Understanding Organisms and their environment: IGCSE Biology Explained

Every ecosystem runs on a single rule: energy flows in one direction, and nutrients cycle back

That principle sounds simple, but it underpins nearly every question you will face on the Organisms and their environment section of IGCSE Biology. If you understand how energy enters a system, how it passes between organisms, and why most of it is lost at each step, you can reason through unfamiliar exam scenarios rather than relying on memorised answers.

This topic covers four interconnected areas: energy flow, food chains and food webs, nutrient cycles, and populations. Each builds on the last. Energy flow explains why food chains exist. Food chains and webs explain how communities are structured. Nutrient cycles explain how matter is recycled even though energy is not. And population dynamics explain what happens when any of these processes are disrupted.

Energy flow: from sunlight to heat

The Sun is the principal source of energy input to biological systems. Green plants and algae absorb light energy and convert it into chemical energy through photosynthesis. That chemical energy is stored in organic molecules, primarily glucose, and becomes available to every other organism in the ecosystem through feeding.

The sequence is always the same: light energy is absorbed by producers, converted to chemical energy, transferred to consumers when they eat, and eventually released as heat energy through respiration. Heat cannot be recaptured by living organisms, so energy flow through an ecosystem is linear, not cyclical.

Why energy is lost at each trophic level

Not all the energy an organism takes in is available to the next organism that eats it. There are three main reasons for this loss:

• Respiration - all organisms use a proportion of their energy intake for cellular respiration. This energy is released as heat and is no longer available to the food chain.
• Excretion and egestion - energy is lost in urine, faeces, and other waste products. These materials contain chemical energy that is not transferred to the consumer.
• Parts not eaten - bones, roots, bark, and other structures are often not consumed, so the energy they contain does not pass to the next trophic level.

As a rough guide, only about 10% of the energy at one trophic level is transferred to the next. This is why food chains rarely exceed four or five links: there simply is not enough energy left to sustain another level of consumers.

Food chains and food webs

A food chain is a linear sequence showing the transfer of energy from one organism to the next, always beginning with a producer. Each organism in the chain occupies a trophic level.

A food web is a network of interconnected food chains within the same ecosystem. It gives a more realistic picture of feeding relationships because most organisms eat (and are eaten by) more than one species.

Constructing and interpreting food chains

When constructing a food chain from data, follow this method:

1. Identify the producer - the organism that photosynthesises.
2. Identify what eats the producer - this is the primary consumer.
3. Continue the chain by asking "what eats this organism?" at each level.
4. Draw arrows in the direction of energy flow (from prey to predator, not the other way round).

Pyramids of numbers, biomass, and energy

Ecological pyramids are bar diagrams that represent the relative amounts at each trophic level. Each type of pyramid reveals something different.

An inverted pyramid of numbers is not an error. It occurs when the producer is very large (a single tree) or when parasites are involved (many fleas on one dog). Pyramids of energy, however, can never be inverted because the second law of thermodynamics guarantees that energy is lost at each transfer.

Nutrient cycles

Unlike energy, matter is recycled within ecosystems. The carbon cycle and the water cycle are the two nutrient cycles you need to know for IGCSE Biology.

The carbon cycle

Carbon moves between four reservoirs: the atmosphere (as CO2), living organisms (as organic compounds), fossil fuels (as coal, oil, and gas), and the oceans (as dissolved CO2 and carbonates). The key processes that move carbon between these reservoirs are:

• Photosynthesis - removes CO2 from the atmosphere and incorporates carbon into glucose in producers.
• Respiration - releases CO2 back into the atmosphere from all living organisms.
• Combustion - burning fossil fuels or biomass releases stored carbon as CO2.
• Decomposition - bacteria and fungi break down dead organisms, releasing CO2 through their own respiration.
• Fossilisation - over millions of years, dead organisms can become fossil fuels, locking carbon away from the cycle.

The cycle is balanced when the rate of carbon fixation (by photosynthesis) roughly equals the rate of carbon release (by respiration, combustion, and decomposition). Human activities, particularly the burning of fossil fuels, have increased the rate of CO2 release beyond what photosynthesis can absorb, contributing to rising atmospheric CO2 levels.

The water cycle

Water cycles through evaporation from bodies of water and soil, transpiration from plants, condensation into clouds, and precipitation back to the surface. In biological terms, the key processes are:

• Transpiration - water vapour lost from leaf stomata accounts for a significant proportion of water returned to the atmosphere from land ecosystems.
• Absorption - plant roots absorb water from the soil by osmosis.
• Excretion - animals lose water in urine, sweat, and exhaled air.

Populations

A population is a group of organisms of the same species living in the same area at the same time. Population size is determined by four factors: birth rate, death rate, immigration, and emigration.

Factors affecting population size

These factors fall into two categories:

• Abiotic factors - non-living factors such as temperature, light intensity, water availability, and mineral ion concentration. These set the upper limit of what an environment can support.
• Biotic factors - living factors such as food supply, predation, disease, and competition. These cause population sizes to fluctuate around the carrying capacity.

Competition

Organisms compete for resources that are in limited supply. Competition can be intraspecific (within the same species) or interspecific (between different species). Intraspecific competition is typically more intense because individuals of the same species require identical resources.

Plants compete for light, water, minerals, and space. Animals compete for food, water, territory, and mates. The outcome of competition determines which organisms survive and reproduce, driving natural selection.

Predator-prey relationships

Predator and prey populations are linked in a cyclical pattern. When prey numbers increase, predator numbers follow because food is abundant. As predator numbers rise, they consume more prey, causing the prey population to decline. With less food available, predator numbers then fall, allowing the prey population to recover. The cycle repeats.

On a graph, this produces two oscillating curves. The predator curve follows the prey curve with a time lag, typically offset by one or two cycles. Recognising this pattern and explaining the cause-effect logic behind each phase is a frequent exam question.

Common exam mistakes

1. Drawing food chain arrows in the wrong direction. Arrows show the direction of energy transfer (prey to predator), not "who eats whom."
2. Confusing energy flow with nutrient cycling. Energy flows through an ecosystem and is lost as heat. Nutrients are recycled. These are fundamentally different processes.
3. Saying "energy is recycled." Energy is never recycled in an ecosystem. It is transferred and eventually lost as heat through respiration.
4. Assuming pyramids of numbers are always upright. They can be inverted when the producer is large (a tree) or when parasites are involved.
5. Describing predator-prey graphs without cause-effect reasoning. Examiners want to know WHY populations change, not just WHAT happens. Use phrases like "because," "this leads to," and "as a result."
6. Forgetting decomposers in the carbon cycle. Decomposition is a major route for returning carbon to the atmosphere. Leaving decomposers out of a carbon cycle diagram will lose marks.
7. Using vague language for competition. Always specify WHAT organisms are competing for (light, water, food, territory), not just that "they compete."

Self-check questions

1. A grassland food chain runs: grass, grasshopper, frog, snake, hawk. If a disease killed most of the frogs, predict and explain what would happen to the grasshopper and snake populations.
2. Explain why a pyramid of energy can never be inverted, but a pyramid of numbers can.
3. Name three processes that return carbon dioxide to the atmosphere and one process that removes it.
4. A predator-prey graph shows the prey population peaking in March and the predator population peaking in June. Explain the three-month time lag.
5. Two species of bird live in the same woodland and eat the same type of seed. Explain why this interspecific competition might cause one species to decline.