Download BSU Honors Biology Chapter 5 Reading Guide Cell

BSU Honors Biology Chapter 5 Reading Guide
Cell Energy and Enzymes
Cells and Energy
5.1The Flow of Energy in Living Things
The Nature of Energy
Learning Objective 5.1.1Differentiate between kinetic and potential energy.
We are about to begin our discussion of energy and cellular chemistry. Although these subjects may seem difficult at first,
remember that all life is driven by energy. The concepts and processes discussed in the next three chapters are key to
life. We are chemical machines, powered by chemical energy, and for the same reason that a successful race car driver
must learn how the engine of a car works, we must look at cell chemistry. Indeed, if we are to understand ourselves, we
must “look under the hood” at the chemical machinery of our cells and see how it operates.
As described in chapter 2, energy is defined as the ability to do work. It can be considered to exist in two states: kinetic
energy and potential energy. Kinetic energy is the energy of motion. Objects that are not in the process of moving but
have the capacity to do so are said to possess potential energy, or stored energy. The difference in the two states of
energy is being experienced by the young man in figure 5.1. A ball perched on a hilltop (figure 5.1a) has potential
energy; after the man pushes the ball and it begins to roll downhill (figure 5.1b), some of the ball's potential energy is
converted into kinetic energy. All of the work carried out by living organisms also involves the transformation of potential
energy to kinetic energy.
Kinetic Energy
Figure 5.1 Potential and kinetic energy.Objects that have the capacity to move but are not moving have potential energy, while
objects that are in motion have kinetic energy. (a) The energy required to move the ball up the hill is stored as potential energy. (b) This
stored energy is released as kinetic energy as the ball rolls down the hill.
D
Energy exists in many forms: mechanical energy, heat, sound, electric current, light, or radioactive radiation. Because it
can exist in so many forms, there are many ways to measure energy. The most convenient is in terms of heat, because all
other forms of energy can be converted to heat. Thus the study of energy is called thermodynamics, meaning “heat
changes.”
Energy flows into the biological world from the sun, which shines a constant beam of light on the earth. It is estimated that
the sun provides the earth with more than 13 × 1023 calories per year, or 40 million billion calories per second! Plants,
algae, and certain kinds of bacteria capture a fraction of this energy through photosynthesis. In photosynthesis, energy
garnered from sunlight is used to combine small molecules (water and carbon dioxide) into more complex molecules
(sugars). These complex sugar molecules have potential energy due to the arrangement of their atoms. This potential
energy, in the form of chemical energy, does the work in cells. Recall from chapter 2 that an atom consists of a central
nucleus surrounded by one or more orbiting electrons, and a covalent bond forms when two atomic nuclei share
electrons. Breaking such a bond requires energy to pull the nuclei apart. Indeed, the strength of a covalent bond is
measured by the amount of energy required to break it. For example, it takes 98.8 kcal to break 1 mole (6.023 × 1023) of
carbon–hydrogen (C–H) bonds.
Energy Conversions
All the chemical activities within cells can be viewed as a series of chemical reactions between molecules. A chemical
reaction is the making or breaking of chemical bonds—gluing atoms together to form new molecules or tearing molecules
apart and sometimes sticking the pieces onto other molecules.
Key Learning Outcome 5.1 Energy is the capacity to do work, either actively (kinetic energy) or stored for later
use (potential energy). Chemical reactions occur when the covalent bonds linking atoms together are formed or
broken.
Page 109
5.2The Laws of Thermodynamics
Running, thinking, singing, reading these words—all activities of living organisms involve changes in energy. A set of
universal laws we call the laws of thermodynamics govern these and all other energy changes in the universe.
The First Law of Thermodynamics
Learning Objective 5.2.1Defend the proposition that heat is kinetic energy.
The first of these universal laws, the first law of thermodynamics, concerns the amount of energy in the universe. It
states that energy can change from one state to another (from potential to kinetic, for example), but it can never be
destroyed nor can new energy be made. The total amount of energy in the universe remains constant.
