1. Energy Flow in the Life of a Cell
Chapter 6
Energy Flow in the Life of a Cell

2. Chapter 6 At a Glance
6.1 What Is Energy?
6.2 How Does Energy Flow in Chemical Reactions?
6.3 How Is Energy Transported Within Cells?
6.4 How Do Enzymes Promote Biochemical Reactions?
6.5 How Do Cells Regulate Their Metabolic Reactions?

3. 6.1 What Is Energy?
Energy is the capacity to do work
Work is a force acting on an object that causes the object to move

4. 6.1 What Is Energy?
Chemical energy is the energy that is contained in molecules and released by chemical reactions
Molecules that provide chemical energy include sugar, glycogen, and fat
Cells use specialized molecules such as ATP to accept and transfer energy from one chemical reaction to the next

5. There are two fundamental types of energy
6.1 What Is Energy?
There are two fundamental types of energy
Potential energy is stored energy
For example, the chemical energy in bonds, the electrical charge in a battery, and a penguin poised to plunge
Kinetic energy is the energy of movement
For example, light, heat, electricity, and the movement of objects

6. Author Animation: Types of Energy

7. From Potential to Kinetic Energy
Fig. 6-1

8. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy
The laws describe the quantity (the total amount) and the quality (the usefulness) of energy
Energy can neither be created nor destroyed (the first law of thermodynamics), but can change form
The first law is often called the law of conservation of energy
The total amount of energy within a closed system remains constant unless energy is added or removed from the system

9. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy (continued)
The amount of useful energy decreases when energy is converted from one form to another (the second law of thermodynamics)
Entropy (disorder) is the tendency to move toward a loss of complexity and of useful energy and toward an increase in randomness, disorder, and less-useful energy

10. 6.1 What Is Energy?
The laws of thermodynamics describe the basic properties of energy (continued)
Useful energy tends to be stored in highly organized matter, and when energy is used in a closed system (such as the world in which we live), there is an overall increase in entropy
For example, when gasoline is burned, the orderly arrangement of eight carbons bound together in a gasoline molecule are converted to eight randomly moving molecules of carbon dioxide

11. Energy Conversions Result in a Loss of Useful Energy
Combustion
by engine
100 units chemical energy
(concentrated)
75 units heat
energy 
25 units kinetic energy
(motion)
Fig. 6-2

12. 6.1 What Is Energy?
Living things use the energy of sunlight to create the low-entropy conditions of life
The highly organized low-entropy systems of life do not violate the second law of thermodynamics because they are achieved through a continuous influx of usable light energy from the sun
In creating kinetic energy in the form of sunlight, the sun also produces vast entropy as heat

13. 6.2 How Does Energy Flow in Chemical Reactions?
A chemical reaction is a process that forms or breaks the chemical bonds that hold atoms together
Chemical reactions convert one set of chemical substances, the reactants, into another set, the products
All chemical reactions require a net input of energy
Exergonic reactions release energy
Endergonic reactions require an input of energy

14. Author Animation: Exergonic and Endergonic Reactions

15. An Exergonic Reaction
energy +
reactants +
products
Fig. 6-3

16. An Endergonic Reaction
energy +
products +
reactants
Fig. 6-4

17. 6.2 How Does Energy Flow in Chemical Reactions?
Exergonic reactions release energy
Reactants contain more energy than products in exergonic reactions
An example of an exergonic reaction is the burning of glucose
As glucose is burned, the sugar (C6H12O6) combines with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), releasing energy
Because molecules of sugar contain more energy than do molecules of carbon dioxide and water, the reaction releases energy

18. Reactants and End Products of Burning Glucose
energy
C6H12O6
(glucose) 
6 O2
(oxygen)
6 CO2
(carbon
dioxide) 
6 H2O
(water)
Fig. 6-5

19. Activation Energy in Exergonic Reactions
Activation energy needed
to ignite glucose
high
energy level of reactants
glucose + O2
energy
content of
molecules
CO2 + H2O
low
Fig. 6-6
progress of reaction

