Tải bản đầy đủ - 0trang
1 Energy, Life, and Biochemical Reactions
The Generation of Biochemical Energy
Our bodies do not produce energy by burning up a meal all at once because the release of a large quantity of energy (primarily as heat) would be harmful to us. Furthermore, it is difficult to capture energy for storage once it has been converted to heat. We
need energy that can be stored and then released in the right amounts when and where
it is needed, whether we are jogging, studying, or sleeping. We, therefore, have some
specific requirements for energy.
Energy must be released from food gradually.
Energy must be stored in readily accessible forms as glycogen and fat
Release of energy from storage must be finely controlled so that it is available
exactly when and where it is needed.
Just enough energy must be released as heat to maintain constant body
Energy in a form other than heat must be available to drive chemical reactions that
are not favorable at body temperatures.
This chapter looks at some of the ways these requirements for energy regulation
are met. We begin by reviewing basic concepts about energy and then learn about
metabolism. Next, we look at the citric acid cycle and oxidative phosphorylation,
which together form the common pathway for the production of energy.
ConCEPtS to rEViEW Review
entropy, enthalpy, endergonic,
exergonic, and free-energy change in
Chemical reactions either release or absorb energy. Whether a reaction is favorable or
not depends on either the release or absorption of energy as heat (the change in enthalpy, ∆H), together with the increase or decrease in disorder (∆S, the entropy change)
caused by the reaction. The net effect of these changes is given by the free-energy
change of a reaction: ∆G = ∆H - T∆S .
Reactions in living organisms are no different from reactions in a chemistry laboratory. Both follow the same laws, and both have the same energy requirements. Spontaneous reactions—that is, those that are favorable in the forward direction—release
free energy, and the energy released is available to do work. Such reactions, described
as exergonic, are the source of our biochemical energy.
As shown by the energy diagram in Figure 7.3 the products of a favorable, exergonic
reaction are farther downhill on the energy scale than the reactants. That is, the products are more stable than the reactants, and as a result the free-energy change 1 ∆G2
has a negative value. Oxidation reactions, for example, are usually downhill reactions
that release energy. Oxidation of glucose, the principal source of energy for animals,
produces 2870 kJ of free energy per mole of glucose.
C6H12O6 + 6 O2 ¡ 6 CO2 + 6 H2O
∆G = - 2870 kJ>mol
The greater the amount of free energy released, the farther a reaction proceeds
toward product formation before reaching equilibrium.
Reactions in which the products are higher in energy than the reactants can also
take place, but such unfavorable reactions cannot occur without the input of energy
from an external source; such reactions are endergonic.
The free-energy change switches sign for the reverse of a reaction, but the value
does not change. Photosynthesis, the process whereby plants convert CO2 and H2O to
glucose and O2, is the reverse of the oxidation of glucose. Its ∆G is therefore positive
and equal to the value for the oxidation of glucose (see the Chemistry in Action “Plants
and Photosynthesis” on p. 696). The sun provides the necessary external energy for
photosynthesis (2870 kJ>mol of glucose formed).
6CO2 + 6H2O
Energy, Life, and Biochemical Reactions
∆G = + 2870 kJ/mol (endergonic, energy required)
C6H12O6 + 6O2
∆G = −2870 kJ/mol (exergonic, energy released)
Living systems make constant use of this principle in the series of chemical reactions we know as the biochemical pathways. Energy is stored in the products of an
overall endergonic reaction pathway. This stored energy is released as needed in an
overall exergonic reaction pathway that regenerates the original reactants. It is not necessary that every reaction in the pathways between the reactants and products be the
same, so long as the pathways connect the same reactants and products.
Pathway A series of enzymecatalyzed chemical reactions that are
connected by their intermediates, that
is, the product of the first reaction is
the reactant for the second reaction,
and so on.
Worked Example 21.1 Determining Reaction Energy
Are the following reactions exergonic or endergonic?
(a) Glucose 6@phosphate S Fructose 6@phosphate
∆G = + 2.09 kJ>mol
(b) Fructose 6@phosphate + ATP S Fructose 1,6@bisphosphate + ADP
∆G = - 14.2 kJ>mol
AnAlySiS Exergonic reactions release free energy, and ∆G is negative. Endergonic reactions gain free energy,
and so ∆G is positive.
