Metabolism: Energy
and Enzymes
Chapter Concepts
6.1 Energy
• Energy cannot be created nor destroyed; energy
can be changed from one form to another but
there is always a loss of usable energy. 104
6.2 Metabolic Reactions and Energy
Transformations
• In cells the breakdown of ATP, which releases
energy, can be coupled to reactions that require
an input of energy. 106
• ATP goes through a cycle: energy from glucose
breakdown drives ATP buildup and then ATP
breakdown provides energy for cellular work.
107
6.3 Metabolic Pathways and Enzymes
• Cells have metabolic pathways in which every
reaction has a specific enzyme. 108
• Enzymes speed reactions because they have an
active site where a specific reaction occurs. 109
• Environmental factors like temperature and pH
affect the activity of enzymes. 110
• Inhibition of enzymes is a common way for cells
to control enzyme activity. 110
• Cofactors sometimes assist enzymes when
chemical reactions occur in cells. 111
6.4 Metabolic Pathways and Oxidation-Reduction
• Photosynthesis and cellular respiration are
oxidation-reduction pathways that allow a flow
of energy through all living things. 112
Plant cells carry on both photosynthesis in chloroplasts and
aerobic cellular respiration in mitochondria. These metabolic
pathways consist of a number of enzymatic reactions that involve
energy transformations. Without enzymes and energy, cells could
not continue to exist.
103
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Part 1
6-2
Cell Biology
ake a look around the room you're in. How many
things are powered by batteries or plugged into electrical outlets? Just as electricity drives all those appliances, lights, etc., a versatile molecule called ATP provides
cells with the energy to move, build proteins, perform
chemical reactions, and carry out any other necessary
duties. ATP doesn't work solo in the cell, however. Assistants known as enzymes help molecules interact with each
other, speeding the cell's chemistry and making it more
energy-efficient. Together, ATP and enzymes govern a cell's
metabolism, as this chapter will explain.
T
6.1
Energy
Living things can’t grow, reproduce, or exhibit any of the
characteristics of life without a ready supply of energy.
Energy, which is the capacity to do work, occurs in many
forms: light energy comes from the sun; electrical energy
powers kitchen appliances; and heat energy warms our
houses. Kinetic energy is the energy of motion. All moving
objects have kinetic energy. Thrown baseballs, falling water,
and contracting muscles have kinetic energy. Potential energy is stored energy. Water behind a dam, or a rock at the top
of a hill, or ATP, has potential energy that can be converted
to kinetic energy. Chemical energy is in the interactions of
atoms, one to the other, in a molecule. Molecules have varying amounts of potential energy. Glucose has much more
energy than its breakdown products, carbon dioxide and
water.
Two Laws of Thermodynamics
Early researchers who first studied energy and its relationships and exchanges formulated two laws of thermodynamics. The first law, also called the “law of conservation of
energy,” says that energy cannot be created or destroyed but can
only be changed from one form to another. Think of the conversions that occur when coal is used to power a locomotive.
First, the chemical energy of coal is converted to heat energy
and then heat energy is converted to kinetic energy in a
steam engine. Similarly, the potential energy of coal or gas is
converted to electrical energy by power plants. Do energy
transformations occur in the human body? As an example,
consider that the chemical energy in the food we eat is
changed to the chemical energy of ATP, and then this form of
potential energy is converted to the mechanical energy of
muscle contraction (Fig. 6.1).
The second law of thermodynamics says that energy cannot
be changed from one form to another without a loss of usable
energy. Only about 25% of the chemical energy of gasoline
is converted to the motion of a car; the rest is lost as heat.
Heat, of course, is a form of energy, but heat is the most
random form of energy and quickly dissipates into the
environment. When muscles convert the chemical energy
within ATP to the mechanical energy of contraction, some
of this energy becomes heat right away. With conversion
upon conversion, eventually all usable forms of energy
become heat that is lost to the environment. And because
heat dissipates, it can never be converted back to a form of
potential energy. The reading on the next page discusses
how ecosystems also obey the second law of thermodynamics.
Entropy
Entropy is a measure of randomness or disorder. An organized, usable form of energy has a low entropy, whereas an
unorganized, less stable form of energy such as heat has a
high entropy. A neat room has a much lower entropy than a
messy room. We know that a neat room always tends
toward messiness. In the same way, energy conversions
eventually result in heat, and therefore the entropy of the
universe is always increasing.
How does an ordered system such as a neat room or
an organism come about? You know very well that it
takes an input of usable energy to keep your room neat. In
the same way, it takes a constant input of usable energy
from the food you eat to keep you organized. This input
of energy goes through many energy conversions, and the
output is finally heat, which increases the entropy of the
universe.
Figure 6.1
Energy for life.
All of the energy needed to move this athlete is provided by the food
he has eaten. Once food has been processed in the digestive tract,
nutrients are transported about the body, including to the muscles.
The energy of nutrient molecules is converted to that of ATP
molecules which power muscle contraction.
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The laws of thermodynamics explain why the
entropy of the universe spontaneously increases
and why organisms need a constant input of
usable energy to maintain their organization.
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Ecosystems and the Second Law of Thermodynamics
do eat meat we could depend more on range-fed cattle. Cattle
kept close to farmland supply manure that can substitute, in
part, for chemical fertilizer. Biological control, the use of natural
enemies to control pests, would cut down on pesticide use.