A lion eating a giraffe is in the process of acquiring energy. Rather than creating new energy or capturing the energy in
sunlight, the lion is merely transferring some of the potential energy stored in the giraffe's tissues to its own body (just as
the giraffe obtained the potential energy stored in the plants it ate while it was alive). Within any living organism, this
chemical potential energy can be shifted to other molecules and stored in chemical bonds, or it can be converted into
kinetic energy or into other forms of energy such as light or electrical energy. During each conversion, some of the energy
dissipates into the environment as heat energy, a measure of the random motions of molecules (and, hence, a measure
of one form of kinetic energy). Energy continuously flows through the biological world in one direction, with new energy
from the sun constantly entering the system to replace the energy dissipated as heat.
Heat can be harnessed to do work only when there is a heat gradient—that is, a temperature difference between two
areas. This is how a steam engine functions. In old steam locomotives like you see in figure 5.2, heat was used to move
the wheels. First, a boiler (not shown) heats up water to create steam. The steam is then pumped into the cylinder of the
steam engine, where it moves the piston to the right. The moving of this piston then does the work of the steam engine by
moving a lever that turns the wheel. Cells are too small to maintain significant internal temperature differences, so heat
energy is incapable of doing the work of cells. Thus, although the total amount of energy in the universe remains constant,
the energy available to do useful work in a cell decreases, as progressively more of it dissipates as heat.
Figure 5.2 A steam engine.In a steam engine, heat is used to produce steam. The expanding steam pushes against a piston that
causes the wheel to turn.
D
The Second Law of Thermodynamics
Learning Objective 5.2.2State the second law of thermodynamics.
The second law of thermodynamics concerns this transformation of potential energy into heat, or random molecular
motion. It states that the disorder in a closed system like the universe is continuously increasing. Put simply, disorder is
more likely than order. For example, it is much more likely that a column of bricks will tumble over than that a pile will
arrange themselves spontaneously to form a column. In general, energy transformations proceed spontaneously to
convert matter from a more ordered, less stable form to a less ordered, more stable form. Without an input of energy from
the teenager (or a parent), the ordered room in figure 5.3 falls into disorder.
Figure 5.3 Entropy in action.As time elapses, a teenager's room becomes more disorganized. It takes energy to clean it up.
D
Entropy
Learning Objective 5.2.3Define entropy.
Entropy is a measure of the degree of disorder of a system, so the second law of thermodynamics can also be stated
simply as “entropy increases.” When the universe formed 10 to 20 billion years ago, it held all the potential energy it will
ever have. It has become progressively more disordered ever since, with every energy exchange increasing the entropy
of the universe.
Key Learning Outcome 5.2 The first law of thermodynamics states that energy cannot be created or destroyed; it
can only undergo conversion from one form to another. The second law states that disorder (entropy) in the
universe tends to increase.
Page 110
5.3Chemical Reactions
Learning Objective 5.3.1Differentiate between endergonic and exergonic chemical reactions.
In a chemical reaction, the original molecules before the chemical reaction occurs are called reactants, or
sometimes substrates, whereas the molecules that result after the reaction has taken place are called theproducts of
the reaction. Not all chemical reactions are equally likely to occur. Just as a boulder is more likely to roll downhill than
uphill, so a reaction is more likely to occur if it releases energy than if it needs to have energy supplied. Consider how the
chemical reaction proceeds in figure 5.4
. Like when rolling a boulder uphill, energy needs to be supplied. This is
because the product of the reaction contains more energy than the reactant. This type of chemical reaction,
called endergonic, does not occur spontaneously. By contrast, an exergonic reaction, shown in
spontaneously because the product has less energy than the reactant.
, tends to occur
Figure 5.4 Chemical reactions and catalysis.
The products of endergonic reactions contain more energy than the
reactants.
The products of exergonic reactions contain less energy than the reactants, but exergonic reactions do not necessarily
proceed rapidly because it takes energy to get them going. The “hill” in this energy diagram represents energy that must be supplied to
destabilize existing chemical bonds.
Catalyzed reactions occur faster because the amount of activation energy required to initiate
the reaction—the height of the energy hill that must be overcome—is lowered, and the reaction proceeds to its end faster.
D
Activation Energy
Learning Objective 5.3.2Define activation energy.
