5.3 Energy and ecosystems

How is biomass formed in plants?

• Plants make organic compounds from CO₂ during photosynthesis.

• Most synthesised sugars are used as respiratory substrates.

• The rest are used to make other biological molecules (e.g., carbohydrates, lipids, proteins), forming biomass.

How can biomass be measured?

• As mass of carbon.

• Or as dry mass of tissue per given area.

How is dry mass of tissue measured?

• Sample is dried in an oven (e.g., at 100°C), avoiding combustion.

• Sample is weighed and reheated at regular intervals until mass remains constant as (all water evaporated).

Why is dry mass more representative than fresh mass?

• Water volume in wet samples varies, so does not allow for comparison.

• This variation does not affect dry mass.

How is the chemical energy in dry biomass estimated?

• Using calorimetry.

1. A known mass of dry biomass is fully combusted (burnt).

2. The heat energy released heats a known volume of water.

3. The increase in temperature of the water is used to calculate the chemical energy of the biomass.

How does a stirrer in a calorimeter help?

• It evenly distributes heat energy in the water.

How does insulation in a calorimeter help?

• It reduces heat loss and gain to and from the surroundings.

Why is water used in a calorimeter?

• Water has a high specific heat capacity.

What is Gross Primary Production (GPP)?

• The chemical energy store in plant biomass, in a given area or volume, in a given time.

• It is the total energy transferred from light into chemical energy during photosynthesis.

What is Net Primary Production (NPP)?

• The chemical energy store in plant biomass after respiratory losses to the environment are taken into account.

State the formula for NPP.

• NPP = GPP – R

• R = respiratory losses to the environment.

Explain the importance of NPP in ecosystems.

• NPP is available for plant growth and reproduction.

• NPP is also available to other trophic levels, such as herbivores and decomposers.

What is primary or secondary productivity?

• The rate of primary or secondary production, respectively.

What are the units for primary or secondary productivity?

• kJ ha⁻¹ year⁻¹

• (Energy per unit area per year).

Why is productivity measured per unit area?

• Different environments vary in size.

• It standardises results to enable comparison between environments.

Why is productivity measured per year?

• To account for the effect of seasonal variation (e.g., temperature) on biomass.

• It makes data more representative and comparable.

Why is most light falling on producers not used in photosynthesis?

• Light is reflected or is the wrong wavelength.

• Light misses chlorophyll, chloroplasts, or photosynthetic tissue.

• CO₂ concentration or temperature is a limiting factor.

State the formula for net production of consumers (N).

• N = I – (F + R)

• I = chemical energy store in ingested food.

• F = chemical energy lost in faeces and urine.

• R = respiratory losses.

State the formula for efficiency of energy transfer.

• Efficiency = (Energy available after transfer / Energy available before transfer)

• Multiply by 100 for a percentage.

Why is energy transfer between trophic levels inefficient?

• Heat energy is lost via respiration.

• Energy is lost via parts not eaten (e.g., bones).

• Energy is lost via undigested food, lost as faeces.

• Energy is lost via excretion (e.g., urea in urine).

What is the overall goal of crop farming practices?

• To simplify food webs, reducing energy and biomass losses to non-human food chains.

How do herbicides increase energy transfer efficiency in crops?

• They kill weeds, reducing competition (e.g., for light).

• This directs more energy into crop biomass.

How do pesticides increase energy transfer efficiency in crops?

• They kill insect pests.

• This reduces the loss of biomass from crops being eaten.

How do fungicides increase energy transfer efficiency in crops?

• They reduce fungal infections.

• This allows more energy to be used for creating biomass.

How do fertilisers (e.g., nitrates) increase energy transfer efficiency in crops?

• They prevent poor growth caused by a lack of nutrients.

What is the overall goal of livestock farming practices?

• To reduce respiratory losses within the human food chain.

• This means more energy is available for creating biomass.

How does restricting movement and keeping animals warm help?

• It reduces the energy lost as heat from respiration.

Why are animals slaughtered while still young and growing?

• This is when most of their energy is used for growth, maximising biomass production.

How do antibiotics help increase energy transfer efficiency in livestock?

• They prevent the loss of energy due to pathogens (disease).

How does selective breeding increase energy transfer efficiency in livestock?

• It produces breeds with higher growth rates.