How Cells Obtain Energy

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How Cells Obtain Energy


Figure 1: A Hummingbird Takes in Food to Produce Energy

A hummingbird needs energy to maintain prolonged flight. The bird obtains its energy

from taking in food and transforming the energy contained in food molecules into forms

of energy to power its flight through a series of biochemical reactions.

a modification of work by Cory Zanker

Learning Resource



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Chapter Outline

Energy and Metabolism Glycolysis

Citric Acid Cycle and Oxidative Phosphorylation Fermentation

Connections to Other Metabolic Pathways


Virtually every task performed by living organisms requires energy. Energy is needed to

perform heavy labor and exercise, but humans also use energy while thinking and even

during sleep. In fact, the living cells of every organism constantly use energy. Nutrients

and other molecules are imported into the cell, metabolized (broken down) and possibly

synthesized into new molecules, modified if needed, transported around the cell, and

possibly distributed to the entire organism. For example, the large proteins that make up

muscles are built from smaller molecules imported from dietary amino acids. Complex

carbohydrates are broken down into simple sugars that the cell uses for energy. Just as

energy is required to both build and demolish a building, energy is required for the

synthesis and breakdown of molecules as well as the transport of molecules into and out

of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria

and viruses, exporting wastes and toxins, and movement of the cell require energy. From

where, and in what form, does this energy come? How do living cells obtain energy, and

how do they use it? This chapter will discuss different forms of energy and the physical

laws that govern energy transfer. It will also describe how cells use energy and replenish

it, and how chemical reactions in the cell are performed with great efficiency.

Energy and Metabolism



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By the end of this section, you will be able to:

explain what metabolic pathways are

state the first and second laws of thermodynamics

explain the difference between kinetic and potential


describe endergonic and exergonic reactions

discuss how enzymes function as molecular catalysts.

Scientists use the term bioenergetics to describe the concept of energy flow (Figure 2)

through living systems, such as cells. Cellular processes such as the building and breaking

down of complex molecules occur through stepwise 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 that used by the many

energy-requiring chemical reactions that constantly take place. Together, all the chemical

reactions that take place inside cells, including those that consume or generate energy, are

referred to as the cell’s metabolism.



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Figure 2: Bioenergetics, the Energy Flow Through Living Systems

Ultimately, most life forms get their energy from the sun. Plants use photosynthesis to

capture sunlight, and herbivores eat the plants to obtain energy. Carnivores eat the

herbivores, and eventual decomposition of plant and animal material contributes to the

nutrient pool.

Metabolic Pathways



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Figure 3: Two Examples of Metabolic Pathways

Catabolic pathways are those that generate energy by breaking down larger molecules.

Anabolic pathways are those that require energy to synthesize larger molecules. Both

types of pathways are required for maintaining the cell’s energy balance

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.

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:

6CO2 + 6H2 O → C6 H12 O6 + 6O2

Because this process involves synthesizing an energy-storing molecule, 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


C6 H12 O6 + 6O2 → 6H2 O + 6CO2

Both of these reactions involve many steps.



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The processes of making and breaking down sugar molecules illustrate two examples of

metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a

starting molecule and modifies it, step-by-step, through a series of metabolic

intermediates, eventually yielding a final product. In the example of sugar metabolism, the

first metabolic pathway synthesized 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

(building polymers) and catabolic pathways (breaking down polymers into their

monomers), respectively. Consequently, metabolism is composed of synthesis (anabolism)

and degradation (catabolism) (Figure 3).

It 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, 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 that 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.

Biological 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 all things 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


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 into and out

of biological systems, it is important to understand two of the physical laws that govern




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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: that of converting the energy of sunlight to

chemical energy stored within organic molecules (Figure 2). Some examples of energy

transformations are shown in Figure 4.

The challenge for all living organisms is to obtain energy from their surroundings in forms

that they can transfer or transform into usable energy to do work. 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, and contracting muscle fibers to create movement.



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Figure 4: Energy Transformations

Shown are some examples of energy transferred and transformed from one

system to another and from one form to another. The food we consume

provides our cells with the energy required to carry out bodily functions,

just as light energy provides plants with the means to create the chemical

energy they need.

(credit “ice cream”: modification of work by D. Sharon Pruitt; credit “kids”: modification of work by

Max from Providence; credit “leaf”: modification of work by Cory Zanker)



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A living cell’s primary tasks of obtaining, transforming, and using energy to do work may

seem simple. 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 in 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.

An 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.

Potential and Kinetic Energy

When an object is in motion, there is energy 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 (Figure 5). A

speeding bullet, a walking person, and the rapid movement of molecules in the air (which

produces heat) all have kinetic energy.

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 (Figure 5). If the ball were to fall, the

potential energy would be transformed into kinetic energy until all 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.



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Figure 5: Examples of Potential and Kinetic Energy of Water

Still water has potential energy; moving water, such as in a waterfall or a rapidly flowing

river, has kinetic energy.

“dam”: a modification of work by “Pascal”/Flickr; “waterfall”: modification of work by Frank Gualtieri)

Potential energy is not only associated with the location of matter, but also with the

structure of matter. Even a spring on the ground has potential energy if it is compressed;

so does a rubber band that is pulled taut. On a molecular level, the bonds that hold the

atoms of molecules together exist in a particular structure that has potential energy.