Reaction (a), the conversion of glucose 6-phosphate to fructose 6-phosphate has a positive ∆G; therefore, it is
endergonic. Reaction (b), the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate has a negative
∆G; therefore it is exergonic.
KEy ConCEPt ProBlEM 21.1
In a cell, glucose can be oxidized via metabolic pathways. Alternatively, you could
burn glucose in the laboratory. Which of these methods consumes or produces more
energy? (Hint: All of the energy comes from converting the energy stored in the reduced bonds in glucose into the most oxidized form, carbon dioxide.)
KEy ConCEPt ProBlEM 21.2
The overall equation in this section,
6CO2 + 6H2O
C6H12O6 + 6O2,
shows the cycle between photosynthesis and oxidation. Pathways operating in opposite
directions cannot be exergonic in both directions.
(a) Which of the two pathways in this cycle is exergonic and which is endergonic?
(b) Where does the energy for the endergonic pathway come from?
The Generation of Biochemical Energy
CHEMiStry in ACtion
Plants and Photosynthesis
The principal biochemical difference between humans and
plants is that plants derive energy directly from sunlight and
we cannot. In the process of photosynthesis, plants use solar
energy to synthesize oxygen and energy-rich carbohydrates
from energy-poor reactants: CO2 and water. Our metabolism
breaks down energy-rich reactants to extract the useful energy and produce energy-poor products: CO2 and water. Is it
surprising to discover that despite this difference in the direction of their reactions, plants rely on biochemical pathways
very much like our own?
The energy-capturing phase of photosynthesis takes place
mainly in green leaves. Plant cells contain chloroplasts, which,
though larger and more complex in structure, resemble mitochondria. Embedded in membranes within the chloroplasts are
large groups of chlorophyll molecules and the enzymes of an
electron-transport chain. Chlorophyll is similar in structure to
heme but contains magnesium ions 1Mg2+ 2 instead of iron
ions 1Fe2+ 2 .
As solar energy is absorbed, chlorophyll molecules pass it
along to specialized reaction centers, where it is used to boost
the energy of electrons. The excited electrons then give up their
extra energy as they pass down a pair of electron-transport
Some of this energy is used to oxidize water, splitting it into
oxygen, hydrogen ions, and electrons (which replace those
entering the electron-transport chain). At the end of the chain,
the hydrogen ions, together with the electrons, are used to reduce NADP+ to NADPH. Along the way, part of the energy of the
electrons is used to pump hydrogen ions across a membrane
to create a concentration gradient. As in mitochondria, the hydrogen ions can only return across the membrane at enzyme
complexes that convert ADP to ATP. Water needed for these
light-dependent reactions enters the plant through the roots
and leaves, and the oxygen that is formed is released through
openings in the leaves.
The energy-carrying ATP and NADPH enter the fluid interior of
the chloroplasts. Here their energy is used to drive the synthesis
of carbohydrate molecules. So long as ATP and NADPH are available, this part of photosynthesis is light-independent—it can
proceed in the absence of sunlight.
Plants have mitochondria as well as chloroplasts, so they
can also carry out the release of energy from stored carbohydrates. Because the breakdown of carbohydrates continues
in many harvested fruits and vegetables, the goal in storage
is to slow it down. Refrigeration is one measure that is taken,
since (like most chemical reactions) the rate of respiration
decreases at lower temperatures. Another is replacement of
air over stored fruits and vegetables with carbon dioxide or
These flowers are converting the potential energy of the sun into
chemical potential energy stored in the bonds of carbohydrates.
CO2 + H2O
The coupled reactions of photosynthesis.
CiA Problem 21.1 Chlorophyll is similar in structure to heme in
red blood cells but does not have an iron atom. What metal
ion is present in chlorophyll?
CiA Problem 21.2 Photosynthesis consists of both light-dependent
and light-independent reactions. What is the purpose of each
type of reaction?
CiA Problem 21.3 One step of the cycle that incorporates
CO2 into glyceraldehyde in plants is the production
of two 3-phosphoglycerates. ∆G = - 3.5 kJ>mol for this
reaction. Is this process endergonic or exergonic?
CiA Problem 21.4 What general process does refrigeration of
harvested fruits and vegetables slow? What cellular processes
are slowed by refrigeration?
Cells and Their Structure
21.2 Cells and Their Structure
• Describe the eukaryotic cell and explain the function of each structure.
