19. Organisms and their environment
- Syllabus
- 0610–2026–2027
- Section
- 19
- Level
- —
Exam analysis
Most tested topics
19.1 Energy flow
- Syllabus
- 0610–2026–2027
- Topic
- 19.1
- Level
- —
Identify the principal energy source
The Sun is the principal source of energy input to biological systems.
Energy may reach an organism through several feeding steps, but trace the pathway backwards: the original input to the biological system is light energy from the Sun.
Plants and food contain chemical energy, but they are not the principal original input. For the biological systems in this syllabus, answer the Sun or sunlight.
Trace energy from sunlight to the environment
Light energy from the Sun is absorbed by producers during photosynthesis and stored as chemical energy in organic molecules. Feeding transfers chemical energy from one organism to another.
Living organisms release energy during their life processes. Energy is eventually transferred to the environment as heat, including heat produced through respiration.
Follow the form as well as the direction: Sun — light energy → producer — chemical energy in food → consumer — chemical energy → environment — heat energy.
Energy flows through a biological system and eventually leaves as heat; it is not recycled. Light is the input, chemical energy is transferred between organisms, and heat is the eventual transfer to the environment.
19.2 Food chains and food webs
- Syllabus
- 0610–2026–2027
- Topic
- 19.2
- Level
- —
Read a food chain as energy transfer
A food chain shows the transfer of energy from one organism to the next and begins with a producer.
Each arrow points from the organism being eaten to the organism that feeds on it: it shows the direction of energy transfer.
An arrow does not point towards the food. In grass → rabbit, energy moves from grass to rabbit.
Construct and interpret a simple food chain
- Start with the producer. 2. Place the organism that eats it next. 3. Continue in feeding order. 4. Draw every arrow from food to feeder.
To predict a population change, follow both directions: fewer prey can reduce its predator, while fewer predators can allow its prey to increase; then trace further effects along the chain.
A prediction needs a feeding link and a direction. Do not assume every population changes in the same direction.
Interpret an interconnected food web
A food web is a network of interconnected food chains. One organism may have several food sources or predators and may feed at more than one trophic level.
For any question, first trace arrows into the named consumer to find its food, then arrows leaving its prey or entering its predators. Predict direct effects before indirect ones and state the feeding reason.
A food web does not prove that every possible effect will occur. Use only the drawn feeding relationships and qualify indirect predictions when alternatives remain.
Define a producer by nutrient production
A producer is an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis.
Producers convert light energy into chemical energy stored in organic molecules and form the first trophic level of most food chains.
A producer is not merely an organism that provides food; it makes organic nutrients rather than obtaining them by feeding.
Define a consumer by feeding
A consumer is an organism that gets its energy by feeding on other organisms.
A consumer's classification depends on what it eats and its position in a particular food chain or route through a food web.
Consumer does not mean only carnivore: herbivores and animals at higher feeding positions are consumers.
Classify consumers by position
A primary consumer feeds on a producer; a secondary consumer feeds on a primary consumer; a tertiary consumer feeds on a secondary consumer; a quaternary consumer feeds on a tertiary consumer.
In a food web, trace one complete route from a producer. The same organism can occupy different consumer levels on different routes.
The label is determined by position, not by an organism's name or size. A predator can be secondary on one route and tertiary on another.
Define a herbivore
A herbivore is an animal that gets its energy by eating plants.
In a chain beginning with a plant producer, the herbivore is normally the primary consumer.
Herbivore describes the animal's food source; it is not another word for producer.
Define a carnivore
A carnivore is an animal that gets its energy by eating other animals.
A carnivore may be a secondary, tertiary or quaternary consumer, depending on the feeding route.
Carnivore does not identify one fixed trophic level; trace what the animal eats in the stated chain or web.
Define a decomposer
A decomposer is an organism that gets its energy from dead or waste organic material.
Dead material and waste can come from producers, herbivores or carnivores, so decomposers receive energy from material originating at every trophic level.
A decomposer is defined by obtaining energy from dead or waste organic material, not simply by being small or living in soil.
Trace human impacts through a food web
Overharvesting removes a food species faster than it is replaced. Its predators may lose a food source and decline, while organisms it ate may increase; further effects can spread through alternative links.
An introduced foreign species may become a new predator, prey or competitor. Trace which native population it directly changes, then follow the arrows to predict indirect effects.
Do not state that every introduced species causes the same outcome. A justified prediction must identify the new feeding or competition relationship and its direction.
Draw and interpret numbers and biomass pyramids
A pyramid of numbers uses bar width to represent the number of organisms at each trophic level. A pyramid of biomass uses bar width to represent their total biomass, usually per unit area.
| Representation | What each bar measures | Possible shape |
|---|---|---|
| numbers | count of individuals | may be inverted or irregular, for example one tree supporting many insects |
| biomass | total mass of living material | usually decreases at higher trophic levels |
Place the producer at the bottom and higher trophic levels above it. Use a common scale, make bar widths proportional to the data, and label every level.