Remember that anabolic cellular pathways require energy to synthesize complex

molecules from simpler ones and catabolic pathways release energy when complex

molecules are broken down. The fact that energy can be released by the breakdown of

certain chemical bonds implies that those bonds have potential energy. In fact, there is

potential energy stored within the bonds of all the food molecules we eat, which is

eventually harnessed for use. This is because these bonds can release energy when

broken. The type of potential energy that exists within chemical bonds, and is released

when those bonds are broken, is called chemical energy. Chemical energy is responsible

for providing living cells with energy from food. The release of energy occurs when the

molecular bonds within food molecules are broken.

Free and Activation Energy

After learning that chemical reactions release energy when energy-storing bonds are

broken, an important next question is the following: 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



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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.

If 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.



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Art Connection

Figure 6: Examples of Endergonic and Exergonic Processes

Figure 6: Examples of Endergonic and Exergonic Processes is adapted

from Concepts of Biology (

biology/pdf) by OpenStax College, which is available under a Creative Commons

Attribution 3.0 Unported ( license. ©

2013, Rice University.

Shown are some examples of endergonic processes (ones that require energy) and

exergonic processes (ones that release energy).

a: modification of work by Natalie Maynor; credit b: modification of work by USDA; credit c: modification of

work by Cory Zanker; credit d: modification of work by Harry Malsch



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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.


A substance that helps a chemical reaction to occur is called a catalyst, and the molecules

that catalyze biochemical reactions are called enzymes. Most enzymes are proteins and

perform the critical task of lowering the activation energies of chemical reactions inside

the cell. Most of the reactions critical to a living cell happen too slowly at normal

temperatures to be of any use to the cell. Without enzymes to speed up these reactions,

life could not persist. Enzymes do this by binding to the reactant molecules and holding

them in such a way as to make the chemical bond-breaking and bond-forming processes

take place more easily. It is important to remember that enzymes do not change whether a

reaction is exergonic (spontaneous) or endergonic. This is because they do not change the

free energy of the reactants or products. They only reduce the activation energy required

for the reaction to go forward (Figure 7). In addition, an enzyme itself is unchanged by the

reaction it catalyzes. Once one reaction has been catalyzed, the enzyme is able to

participate in other reactions.



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Figure 7: The Effect of an Enzyme on Activation Energy Required in a


Enzymes lower the activation energy of the reaction but do not change the

free energy of the reaction.

The chemical reactants to which an enzyme binds are called the enzyme’s substrates.

There may be one or more substrates, depending on the particular chemical reaction. In

some reactions, a single reactant substrate is broken down into multiple products. In

others, two substrates may come together to create one larger molecule. Two reactants

might also enter a reaction and both become modified, but they leave the reaction as two

products. The location within the enzyme where the substrate binds is called the

enzyme’s active site. The active site is where the “action” happens. Since enzymes are

proteins, there is a unique combination of amino acid side chains within the active site.

Each side chain is characterized by different properties. They can be large or small, weakly

acidic or basic, hydrophilic or hydrophobic, positively or negatively charged, or neutral.

The unique combination of side chains creates a very specific chemical environment

within the active site. This specific environment is suited to bind to one specific chemical

substrate (or substrates).



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Active sites are subject to influences of the local environment. Increasing the

environmental temperature generally increases reaction rates, enzyme-catalyzed or

otherwise. However, temperatures outside of an optimal range reduce the rate at which

an enzyme catalyzes a reaction. Hot temperatures will eventually cause enzymes to

denature, an irreversible change in the three-dimensional shape and therefore the

function of the enzyme. Enzymes are also suited to function best within a certain pH and

salt concentration range, and, as with temperature, extreme pH, and salt concentrations

can cause enzymes to denature.

For many years, scientists thought that enzyme-substrate binding took place in a simple

“lock-and-key” fashion. This model asserted that the enzyme and substrate fit together

perfectly in one instantaneous step. However, current research supports a model called

induced fit (Figure 8). The induced-fit model expands on the lock-and-key model by

describing a more dynamic binding between enzyme and substrate. As the enzyme and

substrate come together, their interaction causes a mild shift in the enzyme’s structure

that forms an ideal binding arrangement between enzyme and substrate.

When an enzyme binds its substrate, an enzyme-substrate complex is formed. This

complex lowers the activation energy of the reaction and promotes its rapid progression

in one of multiple possible ways. On a basic level, enzymes promote chemical reactions

that involve more than one substrate by bringing the substrates together in an optimal

orientation for reaction. Another way in which enzymes promote the reaction of their

substrates is by creating an optimal environment within the active site for the reaction to

occur. The chemical properties that emerge from the particular arrangement of amino acid

R groups within an active site create the perfect environment for an enzyme’s specific

substrates to react.

View an animation of Induced fit :


The enzyme-substrate complex can also lower activation energy by compromising the

bond structure so that it is easier to break. Finally, enzymes can also lower activation

energies by taking part in the chemical reaction itself. In these cases, it is important to

remember that the enzyme will always return to its original state by the completion of the

reaction. One of the hallmark properties of enzymes is that they remain ultimately

unchanged by the reactions they catalyze. After an enzyme has catalyzed a reaction, it

releases its product(s) and can catalyze a new reaction.