20. 6.2 How Does Energy Flow in Chemical Reactions?
Exergonic reactions release energy (continued)
All chemical reactions require an initial energy input (activation energy) to get started
The negatively charged electron shells of atoms repel one another and inhibit bond formation
Molecules need to be moving with sufficient collision speed to overcome electronic repulsion and react
Increasing the temperature will increase kinetic energy and, thus, the rate of reaction

21. 6.2 How Does Energy Flow in Chemical Reactions?
Endergonic reactions require a net input of energy
The reactants in endergonic reactions contain less energy than the products
An example of an endergonic reaction is photosynthesis
In photosynthesis, green plants add the energy of sunlight to the lower-energy reactants water and carbon dioxide to produce the higher-energy product sugar

22. Photosynthesis energy C6H12O6 (glucose)  6 O2 (oxygen) 6 CO2 (carbon
dioxide) 
6 H2O
(water)
Fig. 6-7

23. 6.3 How Is Energy Transported Within Cells?
Most organisms are powered by the breakdown of glucose
Energy in glucose cannot be used directly to fuel endergonic reactions
Energy released by glucose breakdown is first transferred to an energy-carrier molecule

24. 6.3 How Is Energy Transported Within Cells?
Energy-carrier molecules are high-energy, unstable molecules that are synthesized at the site of an exergonic reaction, capturing some of the released energy
These high-energy molecules then transfer the energy to an endergonic reaction elsewhere in the cell

25. 6.3 How Is Energy Transported Within Cells?
ATP is the principal energy carrier in cells
Adenosine triphosphate (ATP) is the most common energy-carrying molecule
ATP is composed of the nitrogen-containing base adenine, the sugar ribose, and three phosphates

26. The Interconversion of ADP and ATP
energy P P P P P  P
ATP
phosphate
ADP
(a) ATP synthesis: Energy is stored in ATP
energy P P P
ATP P P  P
phosphate
ADP
Fig. 6-8
(b) ATP breakdown: Energy is released

27. 6.3 How Is Energy Transported Within Cells?
ATP is the principal energy carrier in cells (continued)
Energy is released in cells during glucose breakdown and is used to combine the relatively low-energy molecules adenosine diphosphate (ADP) and phosphate (P) into ATP
Energy is stored in the high-energy phosphate bonds of ATP
The formation of ATP is an endergonic reaction

28. 6.3 How Is Energy Transported Within Cells?
ATP is the principal energy carrier in cells (continued)
At sites in the cell where energy is needed, ATP is broken down into ADP + P and its stored energy is released
This energy is then transferred to endergonic reactions through coupling
Unlike glycogen and fat, ATP stores energy very briefly before being broken down

29. 6.3 How Is Energy Transported Within Cells?
Electron carriers also transport energy within cells
ATP is not the only energy-carrier molecule in cells
Energy can be transferred to electrons in glucose metabolism and photosynthesis
Electron carriers like nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) transport high-energy electrons
Electron carriers donate their high-energy electrons to other molecules, often leading to ATP synthesis

30. 6.3 How Is Energy Transported Within Cells?
Coupled reactions link exergonic with endergonic reactions
In a coupled reaction, an exergonic reaction provides the energy needed to drive an endergonic reaction
Sunlight energy stored in glucose by plants is transferred to other organisms by the exergonic breakdown of the sugar and its use in endergonic processes such as protein synthesis
The two reactions may occur in different parts of the cell, so energy-carrier molecules carry the energy from one to the other

31. Author Animation: Coupled Reactions

32. Coupled Reactions Within Living Cells
high-energy
reactants
(glucose)
high-energy
products
(protein)
ATP
exergonic
(glucose breakdown)
endergonic
(protein synthesis)
low-energy
products
(CO2, H2O)
ADP  P
low-energy
reactants
(amino acids)
Fig. 6-9

33. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzyme structures allow biochemical reactions to catalyze specific reactions
Enzymes, like all catalysts, lower activation energy
Enzymes control the rate of energy release and capture some energy in ATP

