Cambridge IGCSE Biology 0610: Organisms and their environment – Study Guide

This study guide teaches topic 19, Organisms and their environment, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. It covers syllabus sections 19.1 to 19.4: energy flow, food chains and food webs, nutrient cycles and populations. Everything here is Core unless it is labelled Extended only (Supplement content). Core candidates are examined on Papers 1 and 3; Extended candidates sit Papers 2 and 4, which test Core and Supplement content together.

Useful links: the Cambridge IGCSE Biology hub, the printable 0610 checklist, the condensed revision notes for this topic and the practice set with worked answers. To find your weak spots first, try the Core diagnostic or the Extended diagnostic.

What this topic covers

Section	What you must be able to do	Tier
19.1 Energy flow	State that the Sun is the principal energy source; describe energy flow from light to chemical energy and its transfer to the environment	Core
19.2 Food chains and food webs	Construct and interpret food chains and webs; define producer, consumer, herbivore, carnivore, decomposer, trophic level; draw and interpret pyramids of numbers and biomass; human impact through overharvesting and introduced species	Core
19.2 (continued)	Pyramids of energy; why energy transfer is inefficient; why chains have fewer than five trophic levels; why eating crops is more energy efficient than eating livestock	Extended only
19.3 Nutrient cycles	Describe the carbon cycle	Core
19.3 (continued)	Describe the nitrogen cycle and the roles of microorganisms in it	Extended only
19.4 Populations	Define population, community, ecosystem; factors affecting population growth; phases of the sigmoid curve; interpret population graphs	Core
19.4 (continued)	Explain the factors behind each phase of the sigmoid curve, using limiting factors	Extended only

19.1 Energy flow

The Sun is the principal source of energy input to biological systems. Energy flows through an ecosystem in one direction:

1. Light energy from the Sun is absorbed by chlorophyll in producers.
2. Photosynthesis transfers it into chemical energy in organic compounds such as glucose, starch and proteins.
3. When an animal eats the plant, some of that chemical energy passes to the animal. The same happens at each feeding step.
4. At every stage, energy is also transferred to the environment, mainly as heat released by respiration. Energy in dead material and waste passes to decomposers, which also release it as heat.

Energy is not recycled. It enters as light and eventually leaves as heat. This is the key contrast with nutrients such as carbon and nitrogen, which are recycled (section 19.3).

19.2 Food chains and food webs

Definitions you must learn

• Producer: an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
• Consumer: an organism that gets its energy by feeding on other organisms. Consumers are classed as primary, secondary, tertiary and quaternary by their position in the chain.
• 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 level: the position of an organism in a food chain, food web or ecological pyramid.

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. The arrows point in the direction energy flows, from the organism eaten to the organism that eats it:

grass → grasshopper → shrew → owl
producer → primary consumer → secondary consumer → tertiary consumer

A food web is a network of interconnected food chains. In a web, one organism can occupy more than one trophic level. An owl that eats both grasshoppers and shrews is a secondary consumer in one chain and a tertiary consumer in another.

Interpreting a food web

When one population changes, follow every arrow into and out of that organism. Suppose a web contains: grass → rabbits → foxes, and grass → voles → foxes, and voles → owls.

If disease kills most of the rabbits:

• foxes eat more voles, so the vole population falls;
• owls then have fewer voles, so the owl population may fall;
• grass may increase, because fewer rabbits eat it.

Always give the direction of change and the reason.

Human impact on food chains and webs

• Overharvesting a food species (for example, catching too many of one fish) removes a food source for its predators, whose populations may fall. Its own prey may increase because they are eaten less.
• Introducing a foreign species to a habitat can disrupt a web. The newcomer may have no natural predators, may eat native species, or may compete with them for food. Native populations can fall sharply.

Pyramids of numbers and biomass

A pyramid of numbers shows the number of organisms at each trophic level. A pyramid of biomass shows the dry mass of living material at each level, usually per unit area.

Worked example. A single oak tree supports 2 500 caterpillars, which are eaten by 30 blue tits.

• Pyramid of numbers: the bottom bar (1 tree) is very narrow, the caterpillar bar is very wide, the bird bar is narrow. The shape is not a true pyramid.
• Pyramid of biomass: the tree’s mass far exceeds that of the caterpillars, which exceeds that of the birds. Each bar is narrower than the one below, so it is pyramid-shaped.

Advantages of a pyramid of biomass over a pyramid of numbers: it takes account of the size of organisms, so one large tree is not shown as smaller than thousands of insects; it is almost always pyramid-shaped, so it gives a truer picture of how much living material each level supports.

Pyramids of energy (Extended only)

A pyramid of energy shows the energy transferred to each trophic level, usually per unit area per year (for example kJ m⁻² yr⁻¹). It is always pyramid-shaped, because energy is lost at each transfer.

Advantages over pyramids of numbers and biomass: it shows the energy actually transferred over a period of time, not a snapshot at one moment; it is never inverted; and it allows the efficiency of each transfer to be compared.

Why energy transfer is inefficient (Extended only)

Only a small fraction of the energy at one trophic level reaches the next, because:

• much energy is released as heat in respiration and lost to the environment;
• some material is not eaten (roots, bones, shells);
• some eaten material is not digested and is lost in faeces;
• some energy is lost in excretory products such as urea;
• energy is used for movement and other life processes, and is eventually lost as heat.

