13 - Energy & Ecosystems

• In an ecosystem, which organisms can synthesise their own organic compounds?

  Plants
• What is the original source of carbon for the synthesis of organic compounds?

  Carbon Dioxide
• What is biomass?

  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. These biological molecules form the biomass of the plants.
• How can biomass be measured?

  Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area. The chemical energy store in dry biomass can be estimated using calorimetry.
• What is GPP?

  The chemical energy stored in plant biomass, in a given area or volume. It is the total energy resulting from photosynthesis.
• What is NPP?

  The chemical energy stored in plant biomass taking into account the energy that will be lost due to respiration.
• What's the formula for NPP?

  Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) - Respiration
• To work out the net production of consumers (N), such as animals, the formula below can be used:

  N = I - (F+R)

  I= the chemical energy stored in ingested food

  F = the chemical energy lost to the environment in faeces and urine

  R = respiratory losses.
• Which units are used for rates of productivity and explain why?

  Rates of productivity are recorded using the units kJ ha–1 year–1.

  
  

  kJ is the unit for energy, but these units also include per unit area and per year. It is recorded as per unit area to standardize the results to enable environments to be compared. It takes into account that different environments will vary in size. The units are also per year to take into account the impact seasons will have on rain, light and heat – it provides an annual average to allow fair comparisons between different environments.
• What are the role of saprobionts in decomposition?

  These microbes will respire on dead or waste organic matter. This will cause decay and recycle nutrients into the soil so plants can absorb the nitrates/phosphates via active transport in their root hair cells
• What is the role of mycorrhizae?

  Mycorrhizae are fungal associations between plant roots and beneficial fungi. The fungi entwined around the plant roots provide a larger surface area for water and mineral absorption. The fungi part of the mycorrhizae acts like a sponge and can absorb and hold onto water and minerals surrounding the root.
• What is the role of bacteria in the nitrogen cycle?

  Bacteria are used in saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification to convert nitrogen-containing compounds into other N-containing compounds
• What is Nitrogen-fixing (in terms of the Nitrogen cycle)?

  • Bacteria can break the triple bond between the two nitrogen atoms in nitrogen gas in the atmosphere and fix this nitrogen into ammonium ions.
  • The bacteria are either free-living in the soil or symbiotic (mutualistic relationship), living in the root nodules of plants.
  • This is common in leguminous plants such as clover and beans.
• What if Nitrification (in terms of the Nitrogen cycle)?

  • The ammonium ions in the soil are converted to nitrite and then nitrate ions in the soil by nitrifying bacteria.
  • This is a two-stage oxidation reaction.
• What is Denitrification (in terms of the Nitrogen cycle)?

  • This stage is not useful as it returns the nitrogen in compounds back to nitrogen gas in the atmosphere, so it cannot be absorbed by plants.
  • This is done by anaerobic denitrifying bacteria
• What is Ammonification (in terms of the Nitrogen cycle)?

  • Proteins, urea and DNA can be decomposed in dead matter and waste by saprobionts.
  • These are bacteria and fungi that can digest waste extracellularly and return ammonium ions to the soil; this is saprobiotic nutrition.
• Why do organisms need nitrogen?

  To create amino acids, proteins, DNA, RNA & ATP
• What is phosphorous needed for?

  To create DNA, RNA, ATP and phospholipid bilayers.
• Describe how the phosphorous cycle differs from the nitrogen cycle?

  Phosphorous is not found as a gas in the atmosphere. Instead, it is mainly found as a phosphate ion, in mineral form in sedimentary rocks.
• What cycle is this?

  The Phosphorous cycle

  
• Why are fertilisers needed when growing crops?

  To replace the nitrate and phosphate ions lost when plants are harvested and removed from nutrient cycles as crops.
• What are the two types of fertilisers?

  • Natural (Manure)
  • Artificial (Inorganic chemicals)
• What are the pros and cons of natural fertiliser?

  Pros

  • Cheaper (free if farmer owns animals)

  Cons

  • Exact minerals and proportions cannot be controlled
• What are the pros and cons of artificial fertiliser?

  Pros

  • They are created with the exact proportions of minerals, making more efficient crop growth

  Cons

  • They are more expensive
  • They have higher solubility, so when rainfall washes them away, they have a greater impact on the environment
• What is leaching?

  This is when water-soluble compounds are washed away, often into rivers or ponds.
• What is eutrophication?

  1. This is when nitrates leached from fertilised fields stimulate the growth of algae in the pond.
  2. The excessive growth of algae creates a blanket on the surface of the water which blocks out light. As a result, the plants below cannot photosynthesise and die.
  3. Bacteria within the water feed and respire on the dead plant matter. This results in an increase in bacteria, which are all respiring and using up the oxygen within the water.
  4. Eventually, fish and other aquatic organisms die due to the lack of dissolved oxygen in the water.