipitation.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Nutrient cycles
- A pine forest in Finland takes in carbon dioxide from the air all summer. In autumn the needles and branches fall to the forest floor, where fungi and bacteria slowly break them down. Not far away, a power station burns coal to generate electricity.Explain how carbon in the carbon dioxide of the air can become part of a deer that feeds on the plants in the forest.2 marks
- A farmer in Kenya grows maize for several years in the same field, and the yields fall each year because the soil runs short of nitrates. A neighbour grows beans (a legume) in the field every third year. Bean roots have small swellings called nodules that contain bacteria.Explain why growing beans in the field every third year helps the maize crop that follows.2 marks
- A student investigates how temperature affects the speed at which decomposers break down dead leaves. She places 20 g of dried leaf litter and 100 g of moist soil in each of four sealed jars. She keeps the jars at 5 °C, 15 °C, 25 °C and 35 °C for four weeks, then measures the mass of leaf litter that remains. The percentage of the leaf mass lost was 2% at 5 °C, 9% at 15 °C, 21% at 25 °C and 12% at 35 °C.Identify the independent variable, the dependent variable and two control variables in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).