Cambridge IGCSE Biology 19 Organisms and their Environment Exam Questions

About Cambridge IGCSE Biology 19 Organisms and their Environment

Organisms and their environment is the ecology topic of Cambridge IGCSE Biology. You need to describe how energy from the Sun flows through organisms, construct and interpret food chains and food webs, and use the terms producer, consumer, herbivore, carnivore, decomposer and trophic level. You also draw and interpret pyramids of numbers and biomass, describe the carbon cycle and the factors affecting population growth. Extended candidates add pyramids of energy, why energy transfer between trophic levels is inefficient, the nitrogen cycle with its named bacteria, and the phases of a sigmoid population growth curve. Calculations of energy transfer efficiency are common.

What you need to know

• 19.1 Energy flow The Sun is the principal source of energy input to biological systems. Energy flows through living organisms as chemical energy and is eventually lost to the environment, mainly as heat; it is not recycled.
• 19.2 Food chains and webs A food chain shows the transfer of energy from one organism to the next, starting with a producer. Know producer, primary, secondary, tertiary and quaternary consumers, herbivores, carnivores, decomposers and trophic levels. Human impact on food webs, such as overharvesting, can affect other species.
• 19.2 Pyramids Pyramids of numbers can be inverted, for example one tree supporting many insects. Pyramids of biomass show dry mass at each level. Extended: pyramids of energy are always pyramid shaped, and transfer between levels is inefficient (often about 10%) due to respiration, heat loss, excretion and parts not eaten.
• 19.3 Carbon cycle Carbon is removed from the atmosphere by photosynthesis and returned by respiration, decomposition and combustion. Fossilisation locks carbon in coal, oil and gas for millions of years.
• 19.3 Nitrogen cycle (Extended) Nitrogen-fixing bacteria convert nitrogen gas into compounds plants can use, and lightning also fixes nitrogen. Decomposers break down proteins and urea to ammonium ions; nitrifying bacteria convert ammonium ions to nitrate ions; denitrifying bacteria convert nitrate ions back to nitrogen gas. Plants absorb nitrate ions to make amino acids and proteins.
• 19.4 Populations A population is a group of organisms of one species living in the same area at the same time. Growth depends on food supply, competition, predation and disease. Extended: a sigmoid curve has lag, exponential (log), stationary and death phases. A community is all the populations in an area, and an ecosystem is a community interacting with its environment.

Common mistakes in 19 Organisms and their Environment

• Drawing food chain arrows from predator to prey. Arrows show the direction of energy flow, from the organism eaten to the eater.
• Saying energy is recycled in an ecosystem. Nutrients such as carbon and nitrogen are recycled; energy flows through and is lost as heat.
• Saying decomposers turn dead material into nitrates directly. Decomposers produce ammonium ions; nitrifying bacteria make nitrates.
• Saying energy is used up at each trophic level. Energy is transferred to the environment, mainly as heat from respiration.
• Confusing nitrogen-fixing and nitrifying bacteria. Nitrogen-fixing bacteria use nitrogen gas; nitrifying bacteria convert ammonium ions to nitrate ions.

Try three 19 Organisms and their Environment exam-style questions