This free quiz has 11 real AQA A-level Biology exam questions on nutrient cycles (28 marks, about 35 minutes). It marks itself: the calculation question is marked for you, and for written answers you check your work against the real AQA mark scheme.
What the quiz covers (AQA 3.5.4)
how extra phosphate uptake can increase plant growth
how mycorrhizal fungi benefit from their association with plants
nitrogen-containing biological molecules removed when crops are harvested
the roles of saprobionts and nitrifying bacteria in recycling nitrogen (4 marks)
nitrification and denitrification in waterlogged, anaerobic soils
a calculation on the phosphorus cycle
abiotic factors that speed up nutrient recycling, and why nutrients entering a lake reduce fish populations (4 marks)
evaluating data on nitrogenase activity, and on mycorrhizae and crop yield (5 marks)
How to use it
Write each written answer in full before you open the mark scheme, then tick only the points you really made. Name specific compounds and organisms, such as proteins, ATP, saprobionts or nitrifying bacteria, rather than writing “nutrients” or “decomposers”. For the calculation, show your working on paper.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Biological molecules, not ions. Many students gave inorganic ions such as nitrates or ammonium when asked for nitrogen-containing biological molecules, and vague answers about “nutrients for growth” weren't credited. A common error was to name proteins as a use of phosphate.
✔ Gains credit: a named biological molecule, e.g. amino acids, proteins, DNA, RNA, ATP, NAD and chlorophyll contain nitrogen, while ATP, DNA, RNA and phospholipids contain phosphate.
What saprobionts do. Very few students named a nitrogen-containing compound, some confused ammonification with nitrogen fixation, and some described the whole nitrogen cycle. “Nitrogen-containing compounds” on its own was ignored.
✔ Gains credit: saprobionts use enzymes to decompose a named nitrogen-containing compound, such as protein, DNA or urea, releasing ammonia or ammonium ions (ammonification).
Nitrification order. A significant number of students said ammonia is converted into nitrates and then nitrites, described nitrification as reduction, or confused nitrifying bacteria with nitrogen-fixing bacteria.
✔ Gains credit: nitrifying bacteria oxidise ammonium to nitrite and then to nitrate.
Oxygen and denitrification. Students tried to bend incorrect knowledge of the nitrogen cycle to fit, e.g. saying saprobionts are anaerobic or that nitrates are denitrified into ammonia.
✔ Gains credit: nitrification needs oxygen, while denitrifying bacteria work in anaerobic conditions and reduce nitrate to nitrogen gas.
Abiotic factors in recycling. Fewer than 10% of students gained both marks. Many gave a biotic factor, linked temperature to faster “decomposition”, or linked water to photosynthesis, weathering or denitrification.
✔ Gains credit: each abiotic factor linked to a process, e.g. a high temperature or optimum pH for enzymes or nitrification, water for hydrolysis, and oxygen for respiration by saprobionts.
Eutrophication and oxygen. Weaker answers left out photosynthesis or respiration, referred only to “bacteria” or “decomposers”, or said that the nutrients or decomposition products were toxic.
✔ Gains credit: an algal bloom blocks light, so submerged plants can't photosynthesise and die; saprobionts use oxygen in aerobic respiration, leaving less oxygen for fish to respire.
What mycorrhizae gain. Incorrect answers about how the fungi benefit from plants often mentioned nutrients, water or oxygen. “Products of photosynthesis” and “starch” were ignored.
✔ Gains credit: the fungus gets carbon-containing organic compounds from the plant, such as sugars, amino acids or vitamins.
Keep practising
The other AQA 3.5 self-marking quizzes: Photosynthesis · Respiration · Energy and ecosystems
Next, topic 3.6: the stimuli and receptors quiz
The AQA topic index
weekly AQA group classes: Year 12 on Tuesdays at 5.15pm, Year 13 on Wednesdays or Thursdays at 6.30pm
Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.