Energy and ecosystems (A-level only)
AQA
A-Level
Producers form the entry point of carbon and chemical energy into an ecosystem. In terrestrial ecosystems, plants synthesise organic compounds using carbon dioxide from the atmosphere. In aquatic ecosystems, producers utilise dissolved carbon dioxide and hydrogencarbonate ions. Through photosynthesis, this inorganic carbon is converted into organic compounds, primarily sugars. While some sugars are respired, the remainder are used to synthesise other biological molecules, forming the plant's biomass. This biomass represents the chemical energy available to consumers and decomposers. Therefore, the rate at which producers synthesise these organic compounds directly determines the energy available to the rest of the food web.
41
Objectives
26
Exam Tips
41
Pitfalls
51
Key Terms
53
Mark Points
Subtopics in this area
In any ecosystem, plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide.
Most of the sugars synthesised by plants are used by the plant as respiratory substrates.
The rest are used to make other groups of biological molecules.
These biological molecules form the biomass of the plants.
Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area.
The chemical energy store in dry biomass can be estimated using calorimetry.
Gross primary production ( GPP) is the chemical energy store in plant biomass, in a given area or volume.
This net primary production is available for plant growth and reproduction.
It is also available to other trophic levels in the ecosystem, such as herbivores and decomposers.
The net production of consumers ( N), such as animals, can be calculated as: N = I – (F + R) where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment.
Primary and secondary productivity is the rate of primary or secondary production, respectively.
Energy and ecosystems (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Identify the carbon source for terrestrial and aquatic producers.
- Explain how the synthesis of organic compounds contributes to producer biomass.
- Describe the relationship between producer biomass and the energy available to consumers.
- Explain how a plant uses glucose as a respiratory substrate, naming glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation.
- Link a faster rate of photosynthesis to faster growth through sugar supply, ATP and synthesis of organic materials.
- Justify why respiratory loss makes net primary production smaller than gross primary production.
- Rewrite a statement containing 'energy used in respiration' so that it would gain the mark.
- Name the groups of biological molecules made from photosynthetic sugars, including nucleic acids.
- Explain why a plant needs nitrate and phosphate ions to convert sugars into proteins and nucleic acids.
- Describe how condensation reactions are used to build polymers like starch and cellulose.
- Describe the fate of sugars synthesised during photosynthesis.
- Identify the biological molecules that constitute plant biomass.
- Explain that the biological molecules forming biomass represent the chemical energy store of an organism.
- State two units in which biomass can be expressed and give a correct unit for each.
- Calculate the biomass per square metre of an area from the dry mass of a sampled quadrat.
- Explain why mass of carbon is a valid measure of the organic material an organism contains.
- Evaluate the reliability of a biomass estimate made from a small number of samples.
- Describe how a calorimeter is used to estimate the chemical energy store in a dried sample of biomass.
- Calculate an energy value in kJ g-1 from the mass of water, its temperature rise and the dry mass burnt.
- Explain why the sample must be dried and burnt in excess oxygen before measurement.
- Identify two sources of error in a calorimetry experiment and state the effect of each on the result.
- Define gross primary production as a chemical energy store in plant biomass.
- Explain why only a small percentage of the light energy reaching a leaf appears as gross primary production.
- Identify from units alone whether a printed figure represents an energy store or a rate.
- List reasons why incident light is not converted into chemical energy by producers.
- Explain how net primary production becomes new plant tissue, naming at least two organic molecules that store it.
- Calculate net primary production from gross primary production and respiratory loss, using correct rate units.
- Explain why a higher gross primary production does not necessarily give a higher net primary production.
- Distinguish net primary production from the energy a herbivore assimilates from the same plant.
- Explain three distinct reasons why energy transfer between trophic levels is inefficient.
- Calculate the percentage efficiency of energy transfer between two trophic levels from given data.
- Describe how the energy in dead material, faeces and urine becomes available to saprobionts.
- Rewrite a sentence containing 'energy used in respiration' so that it accurately describes energy loss.
- Calculate the net production of a consumer from given values of I, F and R, and state the unit.
- Explain what each of I, F and R represents in terms of where the energy goes.
- Give three distinct reasons for the low efficiency of energy transfer from one consumer to the next.
