Energy and Metabolism

Lesson Content

Inquire: Bioenergetics


Cells perform life functions through various chemical reactions. A cell’s metabolism refers to the chemical reactions that take place within it. There are metabolic reactions that involve breaking down complex chemicals into simpler ones, such as breaking down large macromolecules. Scientists refer to this process as catabolism, and we associate such reactions with energy release. On the other end of the spectrum, anabolism refers to metabolic processes that build complex molecules out of simpler ones, such as macromolecule synthesis. Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and catabolic pathways, respectively.


Big Question

What are metabolic pathways?

Watch: Anabolism + Catabolism = Metabolism

Read: Metabolic Pathways


DecorativeScientists use the term bioenergetics to describe the concept of energy flow through living systems, such as cells. Cellular processes such as the building up and breaking down of complex molecules occur through stages, or chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish their energy supplies, cells must continually produce more energy to replenish the energy used in energy-requiring chemical reactions. Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism.

Metabolic Pathways

Consider the metabolism of sugar. This is a classic example of one of the many cellular processes that use and produce energy. Living things consume sugars as a major energy source because sugar molecules have a great deal of energy stored within their bonds (a strong force holding atoms together in a molecule). For the most part, photosynthesizing organisms like plants produce these sugars. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarized as:


6 carbon dioxide molecules + 6 water molecules yields (produces) 1 molecule of glucose and 6 molecules of oxygen

Because this process involves synthesizing an energy-storing molecule (glucose), it requires energy input to proceed. During the light reactions of photosynthesis, energy is provided by a molecule called adenosine triphosphate (ATP), which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. In contrast, energy-storage molecules such as glucose are consumed only to be broken down to use their energy. The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarized by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. The reaction is summarized as:


1 molecule of glucose + 6 molecules of oxygen yields (produces) 6 water molecules and 6 carbon dioxide molecules

Both of these reactions involve many steps.

The processes of making and breaking down sugar molecules (metabolism) illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule (such as glucose) and modifies it, step-by-step, through a series of metabolic intermediates, eventually creating or yielding a final product. In the example of sugar metabolism, the first metabolic pathway synthesized (made) sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes — the first requiring energy and the second producing energy — are referred to as anabolic pathways and catabolic pathways, respectively. Consequently, metabolism is composed of production (anabolism) and breakdown (catabolism).

DecorativeIt is important to know that the chemical reactions of metabolic pathways do not take place on their own. Each reaction step is facilitated, or catalyzed (sped up), by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions — those that require energy as well as those that release energy.


Thermodynamics refers to the study of energy and energy transfer involving physical matter. The matter relevant to a particular case of energy transfer is called a system, and everything outside of the matter is called the surroundings. For instance, when heating a pot of water on the stove, the system includes the stove, the pot, and the water. Energy is transferred within the system (between the stove, pot, and water). There are two types of systems: open and closed. In an open system, energy can be exchanged with its surroundings. The stovetop system is open because heat can be lost to the air. A closed system cannot exchange energy with its surroundings.

DecorativeBiological organisms are open systems. Energy is exchanged between them and their surroundings as they use energy from the sun to perform photosynthesis, or consume energy-storing molecules and release energy to the environment by doing work and releasing heat. Like everything in the physical world, energy is subject to physical laws. The laws of thermodynamics govern the transfer of energy in and among all systems in the universe.

In general, energy is defined as the ability to do work or to create some kind of change. Energy exists in different forms. For example, electrical energy, light energy, and heat energy are all different types of energy. To appreciate the way energy flows in and out of biological systems, it is important to understand two of the physical laws that govern energy.

Laws of Thermodynamics

The first law of thermodynamics states that the total amount of energy in the universe is constant and conserved. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy may be transferred from place to place or transformed into different forms, but it cannot be created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs transform electrical energy into light and heat energy. Gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful energy transformations on earth: converting the energy of sunlight to chemical energy stored within organic molecules.

The challenge for all living organisms is to obtain energy from their surroundings in forms that they can transfer or transform into usable energy. Living cells have evolved to meet this challenge. Chemical energy, stored within organic molecules such as sugars and fats, is transferred and transformed through a series of cellular chemical reactions into energy within molecules of ATP. Energy in ATP molecules is easily accessible to do work. Examples of the types of work that cells need to do include building complex molecules, transporting materials, powering the motion of cilia or flagella (which provides locomotion in some cells), and contracting muscle fibers to create movement.

However, the second law of thermodynamics explains why these tasks are harder than they appear. All energy transfers and transformations are never completely efficient. In every energy transfer, some amount of energy is lost to a form that is unusable. In most cases, this form is heat energy. Thermodynamically, heat energy is defined as the energy transferred from one system to another that is not work. For example, when a light bulb is turned on, some of the energy being converted from electrical energy into light energy is lost as heat energy. Likewise, some energy is lost as heat energy during cellular metabolic reactions.

DecorativeAn important concept in physical systems is that of order and disorder. The more energy that is lost by a system to its surroundings, the less ordered and more random the system is. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. Molecules and chemical reactions have varying entropy as well. For example, entropy increases as molecules at a high concentration in one place diffuse and spread out. The second law of thermodynamics says that energy will always be lost as heat in energy transfers or transformations.

Living things are highly ordered, requiring constant energy input to be maintained in a state of low entropy.

Reflect: Research


You are writing a research paper about energy. Which type of energy would you most want to research?

Expand: Types of Energy

Potential and Kinetic Energy

When an object is in motion, energy is associated with that object. Think of a wrecking ball; even a slow-moving wrecking ball can do a great deal of damage to other objects. Energy associated with objects in motion is called kinetic energy. A speeding bullet, a walking person, and the rapid movement of molecules in the air (which produces heat) all have kinetic energy.