Solar and wind energy could be used instead of fossil fuel energy, particularly on the farm. For example, wind-driven irrigation pumps are feasible.
Finally, of course, consumers could help matters. We could
overcome our prejudice against vegetables that have slight
blemishes. We could consume less processed foods and buy
cheaper cuts of beef, which have come from range-fed cattle.
And we could avoid using electrically powered gadgets when
preparing food at home.
solar energy
2% of solar energy
photosynthesizers
herbivores
carnivores
decomposers
er but so do ecosystems. In an ecosystem, the energy stored in
the members of one population is used by another to maintain
the organization of its members. Because of this, energy flows
through an ecosystem (Fig. 6A). As transformations of energy
occur, useful energy is lost to the environment in the form of
heat, until finally useful energy is completely used up. Since
energy cannot recycle, there is a need for an ultimate source of
energy. This source, which continually supplies almost all living
things with energy, is the sun. The entire universe is tending
toward disorder, but in the meantime, solar energy is sustaining
living things.
Human beings are also a population that feeds on other
organisms. We feed directly on plants, such as corn, or on animals like poultry and cattle that have fed on corn. In the United
States, however, much supplemental energy in addition to solar
energy is used to produce food. Even before planting time, there
is an input of fossil fuel energy for the processing of seeds, and
the making of tools, fertilizers, and pesticides. Then, fossil fuel
energy is used to transport these materials to the farm. At the
farm, fuel is needed to plant the seeds, to apply fertilizers and
pesticides, and to irrigate, harvest, and dry the crops. After harvesting, still more fuel is used to process the crops to make the
neatly packaged products we buy in the supermarket. Most of
the food we eat today has been processed in some way. Even
farm families now buy at least some of their food from supermarkets in nearby towns.
Since 1940 the amount of supplemental fuel used in the
American food system has greatly increased until now the
amount of supplemental energy is at least three or four times
that of the caloric content of the food produced! This is partially
due to the trend toward producing more food on less land by
using high-yielding hybrid wheat and corn plants. These plants
require more care and about twice as much supplemental energy as the traditional varieties of wheat and corn. Cattle confined
to feedlots and fed grain that has gone through the whole production process require about twenty times the amount of supplemental energy as do range-fed cattle. Our food system has
been labeled energy-intensive because it requires such a large
input of supplemental energy.
Our energy-intensive food system is a matter for concern
because it increases the cost of food and the burning of fossil
fuels adds pollutants to the atmosphere. What can be done?
First of all, we could grow crops that do not require so much
supplemental energy. And second, we could eat primarily vegetables and grains. It is estimated that only about 10% of the
energy contained in one population is actually taken up by the
next population. (About 90% is lost as heat.) This means that
about ten times the number of people can be sustained on a diet
of vegetables and grain rather than a diet of meat. And when we
heat loss
A cell converts the energy of one chemical molecule into anoth-
top carnivores
heat energy returned
to the atmosphere
Figure 6A
Energy loss in an ecosystem.
Ordinarily about 2% of the solar energy reaching the earth is taken
up by photosynthesizers (plants and algae). This is the energy that
allows them to make their own food. Herbivores obtain their food
by eating plants, and carnivores obtain food by eating other
animals. Whenever the energy content of food is used by
organisms, it is eventually converted to heat. With death and decay
by decomposers, all the energy temporarily stored in organisms
returns as heat to the atmosphere. In order to support a very large
population, human beings supplement solar energy with fossil fuel
energy to grow crops. Usually, humans feed on crops directly or on
animals (herbivores) that have been fed on crops.
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Part 1
6-4
Cell Biology
6.2
Metabolic Reactions and Energy
Transformations M
Metabolism is the sum of all the reactions that occur in a
cell. Reactants are substances that participate in a reaction,
while products are substances that form as a result of a
reaction. In the reaction A B £ C D, A and B are the
reactants while C and D are the products. How would you
know that this reaction will occur spontaneously—that is,
without an input of energy? Using the concept of entropy,
it is possible to state that a reaction will occur spontaneously if it increases the entropy of the universe. But this
is not very helpful in cell biology because we don’t wish to
consider the entire universe. We simply want to consider
this reaction. In such instances, cell biologists use the concept of free energy. Free energy is the amount of
energy available—that is, energy that is still “free”
to do work after a chemical reaction has occurred.
Free energy is denoted by the symbol G after
Josiah Gibbs who first developed the concept. A
a.
negative ∆G (change in free energy) means that
the products have less free energy than the reactants and the reaction will occur spontaneously. In
our reaction, if C and D have less free energy than
A and B, then the reaction will “go.”
Exergonic reactions are ones in which ∆G is
negative and energy is released, while endergonic
reactions are ones in which the products have more
free energy than the reactants. Endergonic reactions
can only occur if there is an input of energy.