- Compare the net production of an endotherm and an ectotherm with the same energy intake, and justify the difference.
- Distinguish production from productivity using their units.
- Convert a production figure into a productivity by dividing by the time period, and quote the unit.
- Classify a given value as gross primary, net primary or secondary productivity from its context.
- Explain why secondary productivity is much lower than net primary productivity in the same ecosystem.
Marking Points
Key points examiners look for in your answers
- Identifying carbon dioxide as the inorganic carbon source for terrestrial plants.
- Identifying dissolved carbon dioxide and hydrogencarbonate ions as the carbon source for aquatic producers.
- Explaining that plants synthesise organic compounds, such as sugars, from these inorganic carbon sources.
- Linking the synthesis of organic compounds to the production of producer biomass.
- Stating that this organic carbon and chemical energy then becomes available to consumers and decomposers in the ecosystem.
- one mark for naming respiration as the process that uses the sugars, with aerobic or anaerobic ignored
- one mark for more sugars being produced and used in respiration when photosynthesis is faster
- one mark for linking faster respiration to more ATP or energy being available for growth
- one mark for the resulting faster synthesis of new organic materials
- one mark for identifying respiratory loss as the reason gross primary production exceeds net primary production
- Naming carbohydrates, lipids, proteins, and nucleic acids as the groups of biological molecules synthesised from photosynthetic sugars.
- Stating that nitrate ions are required to convert sugars into amino acids and proteins.
- Identifying that phosphate and nitrogen are required to synthesise nucleotides and nucleic acids.
- Describing a condensation reaction joining monomers into a named polymer (e.g., glucose into starch).
- Providing a named use for these molecules, such as cellulose in cell walls or lipids in membranes.
- Identify that plants synthesise sugars from carbon dioxide during photosynthesis.
- State that a proportion of these synthesised sugars is used by the plant as respiratory substrates.
- Explain that the remaining sugars are used to synthesise other biological molecules, such as cellulose, starch, proteins, and lipids.
- Define plant biomass as the total mass of these biological molecules, which represents the chemical energy store of the organism.
- State carbon as the element whose mass per given area can express biomass.
- Identify dry mass of tissue per given area as the alternative measure.
- Explain that water is removed by heating to constant mass because water content varies.
- Calculate biomass per square metre by dividing the mean dry mass of a quadrat by the quadrat area (e.g., 20 g / 0.25 m² = 80 g m⁻²).
- Provide a correct unit that includes area, such as g m⁻² or kg C m⁻².
- Describe using random sampling and a mean when scaling a quadrat measurement up to a whole area.
- one mark for naming calorimetry, or a calorimeter, as the technique used
- one mark for burning a known dry mass of the sample completely, in oxygen
- one mark for measuring the temperature rise of a known mass or volume of water
- one mark for calculating energy as mass of water multiplied by specific heat capacity multiplied by temperature change, divided by the dry mass
- one mark for a named source of error, such as heat lost to the surroundings or incomplete combustion
- Defining gross primary production as the total chemical energy store in plant biomass in a given area or volume
- Identifying that only a small percentage of incident light energy is converted into this chemical energy store
- Explaining light loss by stating it is the wrong wavelength, reflected, or transmitted through the leaf
- Stating correct units for a store (e.g., kJ m-2) or a rate (e.g., kJ m-2 year-1) depending on the context
- State that net primary production is the chemical energy remaining after the producer's respiratory losses, NPP = GPP - R.
- State that this energy is available for plant growth and reproduction.
- Name a chemical store into which that energy goes, such as cellulose, protein, starch or lipid.
- Link net primary production to an increase in dry mass, expressed as a rate with units of area and time (e.g. kJ m-2 year-1 or g m-2 year-1).
- Explain that a higher GPP does not necessarily give a higher NPP, because respiratory loss may also be greater.
- State that energy is lost between or at each trophic level.
- Name a valid route of energy loss, such as heat from respiration, excretion or faeces.
- Explain that material not eaten, or eaten but not digested, reduces transfer efficiency.
- Describe decomposers or saprobionts obtaining the chemical energy remaining in dead material, faeces and urine.
- Calculate a correct percentage efficiency of energy transfer from given data.
