The potential chemical energy released by cellular respiration is converted to kinetic energy in the muscles, which do the work of drawing the bow. The potential energy stored in the drawn bow is transformed into kinetic energy as the bowstring pushes the arrow toward its target.
Chapter 7:
Energy and Metabolism
© Cengage Learning 2015
SOLOMON • MARTIN • MARTIN • BERG
BIOLOGY
tenth edition
1
Energy Conversion
Cells obtain energy in many forms, and have mechanisms that convert energy from one form to another
Radiant energy is the ultimate source of energy for life
Photosynthetic organisms capture about 0.02% of the sun’s energy that reaches Earth, and convert it to chemical energy in bonds of organic molecules
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7.1 Biological Work
Matter: anything that has mass and takes up space
Energy: the capacity to do work (change in state or motion of matter)
Expressed in units of work (kilojoules, kJ) or units of heat energy (kilocalories, kcal)
1 kcal = 4.184 kJ
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Potential Energy and Kinetic Energy
Potential energy: capacity to do work as a result of position or state
Kinetic energy: energy of motion is used, work is performed
POTENTIAL
Energy of position
KINETIC
Energy of motion
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Figure 7-1 Potential versus kinetic energy
The potential chemical energy released by cellular respiration is converted to kinetic energy in the muscles, which do the work of drawing the bow. The potential energy stored in the drawn bow is transformed into kinetic energy as the bowstring pushes the arrow toward its target.
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Organisms Carry Out Conversions Between Potential/Kinetic Energy
Most actions involve a series of energy transformations that occur as kinetic energy is converted to potential energy – or potential energy to kinetic energy
Chemical energy: potential energy stored in chemical bonds
Example: Chemical energy of food molecules is converted to mechanical energy in muscle cells
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7.2 The Laws of Thermodynamics
Thermodynamics governs all activities of the universe, from cells to stars
Biological systems are open systems that exchange energy with their surroundings
Closed
system
Closed
system
Surroundings
Surroundings
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Figure 7-2 Closed and open systems
A closed system does not exchange energy with its surroundings.
(b) An open system exchanges energy with its surroundings.
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The First Law of Thermodynamics
Energy cannot be created or destroyed
Energy can be transferred or converted from one form to another, including conversions between matter and energy
The energy of any system plus its surroundings is constant
Organisms must capture energy from the environment and transform it to a form that can be used for biological work
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The Second Law of Thermodynamics
When energy is converted from one form to another, some usable energy (energy available to do work) is converted into heat that disperses into the surroundings
As a result, the amount of usable energy available to do work in the universe decreases over time
Heat: the kinetic energy of randomly moving particles
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Entropy
The measure of the disorder or randomness of energy
Organized, usable energy has a low entropy
Disorganized energy, such as heat, has a high entropy
No energy conversion is ever 100% efficient
The total entropy of the universe always increases over time
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7.3 Energy And Metabolism
Metabolism: all chemical reactions taking place in an organism
Includes many intersecting chemical reactions
Two main types:
Anabolism: pathways in which complex molecules are synthesized from simpler substances
Catabolism: pathways in which larger molecules are broken down into smaller ones
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Enthalpy is the Total Potential Energy of a System
Every specific type of chemical bond has a certain amount of bond energy: the energy required to break that bond
Enthalpy is equivalent to the total bond energy
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Free Energy is Available to do Cell Work
Free energy: the amount of energy available to do work under the conditions of a biochemical reaction
Enthalpy (H), free energy (G), entropy (S); and absolute temperature (T) are related:
H = G + TS
As entropy increases, the amount of free energy decreases
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Changes in Free Energy
Although the total free energy of a system (G) can’t be measured, changes in free energy can be measured
The rearranged equation can be used to predict whether a particular chemical reaction will release energy or require an input of energy:
Δ G = Δ H − T Δ S
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Free Energy Decreases During an Exergonic Reaction
Exergonic reaction: releases energy and is a “downhill” reaction, from higher to lower free energy
ΔG is a negative number for exergonic reactions
A certain amount of activation energy is required to initiate every reaction, even a spontaneous one
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Free Energy Increases During an Endergonic Reaction
Endergonic reaction: a reaction in which there is a gain of free energy
ΔG has a positive value: the free energy of the products is greater than the free energy of the reactants
Requires an input of energy from the environment
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Figure 7-3 Exergonic and endergonic reactions
In an exergonic reaction, there is a net loss of free energy. The products have less free energy than was present in the reactants, and the reaction proceeds spontaneously.