Before learning about metabolism, it is important to see where the energy-generating
reactions take place within the cells of living organisms. There are two main categories
of cells: prokaryotic cells, found in single-celled organisms (e.g., bacteria and bluegreen algae), and eukaryotic cells, found in some single-celled organisms, such as
yeast, and all plants and animals.
Eukaryotic cells are about 1000 times larger than bacterial cells, have a membraneenclosed nucleus that contains their deoxyribonucleic acid (DNA), and include several
other kinds of internal structures known as organelles—small, functional units that perform specialized tasks. A generalized eukaryotic cell is shown in Figure 21.2 with short
descriptions of the functions of some of its major parts. Everything between the cell membrane and the nuclear membrane in a eukaryotic cell, including the various organelles,
is the cytoplasm. The organelles are surrounded by the fluid part of the cytoplasm, the
cytosol, which contains electrolytes, nutrients, and many enzymes, all in aqueous solution.
Cytoplasm The region between the
cell membrane and the nuclear membrane in a eukaryotic cell.
Cytosol The fluid part of the
cytoplasm surrounding the organelles
within a cell, contains dissolved
proteins and nutrients.
(movement of materials)
(absorption of extracellular substances)
(synthesis of ATP)
(replication of DNA)
(synthesis of macromolecules)
Smooth endoplasmic reticulum
(synthesis of lipids and carbohydrates)
Rough endoplasmic reticulum
(protein synthesis and transport)
(breakdown of unwanted molecules and cellular components)
(separates cell contents from exterior; permits exchange of
molecules with exterior ﬂuid and delivers signals to interior)
A generalized eukaryotic cell.
Major cell components are labeled with a description of their primary function.
The mitochondria (singular, mitochondrion), often called the cell’s “power plants,”
are the most important of the organelles for energy production and produce about 90%
of the body’s energy-carrying molecule, ATP.
A mitochondrion is a roughly egg-shaped structure composed of a smooth outer
membrane and a folded inner membrane (Figure 21.3). The space enclosed by the inner membrane is the mitochondrial matrix. Within the matrix, the citric acid cycle
(Section 21.7) and production of most of the body’s adenosine triphosphate (ATP)
take place. The coenzymes and proteins that manage the transfer of energy to the
chemical bonds of ATP (Section 21.8) are embedded in the inner membrane of the
mitochondria) An egg-shaped organelle where small molecules are broken
down to provide the energy for an
Mitochondrial matrix The space surrounded by the inner membrane of a
Adenosine triphosphate (ATP) The
principal energy-carrying molecule,
removal of a phosphoryl group to give
ADP releases free energy.
The Generation of Biochemical Energy
sites of ATP production
Cells have many mitochondria. The citric acid cycle takes place in the matrix. Electron transport and ATP production, the final
stage in biochemical energy generation (described in Section 21.8), take place at the inner surface of the inner membrane. The
numerous folds in the inner membrane—known as cristae—increase the surface area over which these pathways can take place.
Mitochondria contain their own DNA, synthesize some of their own proteins, and
multiply using chemicals moved from the cell cytosol into the mitochondrial matrix.
The number of mitochondria is greatest in eye, brain, heart, and muscle cells, where the
need for energy is greatest. The ability of mitochondria to reproduce is seen in athletes
who put heavy energy demands on their bodies—they develop an increased number of
mitochondria to aid in energy production.
21.3 An Overview of Metabolism and Energy Production
• List the stages in catabolism of food and describe the role of each stage.
Metabolism The sum of all of the
chemical reactions that take place in an
Together, all of the chemical reactions that take place in an organism constitute its
metabolism. Most of these reactions occur in the reaction sequences of metabolic
pathways, a sequence of reactions where the product of one reaction serves as the starting material for the next. Such pathways may be linear (a series of reactions that convert
a reactant into a specific product through a series of intermediate molecules and reactions), cyclic (a series of reactions that regenerates one of the first reactants), or spiral
(the same set of enzymes progressively builds up or breaks down a molecule).
. . .
Catabolism Metabolic reaction pathways that break down food molecules
and release biochemical energy.
As we study metabolism we will encounter each of these types of pathways. Those
pathways that break molecules apart are known collectively as catabolism, whereas
those that put building blocks back together to assemble larger molecules are known
An Overview of Metabolism and Energy Production
collectively as anabolism. The purpose of catabolism is to release energy from food,
and the purpose of anabolism is to synthesize new biomolecules, including those that
Anabolism Metabolic reactions that
build larger biological molecules from
A N A B O L IS M
The overall picture of digestion, catabolism, and energy production is simple: eating
provides fuel, breathing provides oxygen, and our bodies oxidize the fuel to extract energy. The process can be roughly divided into the four stages described here and shown
in Figure 21.4.
