Organisms and Their Environment
This topic is about where energy and nutrients go. Energy flows through an ecosystem and is lost at every step; nutrients such as carbon and nitrogen are recycled round and round. Keeping those two ideas separate makes most of the topic straightforward.
1. Energy flow
The Sun is the principal source of energy input to biological systems.
Energy enters as light, is captured by producers in photosynthesis, and is then passed along food chains as chemical energy in the molecules of food.
Watch the form of energy. Light enters the food chain; chemical energy is transferred between organisms; heat is what is lost. A question asking which row correctly describes energy flow is testing exactly this distinction.
2. Food chains and food webs
A food chain shows the transfer of energy from one organism to the next, beginning with a producer.
A food web is a network of interconnected food chains.
The arrows show the direction of energy flow — they point from the organism being eaten to the organism eating it. Drawing them the wrong way round is a straightforward loss of marks.
The vocabulary
| Term | Definition |
|---|---|
| Producer | An organism that makes its own organic nutrients, usually using light energy in photosynthesis |
| Consumer | An organism that gets its energy by feeding on other organisms |
| Herbivore | An animal that gets its energy by eating plants |
| Carnivore | An animal that gets its energy by eating other animals |
| Decomposer | An organism that gets its energy from dead or waste organic material |
Trophic levels
A trophic level is the position of an organism in a food chain.
| Trophic level | Name |
|---|---|
| 1st | Producer |
| 2nd | Primary consumer (herbivore) |
| 3rd | Secondary consumer |
| 4th | Tertiary consumer |
| 5th | Quaternary consumer |
Producers are always at the first trophic level, in every food chain in a web.
In a food web, one organism can occupy more than one trophic level. If a bird eats both insects (making it a secondary consumer) and fish that ate insects (making it a tertiary consumer), it belongs to both. Questions ask for this directly — check every route into the organism, not just one.
Why food chains are short
Energy is lost at every transfer, through:
- Respiration — energy released as heat
- Excretion — energy in urine and faeces
- Uneaten parts — bones, roots, shells
Typically only about 10% of the energy at one level is passed to the next. After four or five levels there is too little energy left to support another, which is why food chains rarely have more than five trophic levels.
This is also why it is more efficient to feed people on crops than on meat. Eating plants directly makes them primary consumers, so only one energy transfer has occurred. Eating animals that ate plants adds another transfer and another large loss.
3. Pyramids
| Pyramid | Shows |
|---|---|
| Pyramid of numbers | The number of organisms at each trophic level |
| Pyramid of biomass | The mass of living material at each trophic level |
| Pyramid of energy | The energy at each trophic level |
Pyramids of numbers can be odd shapes. One large oak tree supports thousands of insects, so the producer bar is narrow and the primary consumer bar is wide — an inverted step. Parasites give the same effect: many fleas on one dog.
Pyramids of biomass are almost always pyramid-shaped, because biomass decreases at each level.
A pyramid of energy is always pyramid-shaped, with no exceptions, because energy is always lost between levels. If a question asks which pyramid must be a true pyramid, the answer is energy.
4. The carbon cycle
The processes that move carbon:
| Process | What it does |
|---|---|
| Photosynthesis | Removes carbon dioxide from the atmosphere |
| Respiration | Returns carbon dioxide to the atmosphere |
| Feeding | Transfers carbon compounds along the food chain |
| Decomposition | Decomposers respire, returning carbon dioxide |
| Combustion | Burning fossil fuels and wood releases carbon dioxide |
| Fossilisation | Long-term storage of carbon as fossil fuels |
Photosynthesis is the only process that removes carbon dioxide from the air. Everything else in the cycle puts it back. That is why increasing combustion and reducing forest cover both raise atmospheric carbon dioxide.
