Energy and Ecosystems (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

All living organisms need organic compounds, both as respiratory substrates and as the raw materials for building their tissues. In an ecosystem these compounds are first synthesised by plants (primary producers) during photosynthesis, which converts light energy into chemical energy stored in sugars such as glucose.

Most of the sugars made by a plant are used as respiratory substrates, supplying ATP for the plant's own metabolism.

Biomass is the mass of living material present in an organism or tissue sample. It is normally measured as dry mass: the mass of tissue after all the water has been removed, expressed per given area (or volume). Dry mass is used because the water content of tissue varies a great deal and contains no stored energy.

Scaling a sample up to a whole area uses a multiplication factor. Worked idea: if a sample was taken from 0.5 m² and you want the biomass of 200 m², step 1 is to work out how many times 0.5 fits into 200 (200 ÷ 0.5 = 400); step 2 is to multiply the dry mass of the sample by 400.

Method for finding dry mass:

Limitations

• fully dehydrating a sample can take a long time (several days for larger samples);
  • precise equipment may not be available in a school laboratory;
  • and a very precise digital balance is needed to detect very small changes in mass.
• The method also destroys the sample.

Biomass is formed from organic molecules, so it is a measure of the chemical energy stored in an organism or tissue sample. A calorimeter can be used to find the energy in a dry sample: the sample is burned and the heat released warms a known mass of water, and the temperature rise is used to calculate the energy transferred.

The simpler and more basic the calorimeter, the less accurate the estimate, because heat energy from the burning sample may be transferred to the surroundings and not to the water, so the energy value is underestimated.

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

Because GPP is a measure per unit area, answers must always include units containing both a mass or energy and an area.

GPP is biomass (or energy) per unit area.

Practice example

• a patch of rainforest of 1 km² contains 1500 kg of biomass.
• Since 1 km² is 1 000 000 m² and 1500 kg is 1 500 000 g, the GPP is 1 500 000 ÷ 1 000 000 = 1.5 g m^-2.

Plants convert light energy to chemical energy in glucose during photosynthesis; the total stored this way is GPP. Some of that energy is lost to the environment as heat through plant respiration. Net primary production (NPP) is the chemical energy stored in plant biomass after respiratory losses to the environment have been taken into account:

NPP = GPP − R (R = respiratory losses)

GPP is the total energy transferred in photosynthesis; NPP is what remains after respiration. The concept of NPP applies only to producers, not animals, so the NPP equation includes respiratory loss only, not losses in faeces and urine.

NPP is the energy available to other trophic levels: it transfers to consumers when plant biomass is eaten by herbivores, or is passed to decomposers when plant material dies.

Consumers (animals) gain energy from the food they ingest but lose some of it. Their net production is the chemical energy stored in their tissues after losses:

N = I − (F + R)

Method

• write out the equation, substitute the known values, calculate N and give appropriate units.
• Energy is lost in faeces and urine because some ingested material is undigested or cannot be absorbed (and so is egested) and because waste nitrogenous compounds such as urea still contain chemical energy.

Do not confuse net primary production (energy stored in producers after respiration) with net production of consumers (energy stored in consumer tissues after losses).

Production is a total stored in biomass; productivity is the rate of production (production per unit area per unit time, e.g. kJ m^-2 yr^-1).

To find average primary productivity of, for example, a sunflower crop over a time period: step 1 calculate the total primary production over the period; step 2 divide by the number of years (or other time unit) and give units such as kJ m^-2 yr^-1.

Net productivity of consumers uses N = I − (F + R), where I is the chemical energy in ingested food, F the energy lost to the environment in faeces and urine, and R respiratory losses.

% efficiency = (productivity of trophic level ÷ energy available from level below) × 100

Worked example

• calculate the percentage of solar energy converted into chemical energy by photosynthesis.
• Substituting: (4.25 × 10³ ÷ 6 × 10⁵) × 100 = 4250 ÷ 600 000 × 100 = about 0.71 %.
• Only a tiny fraction of the light striking a plant is converted into stored chemical energy.

For a consumer such as a toad: step 1, calculate net productivity of the toads using N = I − (F + R); step 2, write out the equation for percentage efficiency and insert the known values (N divided by the energy available from the level below, which is the energy of the prey eaten, then × 100).

Farming practices can increase production so that more food is available for human consumption.

% yield = (actual yield ÷ theoretical yield) × 100

The percentage yield is never 100 % because limiting factors such as light intensity, temperature, water and mineral availability, plus pests, disease, respiration losses and parts of the plant that are not harvested, mean the maximum theoretical production is not achieved.

When answering energy-transfer calculations, follow a fixed routine: (1) write the correct equation; (2) substitute values with matching units; (3) calculate and round sensibly (usually 2 or 3 significant figures); (4) give units or a % sign. Efficiency between trophic levels is generally low (typically 5 to 20 %) because energy is lost as heat in respiration, as faeces and urine, and because not all of an organism is eaten.