(1) Introduction to energy and biomass transfer in food chains

Biomass is mainly organic matter, derived from recently living organisms, and the source chemical energy production for life. Examples include crops, agricultural residues, sewage waste, but the largest biomass residues are from wood.

It is specified for a given population of a trophic level in a food chain.

Reminder: Sunlight energy is the principal source of energy input into biological systems and this flow of energy through living organisms is transferred as chemical energy which is eventually transferred to the environment.

Biomass is measured as the dry residue material of an organism and the biomass is the source of chemical energy to power all forms of life.

So, what is the origin of biomass?

Plants and algae make food via sunlight energy and photosynthesis and this increases their biomass - which is an important source of food and energy for animals.

Almost all life on Earth depends on plants and algae at the base of a food chain.

This also means that most chemical potential energy for life is indirectly derived from the Sun, which is used to make a range of organic molecules from complex carbohydrates, sugars, proteins and lipids (fats and oils).

The carbohydrates store chemical potential energy which is passed down a food chain from plants/algae to animals when animas eat these photosynthesising organisms.

Only plants, algae like seaweed or phytoplankton and some bacteria can carry out photosynthesis.

Many land based plants depend on insects to pollinate them and without the intervention of bees, moths and butterflies, many species of plants would struggle to reproduce.

Carbon dioxide is slightly soluble in water (as is oxygen) so aquatic organisms in water can carry out photosynthesis near the surface.

This has implications for the food chains from which we derive our own food.

So, if pollination is disrupted, so is our food supply, so the more bees, moths and butterflies we have, the better for our ecosystems including our food supply.

Plants use carbon dioxide and produce oxygen in daylight, but use oxygen and produce carbon dioxide in respiration at night.

Animals use oxygen all the time and produce carbon dioxide all the time from respiration.

Fortunately for animals, plants keep the oxygen level at around 20% in the Earth's atmosphere and respiration by animals keeps the carbon dioxide level at around 0.04%, which is sufficient for plants to thrive.

For more details see Carbon cycle, nitrogen cycle, water cycle, decomposition - decay

It may seem a bit strange, but animals only use a small percentage of this biomass to actually increase their own biomass.

The reasons for inefficient transfer of biomass and its loss are discussed in Part 5

By observing the numbers and sizes of the organisms in food chains we can find out what happens to energy and biomass (mass of a particular living organism) as it passes along the food chain.

It is important to understand that all living things are interdependent on each other, especially through the pathways of food chains, which are effectively energy chains too.

You need to be able to interpret pyramids of biomass or construct them from appropriate information.

Reminder: Biomass is measured as the dry material of an organism.

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Revision Notes: Biomass, Food Chains, and Energy Transfer

1. Understanding Biomass

Definition of Biomass

• Biomass refers to the total dry mass of living or recently living organisms within a given area.

• It consists of organic material such as carbohydrates, proteins, and fats, which store energy.

Biomass in Ecosystems

• Biomass is produced by photosynthetic organisms (producers), which convert light energy into chemical energy through photosynthesis.

• This stored energy is then transferred through food chains.

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2. Food Chains and Energy Transfer

Structure of a Food Chain

A food chain represents the transfer of energy from producers to various levels of consumers.

1. Producer (e.g. grass) → absorbs sunlight and synthesizes food.

2. Primary Consumer (e.g. rabbit) → eats the producer.

3. Secondary Consumer (e.g. fox) → eats the primary consumer.

4. Tertiary Consumer (e.g. eagle) → eats the secondary consumer.

Each stage is known as a trophic level.

How Energy is Transferred

• Energy is transferred from one trophic level to the next when organisms consume biomass.

• However, not all energy is passed on—much is lost in various ways.

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3. Energy Loss in Food Chains

Ways Energy is Lost

Energy is lost at each trophic level due to:

1. Respiration → organisms use some energy for movement and cellular processes.

2. Heat Loss → energy is lost as heat, particularly in warm-blooded animals.

3. Waste (Faeces and Urine) → undigested materials and metabolic waste.

4. Incomplete Consumption → not all parts of the organism are eaten (e.g. bones, fur).

Energy Efficiency

• Energy transfer efficiency between trophic levels is typically 10%.

• This means only about 10% of the biomass from one level is passed to the next.

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4. Biomass Pyramids

What is a Biomass Pyramid?

A biomass pyramid visually represents the biomass at each trophic level.

1. The base (widest part) represents producers, which have the highest biomass.

2. The middle sections represent primary and secondary consumers.

3. The top (narrowest part) represents tertiary consumers, which have the lowest biomass.

Why Do Biomass Pyramids Get Smaller at the Top?

• Due to energy losses at each level, less biomass is available higher up the chain.

• This explains why ecosystems rarely support large numbers of top predators.

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5. Implications for Ecosystems

Impact on Food Webs

• Changes in biomass availability affect the entire food web.

• Human activities such as deforestation, pollution, and overhunting can disrupt biomass production, affecting the food chain.

Sustainability & Conservation

• Maintaining stable biomass levels is essential for biodiversity and ecosystem health.

• Conservation strategies such as sustainable agriculture, protected areas, and reducing waste help maintain energy efficiency.

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Biomass is the stored energy in living organisms, transferred through food chains.

Only about 10% of energy is passed to the next trophic level.

Energy is lost due to respiration, heat loss, waste, and incomplete consumption.

Biomass pyramids show how biomass decreases as you move up trophic levels.

Human activity affects biomass and energy transfer, impacting ecosystems.

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Some learning objectives for these sections

These sections will help you answer questions such as ...

What is a food chain?

How do you construct a food chain? 

What is a food web?

How do you construct food webs? 

What is interdependence?

How do you construct a pyramid of biomass? 

How do you construct a pyramid of numbers? 

How do you present and interpret data on biomass transfer? 

How do you calculate % efficiency of biomass transfer?

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