Cambridge IGCSE Biology 0610 · Topic 19
Organisms and Their Environment
Organisms don’t live in isolation. They obtain energy from their surroundings, feed on other organisms, recycle materials through ecosystems, and form populations whose sizes rise and fall over time. This chapter connects four ideas into one continuous story: energy flow, then food chains and food webs, then nutrient cycles, then populations — and Cambridge tests all four as one connected system, not four separate topics.
Cambridge divides Topic 19 into 19.1 Energy flow, 19.2 Food chains and food webs, 19.3 Nutrient cycles and 19.4 Populations. The thread running through all four is that biological systems need a constant input of energy, that energy is used and lost as it passes between organisms, and that this is why living things depend on feeding relationships, material recycling and population regulation to keep functioning at all. Once you can hold that whole chain in your head — energy in, energy passed on, materials recycled, populations responding — most questions on this chapter stop feeling like isolated recall and start feeling like the same underlying logic asked from a different angle.
Energy flow through living systems
Energy is needed for practically everything an organism does — movement, active transport, growth, cell division, and simply maintaining the body’s normal functions. The idea Cambridge specifically wants you to hold onto for this syllabus point is narrow but important: energy flows through an ecosystem, it does not cycle within it, in the way that carbon and nitrogen do.
The Sun is the principal source of energy input to biological systems. For almost every ecosystem, the sequence begins when producers such as green plants absorb light energy and, through photosynthesis, convert it into chemical energy stored in organic molecules. A simplified version of the pathway looks like this:
light energy from the Sun → chemical energy in producers → chemical energy in consumers → energy transferred to the environment
When one organism feeds on another, some of the chemical energy stored in that food passes to the consumer. From there, the energy can take several routes: it may remain stored temporarily in the organism’s tissues, be transferred to another organism when it is eaten, end up in dead material or waste, or be transferred to the environment — particularly as thermal energy released during respiration. Dead organisms and waste still contain chemical energy, which is exactly what decomposers obtain their own energy from; their respiration also transfers energy on to the environment. Energy therefore does not keep moving indefinitely between organisms. It enters biological systems mainly as light energy, and it eventually leaves them as energy transferred to the environment — usually as heat. That one-way movement is what separates energy flow from the recycling you’ll meet in the carbon cycle and nitrogen cycle, and it’s the idea underneath why less energy is available at every successive trophic level.
What’s in this chapter
| Page | Level | What it covers |
|---|---|---|
| Food chains and food webs | Core | Producers, consumers, decomposers, trophic levels, and how to trace what happens to a whole food web when one population changes. |
| Ecological pyramids and energy transfer | Core & Supplement | Pyramids of numbers, biomass and energy compared, and why so much energy disappears between one trophic level and the next. |
| The carbon cycle | Core | Photosynthesis, feeding, respiration, decomposition, fossil fuels and combustion, joined into one syllabus-exact pathway. |
| The nitrogen cycle | Supplement | Nitrogen fixation, nitrification, decomposition, deamination and denitrification — and what microorganisms actually do at each stage. |
| Population, community and ecosystem | Core | The exact difference between these three terms, which examiners test far more precisely than casual usage suggests. |
| Population growth and the sigmoid curve | Core & Supplement | Food supply, competition, predation and disease, and the lag, exponential, stationary and death phases they produce. |
Core or Supplement?
Food chains and food webs, the pyramids of numbers and biomass, the carbon cycle, population, community and ecosystem, and the factors affecting population growth together with the sigmoid growth curve itself are all Core, required whichever tier you’re sitting. Pyramids of energy, the reasons energy transfer between trophic levels is inefficient, why food chains are usually short, the crop-versus-livestock energy comparison, the entire nitrogen cycle, and the detailed explanation of why each phase of the sigmoid curve occurs are Supplement, required only for the Extended paper. If you’re not certain which tier applies to you, it’s worth checking before spending time on the Supplement-only sections below, since none of the Core content in this chapter depends on them.