Question 1
Paper 1A style
What is the source of energy that iron-oxidising bacteria use to fix carbon dioxide?
- Light absorbed by chlorophyll
- Oxidation of iron(II) ions to iron(III) ions
- Reduction of iron(III) ions to iron(II) ions
- Carbon compounds absorbed from dead organic matter
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Answer: B [1]. Oxidation reactions release energy; these bacteria are chemoautotrophs.
Question 2
Paper 1A style
Which statement describes a closed system?
- Energy and matter can both enter and exit
- Matter can enter and exit but energy cannot
- Energy can enter and exit but matter cannot
- Neither energy nor matter can enter or exit
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Answer: C [1]. A describes an open system such as a natural ecosystem.
Question 3
Paper 1A style
Which statement about production in an ecosystem is correct?
- Secondary production is usually greater than primary production
- Primary production is measured in kJ per individual
- Secondary production is lower than primary production because carbon compounds are lost as CO₂ and water in respiration
- Primary production is the total biomass of the producers at one moment
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Answer: C [1]. B: units are g m⁻² yr⁻¹; D describes biomass, not production (a rate).
Question 4
Paper 1B style
The table shows sample data for the energy flowing through each trophic level of a food chain in a grassland. (a) Calculate the percentage of energy transferred from producers to primary consumers. [1] (b) Calculate the percentage transferred from secondary to tertiary consumers. [1] (c) Explain two reasons why less than 100% of the energy is transferred between trophic levels. [2] (d) Using the data, suggest why this food chain has no quaternary consumer. [2]
| Trophic level | Energy / kJ m⁻² yr⁻¹ |
|---|---|
| Producers | 24 000 |
| Primary consumers | 2 160 |
| Secondary consumers | 194 |
| Tertiary consumers | 15 |
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- (a) 2 160 ÷ 24 000 × 100 = 9.0% [1]
- (b) 15 ÷ 194 × 100 = 7.7% [1] accept 7.73%
- (c) energy converted to heat in cell respiration (lost to environment) [1]
- (c) not all of the organisms / parts eaten (e.g. roots, bones) [1]
- (c) not all food digested/absorbed; lost in faeces [1]
- (d) only 15 kJ m⁻² yr⁻¹ reaches tertiary consumers / about 8% of that would pass on, about 1 kJ m⁻² yr⁻¹ [1]
- (d) too little energy to sustain a population of a further trophic level [1]
- max 6
Question 5
Paper 1B style
The table shows illustrative monthly mean CO₂ concentrations at Mauna Loa in two consecutive years, following the pattern of the Keeling Curve. (a) Calculate the difference between the May and September concentrations in Year 1. [1] (b) Explain why CO₂ concentration falls between May and September. [2] (c) Calculate the rate of increase in mean CO₂ concentration between the January values, giving units. [1] (d) State the main cause of the long-term increase. [1]
| Year | January | May | September |
|---|---|---|---|
| Year 1 | 416.5 | 420.0 | 413.5 |
| Year 2 | 419.0 | 422.5 | 416.0 |
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- (a) 420.0 − 413.5 = 6.5 ppm [1]
- (b) northern hemisphere spring/summer: photosynthesis exceeds respiration [1]
- (b) net uptake of CO₂ by (the large area of) land plants in the northern hemisphere [1]
- (c) 419.0 − 416.5 = 2.5 ppm yr⁻¹ / ppm per year [1] units required
- (d) combustion of fossil fuels (coal, oil, natural gas) / accept deforestation by burning [1]
Question 6
Paper 2A style
Distinguish between the ways in which autotrophs and heterotrophs obtain the carbon compounds they require, and state one process they have in common.
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- autotrophs synthesise carbon compounds from simple inorganic substances / CO₂ [1]
- autotrophs use an external energy source (light or oxidation reactions) [1]
- heterotrophs obtain carbon compounds from other organisms [1]
- heterotrophs digest complex compounds (internally or externally) and assimilate the products to build their own compounds [1]
- both release energy by (oxidising carbon compounds in) cell respiration [1]
- max 4
Question 7
Paper 2B style
Explain how carbon is recycled in ecosystems, and how ecosystems and human activity can change the amount of carbon dioxide in the atmosphere.
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- autotrophs/producers fix CO₂ by photosynthesis into carbon compounds [1]
- carbon compounds pass to consumers by feeding [1]
- dead organisms / faeces / dead parts are broken down by decomposers [1]
- producers, consumers and decomposers release CO₂ by (cell) respiration [1]
- if photosynthesis exceeds respiration, there is net uptake / ecosystem is a carbon sink [1]
- if respiration exceeds photosynthesis, there is net release / ecosystem is a carbon source [1]
- named sink/source, e.g. growing forest / peat bog as sink; drained peatland as source [1]
- combustion of biomass / peat / coal / oil / natural gas releases CO₂ [1]
- fossil fuels / peat are carbon stores formed over long periods (thousands to millions of years) [1]
- natural combustion after lightning strikes occurs, but human activity has greatly increased combustion rates [1]
- decomposers also recycle other elements, not just carbon [1]
- max 7
Question 8
Paper 2A style
Explain why chemical elements can be recycled in ecosystems but energy cannot.
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- elements/atoms are not destroyed / the supply of each element is finite [1]
- decomposers break down dead organic matter, releasing elements in inorganic forms that producers can reuse [1]
- energy is converted to heat in cell respiration / when ATP is made and used [1]
- heat is lost to the environment and cannot be converted back to chemical energy by organisms [1]
- so energy must be continually supplied by sunlight [1]
- max 3