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5.3.3Energy Transfers In and Between Organisms (A Level only)

Biomass — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 15 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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5.3.3 Biomass — AQA A Level Biology 7402 notes: Define biomass and describe how it is measured and expressed in terms of dry mass and chemical energy.Biomass is the mass of living material, or the organic material of organisms, in a given area or at a given trophic level.It is us…

  • calorimetry
  • Exam-style question 1
  • Exam-style question 2
  • Calorimeter
  • Constant mass
  • Standing crop

Section 1 of 8What biomass is

Biomass is the mass of living material, or the organic material of organisms, in a given area or at a given trophic level.

Dry mass
It is usually measured as dry mass because the amount of water in tissue varies a great deal between samples and over time,
  • whereas the dry mass represents the carbon-containing compounds that store energy.
Biomass is usually expressed per unit area, for example g m-2 or kg ha-1, or per unit volume in aquatic systems.

Biomass is an indirect measure of the chemical energy stored in an ecosystem, because the energy stored per gram of dry organic material is fairly constant for similar tissue types. It can be measured for a whole community, for a single trophic level, or for a single species, and is the quantity that is built up as an ecosystem grows.

Section 2 of 8Measuring dry mass

To find the dry mass of a sample of plant material:

1
Take a representative sample, with enough replicates to reduce the effect of variation. Take samples at random from the area.
2
Weigh the fresh sample to get its fresh mass.
3
Heat in an oven at a temperature that dries the material but does not burn it, typically around 80°C,
  • so that water is driven off without combustion of organic matter.
4
Weigh, reheat, and reweigh at intervals until the mass is constant. This shows that all the water has been removed.

Because the process destroys the sample, a large-scale study uses a small sub-sample to calculate the dry mass as a fraction of fresh mass, and applies that ratio to the rest. This respects ethical and conservation concerns by limiting the amount of living organism removed.

Section 3 of 8Measuring chemical energy with a calorimeter

The energy stored in biomass can be found by calorimetry.

Measuring chemical energy with a calorimeter
A known mass of dry biomass is burned completely in oxygen in a sealed chamber (a bomb calorimeter) surrounded by a known mass of water.
The energy released raises the temperature of the water.
The energy transferred is calculated using the specific heat capacity of water, 4.18 J g-1 °C-1:

energy released (J) = mass of water (g) × 4.18 × temperature rise (°C)

Dividing by the mass of the sample gives the energy per gram, which can be scaled to the area sampled to give the energy per unit area, for example kJ m-2. Some energy escapes to the apparatus and surroundings, so the result tends to underestimate the true value. Insulation, a lid and stirring the water reduce this error.

Section 4 of 8Biomass through a food chain

Biomass is highest at the lowest trophic level and generally falls at each subsequent level, because energy is lost at each transfer and so there is less to build new tissue.

Biomass through a food chain
A pyramid of biomass shows the dry mass at each level at a given moment.
It can look misleading in aquatic systems,
  • since phytoplankton are small, reproduce rapidly and have a low standing crop but a high production rate;
    • this can give an inverted pyramid at a single time of sampling.
A pyramid of energy, which shows energy transferred per unit area per year, is never inverted.

Biomass also changes seasonally and with succession: it generally rises during the growing season and, in a developing community, increases until a climax is reached.

Section 5 of 8Evaluating biomass data

When interpreting biomass data, ask what was sampled and when.

1
Seasonal sampling matters: a deciduous wood has a high standing crop in summer and far less foliage in winter.
2
Sample size matters too: too few quadrats give an unrepresentative mean, so a large number of random samples reduces the effect of chance variation.
3
Compare biomass values only when they have the same units and area.
4
Percentage change in biomass over time can be calculated as (final − initial) / initial × 100,
  • and used to compare treatments such as fertilised and unfertilised plots.
5
Always remember that dry mass includes the mass of carbon and other elements,
  • whereas the energy content depends on the compounds present;
    • oil-rich seeds store much more energy per gram than a leaf of the same dry mass.

Section 6 of 8Exam-style questions

  1. Describe how you would determine the dry mass of a sample of grass from a field. [4]4 marks

  2. A sample of 1.5 g of dried seeds was burned in a calorimeter and heated 250 g of water by 17.0 °C. Calculate the energy per gram of seed. [3]3 marks

Section 7 of 8Key terms

TermMeaning
BiomassMass of biological material in an area, usually measured as dry mass.
Dry massMass of tissue after all water has been removed.
CalorimeterApparatus used to measure energy released when a sample is burned.
Constant massMass that no longer changes on repeated drying, showing all water has gone.
Standing cropBiomass present at a particular moment.

Section 8 of 8Exam technique

1
Using fresh mass as biomass. Water content varies, so dry mass is used.
2
Drying at too high a temperature. This burns organic matter and loses mass that is not water.
3
Weighing once after drying. The mass must be shown to be constant.
4
Forgetting units per areabiomass must be stated per unit area to be compared.

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