IGCSE Biology
2026 - 2028
Syllabus Checklist
Topic 19: Organisms and Their Environment
1 Energy Flow
State that the Sun
is the principal source of energy input to biological systems.
Describe the flow of energy
through living organisms, from light energy in the Sun to chemical energy in organisms and its eventual transfer to the environment.
2 Food Chains and Food Webs
Describe a food chain
as showing energy transfer from one organism to the next, beginning with a producer.
Construct and interpret simple food chains.
Describe a food web
as a network of interconnected food chains and interpret food webs.
Describe a producer
as an organism that makes its own organic nutrients, usually by photosynthesis.
Describe a consumer
as an organism that gains energy by feeding on other organisms.
State that consumers may be classified as primary, secondary, tertiary,
and quaternary
according to their position in a food chain.
Describe a herbivore
as an animal that eats plants.
Describe a carnivore
as an animal that eats other animals.
Describe a decomposer
as an organism that feeds on dead or waste organic material.
Use food chains and webs to explain human impacts, such as overharvesting
and introducing foreign species
.
Draw, describe, and interpret pyramids of numbers
and biomass
.
Discuss the advantages of using a pyramid of biomass
instead of a pyramid of numbers.
Define a trophic level
as the position of an organism in a food chain, food web, or pyramid.
Identify trophic levels as: producers
, primary consumers
, secondary consumers
, tertiary consumers
, and quaternary consumers
.
Draw, describe, and interpret pyramids of energy
.
Discuss advantages of using pyramids of energy
over pyramids of numbers or biomass.
Explain why energy transfer between trophic levels is inefficient
.
Explain, in terms of energy loss
, why food chains usually have fewer than five trophic levels.
Explain why it is more energy efficient
for humans to eat crops directly rather than livestock fed on crops.
3 Nutrient Cycles
Describe the carbon cycle
, limited to: photosynthesis, respiration, feeding, decomposition, fossil fuel formation, and combustion.
Describe the nitrogen cycle
including:
▪ Decomposition → ammonium ions
▪ Nitrification
▪ Nitrogen fixation by lightning and bacteria
▪ Absorption of nitrates by plants
▪ Formation of amino acids and proteins
▪ Feeding, digestion, deamination, and denitrification
State the roles of microorganisms
in the nitrogen cycle: decomposition, nitrification, nitrogen fixation, and denitrification.
4 Populations
Define a population
as a group of organisms of one species living in the same area at the same time.
Define a community
as all populations of different species in an ecosystem.
Define an ecosystem
as the community of organisms and their environment interacting together.
Identify factors affecting population growth
: food supply, competition, predation, and disease.
Identify lag, exponential, stationary,
and death phases
in the sigmoid population growth curve.
Interpret graphs and diagrams showing population growth
.
Explain factors leading to each phase in the sigmoid curve, referring to limiting factors
where appropriate.
Free Class
Videos, Notes & Solved Past Papers