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Figure 8: The Induced-Fit Model

The induced-fit model is an adjustment to the lock-and-key model and explains how

enzymes and substrates undergo dynamic modifications during the transition state to

increase the affinity of the substrate for the active site.

It would seem ideal to have a scenario in which all of an organism’s enzymes existed in

abundant supply and functioned optimally under all cellular conditions, in all cells, at all

times. However, a variety of mechanisms ensures that this does not happen. Cellular

needs and conditions constantly vary from cell to cell and change within individual cells

over time. The required enzymes of stomach cells differ from those of fat storage cells,

skin cells, blood cells, and nerve cells. Furthermore, a digestive organ cell works much

harder to process and break down nutrients during the time that closely follows a meal

compared with many hours after a meal. As these cellular demands and conditions vary,

so must the amounts and functionality of different enzymes.

Since the rates of biochemical reactions are controlled by activation energy, and enzymes

lower and determine activation energies for chemical reactions, the relative amounts and

functioning of the variety of enzymes within a cell ultimately determine which reactions

will proceed and at what rates. This determination is tightly controlled in cells. In certain

cellular environments, enzyme activity is partly controlled by environmental factors like

pH, temperature, salt concentration, and, in some cases, cofactors or coenzymes.

Enzymes can also be regulated in ways that either promote or reduce enzyme activity.

There are many kinds of molecules that inhibit or promote enzyme function and various

mechanisms by which they do so. In some cases of enzyme inhibition, an inhibitor

molecule is similar enough to a substrate that it can bind to the active site and simply

block the substrate from binding. When this happens, the enzyme is inhibited through

competitive inhibition, because an inhibitor molecule competes with the substrate for

binding to the active site.



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On the other hand, in noncompetitive inhibition, an inhibitor molecule binds to the

enzyme in a location other than the active site, called an allosteric site, but still manages

to block substrate binding to the active site. Some inhibitor molecules bind to enzymes in

a location where their binding induces a conformational change that reduces the affinity

of the enzyme for its substrate. This type of inhibition is called allosteric inhibition (Figure

9). Most allosterically regulated enzymes are made up of more than one polypeptide,

meaning that they have more than one protein subunit. When an allosteric inhibitor binds

to a region on an enzyme, all active sites on the protein subunits are changed slightly such

that they bind their substrates with less efficiency. There are allosteric activators as well

as inhibitors. Allosteric activators bind to locations on an enzyme away from the active

site, inducing a conformational change that increases the affinity of the enzyme’s active

site(s) for its substrate(s) (Figure 9).

Figure 9 Allosteric Inhibition and Allosteric Activation

Allosteric inhibition works by indirectly inducing a conformational change to the active

site such that the substrate no longer fits. In contrast, in allosteric activation, the

activator molecule modifies the shape of the active site to allow a better fit of the




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Careers In Action

Figure 10: Careers in Action: Pharmaceutical Drug Developer

Have you ever wondered how pharmaceutical drugs are developed?

(photo credit: Deborah Austin)

Enzymes are key components of metabolic pathways. Understanding how enzymes

work and how they can be regulated are key principles behind the development of

many of the pharmaceutical drugs on the market today. Biologists working in this

field collaborate with other scientists to design drugs (Figure 10).

Consider statins for example—statin is the name given to one class of drugs that can

reduce cholesterol levels. These compounds are inhibitors of the enzyme HMG-CoA

reductase, which is the enzyme that synthesizes cholesterol from lipids in the body.

By inhibiting this enzyme, the level of cholesterol synthesized in the body can be

reduced. Similarly, acetaminophen, popularly marketed under the brand name

Tylenol, is an inhibitor of the enzyme cyclooxygenase. While it is used to provide

relief from fever and inflammation (pain), its mechanism of action is still not

completely understood.

How are drugs discovered? One of the biggest challenges in drug discovery is

identifying a drug target. A drug target is a molecule that is literally the target of the

drug. In the case of statins, HMG-CoA reductase is the drug target. Drug targets are

identified through painstaking research in the laboratory. Identifying the target

alone is not enough; scientists also need to know how the target acts inside the cell

and which reactions go awry in the case of disease. Once the target and the

pathway are identified, then the actual process of drug design begins. In this stage,

chemists and biologists work together to design and synthesize molecules that can

block or activate a particular reaction. However, this is only the beginning: If and



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when a drug prototype is successful in performing its function, then it is subjected

to many tests from in vitro experiments to clinical trials before it can get approval

from the U.S. Food and Drug Administration to be on the market.

Many enzymes do not work optimally, or even at all, unless bound to other specific non-

protein helper molecules. They may bond either temporarily through ionic or hydrogen

bonds, or permanently through stronger covalent bonds. Binding to these molecules

promotes optimal shape and function of their respective enzymes. Two examples of these

types of helper molecules are cofactors and coenzymes. Cofactors are inorganic ions such

as ions of iron and magnesium. Coenzymes are organic helper molecules, those with a

basic atomic structure made up of carbon and hydrogen. Like enzymes, these molecules

participate in reactions without being changed themselves and are ultimately recycled and

reused. Vitamins are the source of coenzymes. Some vitamins are the precursors of

coenzymes and others act directly as coenzymes. Vitamin C is a direct coenzyme for

multiple enzymes that take part in building the important connective tissue, collagen.

