4.2 Energy losses and cycling

4.2 – Energy losses

lesson one: Energy losses


Individual study 

Objectives: Understand how energy losses occur in food chains; both within and between trophic levels. Understand how this limits the length of food chains. Recall that energy is lost in food chains, while nutrients can be recycled

Key words:

  • Entropy: the concept that systems tend to become disorganised if left to their own devices. This applies to the flow of energy in food chains, that begins as chemical energy in a highly organised bio-molecules and ends up as highly scattered heat energy moving by convection and radiation.
  • Pyramid of energy: a graphical representation of the energy level in each trophic level.
  • Trophic level: the feeding level eg. producer
  • Nutrients. Anything that is required by the body in order to function, and provided by diet, eg. minerals.
  • Thoracic cavity: Chest area of the body, where many vital organs are found eg. heart and lungs

Introductory activity: watch this amazing video showing heatloss made visible from a human body



The supporting image shows the thermal profile of an entire human body.
Have a discussion about the video with your peers. Try to address the following questions:
  • What phenomena is captured by the infra-red goggles?
  • What are the patterns shown by the images?
  • Where does this phenomenum happen?
  • Why is this relevant to me or others?
  • Are there any surprises for you in watching this video?
continue with the lesson below…
It can be concluded that heat loss by radiation occurs constantly, and is particularly associated with the ‘core of the body’ including the thoracic cavity where many vital organs are found.
This energy is generated by aerobic respiration, and in fact one of the functions of cellular respiration is to maintain a constant body temperature – even it means wasting a little heat energy.
Q) What other part of the body also shows a significant loss of energy, besides the thoracic cavity (shown in the video)
Energy flows in a food chain from producers to consumers to top consumers. Ultimately energy is released to the environment and escapes into the Earth’s atmosphere as heat energy.
image credit: naturalwildandfree.com
image credit: naturalwildandfree.com
This is in contrast to nutrients, which are recycled through the work of decomposers and may pass through the same food chain many times. Producers convert light energy into chemical energy through the process of photosynthesis (some other producers also harness the energy in chemical reactions, they are called chemoautotrophs). Chemical energy flows through food chains through the result of feeding.
Checkpoint: Fill in the blanks
Energy _____________ through ecosystems, and is lost to the atmosphere as ________energy. Nutrients also ___________through ecosystems, but they are ____________.
(words: recyled, heat, flows)

Energy losses

Although this varies in different ecosystems, around 90% of energy is often lost in a feeding relationship between trophic levels. So if a person subsists on apples, only 10% of the energy in the apple trees ends up being available to the human.



Energy losses in ecosystems may occur:
  • 1. Between trophic levels (during feeding)
  • 2. Within trophic levels (energy is consumed by the organism)
1. Between trophic levels
Energy losses that occur between trophic levels are due to the incomplete ingestion, incomplete digestion, and uneaten organisms.
  • Incomplete ingestion – using the context of the apple trees, the human does not eat the whole apple tree. Only the apple. Therefore most of the energy available in the apple trees is not accessed.
  • Incomplete digestion – Using the context of the apple trees, some parts of an apple cannot be digested, such as the seeds. They would count as fibre. Therefore the energy in them is not available to the human.
  • Uneaten organisms – The human does not feed from every apple tree. Some apple trees are not fed upon, and of course even on those that are, some apples are not eaten.