If the change in free energy in both directions is
just about zero, the reaction is reversible and the
reaction is at equilibrium. How could you make a
b.
reversible reaction “go” in one direction or the other? Very often in cells, as soon as a product is
formed, the product is used as a reactant in another
reaction. Such occurrences cause the reaction to go
in the direction of the product.
lar reactions that require an input of energy. Coupling,
which requires that the exergonic reaction and the endergonic reaction be closely tied, can be symbolized like this:
C+D
How is a cell assured of a supply of ATP? Recall that glucose breakdown during aerobic cellular respiration provides
the energy for the buildup of ATP in mitochondria. Only
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ATP
exergonic
ADP + P + energy
Muscle contraction
is endergonic
ATP
ADP + P
muscle contraction
Can the energy released by an exergonic reaction be
used to “drive” an endergonic reaction? In the body
many reactions such as protein synthesis, nerve
conduction, or muscle contraction are endergonic:
they require an input of energy. On the other hand,
the breakdown of ATP to ADP P is exergonic
and energy is released (Fig. 6.2).
In coupled reactions, the energy released by an
exergonic reaction is used to drive an endergonic
reaction. ATP breakdown is often coupled to cellu-
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A+B
Coupling
Coupled Reactions
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ADP + P
ATP
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c.
Figure 6.2
Coupled reactions.
a. The breakdown of ATP is exergonic. b. Muscle contraction is endergonic and
therefore cannot occur without an input of energy. c. Muscle contraction is
coupled to ATP breakdown, making the overall process exergonic. Now muscle
contraction can occur.
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6-5
Chapter 6
Adenosine
Metabolism: Energy and Enzymes
107
Triphosphate
P
P
P
ATP
Energy for
endergonic reactions
(e.g., protein synthesis,
nerve conduction,
muscle contraction)
Energy from
exergonic reactions
(e.g., cellular respiration)
ADP +
P
P
Adenosine
Figure 6.3
P
Diphosphate
The ATP cycle.
In cells, the exergonic breakdown of glucose is coupled to the buildup of ATP, and then the exergonic breakdown of ATP is coupled to endergonic
reactions in cells. When a phosphate group is removed by hydrolysis, ATP releases the appropriate amount of energy for most metabolic
reactions. The high-energy content of ATP comes from the complex interaction of the atoms within the molecule.
39% of the free energy of glucose is transformed to ATP; the
rest is lost as heat. When ATP breaks down to drive the reactions mentioned, some energy is lost as heat and the overall
reaction becomes exergonic.
Chemical work. Supplies the energy needed to
synthesize macromolecules that make up the cell.
ATP: Energy for Cells
Mechanical work. Supplies the energy needed to
permit muscles to contract, cilia and flagella to
beat, chromosomes to move, and so forth.
ATP (adenosine triphosphate) is the common energy currency of cells: when cells require energy, they “spend” ATP.
You may think that this causes our bodies to produce a lot of
ATP, and it does; however, the amount on hand at any one
moment is minimal because ATP is constantly being generated from ADP (adenosine diphosphate) and P (Fig. 6.3).
The use of ATP as a carrier of energy has some advantages: (1) It provides a common energy currency that can be
used in many different types of reactions. (2) When ATP
becomes ADP P , the amount of energy released is just
about enough for the biological purposes mentioned in the
following section, and so little energy is wasted. (3) ATP
breakdown is coupled to endergonic reactions in such a way
that it minimizes energy loss.
Transport work. Supplies the energy needed to pump
substances across the plasma membrane.
Structure of ATP
ATP is a nucleotide composed of the base adenine and the
sugar ribose (together called adenosine) and three phosphate groups. ATP is called a “high-energy” compound
because a phosphate group is easily removed. Under cellular conditions, the amount of energy released when ATP is
1
hydrolyzed to ADP P is about 7.3 kcal per mole.
ATP is a carrier of energy in cells. It is the common
energy currency because it supplies energy for
many different types of reactions.
Function of ATP
Recall that at various times we have mentioned at least three
uses for ATP.
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1
A mole is the number of molecules present in the molecular weight of a substance
(in grams).
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6-6
Cell Biology
Part 1
6.3
Metabolic Pathways and
Enzymes M
Reactions do not occur haphazardly in cells; they are usually a
part of a metabolic pathway, a series of linked reactions.
Metabolic pathways begin with a particular reactant and terminate with an end product. While it is possible to write an
overall equation for a pathway as if the beginning reactant
went to the end product in one step, there are actually many
specific steps in between. In the pathway, one reaction leads to
the next reaction, which leads to the next reaction, and so forth
in an organized, highly structured manner. This arrangement
makes it possible for one pathway to lead to several others,
because various pathways have several molecules in common.
Also, metabolic energy is captured and utilized more easily if
it is released in small increments rather than all at once.
A metabolic pathway can be represented by the following diagram:
E2
E1
A
E3
B
C
E4
D
E5
E
E6
F
G
In this diagram, the letters A–F are reactants and letters B–G
are products in the various reactions. The letters E1–E6 are
enzymes.
An enzyme is a protein molecule2 that functions as an
organic catalyst to speed a chemical reaction. In a crowded
ballroom, a mutual friend can cause particular people to
interact. In the cell, an enzyme brings together particular
molecules and causes them to react with one another.
The reactants in an enzymatic reaction are called the
substrates for that enzyme. In the first reaction, A is the substrate for E1 and B is the product. Now B becomes the substrate for E2, and C is the product. This process continues
until the final product G forms.
Any one of the molecules (A–G) in this linear pathway
could also be a substrate for an enzyme in another pathway.
A diagram showing all the possibilities would be highly
branched.