Bar width represents the stated quantity, not the physical size of one organism.
Explain the advantage of biomass pyramids
A biomass pyramid accounts for the mass of organisms, so one large producer is not represented as equivalent to one tiny consumer merely because both count as one individual.
Biomass gives a better indication of how much living material—and therefore potential food—is present at each trophic level than numbers alone.
Biomass is more informative than organism count, but it is still a snapshot and does not directly show energy transferred over time.
Define trophic level as feeding position
A trophic level is the position of an organism in a food chain, food web or ecological pyramid.
Assign it by tracing a feeding route from the producer, which occupies the first trophic level.
An organism can feed at more than one trophic level in a food web if different routes place it in different positions.
Identify named trophic levels
Producer = trophic level 1; primary consumer = level 2; secondary consumer = level 3; tertiary consumer = level 4; quaternary consumer = level 5.
Start at a producer and count each energy-transfer arrow along one route. Name a consumer from the organism immediately before it: eating a primary consumer makes it a secondary consumer.
Do not count the Sun as a trophic level, and do not assign a web organism one level without checking every relevant route.
Draw and interpret a pyramid of energy
A pyramid of energy represents the energy available at each trophic level over a stated area and time. The producer bar forms the base and each higher bar is narrower.
Order trophic levels from producer upward, use one scale for proportional bar widths, include energy units with area and time when supplied, and read transfer as the difference or ratio between adjacent levels.
A pyramid of energy is always upright because less energy is available at each successive trophic level.
Compare the evidence in energy pyramids
A pyramid of energy shows actual energy transfer and includes a time interval, so it accounts for organisms with different sizes, life spans, reproduction rates and turnover.
| Pyramid | Main limitation avoided by energy data |
|---|---|
| numbers | treats every individual as one regardless of size |
| biomass | gives a standing mass at one time and may miss rapid replacement |
| energy | measures flow per area per time and is always upright |
Energy pyramids need more measurement, but they give the most direct representation of transfer efficiency.
Account for inefficient energy transfer
Transfer is inefficient because not all parts are eaten; some eaten material is not digested or absorbed and leaves as faeces; energy is used in respiration, movement and other life processes and is transferred to the environment as heat; waste products also carry energy away.
Efficiency of transfer (%) = energy transferred to the next trophic level ÷ energy available at the previous trophic level × 100.
Less energy is available at each higher level, so higher trophic levels usually support less biomass and fewer organisms.
Energy is transferred, not destroyed; energy unavailable to the next consumer has moved to waste, decomposers or the environment.
Explain why food chains stay short
Because energy transfer at every trophic step is inefficient, successive losses leave progressively less energy available to support biomass at the next level.
After several transfers, too little energy remains to support another stable consumer population. Food chains therefore usually have fewer than five trophic levels.
Food chains are not short because predators choose to stop feeding; accumulated energy loss limits the biomass that another level could support.
Compare crops with livestock for human food
Humans eating crop plants feed at a lower trophic level than humans eating livestock that first ate those crops.
The livestock uses crop energy in respiration, movement and other processes, and loses energy in waste and uneaten or indigestible material. Eating the crop directly avoids this additional inefficient transfer.
For the same crop energy input, a larger proportion can reach humans through crop → human than through crop → livestock → human.
The claim is specifically about energy-transfer efficiency, not a statement that every crop or diet has the same yield, nutrition or environmental impact.
19.3 Nutrient cycles
- Syllabus
- 0610–2026–2027
- Topic
- 19.3
- Level
- —
Trace carbon through six cycle processes
Photosynthesis removes carbon dioxide from the atmosphere and incorporates its carbon into organic molecules in plants.
Feeding transfers carbon-containing compounds from plants to animals. Respiration by plants, animals and decomposers releases carbon dioxide back to the atmosphere.
After death and waste production, decomposition returns carbon through decomposers and their respiration. Some dead organic material forms fossil fuels over very long periods; combustion of these fuels releases carbon dioxide to the atmosphere.
Carbon cycles between stores, but the six required processes have specific directions: photosynthesis removes atmospheric carbon dioxide, while respiration and combustion add it.
Trace nitrogen through its chemical forms
Atmospheric nitrogen gas becomes usable nitrogen compounds through nitrogen fixation by lightning or bacteria. Nitrification converts ammonium ions through nitrite ions into nitrate ions.
Plant roots absorb nitrate ions. Plants use the nitrogen to produce amino acids and then proteins. Feeding transfers protein to animals; digestion breaks dietary proteins into amino acids that can be used to make animal proteins.
