Ad Banner Placeholder

Cambridge IGCSE Biology · 0610

Chapter 19: Organisms and their environment (Part 2)

The carbon cycle

Carbon is an essential element found in all living molecules. The cycle shows its movement between the atmosphere and organisms:

  1. Photosynthesis: Plants take in carbon dioxide (CO2) from the atmosphere.
  2. Feeding: Carbon is passed through the food chain.
  3. Respiration: Organisms release CO2 back into the air.
  4. Decomposition: Decomposers release CO2 while breaking down dead organic matter.
  5. Fossilisation and combustion: Dead organisms can become fossil fuels over time. When burned (combustion), they release large amounts of CO2 back into the atmosphere.
The carbon cycle showing flow between CO2, plants, animals, dead organic matter, and fossil fuels
Diagram 1: The carbon cycle. The flow of carbon between CO2 in the air/water, plants, animals, dead organic matter, and fossil fuels. Arrows are labelled with photosynthesis, respiration, feeding, death/excretion, fossilisation, and combustion.

Exam Traps

  • Do not say combustion removes CO2 from the atmosphere — it releases CO2.
  • Avoid confusing respiration (releases CO2) with photosynthesis (absorbs CO2).

The nitrogen cycle

Nitrogen is used to make amino acids, DNA, and ATP. Microorganisms are vital to this cycle:

  1. Decomposition: Bacteria and fungi break down protein in waste and dead matter into ammonium ions.
  2. Nitrification: Nitrifying bacteria convert ammonium ions into nitrites, then into nitrates.
  3. Nitrogen fixation: Nitrogen-fixing bacteria (in root nodules or soil) or lightning convert atmospheric nitrogen gas into usable nitrates.
  4. Absorption and feeding: Plants absorb nitrates to make proteins, which are passed through feeding.
  5. Deamination: Excess amino acids are converted to urea for excretion.
  6. Denitrification: In anaerobic conditions, denitrifying bacteria break down nitrates and return nitrogen gas to the atmosphere.
The nitrogen cycle showing atmospheric nitrogen, nitrates in soil, plants, and animals with labelled processes
Diagram 2: The nitrogen cycle. Atmospheric nitrogen (N2), nitrates in the soil, plants, and animals are shown. Processes labelled: nitrogen fixation, nitrification, feeding, decomposition (ammonification), and denitrification.

Exam Traps

  • Do not say plants absorb nitrogen gas directly — they absorb nitrates from the soil.
  • Avoid confusing nitrification (ammonium to nitrates) with denitrification (nitrates to N2).
  • Do not claim all nitrogen fixation is done by lightning — bacteria in root nodules are the main biological route.

Populations

Population
A group of organisms of one species living in the same area at the same time.
Community
All the populations of different species in an ecosystem.
Ecosystem
A unit containing the community and its environment interacting together.
Growth factors
The rate of growth is limited by food supply, competition, predation, and disease.

Exam Traps

  • Do not use population and community interchangeably — population is one species only.

The sigmoid curve of population growth

A population in an environment with limited resources follows a sigmoid (S-shaped) curve:

  1. Lag phase: Initial slow growth as organisms adapt to their environment.
  2. Exponential (log) phase: Rapid increase under optimum conditions with no limiting factors.
  3. Stationary phase: Growth levels out where birth rate equals death rate. The population has reached the carrying capacity—the maximum size the environment can support.
  4. Death phase: Population declines as the death rate exceeds the birth rate due to overpopulation, competition, or toxin build-up.
Sigmoid population growth curve showing lag, exponential, stationary, and death phases
Diagram 3: Sigmoid population growth curve. A graph of number of organisms against time. The four phases are labelled: lag, exponential, stationary, and death.

Exam Traps

  • Do not say carrying capacity is when growth is fastest — that is the exponential phase.
  • Avoid confusing lag phase (slow start) with death phase (population falling).

0/15

Ad Banner Placeholder