Ch 3 · Nutrient Cycles · IGCSE Biology 0610
Topic 3 of 4 in Organisms and their Environment — IGCSE Biology 0610 — 38 lessons.
Introduction
Energy flows through an ecosystem once and is gone, but the atoms that make up living things are used again and again; nutrient cycles describe their journeys. The carbon cycle turns on a single balance: photosynthesis removes carbon dioxide, everything else returns it. The nitrogen cycle (Extended) rewards candidates who can name each conversion and the bacteria responsible.
Key ideas — The carbon cycle
Carbon moves between carbon dioxide in the air and carbon compounds in organisms and fossil fuels. Six processes are needed.
🧑🏫 Teacher's explanation
Think of the carbon cycle like a global bank account. Carbon is the 'currency' that is constantly being moved around. Some organisms 'deposit' carbon into the atmosphere by breathing out, while others 'withdraw' it from the air to build their bodies. If the rate of withdrawals (photosynthesis) matches the rate of deposits (respiration, combustion, etc.), the atmospheric balance stays steady.
📖 Definitions
🔍 Worked example — Easy
A plant uses 6 molecules of CO_2 during photosynthesis to produce one molecule of glucose. If a plant consumes 30 molecules of CO_2, how many molecules of glucose are produced?
🔍 Worked example — Standard
Describe the fate of carbon in a dead organism that is buried deep underground in an anaerobic (oxygen-free) environment.
🔍 Worked example — Exam-style
Explain why the combustion of fossil fuels leads to an increase in atmospheric CO_2 levels.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The nitrogen cycle
🧑🏫 Teacher's explanation
Think of nitrogen like a locked treasure chest. The atmosphere is full of nitrogen gas (N_2), but it is in a 'locked' form that plants and animals cannot use. To get the 'treasure' (nitrogen) into their bodies to build proteins and DNA, they need specialized 'locksmiths'—microorganisms—to convert it into usable forms like nitrate ions.
📖 Definitions
🔍 Worked example — Easy
A farmer observes that his crops are turning yellow, a sign of nitrogen deficiency. He decides to add ammonium ions to the soil. Describe the role of nitrifying bacteria in this process.
🔍 Worked example — Standard
Calculate the percentage increase in soil nitrate concentration if it rises from 12 mg/kg to 30 mg/kg due to nitrification.
🔍 Worked example — Exam-style
Explain why waterlogged soil leads to a loss of nitrogen from the soil to the atmosphere.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why waterlogged soil leads to a decrease in the concentration of nitrates in the soil. [3] (b) Suggest one way the farmer could prevent this loss of nitrates. [1] [4 marks]
Key ideas — The roles of microorganisms
🧑🏫 Teacher's explanation
Think of the nitrogen cycle as a giant recycling program for a city. Most of the nitrogen is floating around in the air as gas, but plants can't use it directly—it's like having a mountain of plastic that hasn't been processed into bottles yet. Microorganisms act as the recycling plants, converting that 'useless' gas or waste into forms that plants can actually build with.
📖 Definitions
🔍 Worked example — Easy
A student observes that after a plant dies, the nitrogen within its protein structures is converted into ammonium ions. Name the group of microorganisms responsible for this process.
🔍 Worked example — Standard
Compare the roles of nitrifying bacteria and denitrifying bacteria in the nitrogen cycle.
🔍 Worked example — Exam-style
A farmer notices that his soil has become waterlogged due to heavy rain. Explain why this condition might lead to a decrease in soil fertility regarding the nitrogen cycle.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the role of bacteria found in the root nodules of legumes. [1] (b) Explain how this practice increases the amount of nitrogen available to the cereal crops in the following season. [3] [4 marks]
Worked example
Question. (Supplement) Dead leaves fall onto the soil in a forest. Describe the roles of microorganisms in making the nitrogen in the protein of these leaves available to living trees. [4]
🔍 Worked example — Easy
Identify the correct sequence of ions produced during the nitrogen cycle from the breakdown of organic matter to the form absorbed by plants.
🔍 Worked example — Standard
A soil sample contains a high concentration of ammonium ions but very low concentrations of nitrates. Explain why this might occur in terms of microbial activity.
🔍 Worked example — Exam-style
Compare the role of decomposers and denitrifying bacteria in the nitrogen cycle.
⚠️ Common errors
Common mistakes
Practice
Answers