Biomass (AQA A Level Biology): Revision Note

Exam code: 7402

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Naomi Holyoak

Updated on

Biomass

  • The term biomass can be defined as:

the mass of living material present in an organism or tissue sample

  • Biomass can be measured in terms of:

    • the dry mass of an organism or tissue sample per given area; this is the mass of tissue after water has been removed

      • The water content of living tissue can vary, so dry mass is a more reliable measure

      • Units for dry mass are mass per unit area, e.g. kg m-2

    • the mass of carbon present in an organism or tissue sample per given area: this is taken to be 50 % of the dry mass

Calculating dry mass 

  • The dry mass of a sample can be scaled up to calculate the biomass of a total population or area

Diagram illustrating fern biomass calculation, showing 1 m² with dry mass of 0.3 kg/m² and scaling to 100 m², resulting in biomass of 30 kg.
It is possible to estimate the biomass of organisms in a larger area using values from a smaller sample area

Worked Example

The dry mass of grass taken from a sample area of 0.5 m2 was found to be 0.1 kg.

Calculate the total dry mass for a field with an area of 200 m2

Answer:

Step 1: determine the multiplication factor

  • Calculate the number of times 0.5 fits into 200

200 ÷ 0.5 = 400

Step 2: multiple the dry mass of the sample by the multiplication factor

400 x 0.1 = 40 kg m-2

Measuring chemical energy

  • The biomass of an organism is formed from organic molecules, so biomass is a measure of the chemical energy stored in an organism or tissue sample

  • The chemical energy stored in dry biomass can be measured using calorimetry

Examiner Tips and Tricks

Remember that biomass is a measure of mass, and not of the number of organisms present, so be careful with the language that you use to describe biomass. E.g. avoid vague phrases such as "amount of organisms" and instead use terms such as mass of living tissue.

Finding the dry mass and energy value of plant biomass

Determining dry mass

  • To find the dry mass of a sample, it must be dried out until it contains no more water

  • This can be accomplished using a crucible, oven and a digital balance as follows:

    1. set an oven to a low temperature

      • If the temperature is too high the sample may burn, which would cause it to lose biomass

    2. weigh the crucible and record its mass

    3. place the sample in the crucible and place the crucible in the oven

      • The crucible has no lid, allowing any moisture leaving the sample to evaporate

    4. remove and weigh the crucible and sample at frequent intervals during the drying process

    5. repeat step 4 until the mass of the crucible and sample stops decreasing

      • At this point the sample is fully dehydrated

    6. subtract the original mass of the crucible, calculated at step 2, from the mass determined in step 5 to find the dry mass of the sample

Limitations

  • It can take a long time to fully dehydrate a plant sample to find its dry mass; the drying process could take several days for larger samples

  • Precise equipment may not be available in a school laboratory

    • A very precise digital balance should be used to detect extremely small changes in mass

Determining stored energy

  • A calorimeter can be used to estimate the chemical energy stored within a dried plant sample as follows:

    1. use a measuring cylinder to measure a set volume of water into a copper beaker

    2. record the mass of the water

    3. record the starting temperature of the water using a thermometer

    4. record the starting mass of the dry sample

    5. place the sample in a crucible and place beneath the beaker of water

    6. set fire to the sample and allow it to burn until it has completely burned away

    7. record the final temperature of the water

    8. calculate the change in temperature of the water

    9. calculate the energy transferred per gram of the dry sample as follows:

Energy transferred per gram = fraction numerator m a s s italic space o f italic space w a t e r italic space stretchy left parenthesis g stretchy right parenthesis italic space italic cross times italic space t e m p e r a t u r e italic space c h a n g e italic space stretchy left parenthesis degree C stretchy right parenthesis italic space italic cross times italic space italic 4 italic. italic 2 over denominator m a s s italic space o f italic space s a m p l e end fraction

  • Energy is measured in joules (J) or kilojoules (kJ)

    • 1 joule is the energy needed to raise the temperature of 0.24 g of water by 1 °C

Diagram of a calorimeter setup with components labelled: thermometer, lid, draught excluder, copper container with water, crucible, and burning plant sample.
A calorimeter can be used to determine the energy stored in a dry tissue sample

Limitations

  • The more simple and basic the calorimeter, the less accurate the energy estimate will be

    • Heat energy from the burning sample may be transferred to the surrounding environment and not to the water

  • A bomb calorimeter can give a highly accurate estimate, but is an expensive piece of equipment for schools

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Alistair Marjot

Author: Alistair Marjot

Expertise: Environmental Systems and Societies & Biology Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Naomi Holyoak

Reviewer: Naomi Holyoak

Expertise: Biology Content Creator

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.

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