35 4.9 Energy Needs of Living Things

Created by: CK-12/Adapted by Christine Miller


Image shows a photo of a sled carrying two men being pulled by 8 huskies.
Figure 4.9.1 All living things require energy to maintain homeostasis. These sled dogs use energy as they pull the sled.

These beautiful sled dogs are a metabolic marvel. While running up to 160 kilometres (about 99 miles) a day, they will each consume and burn about 12 thousand calories — about 240 calories per pound per day, which is the equivalent of about 24 Big Macs! A human endurance athlete, in contrast, typically burns only about 100 calories per pound (0.45 kg) each day. Scientists are intrigued by the amazing metabolism of sled dogs, although they still haven’t determined how they use up so much energy. But one thing is certain: all living things need energy for everything they do, whether it’s running a race or blinking an eye. In fact, every cell of your body constantly needs energy just to carry out basic life processes. You probably know that you get energy from the food you eat, but where does food come from? How does it come to contain energy? And how do your cells get the energy from food?

What Is Energy?

In the scientific world, energy is defined as the ability to do work. You can often see energy at work in living things — a bird flies through the air, a firefly glows in the dark, a dog wags its tail. These are obvious ways that living things use energy, but living things constantly use energy in less obvious ways, as well.

Why Living Things Need Energy

Inside every cell of all living things, energy is needed to carry out life processes. Energy is required to break down and build up molecules, and to transport many molecules across plasma membranes. All of life’s work needs energy. A lot of energy is also simply lost to the environment as heat. The story of life is a story of energy flow — its capture, its change of form, its use for work, and its loss as heat. Energy (unlike matter) cannot be recycled, so organisms require a constant input of energy. Life runs on chemical energy. Where do living organisms get this chemical energy?

How Organisms Get Energy

The chemical energy that organisms need comes from food. Food consists of organic molecules that store energy in their chemical bonds. In terms of obtaining food for energy, there are two types of organisms: autotrophs and heterotrophs.


Autotrophs are organisms that capture energy from nonliving sources and transfer that energy into the living part of the ecosystem. They are also able to make their own food. Most autotrophs use the energy in sunlight to make food in the process of photosynthesis. Only certain organisms — such as plants, algae, and some bacteria — can make food through photosynthesis. Some photosynthetic organisms are shown in Figure 4.9.2.

Image shows a photo of a leafy plant Image shows a photograph of green algae living on the ocean floor
Figure 4.9.2 Photosynthetic autotrophs, which make food using the energy in sunlight, include plants (left), algae (middle), and certain bacteria (right).

Autotrophs are also called producers. They produce food not only for themselves, but for all other living things (known as consumers), as well. This is why autotrophs form the basis of food chains, such as the food chain shown In Figure 4.9.3.

Diagram shows two food pyramids, each with trophic levels labelled.
Figure 4.9.3 Food chains: Aquatic and terrestrial ecosystems.

A food chain shows how energy and matter flow from producers to consumers. Matter is recycled, but energy must keep flowing into the system. Where does this energy come from?

Watch the video “The simple story of photosynthesis and food – Amanda Ooten” from TED-Ed to learn more about photosynthesis:

The simple story of photosynthesis and food – Amanda Ooten, TED-Ed, 2013.


Heterotrophs are living things that cannot make their own food. Instead, they get their food by consuming other organisms, which is why they are also called consumers. They may consume autotrophs or other heterotrophs. Heterotrophs include all animals and fungi, as well as many single-celled organisms. In Figure 4.9.3, all of the organisms are consumers except for the grasses and phytoplankton. What do you think would happen to consumers if all producers were to vanish from Earth?

Energy Molecules: Glucose and ATP

Organisms mainly use two types of molecules for chemical energy: glucose and ATP. Both molecules are used as fuels throughout the living world. Both molecules are also key players in the process of photosynthesis.


Glucose is a simple carbohydrate with the chemical formula C6H12O6. It stores chemical energy in a concentrated, stable form. In your body, glucose is the form of energy that is carried in your blood and taken up by each of your trillions of cells. Glucose is the end product of photosynthesis, and it is the nearly universal food for life.  In Figure 4.9.4, you can see how photosynthesis stores energy from the sun in the glucose molecule and then how cellular respiration breaks the bonds in glucose to retrieve the energy.

Image shows the formula for photosynthesis: Carbon dioxide and water are converted to glucose and oxygen, which is an endothermic reaction drawing its energy from the sun. Cellular respiration carries out the opposite reaction, breaking down glucose in the presence of oxygen to produce carbon dioxide and water, and releasing the energy previously stored in the glucose molecule, which is an exothermic reaction.
Figure 4.9.4 Energy transfer in photosynthesis and cellular respiration.


