OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.3.5 Nutrient Cycles Exam Questions
Free OxfordAQA International A Level Biology revision for 3.3.5 Nutrient Cycles: what you need to know, common mistakes and exam-style questions with answers. Covers decomposers and saprobionts, the carbon cycle, greenhouse gases and climate change, the nitrogen cycle, fertilisers, leaching and eutrophication.
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About OxfordAQA International A Level Biology 3.3.5 Nutrient Cycles
Nutrient cycles in Unit 3 explains how elements are recycled between living organisms and the environment. You need to describe the role of decomposers and saprophytic microorganisms, explain the carbon cycle including the effect of respiration, photosynthesis and human activity on carbon dioxide concentration, and link carbon dioxide and methane to the enhanced greenhouse effect and climate change. The specification also includes the effects of climate change on crop yields and the life cycles and numbers of insect pests. You also need to describe the roles of bacteria in the nitrogen cycle, the use of fertilisers, and the environmental problems of leaching and eutrophication.
What you need to know
- Decomposition Saprophytic microorganisms (saprobionts) secrete enzymes and absorb the products of digestion. This releases elements as simple inorganic ions and molecules that producers can take up again.
- Carbon cycle Photosynthesis removes carbon dioxide from the air; respiration and combustion return it. Carbon dioxide concentration fluctuates daily and seasonally because of photosynthesis and respiration, and has risen long term because of burning fossil fuels and deforestation.
- Greenhouse effect Carbon dioxide and methane absorb and re-emit infrared radiation, enhancing the greenhouse effect and leading to global warming. Climate change can alter crop yields and the life cycles, distribution and numbers of insect pests.
- Nitrogen cycle Nitrogen fixation by bacteria such as Rhizobium in root nodules converts nitrogen gas into ammonium compounds. Ammonification by saprobionts produces ammonia from organic nitrogen. Nitrification converts ammonium to nitrite and then nitrate. Denitrification in anaerobic conditions converts nitrate back to nitrogen gas.
- Fertilisers Harvesting crops and removing livestock remove nutrients, so natural fertilisers (manure) or artificial fertilisers are added to replace them and increase yield.
- Leaching and eutrophication Nitrates can be leached into water. Algae grow rapidly and block light, plants die, and saprobionts decomposing them use up oxygen in aerobic respiration, so fish and other aerobic organisms die.
Common mistakes in 3.3.5 Nutrient Cycles
- Saying decomposers break down dead material by eating it. Saprobionts secrete enzymes and absorb the soluble products by extracellular digestion.
- Mixing up nitrification and nitrogen fixation. Nitrogen fixation converts nitrogen gas to ammonium; nitrification converts ammonium to nitrate.
- Saying algae use up the oxygen in eutrophication. It is mainly the aerobic respiration of saprobionts decomposing dead organisms.
- Explaining the daily fall in carbon dioxide over a forest without photosynthesis. During the day photosynthesis is faster than respiration, so carbon dioxide concentration falls.
Try three 3.3.5 Nutrient Cycles exam-style questions
1. Describe the role of bacteria in converting ammonium ions in the soil into nitrate ions. (2 marks)
Show the mark scheme answer
Nitrifying bacteria oxidise ammonium ions to nitrite ions; then nitrite ions to nitrate ions.
2. Explain how fertiliser run-off can lead to the death of fish in a lake. (4 marks)
Show the mark scheme answer
Nitrates leach into the lake and algae grow rapidly; algal bloom blocks light so plants below die; saprobionts decompose dead plants and algae, respiring aerobically; oxygen concentration falls and fish die.
3. Explain why carbon dioxide concentration in the air above a forest is higher at night than during the day. (2 marks)
Show the mark scheme answer
At night there is no photosynthesis; but respiration continues and releases carbon dioxide.
3.3.5 Nutrient Cycles FAQs
What does OxfordAQA International A Level Biology 3.3.5 Nutrient Cycles cover?
It covers decomposers and saprobionts, the carbon cycle, greenhouse gases and climate change, the nitrogen cycle, fertilisers, leaching and eutrophication.
Which unit is 3.3.5 Nutrient Cycles examined in?
It is part of Unit 3, Populations and genes, which is an International A2 paper. The Unit 5 synoptic paper can also draw on this content.
Why are denitrifying bacteria a problem for farmers?
They convert nitrate in the soil into nitrogen gas, reducing the nitrate available for crops. They thrive in waterlogged, anaerobic soil, so ploughing and drainage reduce their activity.