Energy of Activation
Molecules frequently do not react with one another unless
they are activated in some way. In the absence of an enzyme,
activation is very often achieved by heating the reaction flask
to increase the number of effective collisions between molecules. The energy that must be added to cause molecules to
react with one another is called the energy of activation (Ea).
Figure 6.4 compares Ea when an enzyme is not present to
when an enzyme is present, illustrating that enzymes lower
the amount of energy required for activation to occur.
In baseball, a home-run hitter must not only hit the ball
to the fence, but over the fence. When enzymes lower the
energy of activation, it is like removing the fence; then it is
possible to get a home run by simply hitting the ball as far as
the fence was.
Enzyme-Substrate Complexes
The following equation, which is pictorially shown in Figure
6.5, is often used to indicate that an enzyme forms a complex
with its substrate:
2
a.
Figure 6.4
energy of
activation
energy of
reactant
energy of
product
Progress of the reaction
Free Energy
Free Energy
Catalytic RNA molecules are called ribozymes and are not enzymes.
b.
energy of
reactant
energy of
activation
energy of
product
Progress of the reaction
Energy of activation (Ea).
Enzymes speed the rate of chemical reactions because they lower the amount of energy required to activate the reactants. a. Energy of activation
when an enzyme is not present. b. Energy of activation when an enzyme is present. Even spontaneous reactions like this one speed up when an
enzyme is present.
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Chapter 6
enzyme
substrate
enzyme-substrate
complex
product
In most instances only one small part of the enzyme,
called the active site, complexes with the substrate(s). It is
here that the enzyme and substrate fit together, seemingly
like a key fits a lock; however, it is now known that the
active site undergoes a slight change in shape in order to
accommodate the substrate(s). This is called the induced-fit
model because the enzyme is induced to undergo a slight
alteration to achieve optimum fit.
The change in shape of the active site facilitates the reaction that now occurs. After the reaction has been completed,
the product(s) is released, and the active site returns to its
original state, ready to bind to another substrate molecule.
Only a small amount of enzyme is actually needed in a cell
because enzymes are not used up by the reaction.
Some enzymes do more than simply complex with
their substrate(s); they actually participate in the reaction.
Trypsin digests protein by breaking peptide bonds. The
active site of trypsin contains three amino acids with R
groups that actually interact with members of the peptide
bond—first to break the bond and then to introduce the
components of water. This illustrates that the formation of
the enzyme-substrate complex is very important in speeding up the reaction.
Sometimes it is possible for a particular reactant(s) to
produce more than one type of product(s). The presence or
absence of an enzyme determines which reaction takes
substrate
Metabolism: Energy and Enzymes
place. If a substance can react to form more than one product, then the enzyme that is present and active determines
which product is produced.
Every reaction in a cell requires its specific enzyme.
Because enzymes only complex with their substrates, they
are named for their substrates, as in the following examples:
Substrate
Enzyme
Lipid
Urea
Maltose
Ribonucleic acid
Lactose
Lipase
Urease
Maltase
Ribonuclease
Lactase
Most enzymes are protein molecules. Enzymes
speed chemical reactions by lowering the energy
of activation. They do this by forming an enzymesubstrate complex.
Factors Affecting Enzymatic Speed
Enzymatic reactions proceed quite rapidly. Consider, for
example, the breakdown of hydrogen peroxide (H2O2) as
catalyzed by the enzyme catalase: 2 H2O2 £ 2 H2O + O2.
The breakdown of hydrogen peroxide can occur 600,000
times a second when catalase is present. To achieve maximum product per unit time, there should be enough substrate to fill active sites most of the time. Temperature and
optimal pH also increase the rate of an enzymatic reaction.
substrates
product
products
active site
active site
enzyme
enzyme-substrate complex
enzyme
a. Degradative reaction
Figure 6.5
109
enzyme
enzyme-substrate complex
enzyme
b. Synthetic reaction
Enzymatic action.
An enzyme has an active site, which is where the substrates and enzyme fit together in such a way that the substrates are oriented to react.
Following the reaction, the products are released and the enzyme is free to act again. a. Some enzymes carry out degradative reactions in which
the substrate is broken down to smaller molecules. b. Other enzymes carry out synthetic reactions in which the substrates are joined to form a
larger molecule.
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6-8
Cell Biology
0
a.
Figure 6.6
Rate of Reaction
(product per unit of time)
Rate of Reaction
(product per unit of time)
110
10
20 30 40 50 60
Temperature °C
trypsin
pepsin
0
1 2 3
4
5
b.
6 7 8
pH
9 10 11 12
Rate of an enzymatic reaction as a function of temperature and pH.
a. At first, as with most chemical reactions, the rate of an enzymatic reaction doubles with every 10°C rise in temperature. In this graph, the
rate of reaction is maximum at about 40°C; then it decreases until the reaction stops altogether, because the enzyme has become denatured.
b. Pepsin, an enzyme found in the stomach, acts best at a pH of about 2, while trypsin, an enzyme found in the small intestine, performs
optimally at a pH of about 8. The shape that enables these proteins to bind with their substrates is not properly maintained at other pHs.
Enzyme Concentration
Substrate Concentration
Generally, enzyme activity increases as substrate concentration increases because there are more collisions
between substrate molecules and the enzyme. As more
substrate molecules fill active sites, more product results
per unit time. But when the enzyme’s active sites are filled
almost continuously with substrate, the enzyme’s rate of
activity cannot increase anymore. Maximum rate has been
reached.
