Biomass (AQA A Level Biology): Revision Note
Exam code: 7402
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

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:
set an oven to a low temperature
If the temperature is too high the sample may burn, which would cause it to lose biomass
weigh the crucible and record its mass
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
remove and weigh the crucible and sample at frequent intervals during the drying process
repeat step 4 until the mass of the crucible and sample stops decreasing
At this point the sample is fully dehydrated
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:
use a measuring cylinder to measure a set volume of water into a copper beaker
record the mass of the water
record the starting temperature of the water using a thermometer
record the starting mass of the dry sample
place the sample in a crucible and place beneath the beaker of water
set fire to the sample and allow it to burn until it has completely burned away
record the final temperature of the water
calculate the change in temperature of the water
calculate the energy transferred per gram of the dry sample as follows:
Energy transferred per gram =
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

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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