If you remember from section 3.7 Nucleic Acids, ATP (adenosine triphosphate) is the energy-carrying molecule that cells use to power most cellular processes (nerve impulse conduction, protein synthesis and active transport are good examples of cell processes that rely on ATP as their energy source).  ATP is made during the first half of photosynthesis and then used for energy during the second half of photosynthesis, when glucose is made. ATP releases energy when it gives up one of its three phosphate groups (Pi) and changes to ADP (adenosine diphosphate, which has two phosphate groups), as shown in Figure 4.9.5. Thus, the breakdown of ATP into ADP + Pi is a catabolic reaction that releases energy (exothermic). ATP is made from the combination of ADP and Pi, an anabolic reaction that takes in energy (endothermic).

Image shows a diagram of the ATP molecule which consists of adenosine, ribose, and three phosphate groups. When the bond between the second and third phosphate group is broken, energy previously stored in the chemical bonds is released.
Figure 4.9.5 ATP (adenosine TRI phosphate) can be converted to ADP (adensosine DI phosphate) to release the energy stored in the chemical bonds between the second and third phosphate group.

Why Organisms Need Both Glucose and ATP

Why do living things need glucose if ATP is the molecule that cells use for energy? Why don’t autotrophs just make ATP and be done with it? The answer is in the “packaging.” A molecule of glucose contains more chemical energy in a smaller “package” than a molecule of ATP. Glucose is also more stable than ATP. Therefore, glucose is better for storing and transporting energy. Glucose, however, is too powerful for cells to use. ATP, on the other hand, contains just the right amount of energy to power life processes within cells. For these reasons, both glucose and ATP are needed by living things.

How Energy Flows Through Living Things

The flow of energy through living organisms begins with photosynthesis. This process stores energy from sunlight in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need. The process in which glucose is broken down and ATP is made is called cellular respiration.

Photosynthesis and cellular respiration are like two sides of the same coin. This is apparent in Figure 4.9.6. The products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to recycle oxygen in the Earth’s atmosphere.

Image shows a diagram of photosynthesis taking place in chloroplasts and converting carbon dioxide and water into glucose and oxygen. The image also shows how the products of photosynthesis can be transferred into the mitochondria to undergo cellular respiration, converting them back into carbon dioxide and water, and in doing so, releasing the stored energy in the glucose molecule.
Figure 4.9.6 This diagram compares and contrasts photosynthesis and cellular respiration. It also shows how the two processes are related.


4.9 Summary

  • Energy is the ability to do work. It is needed by all living things and every living cell to carry out life processes, such as breaking down and building up molecules, and transporting many molecules across cell membranes.
  • The form of energy that living things need for these processes is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
  • Autotrophs make their own food. Plants, for example, make food by photosynthesis. Autotrophs are also called producers.
  • Heterotrophss obtain food by eating other organisms. Heterotrophs are also known as consumers.
  • Organisms mainly use the molecules glucose and ATP for energy. Glucose is a compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cell processes.
  • The flow of energy through living things begins with photosynthesis, which creates glucose. In a process called cellular respiration, organisms’ cells break down glucose and make the ATP they need.

4.9 Review Questions

  1. Define energy.
  2. Why do living things need energy?
  3. Compare and contrast the two basic ways that organisms get energy.
  4. Describe the roles and relationships of the energy molecules glucose and ATP.
  5. Summarize how energy flows through living things.
  6. Why does the transformation of ATP to ADP release energy?

4.9 Explore More

Learn Biology: Autotrophs vs. Heterotrophs, Mahalodotcom, 2011.

Energy Transfer in Trophic Levels, Teacher’s Pet, 2015.


Figure 4.9.1
Three Airmen participate in dog-sled expedition by U.S. Air Force photo by Tech. Sgt. Dan Rea is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.9.2

Figure 4.9.3

Biomass_Pyramid by Swiggity.Swag.YOLO.Bro on Wikipedia is used and adapted by Christine Miller under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 4.9.4

Photosynthesis and respiration by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.9.5

Photo synthesis and cellular respiration by Lady of Hats/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution



LadyofHats/CK-12 Foundation. (2016, August 15). Figure 5: Photosynthesis and cellular respiration [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.9). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.9/

Mahalodotcom. (2011, January 14). Learn biology: Autotrophs vs. heterotrophs. YouTube. https://www.youtube.com/watch?v=eDalQv7d2cs

Teacher’s Pet. (2015, March 23). Energy transfer in trophic levels. YouTube. https://www.youtube.com/watch?v=0glkXIj1DgE&feature=emb_logo

TED-Ed. (2013, March 5). The simple story of photosynthesis and food – Amanda Ooten. YouTube. https://www.youtube.com/watch?v=eo5XndJaz-Y&feature=youtu.be



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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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