Therefore, enzyme function is, in part, regulated by the abundance of various cofactors

and coenzymes, which may be supplied by an organism’s diet or, in some cases, produced

by the organism.

Feedback Inhibition in Metabolic Pathways

Molecules can regulate enzyme function in many ways. The major question remains,

however: What are these molecules and where do they come from? Some are cofactors

and coenzymes, as you have learned. What other molecules in the cell provide enzymatic

regulation such as allosteric modulation and competitive and noncompetitive inhibition?

Perhaps the most relevant sources of regulatory molecules, with respect to enzymatic

cellular metabolism, are the products of the cellular metabolic reactions themselves. In a

most efficient and elegant way, cells have evolved to use the products of their own

reactions for feedback inhibition of enzyme activity. Feedback inhibition involves the use

of a reaction product to regulate its own further production (Figure 11). The cell responds

to an abundance of the products by slowing down production during anabolic or catabolic

reactions. Such reaction products may inhibit the enzymes that catalyzed their production

through the mechanisms described above.



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Figure 11: Feedback Inhibition

Metabolic pathways are a series of reactions catalyzed by multiple enzymes. Feedback

inhibition, where the end product of the pathway inhibits an upstream process, is an

important regulatory mechanism in cells.

The production of both amino acids and nucleotides is controlled through feedback

inhibition. Additionally, ATP is an allosteric regulator of some of the enzymes involved in

the catabolic breakdown of sugar, the process that creates ATP. In this way, when ATP is

in abundant supply, the cell can prevent the production of ATP. On the other hand, ADP

(adenosine diphosphate) serves as a positive allosteric regulator (an allosteric activator) for

some of the same enzymes that are inhibited by ATP. Thus, when relative levels of ADP

are high compared to ATP, the cell is triggered to produce more ATP through sugar



By the end of this section, you will be able to:

• explain how ATP is used by the cell as an energy source

• describe the overall result in terms of molecules produced of the breakdown of

glucose by glycolysis.


Even exergonic, energy-releasing reactions require a small amount of activation energy to

proceed. However, consider endergonic reactions, which require much more energy input

because their products have more free energy than their reactants. Within the cell, where

does energy to power such reactions come from? The answer lies with an energy-



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supplying molecule called adenosine triphosphate, or ATP. ATP is a small, relatively simple

molecule, but within its bonds it contains the potential for a quick burst of energy that can

be harnessed to perform cellular work. This molecule can be thought of as the primary

energy currency of cells in the same way that money is the currency that people exchange

for things they need. ATP is used to power the majority of energy-requiring cellular


ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would

result in an increase of heat in the cell, which would denature enzymes and other proteins

and thus destroy the cell. Rather, a cell must be able to store energy safely and release it

for use only as needed. Living cells accomplish this using ATP, which can be used to fill

any energy need of the cell. How? It functions as a rechargeable battery.

Figure 12: The Structure of ATP

The structure of ATP shows the basic components of a two-ring

adenine, five-carbon ribose, and three phosphate groups.

When ATP is broken down, usually by the removal of its terminal phosphate group,

energy is released. This energy is used to do work by the cell, usually by the binding of the

released phosphate to another molecule, thus activating it. For example, in the mechanical

work of muscle contraction, ATP supplies energy to move the contractile muscle proteins.

ATP Structure and Function



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At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is

composed of an adenine molecule bonded to both a ribose molecule and a single

phosphate group (Figure 12). Ribose is a five-carbon sugar found in RNA, and AMP is one

of the nucleotides in RNA. The addition of a second phosphate group to this core

molecule results in adenosine diphosphate (ADP); the addition of a third phosphate group

forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires a high amount of energy and

results in a high-energy bond. Phosphate groups are negatively charged and thus repel

one another when they are arranged in series, as they are in ADP and ATP. This repulsion

makes the ADP and ATP molecules inherently unstable. The release of one or two

phosphate groups from ATP, a process called hydrolysis, releases energy.


You have read that nearly all the energy used by living things comes to them in the bonds

of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract

energy for cell metabolism. Many living organisms carry out glycolysis as part of their

metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic


Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule

and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists

of two distinct phases. In the first part of the glycolysis pathway, energy is used to make

adjustments so that the six-carbon sugar molecule can be split evenly into two three-

carbon pyruvate molecules. In the second part of glycolysis, ATP and nicotinamide-

adenine dinucleotide (NADH) are produced (Figure 13).

If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP

molecules from one molecule of glucose. For example, mature mammalian red blood cells

are only capable of glycolysis, which is their sole source of ATP. If glycolysis is interrupted,

these cells would eventually die.



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Figure 13: Glycolysis

In glycolysis, a glucose molecule is converted into two pyruvate molecules.

Citric Acid Cycle and Oxidative Phosphorylation



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By the end of this section, you will be able to:

explain what metabolic pathways are

state the first and second laws of thermodynamics

explain the difference between kinetic and potential


describe endergonic and exergonic reactions

discuss how enzymes function as molecular catalysts.