- State that heat lost from respiration (R) reduces the efficiency of energy transfer.
- Identify F as energy lost in faeces (undigested food) and excretion (accepting urine, urea, uric acid, or ammonia).
- Substitute correctly into N = I - (F + R) with working shown to calculate net production.
- Provide a correctly stated unit for net production, such as kJ m⁻² year⁻¹.
- Explain that endotherms have higher respiratory losses (R) than ectotherms to maintain body temperature, reducing their net production (N).
- one mark for productivity being a rate, that is production per unit time
- one mark for a unit including both area and time, such as kJ m-2 year-1
- one mark for primary productivity referring to producers and secondary productivity to consumers
- one mark for a named factor that changes productivity, with the direction of the effect stated
Examiner Tips
Expert advice for maximising your marks
- 💡For aquatic producers, always specify dissolved carbon dioxide or hydrogencarbonate ions rather than just carbon dioxide.
- 💡When the question is about ecosystems, explicitly link the synthesis of organic compounds to the creation of producer biomass and energy availability for consumers.
- 💡If asked what happens to sugars made in photosynthesis, respiration is the expected one-word answer; adding aerobic or anaerobic neither gains nor loses the mark.
- 💡Whenever energy and respiration appear in the same sentence, check the direction: ATP is made, energy is released, never used in respiration.
- 💡Link faster photosynthesis to growth in three steps: more sugar, more ATP, more synthesis of new organic material.
- 💡If asked why a plant needs nitrate, answer with specific molecules like amino acids, proteins, nucleotides, ATP, and chlorophyll, rather than just 'for growth'.
- 💡Remember to include nucleic acids when discussing the biological molecules that make up plant biomass.
- 💡When asked what happens to the products of photosynthesis, always distinguish between sugars used for respiration and those used to synthesise other biological molecules.
- 💡Remember to explicitly use the phrase 'chemical energy store' when discussing the biological molecules that make up plant biomass.
- 💡Check that a rate unit for productivity has three parts: mass, area, and time (e.g., g m⁻² yr⁻¹), whereas biomass just has mass and area.
- 💡When describing how to estimate dry mass, always specify heating the sample to constant mass to ensure all water is removed.
- 💡Use random coordinates and a mean of several quadrats when scaling dry mass up to a field or pond.
- 💡Write 'calorimeter' clearly and distinguish it from 'colorimeter', which is a different instrument.
- 💡If asked why the measured energy value is lower than the true value, answer heat lost to the surroundings or incomplete combustion.
- 💡State that the sample is dried to constant mass before burning whenever you write the method.
- 💡Read the printed units carefully: a kJ m-2 value represents a store, whereas a kJ m-2 year-1 value represents a rate.
- 💡Use the words 'store' and 'transferred' rather than 'made' or 'produced' whenever you write about energy in ecosystems.
- 💡Quote NPP = GPP - R even in a description question; it earns the definition mark quickly and clearly.
- 💡Always include 'per year' or another time unit when giving units for NPP, as it is a rate (e.g. kJ m-2 year-1).
- 💡Explain growth in linked steps: more sugar fixed, more organic material synthesised, greater increase in dry mass.
- 💡Always use the phrase 'lost as heat from respiration' rather than 'used in respiration' to ensure clarity about energy flow.
- 💡If asked for multiple reasons for inefficient energy transfer, provide distinct routes (e.g., heat from respiration, faeces, and parts not eaten) rather than grouping them.
- 💡Write the equation out and label each term with the figure from the question before calculating anything.
- 💡Questions about mammals and birds almost always want R, because they respire heavily to maintain body temperature.
- 💡Look at the unit before naming the quantity: if it contains 'per year', the answer is a productivity.
- 💡Use the word rate explicitly when defining productivity.
Common Mistakes
Pitfalls to avoid in your exam answers
- Treating the carbon source as atmospheric carbon dioxide only; correction: aquatic producers use dissolved carbon dioxide and hydrogencarbonate ions.
- Describing the detailed biochemistry of the Calvin cycle when asked about energy transfer; correction: focus on the synthesis of organic compounds and biomass for ecosystem questions.
- Assuming all synthesised organic compounds become biomass; correction: a significant proportion of the sugars produced are used in respiration by the plant itself.
