# Energy Flow, Food Chains, Food Webs and Pyramids

> Biology · CIE IGCSE
> Source: https://www.owlsprep.com/study/cie-0610-u14-energy-flow-food-chains-food/

This guide covers energy flow through ecosystems, construction of food chains and webs, and interpretation of ecological pyramids for CIE IGCSE Biology 0610, including all required Core and Extended tier content.

**Prerequisites:** [Basic ecosystem structure and organism classification](https://www.owlsprep.com/study/cie-0610-u14-ecosystem-basics/)

## Learning objectives

- Describe how energy enters and flows through terrestrial and aquatic ecosystems
- Construct and interpret food chains and food webs, identifying trophic levels correctly
- Explain why energy transfer between trophic levels is ~10% efficient, listing sources of energy loss
- Interpret and draw pyramids of number, biomass and energy, distinguishing expected shapes for each type
- Calculate percentage energy transfer between trophic levels for Extended tier questions

## Energy Flow in Ecosystems (Core)

**Energy Flow** — The unidirectional transfer of energy from one organism to the next in an ecosystem, starting with light energy from the Sun absorbed by producers via photosynthesis.

Only ~1% of the Sun’s light energy reaching Earth’s surface is captured by producers. This energy is converted to chemical energy stored in glucose and other organic molecules, which is then passed between organisms when they feed on other organisms.

**Worked example:** State the original source of energy for a grassland ecosystem, and name the process that producers use to capture this energy.

1. 1. Identify the original energy source: Light energy from the Sun (not producers, which only capture the energy).
2. 2. Name the capture process: Photosynthesis, which converts light energy to stored chemical energy in organic molecules.

> **Exam tip:** Always name the Sun as the original energy source for standard ecosystems in Core questions to gain full marks; answers listing only producers will be marked incorrect.

## Food Chains and Food Webs (Core)

**Food Chain** — A linear sequence of organisms showing the transfer of energy from one organism to the next, always starting with a producer at the first trophic level.

*Notation:* Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer

*Example:* Grass → Grasshopper → Frog → Hawk

A food web is a network of interconnected food chains that shows all possible feeding relationships in an ecosystem, which is a more accurate representation of real-world feeding patterns than isolated food chains.

> **tip**
>
> Arrows in food chains and webs always point in the direction of energy flow, from the organism being eaten to the organism that consumes it.

**Worked example:** From the food chain: Algae → Zooplankton → Small Fish → Heron, identify the secondary consumer and state its trophic level.

1. 1. List trophic levels in order: Level 1 = Algae (Producer), Level 2 = Zooplankton (Primary Consumer), Level 3 = Small Fish (Secondary Consumer), Level 4 = Heron (Tertiary Consumer).
2. 2. Secondary consumer is the small fish, occupying the 3rd trophic level.

## Energy Transfer Efficiency (Extended Only)

Approximately 90% of energy is lost between consecutive trophic levels, so only ~10% of energy is passed to the next level. Energy is lost as heat from respiration, in waste products (faeces, urine), and in uneaten body parts (e.g. bones, woody plant stems). To calculate percentage energy transfer, use the formula:

$$\text{Percentage energy transfer} = \frac{\text{Energy in higher trophic level}}{\text{Energy in lower trophic level}} \times 100$$

**Worked example:** If 25,000 kJ of energy is stored in grass (producers) in a grassland ecosystem, calculate the amount of energy available to tertiary consumers in the food chain: Grass → Rabbit → Fox → Eagle.

1. 1. Calculate energy at primary consumer (rabbit) level: 10% of 25,000 kJ = 2,500 kJ.
2. 2. Calculate energy at secondary consumer (fox) level: 10% of 2,500 kJ = 250 kJ.
3. 3. Calculate energy at tertiary consumer (eagle) level: 10% of 250 kJ = 25 kJ.
4. 4. Final answer: 25 kJ of energy is available to the eagle population.