The Citric Acid Cycle

In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are

transported into mitochondria, which are sites of cellular respiration. If oxygen is available,

aerobic respiration will go forward. In mitochondria, pyruvate will be transformed into a

two-carbon acetyl group (by removing a molecule of carbon dioxide) that will be picked up

by a carrier compound called coenzyme A (CoA), which is made from vitamin B5. The

resulting compound is called acetyl CoA. (Figure 14). Acetyl CoA can be used in a variety

of ways by the cell, but its major function is to deliver the acetyl group derived from

pyruvate to the next pathway in glucose catabolism.



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Figure 14: Pyruvate is Converted into Acetyl-CoA in the Glucose Catabolic Pathway

Pyruvate is converted into acetyl-CoA before entering the citric acid cycle.

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle in eukaryotic cells

takes place in the matrix of the mitochondria. Unlike glycolysis, the citric acid cycle is a

closed loop: the last part of the pathway regenerates the compound used in the first step.

The eight steps of the cycle are a series of chemical reactions that produces two carbon

dioxide molecules, one ATP molecule (or an equivalent), and reduced forms (NADH and

FADH2) of NAD+ and FAD+, important coenzymes in the cell. Part of this is considered an

aerobic pathway (oxygen-requiring) because the NADH and FADH2 produced must

transfer their electrons to the next pathway in the system, which will use oxygen. If

oxygen is not present, this transfer does not occur.

Two carbon atoms come into the citric acid cycle from each acetyl group. Two carbon

dioxide molecules are released on each turn of the cycle; however, these do not contain

the same carbon atoms contributed by the acetyl group on that turn of the pathway. The

two acetyl-carbon atoms will eventually be released on later turns of the cycle; in this

way, all six carbon atoms from the original glucose molecule will be eventually released as

carbon dioxide. It takes two turns of the cycle to process the equivalent of one glucose

molecule. Each turn of the cycle forms three high-energy NADH molecules and one high-

energy FADH2 molecule. These high-energy carriers will connect with the last portion of

aerobic respiration to produce ATP molecules. One ATP (or an equivalent) is also made in

each cycle. Several of the intermediate compounds in the citric acid cycle can be used in

synthesizing nonessential amino acids; therefore, the cycle is both anabolic and catabolic.

Oxidative Phosphorylation



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You have just read about two pathways in glucose catabolism—glycolysis and the citric

acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism

of glucose, however, is not generated directly from these pathways. Rather, it derives

from a process that begins with passing electrons through a series of chemical reactions

to a final electron acceptor, oxygen. These reactions take place in specialized protein

complexes located in the inner membrane of the mitochondria of eukaryotic organisms

and on the inner part of the cell membrane of prokaryotic organisms. The energy of the

electrons is harvested and used to generate an electrochemical gradient across the inner

mitochondrial membrane. The potential energy of this gradient is used to generate ATP.

The entirety of this process is called oxidative phosphorylation.

The electron transport chain (Figure 15a) is the last component of aerobic respiration and

is the only part of metabolism that uses atmospheric oxygen. Oxygen continuously

diffuses into plants for this purpose. In animals, oxygen enters the body through the

respiratory system. Electron transport is a series of chemical reactions that resembles a

bucket brigade in that electrons are passed rapidly from one component to the next, to

the endpoint of the chain where oxygen is the final electron acceptor and water is

produced. There are four complexes composed of proteins (labeled I through IV in Figure

15c), and the aggregation of these four complexes, together with associated mobile,

accessory electron carriers, is called the electron transport chain. The electron transport

chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and

in the plasma membrane of prokaryotes. In each transfer of an electron through the

electron transport chain, the electron loses energy, but with some transfers, the energy is

stored as potential energy by using it to pump hydrogen ions across the inner

mitochondrial membrane into the intermembrane space, creating an electrochemical




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Art Connection

Figure 15: Art Connection: The Electron Transport Chain

a) The electron transport chain is a set of molecules that supports a series of

oxidation-reduction reactions. (b) ATP synthase is a complex, molecular machine

that uses an H+ gradient to regenerate ATP from ADP. (c) Chemiosmosis relies on

the potential energy provided by the H+ gradient across the membrane.

Electrons from NADH and FADH2 are passed to protein complexes in the electron

transport chain. As they are passed from one complex to another (there are a total of

four), the electrons lose energy, and some of that energy is used to pump hydrogen ions

from the mitochondrial matrix into the intermembrane space. In the fourth protein

complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its



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extra electrons then combines with two hydrogen ions, further enhancing the

electrochemical gradient, to form water. If there were no oxygen present in the

mitochondrion, the electrons could not be removed from the system, and the entire

electron transport chain would back up and stop. The mitochondria would be unable to

generate new ATP in this way, and the cell would ultimately die from lack of energy. This

is the reason we must breathe to draw in new oxygen.

In the electron transport chain, the free energy from the series of reactions just described

is used to pump hydrogen ions across the membrane. The uneven distribution of H+ ions

across the membrane establishes an electrochemical gradient, owing to the H+ ions’

positive charge and their higher concentration on one side of the membrane.