- writing that energy is 'used in respiration', which examiners reject; energy is released from glucose and transferred to ATP
- saying respiration produces or generates energy rather than transferring it
- assuming plants respire only at night and photosynthesise only in the day
- treating all the sugar made as biomass, which ignores the respiratory losses separating GPP from NPP
- naming glucose as the respiratory substrate but never linking ATP to a named use in the plant
- Defining organic molecules merely as 'containing carbon'; correction: organic molecules are carbon-based compounds, typically containing carbon-hydrogen bonds, whereas carbon dioxide contains carbon but is inorganic.
- Stating that all products of photosynthesis are organic; correction: oxygen is an inorganic product of photosynthesis.
- Saying a plant makes protein directly from glucose; correction: plants require a nitrogen source, such as nitrate ions, to form amino acids first.
- Omitting nucleic acids when listing the biological molecules synthesised by plants; correction: include DNA and RNA alongside carbohydrates, lipids, and proteins.
- Assuming all photosynthetic products become biomass; correction: explicitly state that a significant proportion of sugars is used in respiration and lost as carbon dioxide, so only the remainder forms biomass.
- Confusing biomass with total plant mass including water; correction: biomass consists of the organic biological molecules, excluding water which does not store chemical energy.
- Failing to link biological molecules to energy transfer; correction: explicitly state that these biological molecules act as a chemical energy store that can be passed to the next trophic level.
- Answering 'biomass' or 'organic matter' when asked what specific element is measured per given area; correction: the required answer is carbon.
- Quoting a mass with no area in the unit; correction: include area (e.g., m⁻²) so values can be compared between sites of different sizes.
- Assuming carbon mass and dry mass are the same figure; correction: remember carbon is only a fraction (roughly half) of dry mass.
- Sampling a single quadrat and scaling it up without repeats; correction: use multiple random quadrats and calculate a mean for reliability.
- writing colorimeter instead of calorimeter; phonetic spellings of calorimeter are accepted but colorimeter is rejected outright
- burning fresh rather than dried material, so energy is absorbed evaporating water
- giving an answer in joules without dividing by the sample mass, so values cannot be compared
- ignoring heat lost to the apparatus and surroundings when asked why the value is an underestimate
- assuming the temperature rise alone is the energy value, without using the specific heat capacity of water
- Defining gross primary production as the energy available to herbivores; correction: it is the total energy fixed, not the energy available after plant respiration.
- Writing 'energy produced by the plant'; correction: energy is transferred and stored in organic molecules, never created or produced.
- Assuming most light reaching a leaf becomes gross primary production; correction: only one to three per cent is converted, as much misses the chlorophyll or is reflected.
- Writing that net primary production is the total energy fixed by the plant, instead of the energy remaining after respiration. Correction: NPP is the energy left after respiratory losses (R) have been subtracted from GPP.
- Quoting a net primary production figure without a unit of time, treating it as a static store rather than a rate. Correction: NPP is a rate of biomass production, so it must include time and area units, such as kJ m-2 year-1.
- Answering only that 'the plant grows', without naming organic molecules or the increase in dry mass. Correction: Name specific organic molecules such as cellulose or starch, and refer to the increase in dry mass.
- Assuming high gross primary production always means high net primary production. Correction: NPP depends on the balance GPP - R, so a plant with high GPP but high respiration may have low NPP.
- Writing 'energy is used in respiration' instead of the accurate phrase 'energy is lost as heat during respiration'.
- Stating that energy is recycled in the ecosystem; matter cycles, but energy flows through and is ultimately lost as heat.
- Forgetting that inedible parts such as roots and wood are never ingested at all.
- Stating only that 'energy is lost' without naming the specific mechanism or where it goes.
- Subtracting only F or only R; correction: subtract the sum of (F + R) from I.
- Counting food that was never eaten within I; correction: I represents ingested food only, excluding uneaten parts.
- Writing 'energy used in respiration'; correction: specify 'energy lost as heat from respiration' to explain the loss to the environment.
- using production and productivity interchangeably, so the rate mark is not awarded
- quoting kJ m-2 for a productivity value, leaving the time out of the unit
- defining production as standing biomass, rather than the new biomass or energy generated