Temperature and pH
As the temperature rises, enzyme activity increases (Fig.
6.6a). This occurs because as the temperature rises there are
more effective collisions between enzyme and substrate.
However, if the temperature rises beyond a certain point,
enzyme activity eventually levels out and then declines
rapidly because the enzyme is denatured. An enzyme’s
shape changes during denaturation, and then it can no
longer bind its substrate(s) efficiently.
Each enzyme also has an optimal pH at which the rate of
the reaction is highest. Figure 6.6b shows the optimal pH for
the enzymes pepsin and trypsin. At this pH value, these
enzymes have their normal configurations. The globular
structure of an enzyme is dependent on interactions, such as
hydrogen bonding, between R groups. A change in pH can
alter the ionization of these side chains and disrupt normal
interactions, and under extreme conditions of pH, denaturation eventually occurs. Again, the enzyme has an
altered shape and is then unable to combine efficiently with
its substrate.
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Since enzymes are specific, a cell regulates which enzymes
are present and/or active at any one time. Otherwise
enzymes may be present that are not needed, or one pathway may negate the work of another pathway.
Genes must be turned on to increase the concentration
of an enzyme and must be turned off to decrease the concentration of an enzyme.
Another way to control enzyme activity is to activate or
deactivate the enzyme. Phosphorylation is one way to activate an enzyme. Molecules received by membrane receptors
often turn on kinases, which then activate enzymes by phosphorylating them:
kinase
protein
P
PP
P
protein
Enzyme Inhibition
Actually, enzyme inhibition is a common means by which
cells regulate enzyme activity. In competitive inhibition,
another molecule is so close in shape to the enzyme’s substrate that it can compete with the true substrate for the
enzyme’s active site. This molecule inhibits the reaction
because only the binding of the true substrate results in a
product. In noncompetitive inhibition, a molecule binds to an
enzyme, but not at the active site. The other binding site is
called the allosteric site. In this instance, inhibition occurs
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Metabolism: Energy and Enzymes
Chapter 6
when binding of a molecule causes a shift in the threedimensional structure so that the substrate cannot bind to
the active site.
The activity of almost every enzyme in a cell can be
regulated by its product. When a product is in abundance,
it binds competitively with its enzyme’s active site; as the
product is used up, inhibition is reduced and more product can be produced. In this way, the concentration of the
product is always kept within a certain range. Most metabolic pathways are regulated by feedback inhibition, but
the end product of the pathway binds at an allosteric site
on the first enzyme of the pathway (Fig. 6.7). This binding
shuts down the pathway, and no more product is
produced.
In inhibition, a product binds to the active site or
binds to an allosteric site on an enzyme.
Poisons are often enzyme inhibitors. Cyanide is an
inhibitor for an essential enzyme (cytochrome c oxidase) in
all cells, which accounts for its lethal effect on humans.
Penicillin blocks the active site of an enzyme unique to bacteria. When penicillin is taken, bacteria die but humans are
unaffected.
active site of
enzyme where
reactant A binds
allosteric site
of enzyme
where end
product F binds
E1
E1
E2
first
reactant
A
111
B
E3
E4
C
E5
D
E
end
product
F
Overall view of pathway
first
reactant
A
E1
A
allosteric site of
enzyme where
end product
F binds
E1
E2
B
E3
E4
C
E5
E
D
end
product
F
View of active pathway
Enzyme Cofactors
Many enzymes require an inorganic ion or organic but nonprotein molecule to function properly; these necessary ions
or molecules are called cofactors. The inorganic ions are
metals such as copper, zinc, or iron. The organic, nonprotein
molecules are called coenzymes. These cofactors assist the
enzyme and may even accept or contribute atoms to the
reactions.
It is interesting that vitamins are often components of
coenzymes. Vitamins are relatively small organic molecules
that are required in trace amounts in our diet and in the diet
of other animals for synthesis of coenzymes that affect
health and physical fitness. The vitamin becomes a part of
the coenzyme’s molecular structure. These vitamins are necessary to formation of the coenzymes listed:
Vitamin
Coenzyme
Niacin
B2 (riboflavin)
B1 (thiamine)
Pantothenic acid
B12 (cobalamin)
NAD
FAD
Thiamine pyrophosphate
Coenzyme A (CoA)
B12 coenzymes
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end
product
F
E1
E1
first
reactant
A
X
View of inhibited pathway
Figure 6.7
A deficiency of any one of these vitamins results in a lack of
the coenzyme listed and therefore a lack of certain enzymatic actions. In humans, this eventually results in vitamindeficiency symptoms: niacin deficiency results in a skin
disease called pellagra, and riboflavin deficiency results in
cracks at the corners of the mouth.
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active site of
enzyme where
reactant A binds
Feedback inhibition.
This hypothetical metabolic pathway is regulated by feedback
inhibition. When reactant A binds to the active site of E1, the pathway
is active and the end product is produced. Once there is sufficient
end product, some binds to the allosteric site of E1. Now a change of
shape prevents reactant A from binding to the active site of E1 and
the end product is no longer produced.
Enzymes speed a reaction by forming a complex
with the substrate. Various factors affect enzymatic
speed, including substrate concentration,
temperature, pH, enzyme concentration, the
presence of inhibitors or necessary cofactors.