Worked example. Grass transfers 21 000 kJ m⁻² yr⁻¹ into new biomass. Rabbits gain 1 470 kJ m⁻² yr⁻¹. Foxes gain 126 kJ m⁻² yr⁻¹. Calculate the efficiency of each transfer.

efficiency = (energy at higher level ÷ energy at lower level) × 100

grass → rabbits: (1470 ÷ 21 000) × 100 = 7.0 %
rabbits → foxes: (126 ÷ 1470) × 100 = 8.57… = 8.6 %

Why food chains are short (Extended only)

Because so much energy is lost at each step, the energy reaching each level falls rapidly. By the fourth or fifth level there is too little energy left to support a population large enough to survive. That is why food chains usually have fewer than five trophic levels.

Eating crops versus eating livestock (Extended only)

If humans eat crop plants, they are primary consumers and receive energy after one transfer. If the crop is fed to livestock and humans eat the livestock, there are two transfers, and energy is lost as heat, in faeces and in movement by the animals. A given area of crops therefore feeds more people if the crop is eaten directly.

19.3 Nutrient cycles

The carbon cycle (Core)

The syllabus limits the carbon cycle to six processes:

Process	What happens to carbon
Photosynthesis	Carbon dioxide from the air becomes carbohydrate in producers
Feeding	Carbon compounds pass from plants to animals, and from animal to animal
Respiration	Plants, animals and decomposers release carbon dioxide to the air
Decomposition	Decomposers break down dead organisms and waste, releasing carbon dioxide through respiration
Formation of fossil fuels	Where dead organisms do not fully decompose, over millions of years they form coal, oil and gas
Combustion	Burning fossil fuels and wood releases carbon dioxide to the air

In a description, name the process and say which way the carbon moves.

The nitrogen cycle (Extended only)

Nitrogen gas makes up most of the air, but plants cannot use it directly. They absorb nitrate ions from the soil. The syllabus lists these stages:

1. Decomposition: decomposers break down protein in dead plants and animals (and in waste) to ammonium ions.
2. Nitrification: nitrifying bacteria convert ammonium ions to nitrate ions.
3. Nitrogen fixation: nitrogen gas is converted to nitrogen compounds by lightning and by nitrogen-fixing bacteria (some free in soil, some in root nodules of plants such as peas and beans).
4. Absorption: plant roots absorb nitrate ions.
5. Production of amino acids and proteins: plants use nitrate to make amino acids, then proteins.
6. Feeding and digestion: animals eat plants, digest the protein to amino acids and build their own proteins.
7. Deamination: excess amino acids are deaminated in the liver, forming urea, which is excreted and returns to the soil.
8. Denitrification: denitrifying bacteria convert nitrate ions back to nitrogen gas.

Roles of microorganisms (you do not need names such as Rhizobium): decomposition, nitrification, nitrogen fixation and denitrification. Lightning is the only listed fixation route that is not biological.

19.4 Populations

Definitions

• Population: a group of organisms of one species, living in the same area, at the same time.
• Community: all of the populations of different species in an ecosystem.
• Ecosystem: a unit containing the community of organisms and their environment, interacting together.

Factors affecting the rate of population growth

The syllabus limits these to food supply, competition, predation and disease. More food increases the growth rate; more competition, more predators or more disease reduce it.

The sigmoid growth curve

A population growing in an environment with limited resources follows an S-shaped (sigmoid) curve, followed by a decline:

Phase	What the graph shows	Explanation (Extended only)
Lag	Numbers rise slowly	Few individuals are reproducing; organisms are adjusting to the conditions or maturing
Exponential (log)	Numbers rise steeply	Plenty of food and space; little competition, predation or disease; births far exceed deaths
Stationary	Numbers level off	Limiting factors (food, space, oxygen, build-up of waste) take effect; births equal deaths
Death	Numbers fall	Food runs out or toxic waste accumulates; deaths exceed births

For the Extended explanation, name the limiting factor and link it to birth and death rates. “Competition for food increases, so birth rate falls until it equals death rate” is the level expected.

Interpreting population graphs

Worked example. A population of water fleas in a tank rises from 150 to 870 between day 8 and day 12. Calculate the mean rate of population growth.

rate = change in numbers ÷ time
     = (870 − 150) ÷ (12 − 8)
     = 720 ÷ 4
     = 180 water fleas per day

Read values carefully from the axes, show the subtraction, and give the unit (individuals per unit time).

Common errors

• Drawing food-chain arrows from predator to prey. Arrows show energy flow, so they point to the eater.
• Saying energy is “recycled”. Energy flows through and is lost as heat; only matter is recycled.
• Calling a pyramid of numbers a pyramid of biomass because it is pyramid-shaped. Check the label and units.
• Writing that energy is “used up” or “made”. Energy is transferred to the environment as heat.
• Confusing nitrification (ammonium → nitrate) with nitrogen fixation (nitrogen gas → nitrogen compounds) or with denitrification (nitrate → nitrogen gas).
• Giving “the population stops growing” for the stationary phase without saying births equal deaths or naming a limiting factor.

Where to go next

Condense this into the revision notes, then try the practice questions.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge University Press & Assessment (Cambridge International Education). Topic 19, sections 19.1 to 19.4.