Energy flow

1. Overview

Energy flow is the process by which energy enters an ecosystem and moves through different living organisms. Understanding this topic is vital because it explains how the Sun sustains almost all life on Earth and why there are limits to the number of organisms a habitat can support.

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Key Definitions

• Producer: An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
• Consumer: An organism that gets its energy by feeding on other organisms.
• Herbivore: An animal that gets its energy by eating plants.
• Carnivore: An animal that gets its energy by eating other animals.
• Decomposer: An organism that gets its energy from dead or waste organic matter.
• Ecosystem: A unit containing all of the organisms and their environment, interacting together, in a given area.

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Core Content

The Sun: The Primary Source

• The Sun is the principal source of energy input to biological systems.
• While a massive amount of solar radiation reaches Earth, only a small percentage (approximately 1%) is captured by plants for growth.

The Flow of Energy

Energy does not cycle through an ecosystem; instead, it flows in one direction from the Sun, through organisms, and eventually out into the environment.

Step-by-Step Energy Transfer:

1. Light to Chemical: Producers (green plants/algae) capture light energy from the Sun and convert it into chemical energy during photosynthesis. This energy is stored in organic molecules like glucose and starch.
2. Transfer between Organisms: When a consumer eats a producer, the stored chemical energy is transferred to the consumer. This continues as primary consumers are eaten by secondary consumers.
3. Energy Loss: At every "trophic level" (step in a food chain), energy is lost. It is not 100% efficient.
4. Transfer to Environment: Energy is eventually transferred to the environment as heat energy. This happens through:
   • Respiration: Organisms use chemical energy for movement and metabolism, releasing heat as a byproduct.
   • Excretion/Egestion: Energy remains in waste products (feces/urine).
   • Death: Energy remains in uneaten parts (bones/fur), which is eventually processed by decomposers.

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Extended Content (Extended Only)

Note: According to the current syllabus objectives provided, there is no specific "Supplement" content for sub-topic 19.1. All students should focus on the Core objectives listed above.

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Key Equations

While there are no complex mathematical formulas for this specific sub-topic, it is helpful to remember the relationship between energy intake and loss:

Energy available to next level = Energy received - Energy lost (respiration + waste)

• Units: Energy in ecosystems is usually measured in Kilojoules per square meter per year (kJ/m²/yr).

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that energy is recycled in an ecosystem.
• ✅ Right: Energy is not recycled; it flows in one direction and is lost as heat. (Note: Nutrients like carbon and nitrogen are recycled, but energy is not).
• ❌ Wrong: Drawing food chain arrows pointing to the organism being eaten.
• ✅ Right: Arrows must point in the direction of energy flow (from the food into the mouth of the consumer).
• ❌ Wrong: Stating that the Sun provides chemical energy.
• ✅ Right: The Sun provides light energy; plants convert this into chemical energy.

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Exam Tips

• Command Words: If a question asks you to "Describe" the flow of energy, ensure you mention the Sun, the conversion of light to chemical energy, and the eventual loss of heat.
• Typical Contexts: You will often see "Energy Flow" questions linked to environmental food webs or specific biological organisms like forest or pond ecosystems.
• Numerical Values: Be prepared to see values like 1% (efficiency of light capture) and 10% (average energy transferred from one trophic level to the next).
• The "Final" Stage: Always remember to state that energy is "lost to the environment" at the end of the process. If you just say "lost," you may lose marks for lack of detail. It is lost specifically as heat.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State the principal source of energy input to biological systems. [1]

(b) Describe how energy flows from this source to a herbivore. [3]

(c) What eventually happens to the energy that was initially captured by the herbivore? [1]

Worked Solution:

(a)

1. The Sun. This identifies the fundamental energy source.

How to earn full marks:

• State "The Sun" or "Sunlight".
• Do not write "light" only.

(b)

1. Light energy from the Sun is absorbed by producers (plants) through photosynthesis. This describes the initial capture of energy.

2. The light energy is converted into chemical energy in the form of glucose (or other organic molecules). This describes the energy conversion.

3. The herbivore consumes the producer, transferring the chemical energy to the herbivore. This describes the energy transfer to the herbivore.

How to earn full marks:

• Mention producers (e.g., plants).
• Mention photosynthesis.
• Mention chemical energy (e.g., glucose).

(c)

1. It is eventually transferred to the environment as heat. This describes the ultimate fate of the energy.

How to earn full marks:

• State "heat" or "thermal energy".
• Do not write "lost".

Common Pitfall: Many students simply say "the energy is lost." Remember that energy isn't truly lost, but converted to less usable forms, primarily heat, which dissipates into the environment. Be specific about the form the energy takes as it leaves the system.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Define the term "energy flow" in an ecosystem. [2]

(b) Describe how energy flow is different between a food chain and a food web. [4]

Worked Solution:

(a)

1. Energy flow is the transfer of energy. This introduces the basic concept.

2. From one organism to another in an ecosystem. This specifies the context of the transfer.

How to earn full marks:

• Mention energy transfer.
• Mention organisms or ecosystem.