34. 6.4 How Do Enzymes Promote Biochemical Reactions?
At body temperatures, spontaneous reactions proceed too slowly to sustain life
Reaction speed is generally determined by the activation energy required; that is, how much energy is required to start the process
Reactions with low activation energies proceed rapidly at body temperature
Reactions with high activation energies (e.g., sugar breakdown) move very slowly at body temperature, even if exergonic overall

35. 6.4 How Do Enzymes Promote Biochemical Reactions?
At body temperatures, spontaneous reactions proceed too slowly to sustain life (continued)
Enzymes are employed to catalyze (speed up) chemical reactions in cells by lowering the activation energy needed to start the reaction
Enzymes are biological catalysts and regulate all the reactions in living cells

36. 6.4 How Do Enzymes Promote Biochemical Reactions?
Catalysts reduce activation energy
Catalysts speed up the rate of a chemical reaction without themselves being used up
All catalysts have three important properties:
They speed up reactions by lowering the activation energy required for the reaction to begin
They speed up only exergonic reactions
They are not consumed or changed by the reactions they promote

37. 6.4 How Do Enzymes Promote Biochemical Reactions?
Catalysts reduce activation energy (continued)
Catalytic converters in cars facilitate the conversion of carbon monoxide (CO) to carbon dioxide (CO2)
2 CO + O2  2 CO2 + heat energy

38. Author Animation: Activation Energy

39. Catalysts Such As Enzymes Lower Activation Energy
high
Activation energy
without catalyst
Activation energy
with catalyst
energy
content of
molecules
reactants
products
low
Fig. 6-10
progress of reaction

40. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzymes are biological catalysts
Enzymes are composed primarily of protein synthesized by living organisms and may require small nonprotein helper molecules called coenzymes in order to function
Many water-soluble vitamins (certain B vitamins) are essential to humans because they are used by the body to synthesize coenzymes
Enzymes orient, distort, and reconfigure molecules in the process of lowering activation energy

41. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzymes are biological catalysts (continued)
Enzymes (proteins) have two attributes that set them apart from nonbiological catalysts:
Enzymes are very specific for the reactions they catalyze
Enzyme activity is regulated

42. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzyme structures allow them to catalyze specific reactions
Each enzyme has a pocket called an active site into which one or more reactant molecules, called substrates, can enter
The amino acid sequence of the enzyme protein and the way the protein chains are folded create in the active site a distinctive shape and distribution of electrical charge
The distinctive shape of the active site is both complementary and specific to the substrate
Active site amino acids bind to the substrate and distort bonds to facilitate a reaction

43. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzyme structures allow them to catalyze specific reactions (continued)
There are three steps of enzyme catalysis
Both the shape and the charge of the active site allow substrates to enter the enzyme only in specific orientations
Upon binding, the substrates and active site change shape to promote a reaction
When the reaction between the substrates is finished, the product(s) no longer properly fit into the active site and drift away

44. Author Animation: Enzymes and Substrates

45. The Cycle of Enzyme-Substrate Interactions
substrates
active site
of enzyme
enzyme 1
Substrates enter
the active site in a
specific orientation 3
The substrates, bonded
together, leave the enzyme;
the enzyme is ready for a
new set of substrates
The substrates and
active site change shape,
promoting a reaction
between the substrates 2
Fig. 6-11

46. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzymes, like all catalysts, lower activation energy
The breakdown or synthesis of a molecule within a cell usually occurs in many small steps, each catalyzed by a different enzyme
Each of the enzymes lowers the activation energy for its particular reaction, allowing the reaction to occur readily at body temperature

47. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzymes control the rate of energy release and capture some energy in ATP
Thanks to a series of chemical transformations, each catalyzed by a different enzyme, the energy stored in sugar is released gradually during its breakdown
Some energy is lost as heat, while some is harnessed to power endergonic reactions that lead to ATP synthesis