Hydrogen ions diffuse through the inner membrane through an integral membrane protein

called ATP synthase (Figure 15b). This complex protein acts as a tiny generator, turned by

the force of the hydrogen ions diffusing through it, down their electrochemical gradient

from the intermembrane space, where there are many mutually repelling hydrogen ions, to

the matrix, where there are few. The turning of the parts of this molecular machine

regenerate ATP from ADP. This flow of hydrogen ions across the membrane through ATP

synthase is called chemiosmosis.

Chemiosmosis (Figure 15c) is used to generate 90 percent of the ATP made during

aerobic glucose catabolism. The result of the reactions is the production of ATP from the

energy of the electrons removed from hydrogen atoms. These atoms were originally part

of a glucose molecule. At the end of the electron transport system, the electrons are used

to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen ions

attract hydrogen ions (protons) from the surrounding medium, and water is formed. The

electron transport chain and the production of ATP through chemiosmosis are collectively

called oxidative phosphorylation.

ATP Yield

The number of ATP molecules generated from the catabolism of glucose varies. For

example, the number of hydrogen ions that the electron transport chain complexes can

pump through the membrane varies between species. Another source of variance stems

from the shuttle of electrons across the mitochondrial membrane. The NADH generated

from glycolysis cannot easily enter mitochondria. Thus, electrons are picked up on the

inside of the mitochondria by either NAD+ or FAD+. Fewer ATP molecules are generated

when FAD+ acts as a carrier. NAD+ is used as the electron transporter in the liver and

FAD+ in the brain, so ATP yield depends on the tissue being considered.



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Another factor that affects the yield of ATP molecules generated from glucose is that

intermediate compounds in these pathways are used for other purposes. Glucose

catabolism connects with the pathways that build or break down all other biochemical

compounds in cells, and the result is somewhat messier than the ideal situations described

thus far. For example, sugars other than glucose are fed into the glycolytic pathway for

energy extraction. Other molecules that would otherwise be used to harvest energy in

glycolysis or the citric acid cycle may be removed to form nucleic acids, amino acids,

lipids, or other compounds. Overall, in living systems, these pathways of glucose

catabolism extract about 34 percent of the energy contained in glucose.

Careers In Action

Mitochondrial Disease Physician

What happens when the critical reactions of cellular respiration do not proceed

correctly? Mitochondrial diseases are genetic disorders of metabolism.

Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA,

and they result in the production of less energy than is normal in body cells.

Symptoms of mitochondrial diseases can include muscle weakness, lack of

coordination, stroke-like episodes, and loss of vision and hearing. Most affected

people are diagnosed in childhood, although there are some adult-onset diseases.

Identifying and treating mitochondrial disorders is a specialized medical field. The

educational preparation for this profession requires a college education, followed by

medical school with a specialization in medical genetics. Medical geneticists can be

board certified by the American Board of Medical Genetics and go on to become

associated with professional organizations devoted to the study of mitochondrial

disease, such as the Mitochondrial Medicine Society and the Society for Inherited

Metabolic Disease.




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By the end of this section, you will be able to:

• discuss the fundamental difference between anaerobic

cellular respiration and fermentation

• describe the type of fermentation that readily occurs in

animal cells and the conditions that initiate that fermentation.

In aerobic respiration, the final electron acceptor is an oxygen molecule, O2. If aerobic

respiration occurs, then ATP will be produced using the energy of the high-energy

electrons carried by NADH or FADH2 to the electron transport chain. If aerobic

respiration does not occur, NADH must be reoxidized to NAD+ for reuse as an electron

carrier for glycolysis to continue. How is this done? Some living systems use an organic

molecule as the final electron acceptor. Processes that use an organic molecule to

regenerate NAD+ from NADH are collectively referred to as fermentation. In contrast,

some living systems use an inorganic molecule as a final electron acceptor; both methods

are a type of anaerobic cellular respiration. Anaerobic respiration enables organisms to

convert energy for their use in the absence of oxygen.

Lactic Acid Fermentation

The fermentation method used by animals and some bacteria like those in yogurt is lactic

acid fermentation (Figure 16). This occurs routinely in mammalian red blood cells and in

skeletal muscle that has insufficient oxygen supply to allow aerobic respiration to

continue (that is, in muscles used to the point of fatigue). In muscles, lactic acid produced

by fermentation must be removed by the blood circulation and brought to the liver for

further metabolism. The chemical reaction of lactic acid fermentation is the following:

Pyruvic acid + NADH ↔ lactic acid + NAD+

The enzyme that catalyzes this reaction is lactate dehydrogenase. The reaction can

proceed in either direction, but the left-to-right reaction is inhibited by acidic conditions.

This lactic acid buildup causes muscle stiffness and fatigue. Once the lactic acid has been

removed from the muscle and is circulated to the liver, it can be converted back to pyruvic

acid and further catabolized for energy.



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Art Connection

Figure 16: Lactic Acid Fermentation

Lactic acid fermentation is common in muscles that have become exhausted by use.

Alcohol Fermentation

Another familiar fermentation process is alcohol fermentation (Figure 17), which produces

ethanol, an alcohol. The alcohol fermentation reaction is the following:



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Figure 17: Alcohol Fermentation




The reaction resulting in alcohol fermentation is shown.

