19. Organisms and their environment

19.1 Energy flow

Core

• State that the Sun is the principal source of energy input to biological systems 

Supplement 

• Describe the flow of energy through living organisms including light energy from the Sun and chemical energy in organisms and its eventual transfer to the environment 

19.2 Food chains and food webs

Core 

• Define a food chain as showing the transfer of energy from one organism to the next, beginning with a producer 

• State that energy is transferred between organisms in a food chain by ingestion 

• Construct simple food chains 

Supplement 

• Describe how energy is transferred between trophic levels 

• Define trophic level as the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass 

• Explain why the transfer of energy from one trophic level to another is inefficient 

• Explain why food chains usually have fewer than five trophic levels 

19.2 Food chains and food webs continued

Core 

• Define a food web as a network of interconnected food chains 

• Define producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis 

• Define consumer as an organism that gets its energy by feeding on other organisms 

• State that consumers may be classed as primary, secondary and tertiary according to their position in a food chain 

• Define herbivore as an animal that gets its energy by eating plants 

• Define carnivore as an animal that gets its energy by eating other animals 

• Define decomposer as an organism that gets its energy from dead or waste organic material 

• Interpret food chains and food webs in terms of identifying producers and consumers 

• Use food chains and food webs to describe the impacts humans have through over-harvesting of food species and through introducing foreign species to a habitat 

• Draw, describe and interpret pyramids of numbers 

Supplement 

• Explain why there is a greater efficiency in supplying plants as human food, and that there is a relative inefficiency in feeding crop plants to livestock that will be used as food 

• Identify producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers as the trophic levels in food webs, food chains, pyramids of numbers and pyramids of biomass 

• Draw, describe and interpret pyramids of biomass 

• Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain 

19.3 Nutrient cycles

Core

• Describe the carbon cycle, limited to photosynthesis, respiration, feeding, decomposition, fossilisation and combustion 

• Discuss the effects of the combustion of fossil fuels and the cutting down of forests on the carbon dioxide concentrations in the atmosphere 

• Describe the water cycle, limited to evaporation, transpiration, condensation and precipitation 

Supplement 

• Describe the nitrogen cycle in terms of: 

– decomposition of plant and animal protein to ammonium ions 

– nitrification 

– nitrogen fixation by lightning and bacteria 

– absorption of nitrate ions by plants 

– production of amino acids and proteins 

– feeding and digestion of proteins 

– deamination 

– denitrification 

• State the roles of microorganisms in the nitrogen cycle, limited to decomposition, nitrification, nitrogen fixation and denitrification (generic names of individual bacteria, e.g. Rhizobium, are not required) 

19.4 Population size

Core 

• Define population as a group of organisms of one species, living in the same area, at the same time 

• Identify and state the factors affecting the rate of population growth for a population of an organism, limited to food supply, predation and disease 

• Discuss the increase in human population size over the past 250 years and its social and environmental implications 

• Interpret graphs and diagrams of human population growth 

Supplement 

• Define community as all of the populations of different species in an ecosystem 

• Define ecosystem as a unit containing the community of organisms and their environment, interacting together, e.g. a decomposing log, or a lake 

• Identify the lag, exponential (log), stationary and death phases in the sigmoid population growth curve for a population growing in an environment with limited resources 

• Explain the factors that lead to each phase in the sigmoid curve of population growth, making reference, where appropriate, to the role of limiting factors