5. The nitrogen cycle
Plants and animals cannot use nitrogen gas directly — it is very unreactive. It must be converted into usable compounds first.
| Process | Change | Carried out by |
|---|---|---|
| Nitrogen fixation | Nitrogen gas → nitrogen compounds | Nitrogen-fixing bacteria (in root nodules and soil), and lightning |
| Decomposition | Dead organisms and waste → ammonium compounds | Decomposers |
| Nitrification | Ammonium → nitrate | Nitrifying bacteria |
| Denitrification | Nitrate → nitrogen gas | Denitrifying bacteria |
| Absorption | Nitrate taken up by plant roots (by active transport) | Plants |
Plants use nitrate to make amino acids, and from those, proteins. Animals obtain nitrogen compounds by eating plants or other animals.
How to keep the names straight:
- Nitrogen fixation — fixing nitrogen gas into a usable form
- Nitrification — making nitrate
- Denitrification — the de- prefix means undoing it: nitrate back to nitrogen gas
Watch the direction of the arrow in a diagram before naming the process. Nitrogen gas → nitrate is fixation; nitrate → nitrogen gas is denitrification. They are opposites and are constantly swapped.
A scope check worth doing: the nitrogen cycle is not on every version of this syllabus, and has been trimmed in some recent ones. The carbon cycle is core. Check your own syllabus document before spending revision time on the nitrogen cycle in detail.
6. Population growth
A population is a group of organisms of one species, living in the same area at the same time.
A population growth curve has four phases:
| Phase | What is happening | Why |
|---|---|---|
| Lag | Slow growth | Few individuals; they are adapting to conditions and reproducing slowly |
| Exponential (log) | Rapid growth | Plentiful resources, few limiting factors; birth rate far exceeds death rate |
| Stationary | Population steady | Birth rate equals death rate; limited by food, space, competition, predation or waste build-up |
| Death | Population falls | Resources exhausted, or waste products have accumulated to toxic levels |
Factors affecting the rate of population growth: food supply, competition, predation, and disease.
The stationary phase is not “no births and no deaths.” Births and deaths are both happening — they are simply equal, so the population size does not change. This mirrors the “no net movement” point in diffusion, and is marked the same way.
7. Mistakes that cost marks
Drawing food chain arrows the wrong way. They point in the direction of energy flow — towards the eater.
Saying energy is “lost” without saying how. Respiration as heat, excretion, uneaten parts.
Saying energy is recycled. Energy flows through and is lost; nutrients are recycled.
Giving only one trophic level for an organism in a food web that occupies several.
Saying a pyramid of numbers is always pyramid-shaped. Only the pyramid of energy always is.
Confusing nitrogen fixation with nitrification.
Saying the stationary phase means no births or deaths. They are equal.
Saying plants absorb nitrogen gas. They absorb nitrate ions, by active transport.
Frequently asked questions
What is a trophic level? The position of an organism in a food chain. Producers are always the first.
Why are food chains usually less than five levels long? Energy is lost at every transfer — mainly as heat from respiration — so there is eventually too little to support another level.
Why is it more efficient to eat plants than meat? Fewer energy transfers, so less energy is lost before it reaches humans.
Which pyramid is always a true pyramid shape? The pyramid of energy.
Why can a pyramid of numbers be inverted? Because it counts organisms rather than mass — one tree can support thousands of insects.
What is the only process that removes carbon dioxide from the atmosphere? Photosynthesis.
Why can’t plants use nitrogen gas from the air? It is too unreactive. It must first be converted into nitrogen compounds by nitrogen fixation.
What happens during the stationary phase? Birth rate equals death rate, so population size stays constant.
Quick revision checklist
- I know the Sun is the principal energy source, and can distinguish light, chemical and heat energy
- I can define producer, consumer, herbivore, carnivore and decomposer
- I draw food chain arrows in the direction of energy flow
- I can assign trophic levels, including where an organism occupies more than one
- I can explain why energy is lost between levels, and name the routes
- I can explain why food chains are short and why eating plants is more efficient
- I know the three pyramid types and which is always a true pyramid
- I can explain why a pyramid of numbers may be inverted
- I can name every process in the carbon cycle and its direction
- I know photosynthesis is the only process removing carbon dioxide
- I can name the four nitrogen cycle processes and tell fixation from nitrification
- I can name and explain all four phases of a population growth curve
- I know the stationary phase means births equal deaths
These notes cover topic 19 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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