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Part 1
6-10
Cell Biology
6.4
Metabolic Pathways and
Oxidation-Reduction
As we have noted, chemical reactions can involve energy
transformations from one molecule to another, as when
the potential energy stored in ATP molecules is used to
synthesize macromolecules. In oxidation-reduction
(redox) reactions, electrons also pass from one molecule to
another. Oxidation is the loss of electrons and reduction is
the gain of electrons. Oxidation and reduction always
takes place at the same time because one molecule accepts
the electrons given up by another. Oxidation-reduction
reactions occur during photosynthesis and aerobic cellular
respiration.
Photosynthesis
In living things, hydrogen ions often accompany electrons,
and if so, oxidation is a loss of hydrogen atoms
(e H) and reduction is a gain of hydrogen atoms. For
example, the overall reaction for photosynthesis can be written like this:
6CO2 6H2O Energy £ C6H12O6 6 O2
Carbon
dioxide
Water
Glucose
Oxygen
This equation shows that during photosynthesis hydrogen
atoms are transferred from water to carbon dioxide and
glucose is formed. Therefore, water has been oxidized and
carbon dioxide has been reduced. Since water is a lowenergy molecule and glucose is a high-energy molecule,
energy is needed to form glucose. This energy is supplied
by solar energy. Chloroplasts are able to capture solar
energy, and convert it to chemical energy of ATP molecules that are used along with hydrogen atoms to reduce
glucose.
A coenzyme of oxidation-reduction called NADP
(nicotinamide adenine dinucleotide phosphate) is active
during photosynthesis. NADP carries a positive charge and,
therefore, is written as NADP. During photosynthesis,
NADP accepts electrons and hydrogen ions derived from
water and passes by way of a metabolic pathway to carbon
dioxide.
Aerobic Cellular Respiration
The overall equation for aerobic cellular respiration is the
opposite of the one we used to represent photosynthesis:
C6H12O6 6 O2
Glucose
Oxygen
Since glucose is a high-energy molecule and water is a lowenergy molecule, energy has been released. You will remember that mitochondria in cells use the energy released from
glucose breakdown to build ATP molecules.
In metabolic pathways, most oxidations such as those
that occur during aerobic cellular respiration involve a coenzyme called NAD (nicotinamide adenine dinucleotide).
NAD is a coenzyme of oxidation-reduction that accepts electrons from glucose products and then later passes them on
to a metabolic pathway that reduces oxygen to water. NAD
carries a positive charge and therefore is represented as
NAD. During oxidation reactions, NAD accepts two electrons but only one hydrogen ion. The reaction is:
NAD 2H
The Cycling of Matter and the Flow of
Energy
During photosynthesis, chloroplasts, present in plants, capture solar energy and use it to convert water and carbon
dioxide into carbohydrates which serve as food for all living
things. Oxygen is a by-product of photosynthesis (Fig. 6.8).
Mitochondria, present in both plants and animals, complete the breakdown of carbohydrates and use the released
energy to build ATP molecules. Aerobic cellular respiration
consumes oxygen and produces carbon dioxide and water,
the very molecules taken up by chloroplasts.
This cycling of molecules between chloroplasts and
mitochondria allows a flow of energy from the sun through
all living things. This flow of energy maintains the levels of
biological organization from molecules to ecosystems. In
keeping with the laws of thermodynamics, energy is dissipated with each chemical transformation and eventually the
solar energy captured by plants is lost in the form of heat.
Therefore, most living things are dependent upon an input
of solar energy.
Human beings are also involved in the cycling of molecules between plants and animals and in the flow of energy
from the sun. We inhale oxygen and eat plants and their
stored carbohydrates, or we eat other animals that have eaten plants. Oxygen and nutrient molecules enter our mitochondria which produce ATP and release carbon dioxide
and water, the molecules used by plants to produce carbohydrates. Without a supply of energy-rich molecules produced by plants, we could not produce the ATP molecules
needed to maintain our bodies.
£ 6CO2 6H2O Energy
Carbon
dioxide
Water
In this reaction glucose has lost hydrogen atoms (been oxidized) and oxygen has gained hydrogen atoms (been
reduced). When oxygen gains electrons it becomes water.
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£ NADH H
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Oxidation-reduction reactions are involved in the
pathways of photosynthesis, which take place in
chloroplasts, and of aerobic cellular respiration,
which take place in mitochondria. These pathways
permit a flow of energy from the sun through all
living things.
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Chapter 6
Photosynthesis
Metabolism: Energy and Enzymes
113
Cellular respiration
heat
carbohydrate
O2
Chloroplast
Mitochondrion
heat
CO2 + H2O
ATP for synthetic
reaction, active
transport, muscle
contraction,
nerve impulse
heat
Figure 6.8
Relationship of chloroplasts to mitochondria.
Chloroplasts produce energy-rich carbohydrates. These carbohydrates are broken down in mitochondria, and the energy released is used for the
buildup of ATP. There is a loss of usable energy due to the energy conversions of photosynthesis and aerobic respiration; and then, when ATP is
used as an energy source, all usable energy is converted to heat.