StAgE 1: Digestion Enzymes in saliva, the stomach, and the small intestine convert the
large molecules of carbohydrates, proteins, and lipids to smaller molecules. Carbohydrates are broken down to glucose and other sugars; proteins are broken down to amino
acids; and triacylglycerols, the lipids commonly known as fats and oils, are broken
down to glycerol plus long-chain carboxylic acids, termed fatty acids. These smaller
molecules are transferred into the blood for transport to cells throughout the body.
StAgE 2: Acetyl-coenzyme A production The small molecules from digestion follow
separate pathways that separate their carbon atoms into two-carbon acetyl groups. The
acetyl groups are attached to coenzyme A by a high-energy bond between the sulfur
atom of the thiol 1 ¬ SH2 group at the end of the coenzyme A molecule and the carbonyl carbon atom of the acetyl group.
See the chemical structure of
coenzyme A in Figure 19.10.
Acetyl-coenzyme A (acetyl-CoA)
Acetyl-substituted coenzyme A—the
common intermediate that carries acetyl groups into the citric acid cycle.
Attachment of acetyl group to coenzyme A
The resultant compound, acetyl-coenzyme A, which is abbreviated acetyl-CoA, is an
intermediate in the breakdown of all classes of food molecules. It carries the acetyl
groups into the common pathways of catabolism—Stage 3, the citric acid cycle and
Stage 4, electron transport and ATP production.
StAgE 3: Citric acid cycle Within mitochondria, the acetyl-group carbon atoms are
oxidized to the carbon dioxide that we exhale. Most of the energy released in the
oxidation leaves the citric acid cycle in the chemical bonds of reduced coenzymes
(NADH, FADH2). Some energy also leaves the cycle stored in the chemical bonds of
ATP or a related triphosphate.
StAgE 4: ATP production Electrons from the reduced coenzymes are passed from
molecule to molecule down an electron-transport chain. Along the way, their energy
is harnessed to produce more ATP. At the end of the process, these electrons—along
with hydrogen ions from the reduced coenzymes—combine with oxygen we breathe in
to produce water. Thus, the reduced coenzymes are in effect oxidized by atmospheric
oxygen, and the energy that they carried is stored in the chemical bonds of ATP
conversion of food molecules to acetylCoA, Stages 1 and 2 in Figure 21.4,
occur by different metabolic pathways
for carbohydrates, lipids, and proteins.
Each of these pathways is discussed
separately in later chapters: carbohydrate metabolism in Chapter 22, lipid
metabolism in Chapter 24, and protein
metabolism in Chapter 25.
The Generation of Biochemical Energy
Pathways for the digestion of food and
the production of biochemical energy.
This diagram summarizes pathways
covered in this chapter (the citric acid
cycle and electron transport) and also
the pathways discussed in Chapter 22
for carbohydrate metabolism, in
Chapter 24 for lipid metabolism, and in
Chapter 25 for protein metabolism.
Stage 1. Digestion
Bulk food is digested in
the mouth, stomach, and
small intestine to yield
Stage 2. Acetyl-CoA Production
Sugar and amino acid molecules
are degraded in the cytoplasm of
cells to yield acetyl-CoA. Fatty acid
molecules are degraded in the
mitochondria of cells to yield acetyl-CoA.
Stage 3. Citric Acid Cycle
Acetyl-CoA is oxidized
inside mitochondria by
the citric acid cycle to
yield CO2 and reduced
Stage 4. ATP Production
The energy transferred to the
reduced coenzymes in stage
3 is used to make ATP by the
coupled pathways of electron
transport and oxidative
Worked Example 21.2 Identifying Metabolic Pathways That Convert Basic Molecules to Energy
(a) In Figure 21.4, identify the stages in the catabolic pathway in which lipids ultimately yield ATP.
(b) In Figure 21.4, identify the place at which the products of lipid catabolism can join the common
AnAlySiS Look at Figure 21.4 and find the pathway for lipids. Follow the arrows to trace the flow of energy.
