AQA A-level Biology: Energy Transfer and Productivity in Ecosystems (7402) – Revision Notes

These are condensed revision notes for section 3.5.3, Energy and ecosystems, in the AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015. The section is A-level only. For full explanations and longer worked examples read the study guide, then test yourself with the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. If you are unsure which topics to revise first, use the free diagnostics.

Definitions to learn word for word

Term	Meaning
Biomass	Mass of the biological molecules in organisms, measured as mass of carbon or as dry mass of tissue per given area
Gross primary production (GPP)	The chemical energy store in plant biomass, in a given area or volume
Net primary production (NPP)	The chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
Primary productivity	The rate of primary production: biomass in a given area in a given time
Secondary productivity	The rate of secondary production, with the same kind of unit
R	Respiratory loss to the environment
I	Chemical energy store in ingested food
F	Chemical energy lost to the environment in faeces and urine
N	Net production of a consumer

Formulae

Quantity	Equation	Units
Net primary production	NPP = GPP - R	kJ ha⁻¹ year⁻¹
Respiratory loss of plants	R = GPP - NPP	same
Net production of consumers	N = I - (F + R)	same
Respiratory loss of consumers	R = I - F - N	same
Energy from calorimetry	mass of water × 4.18 × temperature rise	J
Energy per gram	energy ÷ mass of dry tissue burned	J g⁻¹ or kJ g⁻¹
Efficiency of transfer	energy in next level ÷ energy available × 100	%
Area conversion	1 ha = 10 000 m²

Where the energy goes

Plants make organic compounds from carbon dioxide. Most of the sugar is used as a respiratory substrate. The rest is made into other biological molecules, and these form the plant biomass. NPP is available for plant growth and reproduction, and to herbivores and decomposers.

For consumers, the energy in I ends up in one of three places: faeces and urine (F), heat from respiration (R), or new biomass (N). The three must add up to I. Use that as a check on any answer.

Method boxes

Dry mass to biomass per area

1. Weigh fresh sample and note the area sampled.
2. Dry in an oven without burning, re-weigh until constant mass.
3. Dry mass ÷ area gives g m⁻². Multiply by 10 000 for g ha⁻¹.
4. Take samples from many places, as drying destroys the tissue.

Calorimetry

1. Weigh dried tissue and a known mass of water.
2. Record the starting water temperature.
3. Burn the tissue under the water, stirring, and record the highest temperature.
4. Energy = water mass × 4.18 × rise. Divide by the tissue mass.
5. Expect an underestimate, because of heat loss and incomplete combustion.

Efficiency

1. Put both energy values in the same unit, area and time.
2. Divide the amount in the next level by the amount available.
3. Multiply by 100 and round sensibly, in line with the data given.

Worked reminders

NPP. GPP is 920 kJ m⁻² year⁻¹ and R is 340. NPP = 920 - 340 = 580 kJ m⁻² year⁻¹, which is 63.0% of GPP.

Consumer. A beetle population ingests 1 500, loses 520 in faeces and 640 in respiration (kJ m⁻² year⁻¹). N = 1 500 - (520 + 640) = 340 kJ m⁻² year⁻¹. N ÷ I = 22.7%.

Calorimetry. 25.0 g of water rises by 12.0 °C when 0.40 g of tissue burns. Energy = 25.0 × 4.18 × 12.0 = 1 254 J. Per gram = 1 254 ÷ 0.40 = 3 135 J g⁻¹, or 3.14 kJ g⁻¹.

Carbon. If 46% of dry mass is carbon (a value you would be given), 12 400 kg of dry biomass contains 12 400 × 0.46 = 5 704 kg of carbon.

Units checklist

• Energy store per area: kJ m⁻² or kJ ha⁻¹. For a water volume: kJ m⁻³.
• Productivity adds a time: kJ m⁻² year⁻¹, kJ ha⁻¹ year⁻¹ or kJ m⁻² day⁻¹.
• Dry mass productivity: g m⁻² year⁻¹ or kg ha⁻¹ year⁻¹.
• m² to ha: 1 ha = 10 000 m², so a value per m² is 10 000 times larger per ha.
• Year to day: divide by 365. For example, 6 800 kJ m⁻² year⁻¹ is 18.6 kJ m⁻² day⁻¹.
• Efficiency and percentage yield have no unit, only the % sign.