(b) In an endergonic reaction, there is a net gain of free energy. The products have more free energy than was present in the reactants.
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Diffusion is an Exergonic Process
Randomly moving particles diffuse down their own concentration gradient
Free energy decreases as entropy increases
Concentration gradient: an orderly state with a region of higher concentration and another region of lower concentration
A cell must expend energy to produce a concentration gradient
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Free-Energy Changes and the Concentrations of Reactants/Products
Free-energy changes in a chemical reaction depend on the difference in bond energies between reactants and products
Also depends on concentrations of both reactants and products
A reaction that proceeds forward and in reverse at the same time eventually reaches dynamic equilibrium
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Changes in Free Energy (cont’d.)
If the reactants have much greater free energy than the products, most of the reactants are converted to products and vice-versa
If the concentration of reactants is increased, the reaction will “shift to the right” and vice-versa
The reaction always shifts to reestablish equilibrium
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Cells Drive Endergonic Reactions by Coupling Them
Endergonic reactions are coupled to exergonic reactions
Coupled reactions: thermodynamically favorable exergonic reaction provides energy required to drive a thermodynamically unfavorable endergonic reaction
In a living cell the exergonic reaction often involves the breakdown of ATP
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Coupled Reactions (cont’d.)
Two reactions taken together are exergonic:
(1) A → B ΔG = +20.9 kJ/mol (+5 kcal/mol)
(2) C → D ΔG = −33.5 kJ/mol (−8 kcal/mol)
Overall ΔG = −12.6 kJ/mol (−3 kcal/mol)
Reactions are coupled if pathways are altered for a common intermediate link:
(3) A + C → I ΔG = −8.4 kJ/mol (−2 kcal/mol)
(4) I → B + D ΔG = −4.2 kJ/mol (−1 kcal/mol)
Overall ΔG = −12.6 kJ/mol (−3 kcal/mol)
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7.4 ATP, Energy Currency of the Cell
Adenosine triphosphate (ATP): Nucleotide consisting of adenine, ribose, and three phosphate groups
The cell uses energy that is temporarily stored in ATP
Hydrolysis of ATP yields ADP and inorganic phosphate
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ATP Donates Energy
Hydrolysis of ATP can be coupled to endergonic reactions in cells, such as the formation of sucrose
ATP + H2O → ADP + Pi
ΔG = −32 kJ/mol (or −7.6 kcal/mol)
glucose + fructose → sucrose + H2O
ΔG = +27 kJ/mol (or +6.5 kcal/mol)
glucose + fructose + ATP → sucrose + ADP + Pi
ΔG = −5 kJ/mol (−1.2 kcal/mol)
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ATP Donates Energy (cont’d.)
The intermediate reaction in the formation of sucrose is a phosphorylation reaction: phosphate group is transferred to glucose to form glucose-P
glucose + ATP → glucose-P + ADP
glucose-P + fructose → sucrose + Pi
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ATP Links Exergonic and Endergonic Reactions
Exergonic reactions
release energy
Energy released drives
endergonic reactions
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Figure 7-6 ATP links exergonic and endergonic reactions
Exergonic reactions in catabolic pathways (top) supply energy to drive the endergonic formation of ATP from ADP. Conversely, the exergonic hydrolysis of ATP supplies energy to endergonic reactions in anabolic pathways (bottom).
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The Cell Maintains a Very High Ratio of ATP to ADP
A typical cell contains more than 10 ATP molecules for every ADP molecule
High levels of ATP makes its hydrolysis reaction more strongly exergonic, and more able to drive coupled endergonic reactions
The cell cannot store large quantities of ATP
ATP is constantly used and replaced
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7.5 Energy Transfer in Redox Reactions
Energy is transferred through the transfer of electrons from one substance to another
Oxidation: substance loses electrons
Reduction: substance gains electrons
Redox reactions often occur in a series of electron transfers
For cellular respiration, photosynthesis, and many other chemical processes
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Electron Carriers Transfer Hydrogen Atoms
Redox reactions in cells usually involve the transfer of a hydrogen atom
An electron, along with its energy, is transferred to an acceptor molecule such as nicotinamide adenine dinucleotide (NAD+), which is reduced to NADH
XH2 + NAD+ → X + NADH + H+
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NAD+ (oxidized)
NADH (reduced)
Nicotinamide
Ribose
Adenine
Ribose
Phosphate
Phosphate
Figure 7-7 NAD+ and NADH
NAD+ consists of two nucleotides, one with adenine and one with nicotinamide, that are joined at their phosphate groups. The oxidized form of the nicotinamide ring in NAD+ (left) becomes the reduced form in NADH (right) by the transfer of 2 electrons and 1 proton from another organic compound (XH2), which becomes oxidized (to X) in the process.
