Organisms and their environment

• Transfer of Energy

  • The Sun is the principal source of energy input to biological systems
  • Energy flows through living organisms
  • Including light energy from the Sun and chemical energy in organisms
  • Energy is eventually transferred to the environment eg. as heat
• Food Chains

  • A food chain shows the transfer of energy from one organism to the next, starting with a producer
  • The source of all energy in a food chain is light energy from the Sun
  • The arrows in a food chain show the transfer of energy from one trophic level to the next
  • Energy is transferred from one organism to another by ingestion (eating)
• Food Webs

  • A food web is a network of interconnected food chains
  • Food webs are more realistic ways of showing connections between organisms within an ecosystem
  • Food webs give us a lot more information about the transfer of energy in an ecosystem
  • They also show interdependence - how the change in one population can affect others within the food web
• Pyramid of Numbers


  • A pyramid of numbers shows how many organisms we are talking about at each level of a food chain
  • The width of the box indicates the number of organisms at that trophic level
  • A pyramid of numbers doesn't always have to be pyramid-shaped
  • The size of the organism is also important - one large organism can contain enough energy to support many smaller organisms
• Pyramid of Biomass

  • A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms (their 'dry mass')
  • Pyramids of biomass are ALWAYS pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like
  • Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain
• Pyramid of Energy

  • Only the energy that is made into new cells remains with the organism to be passed on
  • The majority of the energy an organism receives gets 'lost' (or 'used') through making waste products, movement, heat, and undigested waste
  • This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long
• It is more energy efficient within a crop food chain for humans to be the herbivores rather than the carnivores
• Pyramid of energy


  1. Humans are omnivores, obtaining energy from both plants and animals

  2. Humans have a choice of what they eat

  3. Choices have an impact on what we grow and how we use ecosystems
• Nutrients such as carbon and nitrogen are not endless resources
• The Carbon Cycle


  1. Carbon is taken out of the atmosphere by photosynthesis

  2. Passed on to animals and decomposers by feeding

  3. Returned by respiration in plants, animals and decomposing microorganisms

  4. Returned by combustion of fossil fuels
• Increased use of fossil fuels is contributing to an increase in the carbon dioxide content of the atmosphere
• Mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis
• Deforestation for land release more carbon dioxide into the atmosphere
• Nitrogen


  Required to make proteins
• Nitrogen Cycle

  1. Nitrogen fixing bacteria convert N2 gas into ammonium compounds

  2. Lightning splits N2 into nitrous oxides

  3. Plants absorb nitrogen as nitrates

  4. Animals get nitrogen from plant proteins

  5. Waste and decomposition return nitrogen to soil as ammonium

  6. Nitrifying bacteria convert ammonium to nitrites and nitrates

  7. Denitrifying bacteria convert nitrates back to N2 gas
• Population


  A group of organisms of one species, living in the same area at the same time
• Community


  All of the populations of different species in an ecosystem
• Ecosystem


  A unit containing the community of organisms and their environment, interacting together
• Factors controlling population growth

  • Food supply
  • Predation
  • Disease
• Population Growth Curve


  1. Lag phase

  2. Log/Exponential phase

  3. Stationary phase

  4. Death phase
• Organisms in a natural environment are unlikely to show population growth like a sigmoid growth curve because they are affected by many other factors