Must-know distinctions

• GPP and NPP. GPP is the total energy fixed. NPP is what remains after plant respiration.
• Production and productivity. Production is an amount stored. Productivity is a rate, so it always has a time unit.
• Primary and secondary. Primary refers to producers. Secondary refers to consumers.
• R, F and N. R is lost as heat from respiration, F is chemical energy still in the waste, and N is stored in biomass.
• Fresh and dry mass. Fresh mass includes variable water. Biomass uses dry mass.
• Energy and mass. Energy values come from calorimetry or from data. Do not mix them with dry mass in one efficiency calculation.

Farming practices

Practice	How it raises efficiency	Cost to mention
Herbicides	Kill competing weeds, so more NPP is stored in the crop	Loss of plant diversity
Pesticides	Kill pests that eat the crop, so less energy goes to a non-human food chain	Harm to non-target species, pollution
Restricting movement or housing animals	Less respiration, so larger N and more growth	Animal welfare
Warm housing	Less energy spent keeping body temperature constant	Cost of heating

Both groups serve the same aim: put more energy into human food. The first two simplify food webs to reduce energy losses to non-human food chains. The last two reduce respiratory losses within a human food chain.

Quick self-test

1. Calculate NPP when GPP is 1 260 kJ m⁻² year⁻¹ and R is 455 kJ m⁻² year⁻¹.
2. A consumer ingests 3 200 kJ, loses 1 050 kJ in faeces and urine and 1 480 kJ in respiration. Calculate N and N as a percentage of I.
3. Convert 1.8 × 10³ kJ m⁻² year⁻¹ to kJ ha⁻¹ year⁻¹.
4. A sample has fresh mass 80 g and dry mass 22 g. What percentage of the fresh mass was water?
5. Why is biomass measured as dry mass and not fresh mass?
6. State two reasons why calorimetry underestimates the energy in biomass.
7. Primary consumers have net production of 2 160 kJ m⁻² year⁻¹ from NPP of 24 000. Calculate the efficiency of transfer.
8. NPP is 7.5 × 10⁶ kJ ha⁻¹ year⁻¹ and R is 2.9 × 10⁶. Calculate GPP.
9. A tissue sample burned heats 40.0 g of water by 15.0 °C. The sample had mass 0.60 g. Calculate the energy per gram in kJ g⁻¹.
10. Explain how using pesticides on a crop increases the efficiency of energy transfer to humans.
11. Explain how keeping cattle in a warm barn increases their net production.
12. A population of secondary consumers has I of 8.0 × 10⁵ and N of 1.2 × 10⁵ kJ ha⁻¹ year⁻¹. Calculate N ÷ I as a percentage.

Answers

1. NPP = 1 260 - 455 = 805 kJ m⁻² year⁻¹.
2. N = 3 200 - (1 050 + 1 480) = 670 kJ; 670 ÷ 3 200 × 100 = 20.9%.
3. 1.8 × 10³ × 10 000 = 1.8 × 10⁷ kJ ha⁻¹ year⁻¹.
4. Water = 80 - 22 = 58 g; 58 ÷ 80 × 100 = 72.5%.
5. Fresh mass includes water, which varies with conditions and holds no usable energy. Dry mass measures the biological molecules.
6. Any two: heat lost to the surroundings or apparatus; incomplete combustion; heat absorbed by the container or thermometer.
7. 2 160 ÷ 24 000 × 100 = 9.0%.
8. GPP = NPP + R = 7.5 × 10⁶ + 2.9 × 10⁶ = 1.04 × 10⁷ kJ ha⁻¹ year⁻¹.
9. Energy = 40.0 × 4.18 × 15.0 = 2 508 J; 2 508 ÷ 0.60 = 4 180 J g⁻¹ = 4.18 kJ g⁻¹.
10. Pesticides kill insects that would eat the crop. Less of the crop’s NPP is lost to a non-human food chain, so more is stored in the harvested product.
11. Cattle in a barn move less and need less energy to keep warm, so R is smaller. Since N = I - (F + R), N is larger and more energy goes into growth.
12. 1.2 × 10⁵ ÷ 8.0 × 10⁵ × 100 = 15%.

Where marks are usually lost

• Subtracting F but forgetting R, or the reverse, in N = I - (F + R).
• Writing GPP + R when asked for NPP, or the reverse.
• Quoting an answer with no unit, or without the “per year”.
• Dividing the wrong way round in efficiency, so the answer is over 100%.
• Describing farming methods without linking them to energy: say which loss is reduced.
• Writing “less energy is wasted” instead of naming respiration, faeces or a competing food chain.
• Giving a calorimetry result without stating that it is probably an underestimate.
• Using area values in m² and ha in the same calculation.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), first teaching 2015, section 3.5.3 Energy and ecosystems (A-level only).