Note that Stage 3 is the point at which the products of lipid, carbohydrate, and protein catabolism all feed into
a central, common metabolic pathway, the citric acid cycle. The lipid molecules that feed into Stage 3 do so
via acetyl-CoA (Stage 2). Note also that most products of Stage 3 catabolism feed into Stage 4 catabolism to
The lipids in food are broken down in Stage 1 (digestion) to fatty acids and glycerol. Stage 2 (acetyl-CoA
production) results in fatty acid oxidation to acetyl-CoA. In Stage 3 (citric acid cycle), acetyl-CoA enters
the citric acid cycle (the common metabolism pathway), which produces ATP, reduced coenzymes, and
CO2. In Stage 4 (ATP production), the energy stored in the reduced coenzymes (from the citric acid cycle) is
converted to ATP energy.
Strategies of Metabolism: ATP and Energy Transfer
(a) In Figure 21.4, identify the stages in the pathway for the conversion of the energy
from carbohydrates to energy stored in ATP molecules.
(b) In Figure 21.4, identify the three places at which the products of amino acid
catabolism can join the central metabolism pathway.
21.4 Strategies of Metabolism: ATP and Energy Transfer
• Describe the role of ATP in energy transfer.
ATP is the body’s energy-transporting molecule. What exactly does that mean? Consider that the molecule has three ¬ PO3- groups.
in hydrolysis to ADP
Adenosine triphosphate (ATP)
Removal of the terminal ¬ PO3 - 2 group from ATP by hydrolysis gives adenosine
diphosphate (ADP). The ATP S ADP reaction is exergonic; it releases chemical energy that was held in the bond to the ¬ PO32- group.
ATP + H2O ¡ ADP + HOPO32- + H +
∆G = - 30.5 kJ>mol
The reverse of ATP hydrolysis—a phosphorylation reaction—is endergonic.
ADP + HOPO32- + H + ¡ ATP + H2O
∆G = + 30.5 kJ>mol
(In equations for biochemical reactions, we represent ATP and other energycarrying molecules in red and their lower-energy equivalent molecules in blue.)
ATP is an energy transporter because its production from ADP requires an input
of energy that is released when the reverse reaction occurs. Biochemical energy is
gathered from exergonic reactions and stored in the bonds of the ATP molecule. ATP
hydrolysis releases energy for energy-requiring work. Biochemical energy production,
transport, and use, all depend upon the ATP ÷ ADP interconversion.
P P P
The hydrolysis of ATP to give ADP and its reverse, the phosphorylation of ADP,
are reactions perfectly suited to their role in metabolism for two major reasons. Firstly,
ATP hydrolysis occurs slowly in the absence of a catalyst, so the stored energy is released only in the presence of the appropriate enzymes.
The Generation of Biochemical Energy
Secondly, the free energy of hydrolysis of ATP is an intermediate value for energy
carriers (Table 21.1). Since the primary metabolic function of ATP is to transport energy, it is often referred to as a “high-energy” molecule or as containing “high-energy”
phosphorus–oxygen bonds. These terms are misleading because they promote the idea
that ATP is somehow different from other compounds. The terms mean only that ATP
is reactive and that a useful amount of energy is released when a phosphoryl group is
removed from it by hydrolysis.
table 21.1 Free Energies of Hydrolysis of Some Phosphates
Final intermediate in conversion of
glucose to pyruvate (glycolysis)—
Stage 2, Figure 21.5
Another intermediate in glycolysis
Energy storage in muscle cells
Principal energy carrier
First intermediate in breakdown of
carbohydrates stored as starch or
First intermediate in glycolysis
Second intermediate in glycolysis
ATP 1 ¡ ADP2
In fact, if removal of a phosphoryl group from ATP released unusually large
amounts of energy, other reactions would not be able to provide enough energy to convert ADP back to ATP. ATP is a convenient energy carrier in metabolism because its
free energy of hydrolysis has an intermediate value among high energy carriers. For
this reason, the phosphorylation of ADP can be driven by coupling this reaction with a
more exergonic reaction.
Acetyl phosphate, whose structure is given here, is another compound with a relatively
high free energy of hydrolysis.
Using structural formulas, write the equation for the hydrolysis of this phosphate.
A common metabolic strategy is the lack of reactivity—that is, the slowness to react—
of compounds whose breakdown is exergonic. For example, hydrolysis of ATP to ADP
or adenosine monophosphate (AMP) is exergonic but does not take place without an
appropriate enzyme present. Why would the cell use this metabolic strategy?