Biomass and productivityAQA A-Level Biology: Revision notes

Section 1

From carbon dioxide to biomass

In any ecosystem, plants synthesise organic compounds from atmospheric or aquatic carbon dioxide by photosynthesis. Most of the sugars made are used by the plant as respiratory substrates, releasing energy for the plant's processes (and eventually lost to the environment as heat). The rest are used to make other biological molecules, such as cellulose, lipids and proteins, which form the plant's biomass.

Key termsbiomassrespiratory substrate

Section 2

Measuring biomass

Biomass can be measured as the mass of carbon or the dry mass of tissue in a given area.

Fresh mass is unreliable because the amount of water varies. A sample is dried in an oven at a low temperature, reweighed at intervals, and the mass is constant when all the water has gone.
Mass is given per unit area, e.g. g m⁻² or kg ha⁻¹. For 240 g in 1 m², 240 × 10 000 = 2 400 000 g, which is 2400 kg ha⁻¹.
Obtaining dry mass destroys the sample, so it is taken from a representative sample.

Key termsdry massconstant mass

Common mistake

Using fresh mass as 'biomass'. It includes water, which is not organic material and varies between samples.

Section 3

Calorimetry

The chemical energy store in dry biomass can be estimated by calorimetry. A known mass of dried material is burned beneath a known mass of water, and the temperature rise is measured.

$E = m_{\text{water}} \times c \times \Delta T$

Example: 100 g of water with $c = 4.2\ \text{J g}^{-1}\ ^\circ\text{C}^{-1}$ rising by 19.0 °C gives $E = 100 \times 4.2 \times 19.0 = 7980$ J. For 0.50 g of sample, the energy store is $7.98 \div 0.50 = 16$ kJ g⁻¹.

Results tend to be underestimates because heat is lost to the surroundings and combustion may be incomplete; insulation or a bomb calorimeter improves accuracy.

Key termscalorimetryspecific heat capacity

Exam tip

Show the full calculation: energy = mass of water × 4.2 × temperature rise, then divide by the mass of sample.

Section 4

Gross and net primary production

Gross primary production (GPP) is the chemical energy store in plant biomass, in a given area or volume, made by photosynthesis.

$\text{NPP} = \text{GPP} - R$

where R is the energy lost to the environment through respiration. Net primary production (NPP) is the energy that remains. It is available for plant growth and reproduction and to other trophic levels, such as herbivores and decomposers.

Example: GPP 21 000 and R 8 400 kJ m⁻² year⁻¹ give NPP = 12 600 kJ m⁻² year⁻¹, so 40 % of GPP was lost in respiration.

Key termsGPPNPPrespiratory loss

Common mistake

Saying that NPP is the energy 'used' by the plant. It is the energy stored in new biomass after respiratory losses.

Section 5

Productivity as a rate

Primary productivity is the rate of primary production, and secondary productivity is the rate of production by consumers. Each is measured as biomass (or energy) in a given area in a given time, for example kJ ha⁻¹ year⁻¹.

A store (kJ m⁻²) becomes a rate when divided by time, so check the units in a question to decide whether it asks for production or productivity.

Key termsproductivityprimary productionsecondary production

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Biomass and productivity

An ecologist estimates the biomass of a meadow. She cuts all the vegetation from several 1.0 m² quadrats, dries it in an oven at 80 °C and reweighs it at intervals until the mass stops changing. The mean dry mass of vegetation per quadrat is 240 g.
The ecologist's quadrats are 1.0 m² and 1 ha = 10 000 m². Calculate the mean dry mass of the meadow vegetation in kg ha⁻¹.2 marks
A student measures the chemical energy stored in dried meadow plants using calorimetry. She burns 0.50 g of dried, ground plant material beneath a boiling tube containing 100 g of water. The temperature of the water rises by 19.0 °C. The specific heat capacity of water is 4.2 J g⁻¹ °C⁻¹.
Calculate the energy released per gram of dried plant material, in kJ g⁻¹.2 marks
A conifer plantation of area 1 ha has a gross primary production (GPP) of 21 000 kJ m⁻² year⁻¹. The trees lose 8 400 kJ m⁻² year⁻¹ to the environment as a result of respiration. A second plantation (B) of the same area has the same GPP but a net primary production (NPP) of 15 750 kJ m⁻² year⁻¹.
Calculate the net primary production (NPP) of the first plantation in kJ m⁻² year⁻¹, and state what this energy is available for.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).