If all chemical reactions that release energy tend to occur spontaneously, it is fair to ask, “Why haven't all exergonic
reactions occurred already?” Clearly they have not. If you ignite gasoline, it burns with a release of energy. So why
doesn't all the gasoline in all the automobiles in the world just burn up right now? It doesn't because the burning of
gasoline, and almost all other chemical reactions, requires an input of energy to get it started—a kick in the pants such as
a match or spark plug to break existing chemical bonds in the reactants. The extra energy required to destabilize existing
chemical bonds and so initiate a chemical reaction is called activation energy, indicated by brackets in figure
5.4
and
. You must first nudge a boulder out of the hole it sits in before it can roll downhill. Activation energy is
simply a chemical nudge.
Catalysis
Learning Objective 5.3.3Describe the effect of catalysis on activation energy.
One way to make an exergonic reaction more likely to happen is to lower the necessary activation energy. Like digging
away the ground below your boulder, lowering activation energy reduces the nudge needed to get things started. The
process of lowering the activation energy of a reaction is called catalysis. Catalysis cannot make an endergonic reaction
occur spontaneously—you cannot avoid the need to supply energy—but it can make a reaction, endergonic or exergonic,
proceed much faster. Compare the activation energy levels (the red arched arrows) in
and
below: The catalyzed
reaction has a lower barrier to overcome.
Key Learning Outcome 5.3 Endergonic reactions require an input of energy. Exergonic reactions release energy.
Activation energy that initiates chemical reactions can be lowered by catalysis.
Enzymes
5.4How Enzymes Work
The Importance of Enzyme Shape
Learning Objective 5.4.1Differentiate between an enzyme's active site and its binding site.
Enzymes, which can be made of proteins or nucleic acids, are the catalysts used by cells to touch off particular chemical
reactions. By controlling which enzymes are present and when they are active, cells are able to control what happens
within themselves, just as a conductor controls the music an orchestra produces by dictating which instruments play
when.
An enzyme works by binding to a specific molecule and stressing the bonds of that molecule in such a way as to make a
particular reaction more likely. The key to this activity is the shape of the enzyme. An enzyme is specific for a particular
reactant, or substrate, because the enzyme surface provides a mold that very closely fits the shape of the desired
reactant. For example, the blue-colored lysozyme enzyme infigure 5.5 is contoured to fit a specific sugar molecule (the
yellow reactant). Other molecules that fit less perfectly simply don't adhere to the enzyme's surface. The site on the
enzyme surface where the reactant fits is called the active site (panel 1 below). The site on the reactant that binds to an
enzyme is called thebinding site. Enzymes are not rigid. The binding of the reactant induces the enzyme to change its
shape slightly. In figure 5.5b and in panel 2 of the Key Biological Process illustration below, the edges of the enzyme
now hug the reactant(s), leading to an “induced fit” between the enzyme and its reactant, like a hand wrapping around a
baseball.
Catalysis
Figure 5.5 Enzyme shape determines its activity.(a) A groove runs through the lysozyme enzyme (blue in this diagram) that fits
the shape of the reactant (in this case, a chain of sugars). (b) When such a chain of sugars, indicated in yellow, slides into the groove, it
induces the protein to change its shape slightly and embrace the substrate more intimately. This induced fit causes a chemical bond
between two sugar molecules within the chain to break.
D
An enzyme lowers the activation energy of a particular reaction. In the case of lysozyme, an enzyme found in human
tears, the enzyme has an antibacterial function, encouraging the breaking of a particular chemical bond in molecules that
make up the cell wall of bacteria (figure 5.5). The enzyme weakens the bond by drawing away some of its electrons.
Alternatively, an enzyme may encourage the formation of a link between two reactants, like the blue- and red-colored
molecules in panel 2 below, by holding them near each other. Regardless of the type of reaction, the enzyme is not
affected by the chemical reaction and is available to be used again.
How Enzymes Work
KEY BIOLOGICAL PROCESS: How Enzymes Work
Page 112
Biochemical Pathways
Learning Objective 5.4.2Distinguish between a chemical reaction and a biochemical pathway.
Every organism contains thousands of different kinds of enzymes that together catalyze a bewildering variety of reactions.
