Energy transfers

• What is the law of conservation of energy?

  The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
• Plants are producers, as they convert light energy into chemical energy through photosynthesis. Energy is transferred through an ecosystem when organisms eat other organisms (heterotrophs) - plants are eaten by primary consumers, and they are eaten by secondary consumers. Each stage is a trophic level. Food chains/webs show how energy is transferred through an ecosystem
• What is the biomass of an organism?

  • The mass of living material of the organism/tissue
  • The chemical energy that is stored within the organism/tissue
• How can biomass be measured?

  • The dry mass of an organism/tissue in a given area
  • The mass of carbon that an organism/tissue contains - generally taken to be 50% of the dry mass of the sample
• Why do we use dry mass?

  Dry mass is the mass of the organism/tissue after all the water has been removed - this allows for a reliable comparison
• How do you work out biomass using dry mass?

  Multiply dry mass by a conversion factor. e.g if dry mass of 1 plant = 0.2kg, the biomass of 200 plants = 0.2 x 200 = 40kg
• How can biomass change over time?

  The biomass of deciduous trees decreases over autumn and winter as they lose their leaves - biomass is sometimes given with units of time as well (kg/yr-1)
• What is calorimetry?

  Can be used to estimate the chemical energy stored in biomass:

  1. First burn the sample of dry biomass in a calorimeter
  2. The burning sample heats a known volume of water
  3. The change in temperature of the water provides an estimate of the chemical energy the sample contains
• How to find the dry mass of a plant sample:

  1. Weigh the crucible without the sample
  2. Place the sample in the crucible and place the crucible in the oven
  3. Set the oven to a low temperature - prevents sample from burning causing it to lose biomass
  4. Remove and weigh the crucible at regular intervals until the mass is constant - the sample is now fully dehydrated
  5. Final mass - original mass = dry mass
• How to find the energy released by a sample of plant biomass?

  • A calorimeter burns the dried sample and uses the energy released to heat a known volume of water
  • Measure the change in temperature of the water
  • This temperature change can be used to estimate the chemical energy stored within the sample

  
• Describe a bomb calorimeter:

  .

  
• Limitations of calorimetry

  • It can take a long time to fully dehydrate (dry out) a plant sample to find its dry mass
  • The sample has to be heated at a relatively low temperature to ensure it doesn't burn
  • Depending on the size of the sample, the drying process could take several days
  • Precise equipment is needed, which may not be available
• Equipment needed for accurate calorimetry

  • A very precise digital balance should be used to measure the mass of the plant sample as it is drying (to detect even extremely small changes in mass)
  • A very precise digital thermometer should be used when measuring the temperature change of the water in the calorimeter (again, to detect even very small temperature changes)
• Simple/basic calorimeter

  Less accurate estimate of the chemical energy contained within the plant sample
• Bomb calorimeter

  • Ensures that almost all the heat energy from the burning sample is transferred to the water, giving a highly accurate estimate
• Why isn't most of the sun's energy converted to biomass?

  Plants only convert between 1 - 3% of the sun's energy into organic matter because:

  • Over 90% of the Sun's energy is reflected back into space by clouds/ absorbed into the atmosphere
  • Not all wavelengths of light can be absorbed
  • Light may not fall on a chlorophyll molecule
  • A factor e.g low CO2 may limit rate of photosynthesis
• How to work out net primary production:

  Gross primary production - respiratory losses

  Gross primary production = total quantity of the chemical energy store in plant biomass, in a given area/volume

  Plants use 20-50% of this energy in respiration, therefore the chemical energy that is left after respiratory losses is Net Primary Productivity
• Why is a low percentage of energy transferred at each trophic level?

  Less than 10% of NPP in plants is used by primary consumers for growth - secondary consumers are slightly more efficient, and transfer 20% of the energy from their prey into their own bodies. This is because:

  • Some of the organism isn't consumed
  • Some parts are consumed, but cannot be digested
  • Some of the energy is lost in excretion
  • Some energy losses occur as heat from respiration and lost to the environment - losses are high in mammals due to their high body temperature
• How can net primary productivity be calculated?

  N = I - (F+R)

  N = net production

  I = chemical energy store of ingested food

  F = energy lost in faeces and urine

  R = energy lost in respiration
• The relative inefficiency of energy transfer between trophic levels explains why :

  • Most food chains have only 4 or 5 trophic levels, as insufficient energy is available to support a large enough breeding population at trophic levels higher than those
  • Biomass is less at higher trophic levels
  • Total amount of energy available is less as you move up a food chain
• How can you calculate efficiency of energy transfers between trophic levels?

  % efficiency = (energy available after transfer/energy available before transfer) x 100

  Energy is measured in kilojoules per square metre per year (kJ m-2 yr-1)
• Why do we use farming practices?

  Increase yields by increasing the efficiency of energy transfer along the food chains.

  As energy passes along a food chain, only a small percentage passes from one organism to the next, as lots of energy is lost as heat during respiration. Farming practices reduce energy loss, and increase the yield
• Intensive rearing of domestic livestock

  • Converts the smallest possible quantity of food energy into the greatest quantity of animal mass
• Energy conversion efficiency

  1. Ensure energy from respiration goes into growth

  2. Keep animals in confined spaces (factory farming)

  3. Restrict movement so less energy used in muscle contraction

  4. Keep environment warm to reduce heat loss

  5. Control feeding to provide optimum amount/type of food for maximum growth and no wastage

  6. Exclude predators so no loss to other organisms
• Intensive rearing of domestic livestock increases productivity
• Another farming practice that increases efficiency of energy transfer is by simplifying food webs e.g reducing/eliminating organisms that compete with the plant/animal. Weeds compete with crop plants for light, water CO2 etc , meaning less is available for the crop.

  Insect pests may damage the leaves of crops, limiting their ability to photosynthesise, and therefore reducing their productivity, or they may eat the crop itself.

  Many crops are now grown in monoculture