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Electron Carriers (cont’d.)
An electron progressively loses free energy as it is transferred from one acceptor to another
In cellular respiration, NADH transfers electrons to another molecule
Energy is then transferred through a series of reactions that result in formation of ATP
NADP+ is not involved in ATP synthesis
Electrons of NADPH are used to provide energy for photosynthesis
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Other Important Electron Carriers
Flavin adenine dinucleotide (FAD): nucleotide that accepts hydrogen atoms and their electrons
Reduced form is FADH2
Cytochromes: proteins that contain iron
The iron component accepts electrons from hydrogen atoms, then transfers the electrons to some other compound
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Cells regulate rates of chemical reactions with enzymes, which increase speed of a chemical reaction without being consumed by the reaction
Example: Catalase has the highest known catalytic rate; it protects cells by destroying hydrogen peroxide (H2O2)
Most enzymes are proteins, but some types of RNA molecules also have catalytic activity
7.6 Enzymes
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All Reactions Have a Required Energy of Activation
Even a strongly exergonic reaction may be prevented from proceeding by the activation energy required to begin the reaction
Energy of activation (EA) or activation energy: the energy required to break existing bonds and begin a reaction
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Figure 7-10 Activation energy and enzymes
An enzyme speeds up a reaction by lowering its activation energy (EA). In the presence of an enzyme, reacting molecules require less kinetic energy to complete
a reaction.
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An Enzyme Works By Forming an Enzyme–Substrate Complex
An enzyme controls the reaction by forming an unstable intermediate complex with a substrate
When the ES complex breaks up, the product is released
Enzyme molecule is free to form a new ES complex:
enzyme + substrate(s) → ES complex
ES complex → enzyme + product(s)
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Active Sites
Enzymes bind to active sites to position substrates close together to speed up the reaction
Induced fit: binding of substrate to enzyme causes a change in shape to enzyme
Distorts the chemical bonds of the substrate
Proximity and orientation of reactants, plus strains in their chemical bonds, facilitate the breakage/formation of products
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Enzymes are Specific
Due to shape of active site and its relationship to the shape of the substrate
Some are specific only to a certain chemical bond
Example: lipase splits ester linkages in many fats
Scientists classify enzymes into six classes that catalyze similar reactions
Each class is divided into many subclasses
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TABLE 7-1 Important Classes of Enzymes
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Many Enzymes Require Cofactors
Some enzymes have two components: an apoenzyme and a cofactor
Neither alone has catalytic activity, enzyme functions only when the two combined
Cofactors may be a specific metal ion
Iron, copper, zinc, and manganese all function as cofactors
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Coenzymes
Organic, nonpolypeptide compound that binds to the apoenzyme and serves as a cofactor
Most are carrier molecules:
NADH, NADPH, and FADH2 transfer electrons
ATP transfers phosphate groups
Coenzyme A transfers groups derived from organic acids
Most vitamins are coenzymes or components of coenzymes
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Each Enzyme Has an Optimal Temperature
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Figure 7-12 The effects of temperature on enzyme activity
(a) Generalized curves for the effect of temperature on enzyme activity. As temperature increases, enzyme activity increases until it reaches an optimal temperature.
Enzyme activity abruptly falls after it exceeds the optimal temperature because the enzyme, being a protein, denatures.
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Heat-Tolerant Archaea
Certain archaea have enzymes that allow them to survive in extreme habitats
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Figure 7-13 Grand Prismatic Spring in Yellowstone National Park
The world’s third-largest spring, about 61 m (200 ft) in diameter, the Grand Prismatic Spring teems with heat-tolerant archaea. The rings around the perimeter, where the water is slightly cooler, get their distinctive colors from the various kinds of archaea living there.