Often several of these reactions occur in a fixed sequence called a biochemical pathway, the product of one reaction
becoming the substrate for the next. You can see in the biochemical pathway shown in figure 5.6 how the initial substrate
is altered by enzyme 1 so that it now fits into the active site of another enzyme, becoming the substrate for enzyme 2, and
so on until the final product is produced. Because these reactions occur in sequence, the enzymes involved are often
positioned near each other in the cell. The close proximity of the enzymes allows the reactions of the biochemical pathway
to proceed faster. Biochemical pathways are the organizational units of metabolism.
Figure 5.6 A biochemical pathway.The original substrate is acted on by enzyme 1, changing the substrate to a new form
recognized by enzyme 2. Each enzyme in the pathway acts on the product of the previous stage.
A Biochemical Pathway
D
Factors Affecting Enzyme Activity
Learning Objective 5.4.3Explain the influence of temperature on an enzyme-catalyzed reaction.
Enzyme activity is affected by any change in condition that alters the enzyme's three-dimensional shape.
Temperature When the temperature increases, the bonds that determine enzyme shape are too weak to hold the
enzyme's peptide chains in the proper position, and the enzyme denatures. As a result, enzymes function best within an
optimum temperature range, which is relatively narrow for most human enzymes. In the human body, enzymes work best
at temperatures near the normal body temperature of 37°C, as shown by the brown curve in figure 5.7a. Notice that the
rates of enzyme reactions tend to drop quickly at higher temperatures, when the enzyme begins to unfold. This is why an
extremely high fever in humans can be fatal. However, the shapes of the enzymes found in hot springs bacteria (the red
curve) are more stable, allowing the enzymes to function at much higher temperatures. This allows the bacteria to live in
water that is near 70°C.
Figure 5.7 Enzymes are sensitive to their environment.The activity of an enzyme is influenced by both (a) temperature and
(b) pH. Most human enzymes work best at temperatures of about 40°C and within a pH range of 6 to 8.
D
pH In addition, most enzymes also function within an optimal pH range, because the shape-determining polar interactions
of enzymes are quite sensitive to hydrogen ion (H+) concentration. Most human enzymes, such as the protein-degrading
enzyme trypsin (the dark blue curve in figure 5.7b), work best within the range of pH 6 to 8. Blood has a pH of 7.4.
However, some enzymes, such as the digestive enzyme pepsin (the light blue curve), are able to function in very acidic
environments such as the stomach but can't function at higher pHs.
Key Learning Outcome 5.4 Enzymes catalyze chemical reactions within cells and can be organized into
biochemical pathways. Enzymes are sensitive to temperature and pH because both of these variables influence
enzyme shape.
Page 113
KEY BIOLOGICAL PROCESS: Allosteric Enzyme Regulation
5.5How Cells Regulate Enzymes
Learning Objective 5.5.1Distinguish between competitive and noncompetitive allosteric feedback inhibition.
Because an enzyme must have a precise shape to work correctly, it is possible for the cell to control when an enzyme is
active by altering its shape. Many enzymes have shapes that can be altered by the binding of “signal” molecules to their
surfaces. Such enzymes are called allosteric (Latin, other shape). Enzymes can be inhibited or activated by the binding of
signal molecules. For example, the upper tan panels in the Key Biological Process illustration above show an enzyme that
is inhibited. The binding of a signal molecule, called a repressor (panel 2), alters the shape of the enzyme's active site
such that it cannot bind the substrate. In other cases, the enzyme may not be able to bind the reactants unless the signal
molecule is bound to the enzyme. The lower set of panels shows a signal molecule serving as anactivator. The red
substrate cannot bind to the enzyme's active site unless the activator (the yellow molecule) is in place, altering the shape
of the active site. The site where the signal molecule binds to the enzyme surface is called the allosteric site.
Enzymes are often regulated by a mechanism called feedback inhibition, where the product of the reaction acts as the
repressor. Feedback inhibition can occur in two ways: competitive inhibitors andnoncompetitive inhibitors. The blue
molecule in figure 5.8a functions as a competitive inhibitor, blocking the active site so that the substrate cannot bind. The
yellow molecule in figure 5.8b functions as a noncompetitive inhibitor. It binds to an allosteric site, changing the shape of
the enzyme such that it is unable to bind to the substrate.