An (apparently unused) office building near our condo recently disappeared, thanks to this guy

Now what if that same motionless wrecking ball is lifted two stories above ground with a crane? If the suspended wrecking ball is unmoving, is there energy associated with it? The answer is yes. The energy that was required to lift the wrecking ball did not disappear, but is now stored in the wrecking ball by virtue of its position and the force of gravity acting on it. This type of energy is called potential energy. If the ball were to fall, the potential energy would be transformed into kinetic energy until all of the potential energy was exhausted when the ball rested on the ground. Wrecking balls also swing like a pendulum; through the swing, there is a constant change of potential energy (highest at the top of the swing) to kinetic energy (highest at the bottom of the swing). Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.

Free and Activation Energy

DecorativeAfter learning that chemical reactions release energy when energy-storing bonds are broken, you may ask the following questions: How is the energy associated with these chemical reactions quantified and expressed? How can the energy released from one reaction be compared to that of another reaction?

A measurement of free energy is used to quantify these energy transfers. Recall that, according to the second law of thermodynamics, all energy transfers involve the loss of some amount of energy in an unusable form, such as heat. Free energy specifically refers to the energy associated with a chemical reaction that is available after the losses are accounted for. In other words, free energy is usable energy, or energy that is available to do work.

If energy is released during a chemical reaction, then the change in free energy, signified as ∆G (delta G) will be a negative number. A negative change in free energy also means that the products of the reaction have less free energy than the reactants, because they release some free energy during the reaction. Reactions that have a negative change in free energy and consequently release free energy are called exergonic reactions. Think: exergonic means energy is exiting the system. These reactions are also referred to as spontaneous reactions, and their products have less stored energy than the reactants. An important distinction must be drawn between the term spontaneous and the idea of a chemical reaction occurring immediately. Contrary to the everyday use of the term, a spontaneous reaction is not one that suddenly or quickly occurs. The rusting of iron is an example of a spontaneous reaction that occurs slowly, little by little, over time.

DecorativeIf a chemical reaction absorbs energy rather than releases energy on balance, then the ∆G for that reaction will be a positive value. In this case, the products have more free energy than the reactants. Thus, the products of these reactions can be thought of as energy-storing molecules. These chemical reactions are called endergonic reactions and they are non-spontaneous. An endergonic reaction will not take place on its own without the addition of free energy.

There is another important concept that must be considered regarding endergonic and exergonic reactions. Exergonic reactions require a small amount of energy input to get going, before they can proceed with their energy-releasing steps. These reactions have a net release of energy, but still require some energy input in the beginning. This small amount of energy input necessary for all chemical reactions to occur is called the activation energy.

Check Your Knowledge

Use the quiz below to check your understanding of this lesson’s content. You can take this quiz as many times as you like.

Lesson Resources

Lesson Toolbox

Additional Resources and Readings

ATP & Respiration: Crash Course Biology #7

A Crash Course video covering cellular respiration

Metabolism & Nutrition, part 1: Crash Course A&P #36

A Crash Course video covering metabolism and energy conversion in the body

Metabolism & Nutrition, part 2: Crash Course A&P #37

A Crash Course video continuing the discussion of metabolism and energy conversion in the body

Lesson Glossary


AJAX progress indicator
  • activation energy
    the small amount of energy input necessary for all chemical reactions to occur
  • adenosine triphosphate (ATP)
    a very high-energy molecule that is the primary energy currency of all cells
  • anabolic
    (or anabolism) pathways that require an energy input to synthesize complex molecules from simpler ones
  • bioenergetics
    study of energy flowing through living systems
  • catabolic
    (or catabolism) pathways in which complex molecules break down into simpler ones
  • endergonic
    describes chemical reactions that require energy input
  • entropy
    a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system
  • enzyme
    a protein that facilitates or catalyzes biological reactions
  • exergonic
    describes chemical reactions that release free energy
  • free energy
    usable energy, or energy that is available to do work
  • heat energy
    the energy transferred from one system to another that is unable to do work
  • kinetic energy
    energy type that takes place with objects or particles in motion
  • metabolic pathway
    a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products
  • metabolism
    all the chemical reactions that take place inside cells, including anabolism and catabolism
  • potential energy
    energy type that has the potential to do work; stored energy
  • thermodynamics
    study of energy and energy transfer involving physical matter

License and Citations

Content License

Lesson Content:

Authored and curated by Jill Carson for The TEL Library. CC BY NC SA 4.0

Adapted Content:

Title: Biology – 6.1 Energy and Metabolism – Carbohydrate Metabolisms; Metabolic Pathways, OpenStax CNX. License: CC BY 4.0

Title: Biology – 6.3 The Laws of Thermodynamics – Introduction, OpenStax CNX. License: CC BY 4.0

Title: Biology – 6.2 Potential, Kinetic, Free, and Activation Energy- Energy Types, OpenStax CNX. License: CC BY 4.0

Title: Biology – 4.1 Energy and Metabolism – Metabolic Pathways; Energy; Thermodynamics; Oxidative Phosphorylation, OpenStax CNX. License: CC BY 4.0

Media Sources

DecorativeCatabolismTimVickersWikimedia CommonsPublic Domain
Decorativeaged brown chainMiguel Á. PadriñánPexelsCC 0
Decorativeadrenaline jumping jumpsuitPixabayPexelsCC 0
DecorativeWrecking ball – Nov. 2011bradleypjohnsonWikimedia CommonsCC BY 2.0
Decorativebright bulb close-upPixabayPexelsCC 0
DecorativePan Water KitchenThree-shotsPixabayCC 0
DecorativeMetabolic PathwaysOpenStaxOpenStaxCC BY 4.0
DecorativeEnergy and MetabolismOpenStaxOpenStaxCC BY 4.0