(b)

1. A food chain represents a single pathway of energy flow. This describes food chain energy flow.

2. Energy flows linearly from one organism to the next. This clarifies the linear nature.

3. A food web represents multiple interconnected food chains. This describes food web energy flow.

4. Energy flows through many different pathways, showing a more complex network of feeding relationships. This highlights the complex nature of the network.

How to earn full marks:

• State food chain is a single pathway.
• State food web is multiple interconnected pathways.
• Mention the linear nature of food chains.
• Mention the complex network of food webs.

Common Pitfall: It's easy to confuse food chains and food webs. Remember that a food chain is a simplified, linear representation, while a food web is a more realistic and complex network showing all the interconnected feeding relationships in an ecosystem.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student sets up an experiment to investigate energy transfer in a small ecosystem consisting of grass, grasshoppers, and a lizard. The grasshoppers eat the grass, and the lizard eats the grasshoppers. The student measures the energy content of each trophic level over a period of one week. The results are shown in the table below:

Trophic Level	Energy Content (kJ)
Grass	10000
Grasshoppers	1500
Lizard	150

(a) Calculate the percentage of energy transferred from the grass to the grasshoppers. [2]

(b) Explain why the energy transfer between trophic levels is not 100% efficient. [4]

(c) Suggest one way the student could improve the accuracy of their energy content measurements. [2]

Worked Solution:

(a)

1. Calculate the percentage energy transfer: $ (Energy , in , grasshoppers / Energy , in , grass) \times 100 $ This states the formula for percentage energy transfer.

2. $ (1500 / 10000) \times 100 = 15%$ This calculates the percentage.

How to earn full marks:

• Show the correct formula (or equivalent understanding).
• Calculate the correct percentage: $\boxed{15%}$
• Include the % sign in the answer.

(b)

1. Energy is lost as heat during respiration by both the grass and the grasshoppers. This identifies one mode of energy loss.

2. Some parts of the grass are not eaten by the grasshoppers (e.g., roots, stems). This identifies another mode of energy loss.

3. Some of the grasshoppers' food is undigested and egested as faeces. This identifies a third mode of energy loss.

4. Energy is used for movement and other life processes by the grasshoppers, and not all of this energy is transferred to the lizard. This explains that energy is used and not transferred.

How to earn full marks:

• Mention heat loss due to respiration.
• Mention undigested food or egestion/excretion.
• Mention parts of the organism not consumed.
• Mention energy used for movement/life processes.

(c)

1. Increase the sample size of organisms measured at each trophic level. This suggests a valid improvement.

2. Measure the energy content more frequently (e.g., daily instead of weekly) to account for short-term fluctuations. This suggests another valid improvement.

How to earn full marks:

• Suggest increasing sample size.
• Suggest measuring energy content more frequently.
• Any reasonable and plausible improvement is accepted.

Common Pitfall: When calculating percentage energy transfer, make sure you're dividing the energy of the consumer by the energy of the producer. Also, remember that energy losses occur at each trophic level due to respiration, excretion, and uneaten parts.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A remote island ecosystem consists of plants, insects, small birds that eat insects, and larger predatory birds that eat the small birds.

(a) Draw a food web for this ecosystem. [3]

(b) Explain how the energy flow in this ecosystem would be affected if a disease drastically reduced the insect population. [4]

(c) The island is also home to a species of decomposer bacteria. Describe the role of these bacteria in the island's ecosystem. [2]

Worked Solution:

(a)

1. 📊A food web diagram showing: Plants being eaten by Insects. Insects being eaten by Small Birds. Small Birds being eaten by Large Birds. Arrows must show the correct direction of energy flow (i.e., from food to consumer). Plants also have an arrow pointing to Decomposers. All labels must be correct and clear.

How to earn full marks:

• Show plants, insects, small birds, and large birds.
• Show correct feeding relationships (arrows in the right direction).
• Show decomposers linked to plants or other organisms.

(b)

1. The small bird population would decrease due to a reduced food supply (fewer insects). This describes the impact on the small birds.

2. This would lead to increased competition for the remaining insects among the small birds. This explains the increased competition.

3. The large predatory bird population would also eventually decrease due to a reduced food supply (fewer small birds). This describes the impact on the large birds.

4. Overall energy flow through the ecosystem would be reduced, as less energy is being transferred from the plants to the insects, and subsequently to the higher trophic levels. This summarizes the overall effect on energy flow.

How to earn full marks:

• Explain the impact on small birds (reduced food, population decrease).
• Explain the impact on large birds (reduced food, population decrease).
• Mention increased competition.
• Mention the overall reduction in energy flow.

(c)

1. Decomposers break down dead organisms and waste products. This describes the basic function of decomposers.

2. This releases nutrients back into the ecosystem, which can then be used by plants for growth. This explains the importance of nutrient recycling.

How to earn full marks:

• Mention breaking down dead organisms/waste.
• Mention releasing nutrients.
• Mention nutrients being used by plants.

Common Pitfall: When describing the impact of a population change on a food web, trace the effects up and down the trophic levels. Don't just focus on one organism; consider how the entire ecosystem is affected. Also, remember that decomposers recycle nutrients, which is essential for plant growth and overall ecosystem health.