Feedback Inhibition of Biochemical Pathways
Figure 5.8 How enzymes can be inhibited.(a) In competitive inhibition, the inhibitor interferes with the active site of the enzyme.
(b) In noncompetitive inhibition, the inhibitor binds to the enzyme at a place away from the active site, effecting a conformational change
in the enzyme so that it can no longer bind to its substrate. In feedback inhibition, the inhibitor molecule is the product of the reaction.
D
Many drugs and antibiotics work by inhibiting enzymes. Statin drugs like Lipitor lower cholesterol by inhibiting a key
enzyme cells use to make cholesterol.
Key Learning Outcome 5.5 An enzyme's activity can be affected by signal molecules that bind to it, changing its
shape.
How Cells Use Energy
5.6ATP: The Energy Currency of the Cell
Cells use energy to do all those things that require work, but how does the cell use energy from the sun or the potential
energy stored in molecules to power its activities? The sun's radiant energy and the energy stored in molecules are
energy sources, but like money that is invested in stocks and bonds or real estate, these energy sources cannot be used
directly to run a cell. To be useful, the energy from the sun or food molecules must first be converted to a source of
energy that a cell can use, like someone converting stocks and bonds to ready cash. The “cash” molecule in the body
is adenosine triphosphate (ATP). ATP is the energy currency of the cell.
Structure of the ATP Molecule
Learning Objective 5.6.1Explain how the chain of three phosphate groups in ATP stores potential energy.
Each ATP molecule is composed of the three parts shown in figure 5.9: (1) a sugar (colored blue) serves as the
backbone to which the other two parts are attached, (2) adenine (colored peach) is one of the four nitrogenous bases in
DNA and RNA, and (3) a chain of three phosphates (colored yellow) contain high-energy bonds.
Figure 5.9 The parts of an ATP molecule.
D
As you can see in the figure, the phosphates carry negative electrical charges, and so it takes considerable chemical
energy to hold the line of three phosphates next to one another at the end of ATP. Like a coiled spring, the phosphates
are poised to push apart. It is for this reason that the chemical bonds linking the phosphates are such chemically reactive
bonds.
When the endmost phosphate is broken off an ATP molecule, a sizable packet of energy is released. The reaction
converts ATP to adenosine diphosphate, ADP. The second phosphate group can also be removed, yielding additional
energy and leaving adenosine monophosphate (AMP). Most energy exchanges in cells involve cleavage of only the
outermost bond, converting ATP into ADP and Pi, inorganic phosphate:
Exergonic reactions require activation energy, and endergonic reactions require the input of even more energy, and so
these reactions in the cell are usually coupled with the breaking of the phosphate bond in ATP, called coupled reactions.
Because almost all chemical reactions in cells require less energy than is released by this reaction, ATP is able to power
many of the cell's activities, producing heat as a by-product. Table 5.1 introduces you to some of the key cellular activities
powered by the breakdown of ATP. ATP is continually recycled from ADP and Pi through the ATP-ADP cycle.
TABLE 5.1
HOW CELLS USE ATP ENERGY TO POWER CELLULAR WORK
Contraction
Chemical Activation
Cells use the energy released from
the exergonic hydrolysis of ATP to
drive endergonic reactions like
those of protein synthesis, an
approach called energy coupling.
In muscle cells, filaments of
protein repeatedly slide past each
other to achieve contraction of the
cell. An input of ATP is required
for the filaments to reset and slide
again.
Proteins can become activated
when a high-energy phosphate
from ATP attaches to the protein,
activating it. Other types of
molecules can also become
phosphorylated by transfer of a
phosphate from ATP.
Importing Metabolites
Active Transport: Na+−K+ Pump
Metabolite molecules such as
amino acids and sugars can be
transported into cells against their
concentration gradients by
coupling the intake of the
metabolite to the inward movement
of an ion moving down its
concentration gradient, this ion
gradient being established using
ATP.
Most animal cells maintain a low
internal concentration of
Na+relative to their surroundings,
and a high internal concentration of
K+. This is achieved using a protein
called the sodium-potassium pump,
which actively pumps Na+ out of
the cell and K+in, using energy
from ATP.