In the first reaction, a carboxyl group is removed from pyruvic acid, releasing carbon

dioxide as a gas. The loss of carbon dioxide reduces the molecule by one carbon atom,

making acetaldehyde. The second reaction removes an electron from NADH, forming

NAD+ and producing ethanol from the acetaldehyde, which accepts the electron. The

fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages

(Figure 18). If the carbon dioxide produced by the reaction is not vented from the

fermentation chamber, for example in beer and sparkling wines, it remains dissolved in the

medium until the pressure is released. Ethanol above 12 percent is toxic to yeast, so

natural levels of alcohol in wine occur at a maximum of 12 percent.



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Figure 18: Wine Fermentation Tanks

Fermentation of grape juice to make wine produces CO2 as a byproduct.

Fermentation tanks have valves so that pressure inside the tanks can be


Anaerobic Cellular Respiration

Certain prokaryotes, including some species of bacteria and Archaea, use anaerobic

respiration. For example, the group of Archaea called methanogens reduces carbon

dioxide to methane to oxidize NADH. These microorganisms are found in soil and in the

digestive tracts of ruminants, such as cows and sheep. Similarly, sulfate-reducing bacteria

and Archaea, most of which are anaerobic (Figure 19), reduce sulfate to hydrogen sulfide

to regenerate NAD+ from NADH.



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Figure 19: Anaerobic Cellular Respiration

The green color seen in these coastal waters is from an eruption of

hydrogen sulfide. Anaerobic, sulfate-reducing bacteria release hydrogen

sulfide gas as they decompose algae in the water.

NASA image courtesy Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC

Other fermentation methods occur in bacteria. Many prokaryotes are facultatively

anaerobic. This means that they can switch between aerobic respiration and fermentation,

depending on the availability of oxygen. Certain prokaryotes, like Clostridia bacteria, are

obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen.

Oxygen is a poison to these microorganisms and kills them upon exposure. It should be

noted that all forms of fermentation, except lactic acid fermentation, produce gas. The

production of particular types of gas is used as an indicator of the fermentation of specific

carbohydrates, which plays a role in the laboratory identification of the bacteria. The

various methods of fermentation are used by different organisms to ensure an adequate

supply of NAD+ for the sixth step in glycolysis. Without these pathways, that step would

not occur, and no ATP would be harvested from the breakdown of glucose.

Connections to Other Metabolic Pathways



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By the end of this section, you will be able to:

• discuss the way in which carbohydrate metabolic

pathways, glycolysis, and the citric acid cycle interrelate with

protein and lipid metabolic pathways

• explain why metabolic pathways are not considered

closed systems.

You have learned about the catabolism of glucose, which provides energy to living cells.

But living things consume more than just glucose for food. How does a turkey sandwich,

which contains protein, provide energy to your cells? This happens because all the

catabolic pathways for carbohydrates, proteins, and lipids eventually connect into

glycolysis and the citric acid cycle pathways (Figure 20). Metabolic pathways should be

thought of as porous—that is, substances enter from other pathways, and other

substances leave for other pathways. These pathways are not closed systems. Many of

the products in a particular pathway are reactants in other pathways.

Connections of Other Sugars to Glucose Metabolism

Glycogen, a polymer of glucose, is a short-term energy storage molecule in animals. When

there is adequate ATP present, excess glucose is converted into glycogen for storage.

Glycogen is made and stored in the liver and muscle. Glycogen will be taken out of storage

if blood sugar levels drop. The presence of glycogen in muscle cells as a source of glucose

allows ATP to be produced for a longer time during exercise.

Sucrose is a disaccharide made from glucose and fructose bonded together. Sucrose is

broken down in the small intestine, and the glucose and fructose are absorbed separately.

Fructose is one of the three dietary monosaccharides, along with glucose and galactose

(which is part of milk sugar, the disaccharide lactose), that are absorbed directly into the

bloodstream during digestion. The catabolism of both fructose and galactose produces the

same number of ATP molecules as glucose.

Connections of Proteins to Glucose Metabolism



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Proteins are broken down by a variety of enzymes in cells. Most of the time, amino acids

are recycled into new proteins. If there are excess amino acids, however, or if the body is

in a state of famine, some amino acids will be shunted into pathways of glucose

catabolism. Each amino acid must have its amino group removed prior to entry into these

pathways. The amino group is converted into ammonia. In mammals, the liver synthesizes

urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the

principal waste product in mammals from the nitrogen originating in amino acids, and it

leaves the body in urine.

Connections of Lipids to Glucose Metabolism

The lipids that are connected to the glucose pathways are cholesterol and triglycerides.

Cholesterol is a lipid that contributes to cell membrane flexibility and is a precursor of

steroid hormones. The synthesis of cholesterol starts with acetyl CoA and proceeds in

only one direction. The process cannot be reversed, and ATP is not produced.

Triglycerides are a form of long-term energy storage in animals. Triglycerides store about

twice as much energy as carbohydrates. Triglycerides are made of glycerol and three fatty

acids. Animals can make most of the fatty acids they need. Triglycerides can be both made

and broken down through parts of the glucose catabolism pathways. Glycerol can be

phosphorylated and proceeds through glycolysis. Fatty acids are broken into two-carbon

units that enter the citric acid cycle.

