Pack 17 - Energy, Ecosystems and Nutrient cycles

Pack 17 - Energy, Ecosystems and Nutrient cycles

1. Food chains and energy transfer:

   • Most plants convert energy from sunlight into chemical energy in organic comounds
     • Nitrates and phosphates are required to make these organic compounds
   • Food chains:
     • Producers:
       • Photosynthetic organisms manufacture organic substances
     • Consumers:
       • Consume other organisms to obtain their energy
       • Primary consumers eat producers
       • Secondary consumers eat primary consumers
     • Saprobionts
       • Extracellular digestion
         • Break down things to absorbable parts
           • Assimilated
   • Trophic levels
     • Energy flows from primary producers to consumers. Each group is said to be at a certain trophic level
     • e.g. grass-->rabbit-->fox
     • There are usually only 4 or 5 trophic levels due to the inefficiency of energy transfer
   • Food webs:
     • In any habitat, many food chains are inter-linked to form food webs
     • Some organisms will be in more than one trophic level
       • The greater the number of interactions, the more stable the food web is

2. Biomass

   • Total mass of living material in a specific material in a specific area at a given time
     • All organisms contain a variable volume of water
   • Dry mass is measured by drying the sample in an oven at 100 degrees until a constant mass is reached.
     • Issues:
       • Organisms must be dead
         • Only small samples can be taken
           • Sample may not be representative
   • Measured in dry mass per given area in a given time
   • Calorimetry can be used to estimate the chemical energy store in dry mass
     • 1. Sample of dry material weighed
       • 2. Burnt in pure oxygen in a sealed chamber
         • 3. Bomb is surrounded by water bath
           • 4. Heat of combustion causes temp. of the water to rise

3. Energy transfer and Productivity

   • 90% of energy from the sun is lost, even more is lost of the energy hitting a leaf:
     • Some energy used for evaporation
     • Some energy is reflected by cuticle
     • Not all light hits the cuticle
     • Not all wavelengths used in photosynthesis
     • Low CO2 Conc. may limit photosynthesis
   • Gross primary production (GPP) and net primary production (NPP)
     • GPP = The chemical energy store in a plant biomass, in a given area volume at a given time
     • NPP = Chemical energy store in plant biomass after respiratory losses to the environment
     • NPP = GPP - R
   • Less than 10% NPP in plants available to consumers
     • Not all plant eaten
     • Some used in respiration for ATP production
     • Some lost in egestion and excretion
     • Plant material is difficult to digest
   • Less than 20% available from secondary to tertiary
     • Not all consumed
     • Some parts are lost in faeces
     • Lost in excretion
   • Net production of consumers
     • N = I - (F+R)
       • I = Chemical energy
       • F = chemical energy lost to environment
       • R = Respiratory loss

4. Productivity and farming practices

   • Increasing the efficiency of energy transfer leads to higher yields
   • Intensive farming techniques:
     • Restrict movement - less energy used in muscle contraction
     • Environment kept warm - reduce heat loss from the body
     • Optimum amount of food for max growth
     • Predators excluded so no loss to other organisms

5. Chemical and biological control of agricultural pests

   • Pesticides
     • Effective ones should:
       • Be specific to only one organism
       • Biodegrade
       • Not accumulate
     • Development of genetic resistance
       • Mutation confers resistance
         • Pests without mutant allele are killed by pesticide
           • Organisms with allele for resistance survive
             • Breed to pass on allele
   • Biological Control
     • Eradicating pests may mean that other pests can multiply
     • Very specific, once introduced the organisms reproduce themselves
     • Pests can't become resistant
   • Integrated control systems
     • Aim to keep pests are acceptable levels
     • Choose varieties of animals/plants that are resistance to pest
       • Providing habitats close to crops for natural predators
         • Monitor crops for signs of pests
           • Remove pests mechanically
             • Use biological agents
               • Use pesticides

6. Nutrient cycles

   • Saprobiotic organisms:
     • Activate a gene that makes digestive enzymes
       • Enzymes are secreted and hydrolyse surrounding material into soluble compounds
         • Compounds are absorbed by cells and assimilated
   • Nitrogen cycle
     • Ammonification
       • Production of ammonia from organic nitrogen containing compounds
       • Carried out by saprobiotic organisms
     • Nitrification
       • Oxidation of ammonium ions to nitrate ions
         • Then oxidation of nitrite ions to nitrate ions
     • Nitrogen Fixation
       • Reduce gaseous nitrogen to ammonia
     • Denitrification
       • Denitrifying bacteria convert soil nitrates into gaseous nitrogen in anaerobic conditions
   • Phosphorus cycle
   • Role of Mycorrhizae
     • This is the association between certain types of fungi and most plants
     • Fungi act as extension of plants to increase surface area
     • Plants provide organic compounds
       • Mycorrhizae provide water and minerals and resist drought
   • Fertilisers
     • Natural fertilisers
       • Dead and decaying remains of animals and plants
       • Animal waste
       • More expensive
     • Artificial fertilisers
       • Mined from rocks and deposits
       • Convert into different forms and blended to give balance of minerals
     • Agricultural ecosystems
       • Intensive food production means minerals are taken away
     • Nitrogen is needed for growth (amino acids)
     • Environmental issues:
       • Leeching occurs:
         • This is the process by which nutrients are removed from the soil
           • Rainwater dissolves soluble nutrients, this is carried into watercourses
         • This can lead to eutrophication
           • 1) Nitrate ion concentration increases due to leaching
             • 2) Algae bloom - upper layers become densely populated
               • 3) Algae absorb light - light cannot penetrate lower levels
                 • 4) Light becomes the limiting factor for plants below - they die
                   • 5) Lack of dead plants no longer the limiting factor for saprobionts, so they grow
                     • 6) Saprobionts require oxygen for respiration, increased oxygen demand
                       • 7) Oxygen concentration in water reduces, nitrates released from decaying organisms
                         • 8) Oxygen now limiting factor for fish - they die
                           • 9) Less competition for anaerobic respiration, so anaerobic organisms increase
                             • 10) Anaerobic organisms continue to decompose material and release nitrate and toxic waste

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