Biosynthesis
Cytoplasmic Transport
Within a cell's cytoplasm, vesicles
or organelles can be dragged
along microtubular tracks using
molecular motor proteins, which
are attached to the vesicle or
organelle with connector proteins.
The motor proteins use ATP to
power their movement.
Flagellar Movements
Cell Crawling
Microtubules within flagella slide
past each other to produce flagellar
movements. ATP powers the
sliding of the microtubules.
Actin filaments in a cell's
cytoskeleton continually assemble
and disassemble to achieve changes
in cell shape and to allow cells to
crawl over substrates or engulf
materials. The dynamic character
of actin is controlled by ATP
molecules bound to actin filaments.
Heat Production
The hydrolysis of the ATP
molecule releases heat. Reactions
that hydrolyze ATP often take
place in mitochondria or in
contracting muscle cells and may
be coupled to other reactions. The
heat generated by these reactions
can be used to maintain an
organism's temperature.
D
Cells use two different but complementary processes to convert energy from the sun and potential energy found in food
molecules into ATP. Some cells convert energy from the sun into molecules of ATP through the process
of photosynthesis. This ATP is then used to manufacture sugar molecules, converting the energy from ATP into potential
energy stored in the bonds that hold the atoms together. All cells convert the potential energy found in food molecules into
ATP through cellular respiration.
Key Learning Outcome 5.6 Cells use the energy in ATP molecules to drive chemical reactions.
Page 116
INQUIRY
& ANALYSIS
Do Enzymes Physically Attach to Their Substrates?
When scientists first began to examine the chemical activities of organisms, no one knew that biochemical reactions were
catalyzed by enzymes. The first enzyme was discovered in 1833 by French chemist Anselme Payen. He was studying
how beer is made from barley: First barley is pressed and gently heated so its starches break down into simple two-sugar
units; then yeasts convert these units into ethanol. Payen found that the initial breakdown requires a chemical factor that
is not alive and that does not seem to be used up during the process—a catalyst. He called this first enzyme diastase (we
call it amylase today).
Did this catalyst operate at a distance, increasing the reaction rate all around it, much as raising the temperature of
nearby molecules might do? Or did it operate in physical contact, actually attaching to the molecules whose reaction it
catalyzed (its “substrate”)?
The answer was discovered in 1903 by French chemist Victor Henri. He saw that the hypothesis that an enzyme
physically binds to its substrate makes a clear and testable prediction: In a solution of substrate and enzyme, there must
be a maximum reaction rate, faster than which the reaction cannot proceed. When all the enzyme molecules are working
full tilt, the reaction simply cannot go any faster, no matter how much more substrate you add to the solution. To test this
prediction, Henri carried out the experiment whose results you see in the graph, measuring the reaction rate (V) of
diastase at different substrate concentrations (S).
1. Making Inferences As S increases, does V increase? If so, in what manner—steadily or by smaller and smaller
amounts? Is there a maximum reaction rate?
2. Drawing Conclusions Does this result provide support for the hypothesis that an enzyme binds physically to its
substrate? Explain. If the hypothesis were incorrect, what would you expect the graph to look like?
3. Further Analysis If the smaller amounts by which V increases are strictly the result of fewer unoccupied enzymes
being available at higher values of S, then the curve in Henri's experiment should show a pure exponential decline
in V—mathematically, meaning a reciprocal plot (1/Vversus 1/S) should be a straight line. If some other factor is also
at work that reacts differently to substrate concentration, then the reciprocal plot would curve upward or downward.
Fill in the reciprocal values in the table to the right, and then plot the values on the lower graph (1/S on the xaxis and
1/V on the y axis). Is a reciprocal plot of Henri's data a straight line?
D
D
Reviewing What You Have Learned
Cells and Energy
The Flow of Energy in Living Things


5.1.1Energy is the ability to do work. Energy exists in two states: kinetic energy and potential energy.
•Kinetic energy is the energy of motion. Potential energy is stored energy, which exists in objects that aren't in motion
but have the capacity to move (figure 5.1). All of the work carried out by living things involves the transformation of
potential energy into kinetic energy.