2. Energy loss within trophic levels

Energy losses that occur within trophic levels are due to the organism digesting the food and then using the energy in respiration for cellular processes. These include:
  • Muscle contraction. Movement of the skeleton, blood in the circulatory system and food in the digestive system is all carried out by muscle contraction, which happens at the cellular level.
  • Active transport. Movement of substances into and out of cells against the concentration gradient, which may happen when food is absorbed across the intestinal wall.
  • Anabolic reactions. Building large molecules from small ones. This happens whenever a part of the body grows eg. amino acids are assembled to make the protein keratin, when hair grows.
Energy storage
Adenosine Tri-Phosphate(ATP)  breaks down into Adenosine Di-phosphate (ADP), yielding a packet of energy that can be instantly used for muscle contraction, molecule synthesis, cellular movement e.t.c.
When more energy is released from sugars in cellular respiration, ADP is once more converted into ATP. However not all the energy released is captured by the ATP, some is released as heat energy.
In this way, ATP acts as an energy storage molecule. You can think of ATP as being like a battery that needs to be charged.
image credit:bioninja.com.au

image credit:bioninja.com.au

The second law of thermodynamics states that energy changes (from one form to another), are never 100% efficient. Ultimately this means that the energy is lost as heat energy ( mostly as radiation).


The energy obtained by an organism tends to be stored as chemical energy, in the body. This not only includes storage carbohydrates such as glycogen (animals), or starch (plants), but proteins and lipids as well, as well structural components like bones and hair. This means that the mass of the organisms in a trophic level, can represent the energy stored at that trophic level.
P= Energy stored at a trophic level, in grams per year.
Pyramids of Energy
A pyramid of energy represents the energy found at a particular trophic level. This consists of a kind of horizontal bar chart, where the energy at each trophic level is shown by the area (length) of the bar. The units of a pyramid of energy are often kilojoules / m2 / year.



Class activities A (don’t need to to this until class)
Problem: Should salmon farming be carried out using fish feed, or soy as a source of food for the salmon

  1. Draw the two possible food chains
  2. Using graph paper, make two pyramids of energy to show each food chain
  3. Discuss the problem, using your pyramids of energy to defend your answer.
Food chain soy
  • Energy in Soy 50,000 KJ m-2yr-1
  • Energy in Salmon 7,000 KJ m-2yr-1
Food chain fish meal
  • Energy in plankton 80,000 KJm-2yr-1
  • Energy in fish meal 2,500 KJm-2yr-1
  • Energy in salmon 200 KJm-2yr-1





Class activities B:
Some students were asked Demonstration of Energy losses using the ‘blind paper toss model’.They stood in four rows, facing the same way. The front row were given paper balls and asked to toss them over their shoulders at the group behind. Any students in the second row who caught them, were considered to have been successful feeders. Any who did not catch a paper ball are effectively eliminated, and have to sit down. This continues until the fourth row.
Discussion of model.
  • Does this model produce a pyramid of energy?
  • Why does this occur?
  • How does this accurately model energy flow through food chains, and therefore explain the pyramid shape shown by ecological pyramids.
  • Conversely, what are the limitations of this model.
  • Can you make suggested refinements to this model, perhaps including energy loss within a trophic level? or the inclusion of decomposers?
Recommended questions IB Oxford 
Page 214 questions 1, 2, 3
Page 216 questions a, b, c
 Page 219 questions 1, 2, 4, 5
Socratic discussion question: Discuss: If a Cappucin monkey eats a fig, at what point will it actually access the energy in the fruit?



Introduction from recent news: the world’s biggest ever Mesocosm. Biosphere 2.


A mesocosm is a mini-ecosystem in closed system. A closed system means that only energy can be exchanged across the boundary, and not matter. This means that light energy can come in, and heat energy out, but you can’t add food or water to a mecososm once it has been set up.

Mesocosms can be aquatic, or terrestrial.

Your challenge? Make a successful mesocosm in the lab. I suggest that you:

-choose the type of mecocosm you want to make (terrestrial or aquatic)

-choose the community you want to include (which species)

-think carefully about how those species might interact in a stable way

-ethics: how can you prevent an organism from suffering as a result of a mesocosm experiment.

-choose the abiotic components

-Hint: what will the community need in order to survive?

Download this document for the mesocosm Project:


Check out this strange story of one of the world’s most successful mecososms.


Figure below: A conceptualised energy flow diagram can be developed (note that energy is ultimately lost to space and not recycled)



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