I
n the United States, solar energy to grow
food is greatly supplemented by fossil
fuel energy. Even before crops are sowed,
there is an input of fossil fuel energy for
the production of seeds, tools, fertilizers,
pesticides, and their transportation to the
farm. At the farm, fuel is needed to plant
the seeds, to apply fertilizers and pesticides, to irrigate, and to harvest and dry
crops. After harvesting, still more fuel is
used to process crops and put it in those
neatly packaged products we buy in the
supermarket.
At this time, the supplemental energy
to grow food is several hundred times its
caloric content because we devote a limited amount of land to agriculture, and we
use high-yielding plants that require more
care anyway. It takes about twenty times
the amount of energy to keep cattle in
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feedlots and feed them grain as it does to
range-feed them. Because the combustion
of fossil fuel energy contributes to environmental problems such as global warming
and air pollution, it behooves us to take
steps to cut down on supplemental energy
to grow food. What can be done? First of
all we could devote as much land as possible to farming and animal husbandry.
Plant breeders could sacrifice some yield
to develop plants that would require less
supplemental energy. And we could
range-feed cattle. If cattle are kept close to
farmland, manure can substitute in part
for chemical fertilizers. Biological control,
the use of natural enemies to control pests,
would cut down on pesticide use and possibly improve the health of farm families.
Solar and wind energy could be used
instead of fossil fuel energy; for example,
Study Guide TOC
wind-driven irrigation pumps are feasible.
Finally. consumers could help. We
could overcome our prejudices against
slight blemishes on our fruits and vegetables. We could cut down on our consumption of processed foods, eat less meat, and
buy cheaper cuts. And we could avoid
using electrically powered gadgets when
preparing food at home.
Questions
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1.
2.
3.
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Are you in favor of taking all possible
steps to reduce the input of supplemental
energy to grow food? Why or why not?
The way we grow food contributes to air,
water, and land pollution. Should this
become common knowledge? Why or
why not?
Are you willing to make sacrifices to
improve the quality of the environment?
Why or why not?
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Part 1
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Cell Biology
Summarizing the Concepts
6.1 Energy
There are two energy laws that are basic to understanding energyuse patterns at all levels of biological organization. The first law
states that energy cannot be created or destroyed, but can only be
transferred or transformed. The second law states that one usable
form of energy cannot be completely converted into another usable
form. As a result of these laws, we know that the entropy of the universe is increasing and that only a constant input of energy maintains the organization of living things.
6.2 Metabolic Reactions and Energy Transformations
Metabolism is a term that encompasses all the chemical reactions
occurring in a cell. Considering individual reactions, only those
that result in a negative free energy difference—that is, the products have less usable energy than the reactants—occur spontaneously. Such reactions, called exergonic reactions, release energy.
Endergonic reactions, which require an input of energy, occur only
in cells because it is possible to couple an exergonic process with an
endergonic process. For example, glucose breakdown is an exergonic metabolic pathway that drives the buildup of many ATP
molecules. These ATP molecules then supply energy for cellular
work. Thus, ATP goes through a cycle in which it is constantly
being built up from, and then broken down, to ADP + P.
6.3 Metabolic Pathways and Enzymes
A metabolic pathway is a series of reactions that proceed in an
orderly, step-by-step manner. Each reaction requires a specific
enzyme. Reaction rates increase when enzymes form a complex
with their substrates. Generally, enzyme activity increases as substrate concentration increases; once all active sites are filled, maximum rate has been achieved.
Any environmental factor, such as temperature and pH,
affects the shape of a protein and, therefore, also affects the ability
of an enzyme to do its job. Cellular mechanisms regulate enzyme
quantity and activity. The activity of most metabolic pathways is
regulated by feedback inhibition. Many enzymes have cofactors or
coenzymes that help them carry out a reaction.
6.4 Metabolic Pathways and Oxidation-Reduction
There is a cycling of molecules between plants and animals
and a flow of energy through all living things. Photosynthesis is a
metabolic pathway in chloroplasts that transforms solar energy to
the chemical energy within carbohydrates, and aerobic respiration
is a metabolic pathway completed in mitochondria that transforms
this energy into that of ATP molecules. Eventually the energy within ATP molecules becomes heat. The world of living things is
dependent on a constant input of solar energy.
Studying the Concepts
1. State the first law of thermodynamics and give an example.
104
2. State the second law of thermodynamics and give an example. 104
3. Explain why the entropy of the universe is always increasing
and why an organized system like an organism requires a
constant input of useful energy. 104
4. What is the difference between exergonic reactions and
endergonic reactions? Why can exergonic but not endergonic
reactions occur spontaneously? 106
5. Define coupling and write an equation that shows an endergonic reaction being coupled to ATP breakdown. 106
6. Why is ATP called the energy currency of cells? What is the
ATP cycle? 107
7. Diagram a metabolic pathway. Label the reactants, products,
and enzymes. 108
8. Why is less energy needed for a reaction to occur when an
enzyme is present? 108
9. Why are enzymes specific, and why can’t each one speed up
many different reactions? 109
10. Name and explain the manner in which at least three factors
can influence the speed of an enzymatic reaction. How do
cells regulate the activity of enzymes? 110–11
11. What are cofactors and coenzymes? 111
12. Describe how oxidation-reduction occurs in cells and discuss
the overall equations for photosynthesis and aerobic cellular
respiration in terms of oxidation-reduction. 112
13. How do chloroplasts and mitochondria permit a flow of
energy through the world of living things? 112
The overall equation for photosynthesis is the opposite of that for
aerobic respiration. Both processes involve oxidation-reduction
reactions. During photosynthesis, NADP is a coenzyme that
reduces carbon dioxide to glucose, and during aerobic respiration,
NAD is a coenzyme that oxidizes glucose products so that carbon
dioxide is released. Redox reactions are a major way in which energy transformation occurs in cells.
