• The?biomass?of an organism (or of a sample of tissue from an organism) is:
  • The mass of?living material?of the organism or tissue
  • The?chemical energy?that is?stored?within the organism or tissue

   

• Biomass can be measured in terms of:
  • The?dry mass?of an organism or tissue (in a?given?area)
  • The?mass of carbon?that an organism or tissue contains
  • The mass of?carbon?that a sample (i.e.?an organism or tissue) contains is generally taken to be?50% of the dry mass?of the sample

   

Dry mass?

• The dry mass is the mass of the organism or tissue?after all the water has been removed
• The dry mass of a sample can be used to?calculate?the biomass of a?total population?of organisms or of a?particular area. For example:
  • If the dry mass of one daffodil plant is found to be?0.1 kg, then the dry mass (i.e.?the biomass) of?200?daffodils would be?20 kg?(0.1 x 200 = 20)

   

It is possible to estimate the biomass of a group of organisms if you know the dry mass of a single organism

• • If the dry mass of the grass from?1 m2?of a field is found to be?0.2 kg, we can say that the grass has a dry mass (i.e.?biomass) of?0.2 kg m?2?(this means 0.2 kg per square?metre). If the grass field is?200 m2?in size, then the biomass of the whole field must be?40 kg?(0.2 x 200 = 40)

   

It is possible to estimate the biomass of organisms in a larger area if you know the dry mass of the organisms in a given (smaller) area

Biomass can?change?over time

• For example, the biomass of deciduous trees decreases over autumn and winter as they lose their leaves
• This means that biomass is sometimes given with?units of time?as well
• This shows the average biomass of an organism within a given area over that time period
• For example, if the average biomass of a group of oaks trees over the course of a?year?is found to be?13,000 kg, the biomass may be given as?13,000 kg y?1?(this means 13,000 kg per year)
• If the average biomass of?1 m2?of a grass field over the course of a year is found to be?0.15 kg m?2, the biomass may be given as?0.15 kg m?2?yr?1?(this means 0.15 kg per square?metre?per year)

Calorimetry?

• Calorimetry can be used to?estimate?the?chemical energy?stored in dry biomass
  • This involves?burning?the sample of dry biomass in a piece of equipment known as a?calorimeter
  • The burning sample heats a?known volume?of water
  • The?change in temperature?of the water provides an estimate of the chemical energy the sample contains

   

• Dry mass?is used to measure the mass of living material in a plant sample
  • The dry mass is the mass of the organism or tissue after?all?the?water?has been?removed
  • To find the dry mass, the sample must first be?dehydrated?(dried out until it contains no more water)

   

• A?calorimeter?can then be used to?estimate?the?chemical energy?stored within the dried plant sample

Apparatus

• A heat-proof, open-topped container
  • This is often referred to as a?crucible
  • It needs to be able to withstand the temperatures inside the oven
  • It is open-topped to allow any moisture leaving the sample can escape and evaporate

   

• Oven
  • To slowly dry the sample

   

• Digital balance
  • For monitoring the mass of the plant sample as it dries out
  • Needs to have a?high level of precision?to detect?small changes?in the mass of the sample

   

• Calorimeter
  • These can be?simple and inexpensive?(easy for students to set up using classroom equipment) or very?precise,?expensive?pieces of apparatus known as?bomb calorimeters?(more commonly found in professional scientific laboratories)

   

Method

Finding the dry mass of a plant sample:

• Weigh the crucible (heat-proof container)?without?the sample first
• Place the sample in the crucible
• Place the crucible in the oven
• Set the oven to a?low?temperature (if the temperature is too high the sample may burn, which would cause it to lose biomass)
• Remove and weigh the crucible (containing the sample) at?regular intervals?during the drying process
• Once the mass of the crucible (and sample)?stops decreasing?and becomes?constant, the sample is?fully dehydrated?(all the water has been removed)
• From this final constant mass,?subtract the original mass of the crucible?(without the sample in it) to find the?dry mass?of the sample

Finding the energy released by a sample of plant biomass:

• To estimate the chemical energy stored in the dried sample, use a?calorimeter
• 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
• This energy is measured in?joules?(J) or?kilojoules?(kJ)
• 1 joule is the amount of heat needed to raise the temperature of?24 g?or?0.24 cm3?or?0.24 ml?of water by?1 °C
• 1 kilojoule (kJ) is the amount of heat needed to raise the temperature of?240 g?or?240 cm3?or?240 ml?of water by?1 °C?(or to raise the temperature of 24 g of water by 10 °C)

A simple, inexpensive version of a calorimeter that can be set up using classroom equipment

An example of a more precise (and much more expensive) version of a calorimeter known as a bomb calorimeter - this type is used in professional scientific laboratories

Limitations

• It can take a?long time?to?fully?dehydrate (dry out) a plant sample to find its dry mass
  • This is partly because 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
  • 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)
  • It is preferable to use a very precise digital thermometer when measuring the temperature change of the water in the calorimeter (again, to detect even very small temperature changes)

   

• The more?simple?and?basic?the calorimeter, the?less accurate?the estimate will be for the chemical energy contained within the plant sample
  • This is due to?heat?energy?from the burning sample being?lost?and not being transferred efficiently to the water
  • A?bomb calorimeter?ensures that almost?all?the heat energy from the burning sample is transferred to the water, giving a?highly accurate estimate