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Each Enzyme has an Optimal pH
Optimal pH for most human enzymes is 6 to 8
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Figure 7-12b The effects of pH on enzyme activity
(b) Enzyme activity is very sensitive to pH. Pepsin is a protein-digesting enzyme in the very acidic stomach juice. Trypsin, secreted by the pancreas into the slightly
basic small intestine, digests polypeptides.
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Enzymes in Metabolic Pathways
Metabolic pathway: the product of one enzyme-controlled reaction serves as substrate for the next in series of reactions
Removal of intermediate and final products drives the sequence of reactions in a particular direction
Enzymes can bind to one another to form a multienzyme complex that transfers intermediates in the pathway from one active site to another
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The Cell Regulates Enzymatic Activity
Gene control: a specific gene directs synthesis of each type of enzyme
Gene may be switched on by a signal from a hormone or other signal molecule
Amounts of enzymes influence overall cell reaction rate
Rate of a reaction can be limited by enzyme concentration or by substrate concentration
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Figure 7-14 The effects of enzyme concentration and substrate concentration on the rate of a reaction.
In this example the rate of reaction is measured at different enzyme concentrations, with an excess of substrate present. (Temperature and pH are constant.) The rate of the reaction is directly proportional to the enzyme concentration.
(b) In this example the rate of the reaction is measured at different substrate concentrations, and enzyme concentration, temperature, and pH are constant. If the
substrate concentration is relatively low, the reaction rate is directly proportional to substrate concentration. However, higher substrate concentrations do not increase the reaction rate because the enzymes become saturated with substrate.
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The Cell Regulates Enzymatic Activity (cont’d.)
The product of one enzymatic reaction may control activity of another enzyme in a sequence of enzymatic reactions
When concentration of a product is low, the sequence of reactions proceeds rapidly
When concentration of a product is high, reactions stop
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The Cell Regulates Enzymatic Activity (cont’d.)
Feedback inhibition
Enzyme regulation in which the formation of a product inhibits an earlier reaction in the sequence
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The Cell Regulates Enzymatic Activity (cont’d.)
Some enzymes have an allosteric site that modifies the enzyme’s activity when an allosteric regulator is bound to it
Allosteric inhibitors keep the enzyme in its inactive shape
Allosteric activators result in a functional active site
Example: cAMP-dependent protein kinase
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Cyclic AMP
Active
site
Allosteric
site
Regulator
(inhibitor)
Substrates
Substrates
Figure 7-16 An allosteric enzyme
(a) Inactive form of the enzyme. The enzyme protein kinase is inhibited by a regulatory protein that binds reversibly to its allosteric site. When the enzyme is in this
inactive form, the shape of the active site is modifed so that the substrate cannot combine with it.
(b) Active form of the enzyme. Cyclic AMP removes the allosteric inhibitor and activates the enzyme.
(c) Enzyme–substrate complex. The substrate can then combine with the active site.
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Enzymes Are Inhibited by Certain Chemical Agents
Substrates
Active site
Active site not suitable
for reception of substrates
Enzyme
Inhibitor
Enzyme
Substrate
Substrate
Inhibitor
Inhibitor binds to
active site
a
b
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Figure 7-17 Competitive and noncompetitive inhibition (Reversible inhibition)
Competitive inhibition. The inhibitor competes with the normal substrate for the active site of the enzyme. A competitive inhibitor occupies the active site only temporarily.
(b) Noncompetitive inhibition. The inhibitor binds with the enzyme at a site other than the active site, altering the shape of the enzyme and thereby inactivating it.
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Enzyme Inhibition (cont’d.)
Irreversible inhibition: inhibitor permanently inactivates or destroys an enzyme when the inhibitor combines with one of the enzyme’s functional groups, either at the active site or elsewhere
Many poisons are irreversible enzyme inhibitors, such as mercury and lead, nerve gases, cyanide
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Some Drugs are Enzyme Inhibitors
Some drugs used to treat bacterial infections directly or indirectly inhibit bacterial enzyme activity
Example: sulfa drugs compete with PABA for the active site of the bacterial enzyme
Example: penicillin and related antibiotics irreversibly inhibit the bacterial enzyme transpeptidase
Drug resistance is a growing problem