Decomposition converts nitrogen in dead plant and animal protein to ammonium ions. In animals, deamination removes the amino group from excess amino acids; nitrogen-containing waste can return through decomposition. Denitrification converts nitrate ions back to nitrogen gas in the atmosphere.
Key direction: nitrogen gas → fixed nitrogen compounds → ammonium/nitrate ions → plant amino acids and proteins → animal proteins → ammonium ions → nitrate ions → nitrogen gas.
Plants absorb nitrate ions, not atmospheric nitrogen gas or ready-made protein. Nitrification forms nitrates; denitrification removes nitrates and returns nitrogen gas.
Assign four microbial roles in the nitrogen cycle
Microorganisms drive four required conversions in the nitrogen cycle; individual bacterial genus names are not required.
| Microbial role | Starting material | Product or effect |
|---|---|---|
| decomposition | protein in dead organisms and waste | ammonium ions |
| nitrification | ammonium ions, through nitrite ions | nitrate ions |
| nitrogen fixation | nitrogen gas | nitrogen compounds available to enter the cycle |
| denitrification | nitrate ions | nitrogen gas returned to the atmosphere |
Identify the role from arrow direction and chemical form: towards nitrate is nitrification; from nitrate to atmospheric nitrogen is denitrification; from atmospheric nitrogen into compounds is fixation.
Do not reverse fixation and denitrification. Decomposition returns organic nitrogen to ammonium ions; it is not the same conversion as nitrification.
19.4 Populations
- Syllabus
- 0610–2026–2027
- Topic
- 19.4
- Level
- —
Define a population with all three conditions
A population is a group of organisms of one species living in the same area at the same time.
Check all three conditions: same species, same area and same time. If any condition is missing, the group is not fully described as a population.
All organisms in an area may include several species, so they form a community rather than one population.
Distinguish a community from a population
A community is all the populations of different species in an ecosystem.
One species in an area forms a population. Several populations of different species together form the community.
A community contains living populations; it does not by itself include the non-living environment.
Define an ecosystem as an interacting unit
An ecosystem is a unit containing a community of organisms and their environment, interacting together.
The community is the living part. Environmental conditions such as temperature, water or oxygen are non-living parts. Their interactions make the whole unit an ecosystem.
A list of species describes a community. It becomes an ecosystem description only when the environment and interactions are included.
Link four limiting factors to population growth
Population growth rate changes when food supply, competition, predation or disease changes.
| Factor | How it can limit population growth |
|---|---|
| food supply | less food increases competition and can reduce survival or reproduction |
| competition | organisms obtain fewer limited resources |
| predation | more individuals are killed and removed from the population |
| disease | illness or death lowers survival and may reduce reproduction |
More available food can increase growth; stronger competition, predation or disease usually decreases growth. State the causal link, not just the factor name.
These are the four required factors. Do not replace them with an unrequested list of environmental variables.
Identify four phases from curve shape
Identify each phase from the direction and steepness of the population curve, not from its letter or position alone.
| Phase | Graph feature | Birth and death relationship |
|---|---|---|
| lag | low population; little increase | growth is slow |
| exponential (log) | steepest rising section | birth rate is greater than death rate |
| stationary | plateau or small fluctuations around a level | birth rate is approximately equal to death rate |
| death | falling section | death rate is greater than birth rate |
A sigmoid curve describes growth under limited resources. The death phase is the falling section, not simply any point where the population is small.
Interpret a population growth graph
Read the axes and units first. Then describe whether population size rises, falls or stays approximately constant, and use the slope to compare rates of change.
Support a description with labelled regions, times or values from the graph. A steeper upward slope means a faster increase; a horizontal section means no net change; a downward slope means a decrease.
Translate the graph into population balance: rising means births exceed deaths, a plateau means births approximately equal deaths, and falling means deaths exceed births.
A curve can fluctuate around a stable level. Do not describe every small rise or fall as a new exponential or death phase without using the overall trend.
Explain the causes of each sigmoid phase
The sigmoid phases result from changes in reproduction, death and limiting factors as population density increases.
| Phase | Explanation |
|---|---|
| lag | the population is small, so the birth rate and total increase are low |
| exponential (log) | resources are abundant and competition, predation or disease have little limiting effect; births exceed deaths |
| stationary | limiting factors prevent further net growth; births approximately equal deaths around the carrying capacity |
| death | resources become insufficient or disease, predation or harmful waste effects increase; deaths exceed births |
Carrying capacity is the population size the environment can support under its current limiting factors. It explains the stable level, not an absolute maximum that can never fluctuate.
Name the phase, describe the curve and connect a relevant limiting factor to the birth–death balance. A factor name without its effect is not a complete explanation.