> **Exam tip:** You may be given actual energy values instead of the 10% rule in exam questions; always use the values provided to calculate transfer efficiency for full marks.

## Ecological Pyramids (Core + Extended)

**Ecological Pyramid** — A diagram that represents the relative amount of energy, biomass, or number of organisms at each trophic level in an ecosystem.

| Pyramid Type | Tier Requirement | Key Shape Notes |
| --- | --- | --- |
| Pyramid of Number | Core | Can be irregular/inverted (e.g. one large tree supports thousands of insects) |
| Pyramid of Biomass | Core | Almost always upright, rare exceptions for aquatic ecosystems |
| Pyramid of Energy | Extended | Always perfectly upright, units are kJ/m²/year |

**Worked example:** Explain why a pyramid of energy can never be inverted, but a pyramid of number can be inverted.

1. 1. Energy is always lost between trophic levels, so the total energy at a higher trophic level can never exceed the total energy of the level below it, making energy pyramids always upright.
2. 2. A pyramid of number counts individual organisms, so a single large producer (e.g. an oak tree) can support hundreds of primary consumer individuals (caterpillars), making the first trophic level bar smaller than the second, inverting the pyramid.

> **Exam tip:** If asked to draw an ecological pyramid in an exam, always label each trophic level clearly and write the units for the pyramid type you are drawing.

## Common pitfalls

- **Wrong:** Drawing food chain arrows pointing from predator to prey
  - Why it fails: Arrows represent the direction of energy flow, not the direction of feeding action.
  - Correct: Always draw arrows pointing from the organism being eaten to the organism that consumes it.
- **Wrong:** Stating producers are the original source of energy in an ecosystem
  - Why it fails: Producers only capture light energy from the Sun, which is the ultimate original source of energy for almost all ecosystems.
  - Correct: Name the Sun as the original energy source for all standard ecosystem questions.
- **Wrong:** Assuming all ecological pyramids are always upright
  - Why it fails: Pyramids of number and biomass can be inverted in specific cases, so this generalisation is incorrect.
  - Correct: Only state that pyramids of energy are always upright, and note possible exceptions for number and biomass pyramids.
- **Wrong:** Calculating energy transfer as 10% of the higher trophic level to get the lower level value
  - Why it fails: Energy decreases as you move up trophic levels, so you always multiply the lower level energy by 0.1 to get the higher level value.
  - Correct: Use the percentage energy transfer formula to cross-check your calculations for Extended questions.
- **Wrong:** Counting primary consumers as the first trophic level
  - Why it fails: Producers are always the first trophic level, so primary consumers occupy the second trophic level, secondary consumers the third, etc.
  - Correct: Always start counting trophic levels from the producer at level 1, moving up one for each consecutive consumer.

## Cheatsheet

| Concept | Key Fact | Exam Answer Reminder |
| --- | --- | --- |
| Original Energy Source | Sun, 1% captured by producers | Always name the Sun first when asked for energy source |
| Food Chain Arrows | Point in direction of energy flow | Draw arrows from prey to predator, never reverse |
| Energy Transfer | ~10% passed between levels, 90% lost | Use 10% rule if no values are given in Extended questions |
| Pyramid of Energy | Always upright, units kJ/m²/year | Never state energy pyramids can be inverted |
| Trophic Levels | 1=Producer, 2=Primary Consumer, 3=Secondary, 4=Tertiary | Count levels starting at producers, not consumers |

## What's next

Now that you have mastered energy flow, food chains, webs and pyramids, you are ready to move on to nutrient cycles and human impacts on ecosystems, the next core topics in the Organisms and Their Environment unit for CIE IGCSE Biology 0610. Practice constructing food webs from given organism lists, calculating energy transfer efficiency (for Extended tier), and interpreting pyramid diagrams to answer exam-style questions, as these are frequent high-mark questions in both Core and Extended papers. Make sure you can clearly explain the reasons for energy loss between trophic levels to gain full marks in extended response questions.

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