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Testing Yourself
Choose the best answer for each question.
1. When ATP becomes ADP + P,
a. some usable energy is lost to the environment.
b. energy is created according to the first law of thermodynamics.
c. an enzyme is required because the reaction does not occur
spontaneously.
d. the entropy of the universe is increased.
e. All of these are correct.
2. If A B £ C D energy occurs in a cell,
a. this reaction is exergonic.
b. an enzyme could still speed the reaction.
c. ATP is not needed to make the reaction go.
d. A and B are reactants; C and D are products.
e. All of these are correct.
3. Which of these does not utilize ATP?
a. synthesis of molecules in cells
b. active transport of molecules across the plasma membrane
c. muscle contraction
d. nerve conduction
e. sweating to lose excess heat
4. Energy of activation
a. is the amount of entropy in a system.
b. is the amount of energy given off by a reaction.
c. converts kinetic energy to potential energy.
d. is the energy needed to start a reaction.
e. is a way for cells to compete with one another.
5. The active site of an enzyme is
a. similar to that of any other enzyme.
b. the part of the enzyme where its substrate can fit.
c. can be used over and over again.
d. not affected by environmental factors like pH and temperature.
e. Both b and c are correct.
6. If you wanted to increase the amount of product per unit
time of an enzymatic reaction, do not increase
a. the amount of substrate.
b. the amount of enzyme.
c. the temperature somewhat.
d. the pH.
e. All of these are correct.
7. An allosteric site on an enzyme is
a. the same as the active site.
b. nonprotein in nature.
c. where ATP attaches and gives up its energy.
d. often involved in feedback inhibition.
e. All of these are correct.
8. Coenzymes
a. have specific functions in reactions.
b. have an active site just like enzymes do.
c. can be a carrier for proteins.
d. always have a phosphate group.
e. are used in photosynthesis but not cellular respiration.
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9. During photosynthesis, carbon dioxide
a. is oxidized to oxygen.
b. is reduced to glucose.
c. gives up water to the environment.
d. is a coenzyme of oxidation-reduction.
e. All of these are correct.
10. The oxygen given off by photosynthesis
a. is used by animal cells, but not plant cells, to carry on
cellular respiration.
b. is used by both plant and animal cells to carry on cellular
respiration.
c. is an example of the flow of energy through living things.
d. is an example of the cycling of matter through living
things.
e. Both b and d are correct.
11. Use these terms to label this diagram: substrates, enzyme
(used twice), active site, product, and enzyme-substrate complex. Explain the importance of an enzyme’s shape to its
activity.
c.
b.
a.
d.
e.
f.
Thinking Scientifically
1. Pepsin is an enzyme that breaks down protein.
a. A student has a test tube that contains pepsin, egg white,
and water. What optimal conditions would you recommend to ensure digestion of the egg white? (page 110)
b. If all the conditions are optimal, how could you increase
the yield (i.e., amount of product—amino acids—per unit
of time)? (page 110)
c. The instructor adds an inhibitor to the test tube. How
could the student tell if inhibition is reversible or
irreversible? (page 110)
2. A lack of oxygen causes death. Explain why by referring to
a. the overall equation for aerobic cellular respiration. (page
112)
b. the necessity of ATP for muscle contraction.
c. the needs of ordinary body cells.
d. brain activity as a test of death.
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Cell Biology
Understanding the Terms
active site 109
ADP (adenosine
diphosphate) 107
ATP (adenosine
triphosphate) 107
chemical energy 104
coenzyme 111
cofactor 111
coupled reactions 106
denatured 110
endergonic reaction 106
energy 104
energy of activation 108
entropy 104
enzyme 108
enzyme inhibition 110
exergonic reaction 106
Using Technology
feedback inhibition 111
free energy 106
induced-fit model 109
kinetic energy 104
laws of thermodynamics 104
metabolic pathway 108
metabolism 106
NAD 112
NADP 112
oxidation 112
potential energy 104
product 106
reactant 106
reduction 112
substrate 108
vitamin 111
Your study of Metabolism: Energy and Enzymes is supported by
these available technologies:
Essential Study Partner CD-ROM
Cells £ Metabolism
Visit the Mader web site for related ESP activities.
Exploring the Internet
The Mader Home Page provides resources and tools as
you study this chapter.
http://www.mhhe.com/biosci/genbio/mader
Virtual Physiology Laboratory CD-ROM
Enzyme Characterisitics
Match the terms to these definitions:
a.
All of the chemical reactions that occur in a cell
during growth and repair.
b.
Nonprotein adjunct required by an enzyme in
order to function; many are metal ions, others are coenzymes.
c.
Energy associated with motion.
d.
Essential requirement in the diet, needed in small
amounts. They are often part of coenzymes.
e.
Loss of an enzyme’s normal shape so that it no
longer functions; caused by an extreme change in pH and
temperature.
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Life Science Animations 3D Video
7 Enzyme Action
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