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Organisms and their Environment - Energy flow

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Sun is the principal source of energy input to biological systems, where light energy is converted into chemical energy by producers through photosynthesis: 6CO2+6H2O→light, chlorophyllC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2.

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Energy flow is non-cyclical and one-way; energy enters ecosystems as light and is eventually lost to the environment as heat, unlike nutrients which are recycled within the system.

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A trophic level refers to the position of an organism in a food chain, food web, or pyramid (e.g., Producers →\rightarrow Primary Consumers →\rightarrow Secondary Consumers).

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Energy is lost between trophic levels (approximately 90%90\% loss) due to metabolic heat generated during respiration, movement, egestion of undigested food, and excretion of waste products like CO2CO_2 and urea.

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The inefficiency of energy transfer limits the number of trophic levels in an ecosystem, usually to no more than four or five, as there is insufficient energy to support higher levels.

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Pyramids of biomass represent the total dry mass of organic matter at each level, usually measured in g⋅m−2g \cdot m^{-2} or J⋅m−2J \cdot m^{-2}. These are typically upright but can be inverted in aquatic ecosystems.

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Net Primary Productivity (NPP) represents the energy actually available to consumers after the producers have met their own respiratory needs: NPP=GPP−RNPP = GPP - R.

📐Formulae

Efficiency of Energy Transfer=(Energy in higher trophic levelEnergy in lower trophic level)×100\text{Efficiency of Energy Transfer} = \left( \frac{\text{Energy in higher trophic level}}{\text{Energy in lower trophic level}} \right) \times 100

NPP=GPP−RNPP = GPP - R

Percentage Energy Loss=Energy available at start−Energy available at endEnergy available at start×100\text{Percentage Energy Loss} = \frac{\text{Energy available at start} - \text{Energy available at end}}{\text{Energy available at start}} \times 100

💡Examples

Problem 1:

In a grassland ecosystem, the producers contain 20,000 kJ⋅m−220,000 \text{ kJ} \cdot m^{-2} of energy. The primary consumers that feed on them contain 2,400 kJ⋅m−22,400 \text{ kJ} \cdot m^{-2}. Calculate the efficiency of energy transfer between these two levels.

Solution:

Efficiency=2,40020,000×100=12%\text{Efficiency} = \frac{2,400}{20,000} \times 100 = 12\%

Explanation:

The efficiency is calculated by taking the energy stored in the consumer level and dividing it by the energy available in the producer level, then converting it to a percentage.

Problem 2:

An ecosystem has a Gross Primary Productivity (GPP) of 45,000 kJ⋅m−2⋅y−145,000 \text{ kJ} \cdot m^{-2} \cdot y^{-1}. If the respiration (R) rate of the plants is 28,000 kJ⋅m−2⋅y−128,000 \text{ kJ} \cdot m^{-2} \cdot y^{-1}, calculate the Net Primary Productivity (NPP).

Solution:

NPP=45,000−28,000=17,000 kJ⋅m−2⋅y−1NPP = 45,000 - 28,000 = 17,000 \text{ kJ} \cdot m^{-2} \cdot y^{-1}

Explanation:

NPP is the energy that remains after the producers have used a portion of the total fixed energy (GPP) for their own cellular respiration (R).

Problem 3:

If a secondary consumer receives 500 J500 \text{ J} of energy, and 450 J450 \text{ J} of that energy is lost as heat through respiration and movement, how much energy is potentially available for the tertiary consumer?

Solution:

500 J−450 J=50 J500 \text{ J} - 450 \text{ J} = 50 \text{ J}

Explanation:

The energy available to the next trophic level is the total energy intake minus the energy used for metabolic processes and lost as heat.

Energy flow - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology