Chapter B3: Living together – food and ecosystems — OCR GCSE Combined Science

Chapter B3: Living together – food and ecosystems explores the complex interactions between organisms and their environment. You'll learn how energy flows through ecosystems via food chains and webs, how nutrients are cycled (particularly carbon and nitrogen), and how human activities can disrupt these delicate balances. This topic is central to understanding biodiversity, conservation, and sustainability — key themes in modern biology.

Why does this matter? Ecosystems provide essential services like pollination, water purification, and climate regulation. By studying food and ecosystems, you'll appreciate how human actions — from deforestation to fertiliser use — can have far-reaching consequences. This knowledge is crucial for making informed decisions about environmental issues and for understanding the interdependence of life on Earth.

In the wider OCR GCSE Combined Science course, B3 builds on cell biology and photosynthesis (B1) and respiration (B2). It connects to later topics like genetics and evolution, as ecosystems drive natural selection. Mastering B3 will also help you with practical skills, such as sampling techniques and interpreting data from ecological studies.

• →Food chains and webs: Understand how energy is transferred from producers (plants) through consumers (herbivores, carnivores, omnivores) to decomposers. Know that only about 10% of energy passes to the next trophic level — the rest is lost as heat, waste, or used for respiration.
• →Carbon cycle: Describe how carbon moves between the atmosphere (as CO₂), organisms (through photosynthesis, respiration, feeding), and long-term stores (fossil fuels, limestone). Be able to explain the impact of burning fossil fuels and deforestation on atmospheric CO₂ levels.
• →Nitrogen cycle: Know the roles of bacteria in nitrogen fixation (converting N₂ to ammonia), nitrification (ammonia to nitrates), and denitrification (nitrates to N₂). Understand why plants need nitrates for protein synthesis and how fertilisers can cause eutrophication.
• →Biodiversity and stability: Explain how high biodiversity makes ecosystems more stable (resistant to change) and resilient (able to recover). Recognise factors that reduce biodiversity, such as habitat destruction, pollution, and climate change.
• →Human impact: Evaluate the effects of agriculture (monoculture, pesticides, fertilisers), deforestation, and global warming on food webs and ecosystem sustainability. Understand the concept of bioaccumulation (e.g., DDT in food chains).

• Photosynthesis as an endothermic process requiring light and chlorophyll.
• The two stages of photosynthesis: splitting water and combining carbon dioxide with hydrogen.
• Enzyme action explained by the lock and key model.
• Factors affecting enzyme activity: substrate concentration, temperature, and pH.
• Inverse square law relating light intensity to distance from a light source.
• Mechanisms of substance transport: diffusion, osmosis, and active transport.
• Role of xylem and phloem in plant transport.
• Interdependence, competition, and the cycling of carbon and water in ecosystems.
• Impact of abiotic and biotic factors on population distribution and abundance.

• 💡Ensure you can interpret graphs showing the effect of limiting factors on photosynthesis.
• 💡Be prepared to perform calculations involving rates of reaction and percentage mass.
• 💡Use the lock and key model to explain enzyme specificity and denaturation.
• 💡Practice drawing and interpreting food webs to explain interdependence.
• 💡Understand how to use a light microscope to observe plant cell structures like stomata.
• 💡When drawing food webs, always start with the producer at the bottom and use arrows to show the direction of energy flow (from prey to predator). Don't forget to include decomposers in written answers, even if not in the diagram.
• 💡For questions on eutrophication, use the acronym 'ALGAE' to remember the sequence: Algae bloom (due to excess nitrates/phosphate) → Light blocked → plants die → decomposers increase → oxygen depleted → fish die. This step-by-step approach gains full marks.
• 💡In data analysis questions, always quote specific numbers from graphs or tables to support your points. For example, 'The population of rabbits decreased by 40% after the introduction of foxes, showing a predator-prey relationship.'

• Confusing the reactants and products of photosynthesis.
• Misunderstanding the role of enzymes as catalysts rather than reactants.
• Incorrectly applying the inverse square law for light intensity.
• Confusing the mechanisms of diffusion, osmosis, and active transport.
• Failing to link environmental factors to limiting rates of photosynthesis.
• Misconception: 'Decomposers are at the top of the food chain.' Correction: Decomposers (bacteria and fungi) break down dead matter at all trophic levels, but they are not part of the linear food chain. They recycle nutrients, not energy.
• Misconception: 'Energy is recycled in ecosystems.' Correction: Energy flows one way through ecosystems — it is not recycled. Only matter (e.g., carbon, nitrogen) is recycled. Energy is lost as heat at each trophic level and must be replenished by the sun.
• Misconception: 'All bacteria are harmful.' Correction: Many bacteria are essential for nutrient cycling. For example, Rhizobium fixes nitrogen in plant roots, and Nitrosomonas converts ammonia to nitrites. Without them, plants couldn't absorb nitrogen.