•Energy flows from the sun to the earth, where it is trapped by photosynthetic organisms and stored in carbohydrates
as potential energy. This energy is transferred during chemical reactions.
The Laws of Thermodynamics




5.2.1The laws of thermodynamics describe changes in energy in our universe. The first law of thermodynamics
explains that energy can never be created or destroyed, only changed from one state to another. The total amount of
energy in the universe remains constant.
•Energy exists in different forms in the universe, such as light, electrical, or heat energy. This energy, such as heat
energy, can be harnessed to do work.
5.2.2The second law of thermodynamics explains that the conversion of potential energy into random molecular
motion is constantly increasing. This conversion of energy progresses from an ordered but less stable form to a
disordered but stable form.
5.2.3Entropy, which is a measure of disorder in a system, is constantly increasing such that disorder is more likely
than order. Energy must be used to maintain order.
Chemical Reactions

5.3.1Chemical reactions involve the breaking or formation of covalent bonds. The starting molecules are called the
reactants, and the molecules produced by the reaction are called the products. Chemical reactions in which the
products contain more potential energy than the reactants are called endergonic reactions. Chemical reactions that
release energy are called exergonic reactions, as shown here from figure 5.4, and are more likely to occur.


5.3.2All chemical reactions require an input of energy. The energy required to start a reaction is called activation
energy.
5.3.3A chemical reaction proceeds faster when its activation energy is lowered, a process called catalysis.
Enzymes
How Enzymes Work




5.4.1Enzymes are macromolecules that lower the activation energy of chemical reactions in the cell. Enzymes are
catalysts.
•An enzyme, like the lysozyme shown here from figure 5.5, binds the reactant, or substrate. The substrate binds to
the enzyme's active site. The actions of the enzyme increase the likelihood that chemical bonds will break or form.
The enzyme is not affected by the reaction and can be used over and over again.
5.4.2Sometimes enzymes work in a series of reactions called a biochemical pathway. The product of one reaction
becomes the substrate for the next reaction. The enzymes that are involved are usually located near each other in
the cell.
5.4.3Factors such as temperature and pH affect enzyme function, and so most enzymes have an optimal
temperature and pH range. Higher temperatures can disrupt the bonds that hold the enzyme in its proper shape,
decreasing its ability to catalyze a chemical reaction. The bonds that hold the enzyme's shape are also affected by
hydrogen ion concentrations, and so increasing or decreasing the pH can disrupt the enzyme's function.
How Cells Regulate Enzymes


5.5.1An enzyme can be inhibited or activated in the cell as a means of regulation by temporarily altering the
enzyme's shape. An enzyme can be inhibited when a molecule, called a repressor, binds to the enzyme, altering the
shape of the active site so that it cannot bind the substrate. Some enzymes need to be activated, or turned on, in
order to bind to their substrate. A molecule called an activator binds to the enzyme, changing the shape of the active
site so that it is able to bind the substrate. Enzymes that are controlled in this way are allosteric enzymes.
•A repressor molecule can bind to the active site of the enzyme, blocking it. This is called a competitive inhibition. In
noncompetitive inhibition, the repressor binds to a different site on the enzyme, altering the shape of the active site
so it cannot bind its substrate. Enzymes are often regulated by feedback inhibition, a process where the product of
the reaction functions as a repressor, shutting down its own synthesis.
How Cells Use Energy
ATP: The Energy Currency of the Cell

5.6.1Cells require energy to do work in the form of ATP. ATP contains a sugar, an adenine, and a chain of three
phosphates, as shown here from figure 5.9. The three phosphates are held together with high-energy bonds. When
the endmost phosphate bond breaks, considerable energy is released. A cell uses this energy to drive reactions in
the cell by coupling the breakdown of ATP with other chemical reactions in the cell.
Page 118
Test Your Understanding









5.1.1The ability to do work is the definition for
a. thermodynamics.
b. radiation.
c. energy.
d. entropy.
•The difference between potential energy and kinetic energy is that
a. potential energy is less powerful than kinetic energy.
b. potential energy is the energy of motion.
c. kinetic energy is less powerful than potential energy.
d. kinetic energy is the energy of motion.
