Energy transfers in and between organisms

• Photosynthesis


  Light is absorbed by chlorophyll and this is linked to the production of ATP
• Respiration


  Various substances are used as respiratory substrates, and the hydrolysis of these respiratory substrates is linked to the production of ATP
• ATP production in respiration and photosynthesis


  Protons diffuse down an electrochemical gradient through molecules of the enzyme ATP synthase, embedded in the membranes of cellular organelles
• The process of photosynthesis is common in all photoautotrophic organisms and the process of respiration is common in all organisms, providing indirect evidence for evolution
• In communities, the biological molecules produced by photosynthesis are consumed by other organisms, including animals, bacteria and fungi. Some of these are used as respiratory substrates by these consumers
• Photosynthesis and respiration are not 100% efficient. The transfer of biomass and its stored chemical energy in a community from one organism to a consumer is also not 100% efficient
• Light-dependent reaction in photosynthesis

  • Chlorophyll absorbs light, leading to photoionisation of chlorophyll
  • Some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP
  • The production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory)
  • Photolysis of water produces protons, electrons and oxygen
• Light-independent reaction in photosynthesis

  • Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP), catalysed by the enzyme rubisco
  • ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate
  • Some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
  • Some of the triose phosphate is converted to useful organic substances
• Students should be able to identify environmental factors that limit the rate of photosynthesis and evaluate data relating to common agricultural practices used to overcome the effect of these limiting factors
• Glycolysis


  1. Phosphorylation of glucose to glucose phosphate, using ATP

  2. Production of triose phosphate

  3. Oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD
• Anaerobic respiration


  If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis
• Aerobic respiration

  • Pyruvate from glycolysis enters the mitochondrial matrix by active transport
  • Pyruvate is oxidised to acetate, producing reduced NAD in the process
  • Acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A
  • Acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle
  • In a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost
  • Synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory)
  • Other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle
• In any ecosystem, plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide. Most of the sugars synthesised by plants are used by the plant as respiratory substrates. The rest are used to make other groups of biological molecules
• Biomass


  The chemical energy store in dry biomass can be estimated using calorimetry
• Gross primary production (GPP)


  The chemical energy store in plant biomass, in a given area or volume
• Net primary production (NPP)

  The chemical energy store in plant biomass after respiratory losses to the environment have been taken into account, ie NPP = GPP - R where GPP represents gross production and R represents respiratory losses to the environment
• Net production of consumers (N)

  N = I - (F + R) where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment
• Primary and secondary productivity

  The rate of primary or secondary production, respectively, measured as biomass in a given area in a given time eg kJ ha–1 year–1
• Students should be able to appreciate the ways in which production is affected by farming practices designed to increase the efficiency of energy transfer by simplifying food webs to reduce energy losses to non-human food chains and reducing respiratory losses within a human food chain
• Nitrogen cycle

  • Role of saprobionts in decomposition
  • Role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants
  • Role of bacteria in the nitrogen cycle in sufficient detail to illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification
• The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock, and the environmental issues arising from the use of fertilisers including leaching and eutrophication