Figure 20: Catabolic Pathways for Carbohydrates

Glycogen from the liver and muscles, together with fats, can feed into the

catabolic pathways for carbohydrate



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Evolution In Action

Pathways of Photosynthesis and Cellular Metabolism

Photosynthesis and cellular metabolism consist of several very complex pathways. It

is generally thought that the first cells arose in an aqueous environment—a “soup” of

nutrients. If these cells reproduced successfully and their numbers climbed steadily,

it follows that the cells would begin to deplete the nutrients from the medium in

which they lived, as they shifted the nutrients into their own cells. This hypothetical

situation would have resulted in natural selection favoring those organisms that

could exist by using the nutrients that remained in their environment and by

manipulating these nutrients into materials that they could use to survive.

Additionally, selection would favor those organisms that could extract maximal

value from the available nutrients.

An early form of photosynthesis developed that harnessed the sun’s energy using

compounds other than water as a source of hydrogen atoms, but this pathway did

not produce free oxygen. It is thought that glycolysis developed prior to this time

and could take advantage of simple sugars being produced, but these reactions were

not able to fully extract the energy stored in the carbohydrates. A later form of

photosynthesis used water as a source of hydrogen ions and generated free oxygen.

Over time, the atmosphere became oxygenated. Living things adapted to exploit this

new atmosphere and allowed respiration as we know it to evolve. When the full

process of photosynthesis as we know it developed and the atmosphere became

oxygenated, cells were finally able to use the oxygen expelled by photosynthesis to

extract more energy from the sugar molecules using the citric acid cycle.

Key Terms

ATP synthase a membrane-embedded protein complex that regenerates ATP from ADP

with energy from protons diffusing through it

ATP (also, adenosine triphosphate) the cell’s energy currency

acetyl CoA the combination of an acetyl group derived from pyruvic acid and coenzyme A

that is made from pantothenic acid (a B-group vitamin)

activation energy the amount of initial energy necessary for reactions to occur



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active site a specific region on the enzyme where the substrate binds

allosteric inhibition the mechanism for inhibiting enzyme action in which a regulatory

molecule binds to a second site (not the active site) and initiates a conformation change in

the active site, preventing binding with the substrate

anabolic describes the pathway that requires a net energy input to synthesize complex

molecules from simpler ones

anaerobic cellular respiration the use of an electron acceptor other than oxygen to

complete metabolism using electron transport-based chemiosmosis

bioenergetics the concept of energy flow through living systems

catabolic describes the pathway in which complex molecules are broken down into

simpler ones, yielding energy as an additional product of the reaction

chemiosmosis the movement of hydrogen ions down their electrochemical gradient across

a membrane through ATP synthase to generate ATP

citric acid cycle a series of enzyme-catalyzed chemical reactions of central importance in

all living cells that harvests the energy in carbon-carbon bonds of sugar molecules to

generate ATP; the citric acid cycle is an aerobic metabolic pathway because it requires

oxygen in later reactions to proceed

competitive inhibition a general mechanism of enzyme activity regulation in which a

molecule other than the enzyme’s substrate is able to bind the active site and prevent the

substrate itself from binding, thus inhibiting the overall rate of reaction for the enzyme

electron transport chain a series of four large, multiprotein complexes embedded in the

inner mitochondrial membrane that accepts electrons from donor compounds and

harvests energy from a series of chemical reactions to generate a hydrogen ion gradient

across the membrane

endergonic describes a chemical reaction that results in products that store more chemical

potential energy than the reactants

enzyme a molecule that catalyzes a biochemical reaction

exergonic describes a chemical reaction that results in products with less chemical

potential energy than the reactants, plus the release of free energy



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feedback inhibition a mechanism of enzyme activity regulation in which the product of a

reaction or the final product of a series of sequential reactions inhibits an enzyme for an

earlier step in the reaction series

fermentation the steps that follow the partial oxidation of glucose via glycolysis to

regenerate NAD+; occurs in the absence of oxygen and uses an organic compound as the

final electron acceptor

glycolysis the process of breaking glucose into two three-carbon molecules with the

production of ATP and NADH

heat energy the energy transferred from one system to another that is not work

kinetic energy the type of energy associated with objects in motion

metabolism all the chemical reactions that take place inside cells, including those that use

energy and those that release energy

noncompetitive inhibition a general mechanism of enzyme activity regulation in which a

regulatory molecule binds to a site other than the active site and prevents the active site

from binding the substrate; thus, the inhibitor molecule does not compete with the

substrate for the active site; allosteric inhibition is a form of noncompetitive inhibition

oxidative phosphorylation the production of ATP by the transfer of electrons down the

electron transport chain to create a proton gradient that is used by ATP synthase to add

phosphate groups to ADP molecules

potential energy the type of energy that refers to the potential to do work

substrate a molecule on which the enzyme acts

thermodynamics the science of the relationships between heat, energy, and work

Chapter Summary

Energy and Metabolism

Cells perform the functions of life through various chemical reactions. A cell’s metabolism

refers to the combination of chemical reactions that take place within it. Catabolic

reactions break down complex chemicals into simpler ones and are associated with energy

release. Anabolic processes build complex molecules out of simpler ones and require




11/6/2020 How Cells Obtain Energy

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