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
Introduction
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
energy
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
as:
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.
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
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 into and out
of biological systems, it is important to understand two of the physical laws that govern
energy.
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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: 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 (http://openstaxcollege.org/textbooks/concepts-of-
biology/pdf) by OpenStax College, which is available under a Creative Commons
Attribution 3.0 Unported (http://creativecommons.org/licenses/by/3.0/) 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.
Enzymes
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
Reaction
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 : http://openstaxcollege.org/l/hexokinase2
(http://openstaxcollege.org/l/hexokinase2)
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
substrate.
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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
catabolism.
Glycolysis
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.
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
reactions.
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.
Glycolysis
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
cells.
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
energy
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
gradient.
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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.
Fermentation
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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
released
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
energy.
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