Answer
5.2.1The first law of thermodynamics
a. says that energy recycles constantly, as organisms use and reuse it.
b. says that entropy continually increases in a closed system.
c. is a formula for measuring entropy.
d. says that energy can change forms but cannot be made or destroyed.
•When a baseball thrown by a pitcher encounters the swinging bat of a slugger, what happens to the ball's kinetic
energy? What happens to the bat's kinetic energy?
•Heat is a form of
a. potential energy.
b. kinetic energy.
Answer
5.2.2The second law of thermodynamics
a. says that energy recycles constantly, as organisms use and reuse it.
b. says that disorder continually increases in a closed system.
c. is a formula for measuring entropy.
d. says that energy can change forms but cannot be made or destroyed.
Answer
5.2.3Entropy is a measure of
a. energy transfer rate.
b. potential energy.
c. the degree of disorder.
d. light.
Answer
5.3.1Chemical reactions that occur spontaneously are called
a. exergonic and release energy.
b. exergonic, and their products contain more energy.
c. endergonic and release energy.
d. endergonic, and their products contain more energy.
•An endergonic reaction is one in which
a. the reactants contain more energy than the products.
b. the products contain more energy than the reactants.
c. energy is released.













d. entropy is increased.
Answer
5.3.2What is activation energy?
a. thermal energy associated with random movements of molecules
b. energy released through breaking chemical bonds
c. the difference in energy between the reactants and products
d. the energy required to initiate a chemical reaction
Answer
5.3.3A is a substance that lowers the activation energy of a reaction.
a. catalyst
b. substrate
c. product
d. reactant
Answer
5.4.1The catalysts that help an organism carry out needed chemical reactions are called
a. hormones.
b. enzymes.
c. reactants.
d. substrates.
•In order for an enzyme to work properly,
a. it must have a particular shape.
b. the temperature must be within certain limits.
c. the pH must be within certain limits.
d. All of the above.
•A restriction endonuclease is an enzyme that cuts DNA at a specific, unique sequence, like GAATTC. How does a
particular restriction enzyme “know” when it has found its target sequence?
•The enzyme sucrase splits the disaccharide sugar sucrose into the monosaccharides glucose and fructose. What
prevents the glucose and fructose molecules from reentering the active site and reforming a sucrose molecule?
•Which of the following is not a property of an enzyme?
a. An enzyme reduces the activation energy of a reaction.
b. The site on an enzyme that binds to the reactant is called the active site.
c. An enzyme is not affected by the reaction and can be used again.
d. An enzyme acts as a catalyst by being highly reactive and able to bind to any molecule in its vicinity.
Answer
5.4.2In terms of lengthy biochemical pathways, which reactions would you expect to have generally evolved first, the
initial reaction in the series or the final reaction? Explain your reasoning.
Answer
5.4.3Factors that affect the activity of an enzyme molecule include
a. the potential energy stored in the reactant molecules.
b. size of the cell.
c. temperature and pH.
d. entropy.
•What happens to a human enzyme when the body's temperature is raised above 40°C? Why?
Answer
5.5.1In competitive inhibition,
a. an enzyme molecule has to compete with other enzyme molecules for the necessary substrate.
b. an enzyme molecule has to compete with other enzyme molecules for the necessary energy.
c. an inhibitor molecule competes with the substrate for the active site on the enzyme.
d. two different products compete for the same binding site on the enzyme.
•An allosteric site is the location on an enzyme's surface where
a. the substrate binds.
b. the signal molecule binds.
c. catalysis takes place.
d. ATP binds.
Answer
5.6.1Which of the following is not a component of ATP?


a. glucose
b. ribose
c. adenine
d. phosphate groups
•Endergonic reactions can occur in the cell because they are coupled with
a. the breaking of phosphate bonds in ATP.
b. uncatalyzed reactions.
c. activators.
d. All of the above.
•Where is the energy stored in a molecule of ATP?
a. in the bonds between nitrogen and carbon
b. in the carbon–carbon bonds found in the ribose
c. in the phosphorus–oxygen double bond
d. in the bonds connecting the two terminal phosphate groups
Answer