19 Organisms and Their Environment
(Core / Extended)

19.1 Energy Flow
What you need to know:

1. State that the Sun is the principal source of energy input to biological systems
   
2. 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

Videos: Ecology-Energy Flow

19.2 Food Chains and Food Webs
What you need to know:

1. Describe a food chain as showing the transfer of energy from one organism to the next, beginning with a producer
2. Construct and interpret simple food chains
3. Describe a food web as a network of interconnected food chains and interpret food webs
4. Describe a producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
5. Describe a consumer as an organism that gets its energy by feeding on other organisms
6. State that consumers may be classed as primary, secondary, tertiary and quaternary according to their position in a food chain
7. Describe a herbivore as an animal that gets its energy by eating plants
8. Describe a carnivore as an animal that gets its energy by eating other animals
9. Describe a decomposer as an organism that gets its energy from dead or waste organic material
10. Use food chains and food webs to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat
11. Draw, describe and interpret pyramids of numbers and pyramids of biomass
12. Draw, describe and interpret pyramids of energy
13. Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain
14. Discuss the advantages of using a pyramid of energy rather than pyramids of numbers or biomass to represent a food chain
15. Describe a trophic level as the position of an organism in a food chain, food web or ecological pyramid​
16. Identify the following as the trophic levels in food webs, food chains and ecological pyramids: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers
17. Explain why the transfer of energy from one trophic level to another is often not efficient
18. Explain, in terms of energy loss, why food chains usually have fewer than five trophic levels
19. Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants

Videos: Food Chains and Food Webs

Videos: Pyramid of Biomass

Videos: Ecological Pyramids

Videos: Pyramid of Biomass & Trophic Levels

19.3 Nutrient Cycles
What you need to know:

1. Describe the carbon cycle, limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion
2. Describe the nitrogen cycle with reference to: (a) decomposition of plant and animal protein to ammonium ions (b)nitrification (c) nitrogen fixation by lightning and bacteria (d) absorption of nitrate ions by plants (e) production of amino acids and proteins (f) feeding and digestion of proteins (g)deamination (h) denitrification
   
3. 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) 
   

Videos: Nutrient Cycles - Overview

Videos: Nitrogen Cycle

Videos: Carbon Cycle

19.4 Population
What you need to know:

1. Describe a population as a group of organisms of one species, living in the same area, at the same time
2. Describe a community as all of the populations of different species in an ecosystem
3. Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together
4. Identify and state the factors affecting the rate of population growth for a population of an organism, limited to food supply, competition, predation and disease
5. Identify the lag, exponential (log), stationary and death phases in the sigmoid curve of population growth for a population growing in an environment with limited resources
6. 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
7. Interpret graphs and diagrams of population growth​

Videos: Key Definitions

Videos: Population Growth Curves

Videos: Population Sampling Fieldwork

EXAMPLE QUESTIONS

Videos: None available at the moment.