A-Level Biology: Energy Transfers and Nutrient Cycles

Energy flow and nutrient recycling close the ecosystems part of the A level biology course, and they are examined in two very different ways: short calculations built on data you have never seen before, and extended explanations of a cycle or of the chain of events that follows a fertiliser spill. This material trains both.

The energy half starts with how biomass is measured, as dry mass dried to constant mass or as chemical energy, and then works through the three relationships that turn up again and again: net primary production as gross primary production minus respiratory losses (NPP = GPP − R), net production of consumers as N = I − (F + R), and the percentage efficiency of transfer from one trophic level to the next. Every calculation uses original data in kJ m⁻² year⁻¹ and asks you to keep the units and the rounding under control. Alongside the numbers come the reasons transfer is inefficient (respiration, parts not eaten, energy lost in faeces and urine) and the ways farming raises the proportion of energy reaching people: simplifying food webs and reducing respiratory losses of livestock.

The nutrient half covers the nitrogen cycle process by process (ammonification by saprobionts, nitrification, nitrogen fixation by free-living bacteria and by Rhizobium in legume root nodules, denitrification in waterlogged soil), the phosphorus cycle with its rock-weathering source and no atmospheric phase, and the role of mycorrhizae in increasing the area for water and mineral uptake. It finishes with natural and artificial fertilisers, nitrate leaching and the full eutrophication sequence from algal bloom to oxygen depletion.

The material offers a multiple-choice quiz with explained answers, a flashcard deck of the key terms and equations, a printable written-work sheet of eight open questions (explanations and two worked calculations) that you answer by hand and have checked, and an oral exam in which an examiner asks one question at a time about ecosystem energetics and nutrient cycles and gives feedback at the end.

The content is based on the energy transfers and ecosystems content of the DfE A level biology subject content, as taught in current exam-board specifications (for example AQA topic 5, energy transfers in and between organisms). It is practice material and does not replace your own board's specification.

• Calculate NPP from GPP and respiratory losses, and consumer net production from N = I − (F + R)
• Calculate and interpret the percentage efficiency of energy transfer between trophic levels
• Explain why energy transfer is inefficient and how farming practices increase efficiency
• Describe the nitrogen cycle, naming each process and the microorganisms involved
• Describe the phosphorus cycle and the role of mycorrhizae
• Explain how fertilisers, leaching and eutrophication are linked

Practice material written by Zestly, based on the DfE A level biology subject content (energy transfers in and between organisms: ecosystems, nutrient cycles).