5.3 Energy Transfers

• How can we measure biomass?

  Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area per given time.
• What are the units for biomass?

  For units you would use gm-2year-1. The units can be changed respectively but try to keep it in grams. For aquatic habitats we will use volume instead: gm-3year-1.
• What is fresh mass unreliable for biomass?

  Fresh mass is unreliable as the amount of water varies between habitats so values can’t be compared, whereas dry mass allows a comparison to be made.
• How can dry mass be determined?

  Dry mass can be determined by heating the species.
• How do we know when a mass of tissue is fully dried?

  We know it is fully dry when the mass of the species stops decreasing.
• Why do we have to be careful to not overheat a mass of tissue that we are drying?

  This is to avoid mass loss due to combustion of the material.
• Why is dry mass unreliable?

  Dry mass is unreliable as it involves killing organisms so samples tend to be small and therefore may not be representative.
• The chemical energy stored in dry biomass can be estimated using calorimetry.
• How is calorimetry carried out to estimate the chemical energy stored in dry biomass?

  1. Weigh the sample of dry material.
  2. Burn the sample in pure oxygen in a sealed chamber surrounded by water of a known volume.
  3. The heat of combustion causes the temperature of the water to increase.
  4. The heat required to increase the temperature of 1g of water by 1°C is one calorie.

  Through this the energy release in kJ can be calculated.
• In calorimetry to calculate the chemical energy stored in dry biomass you need to measure:

  • The dry mass.
  • The water volume.
  • The temperature change of the water.
• What is the units for energy release per mass?

  $kJ/kg^{-1}$
• Label the following diagram:

  A) Thermometer

  B) Water

  C) Air

  D) Stirrer

  
• How do the following equipment ensure an accurate measure of energy?

  • The stirrer can distribute the heat energy.
  • Air insulates to reduce heat loss by conduction.
  • Water has a high specific heat capacity.
• Why is measuring energy better than measuring biomass?

  Measuring energy is better than biomass because two organisms of the same dry biomass may store different amounts of energy. For example, fat stores more energy than the same quantity of carbohydrates.
• What is the equation to calculate energy in calorimetry?

  Signs you may need: Δ.

  q = (mcΔT)/g
• Gross primary production is the rate of biomass production. It's units are 'mass per area/ volume per time', e.g. kgha-1year-1 or gcm-3day-1. Different ecosystems have different gross primary productions.
• Why is most of the Sun’s energy not converted into chemical energy by plants?

  • More than 90% of the energy is reflected back by dust or clouds.
  • Light may be of the wrong wavelength to be absorbed by the pigments the leaves contain.
  • Light may not hit the chloroplast or specifically hit chlorophyll.
  • Other factors may limit the rate of photosynthesis, like carbon dioxide concentration.
• What is the equation for net primary production?

  NPP = GPP - Respiration
• What is the equation for gross primary production?

  GPP = NPP + Respiration
• Why is a low percentage of energy transferred at each stage of the good chain?

  • Some parts of the organism are not eaten, like bones and fur.
  • Some parts of the organism that are eaten are not digested, like fibres in humans.
  • Some energy is lost via excretion, like urine.
  • Heat energy is lost during respiration, like during movement or homeostasis.
• What is net production?

  Net production is the energy in biomass available to the next trophic level.
• What is the equation for net production?

  N = Ingested food - Faeces - Respiration
• Respiration and heat energy are the same!
• What is the equation for percentage efficiency in terms of energy transfer?

  $\frac{\textit{energy available after transfer}}{\textit{energy available before transfer}} \times 100$
• How does the intensive rearing of domestic livestock increase net productivity?

  • Slaughtered when still growing so that more energy is transferred into biomass.
  • They’re fed on a controlled diet so that a higher proportion of digested food is absorbed.
  • Movement is restricted, leading to less respiratory losses.
  • They’re kept inside; providing heating so that less energy is used in respiration.
  • Genetically selected for high productivity.
• Weeds compete with crop plants for water, minerals, e.g. nitratesm, carbon dioxide and light. These factors may limit the rate of photosynthesis of the crop plant, thereby limiting growth and productivity.
• Insect pests can damage the leaves of crop plants or by eating the crop plant they will reduce the biomass and therefore the energy available to the next trophic level.
• Now that many crops are grown in monoculture, care has to be taken to prevent the rapid spread of disease caused by insects and fungi.
• The aim of pest control is to limit the effects of pests on productivity to a commercially acceptable level (cost-benefit).