48. 6.5 How Do Cells Regulate Their Metabolic Reactions?
The sum of all the chemical reactions inside a cell is its metabolism
Many cellular reactions are linked through metabolic pathways
In metabolic pathways, an initial reactant molecule is modified by an enzyme, creating a slightly different intermediate molecule, which is modified by another enzyme, and so on, until a final product is produced

49. Simplified Metabolic Pathways
Initial reactant
Intermediates
Final products
PATHWAY 1 A B C D E
enzyme 1
enzyme 2
enzyme 3
enzyme 4
PATHWAY 2 F G
enzyme 5
enzyme 6
Fig. 6-12

50. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Reaction rates tend to increase as substrate or enzyme levels increase
For a given amount of enzyme, as substrate levels increase, the reaction rate will increase until the active sites of all the enzyme molecules are being continuously occupied by new substrate molecules

51. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Reaction rates tend to increase as substrate or enzyme levels increase (continued)
Metabolic pathways are controlled in several ways
Control of enzyme synthesis, which regulates availability
Control of enzyme activity

52. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Cells regulate enzyme synthesis
Genes that code for specific proteins are turned on and off according to metabolic need
An increase in substrate can trigger increased enzyme production, leading to decreased substrate levels

53. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Cells regulate enzyme activity
Some enzymes are synthesized in inactive form
For example, the protein-digesting enzymes pepsin and trypsin are inactive when synthesized, but become activated in the stomach under acidic conditions (pepsin) or in the small intestine under alkaline conditions (trypsin)

54. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Cells regulate enzyme activity (continued)
Some enzymes are inhibited
In competitive inhibition, a substance that is not the enzyme’s normal substrate binds to the active site of the enzyme, competing with the substrate for the active site
In noncompetitive inhibition, a molecule binds to a site on the enzyme distinct from the active site

55. Competitive and Noncompetitive Enzyme Inhibition
substrate
active site
enzyme
noncompetitive
inhibitor site
(a) A substrate binding to an enzyme
Fig. 6-13a

56. Competitive and Noncompetitive Enzyme Inhibition
A competitive inhibitor
molecule occupies the
active site and blocks
entry of the substrate
(b) Competitive inhibition
Fig. 6-13b

57. Competitive and Noncompetitive Enzyme Inhibition
A noncompetitive
inhibitor molecule
causes the active site
to change shape, so the
substrate no longer fits
noncompetitive
inhibitor molecule
(c) Noncompetitive inhibition
Fig. 6-13c

58. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Cells regulate enzyme activity (continued)
Small regulator molecules can bind to enzymes and enhance or inhibit activity by allosteric regulation
Enzymes that undergo allosteric regulation have a special regulatory binding site on the enzyme that is distinct from the enzyme’s active site and similar to a noncompetitive inhibitor site
Allosteric regulation can either increase or decrease enzyme activity, whereas noncompetitive inhibition only reduces activity
allosteric regulation 變構調節：變構調節是指某些調節物能與酶的調節部位結合使酶分子的構象發生改變，從而改變酶的活性，稱酶的變構調節

59. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Cells regulate enzyme activity (continued)
Feedback inhibition is a negative feedback type of allosteric inhibition that causes a metabolic pathway to stop producing its product when quantities reach an optimum level
An enzyme near the beginning of a metabolic pathway is inhibited allosterically by the end product of the pathway

60. Allosteric Regulation of an Enzyme by Feedback Inhibition
intermediates A B C D
enzyme 1
enzyme 2
enzyme 3
enzyme 4
enzyme 5
As levels of isoleucine rise,
it binds to the regulatory site
on enzyme 1, inhibiting it
threonine
(initial reactant)
enzyme 1
isoleucine
(end product)
isoleucine
Fig. 6-14

61. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Poisons, drugs, and environmental conditions influence enzyme activity
Drugs and poisons often inhibit enzymes by competing with the natural substrate for the active site
This process occurs either by competitive or by noncompetitive inhibition

62. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Poisons, drugs, and environmental conditions influence enzyme activity (continued)
Some inhibitors bind permanently to the enzyme
Some nerve gases and insecticides permanently block the active site of acetylcholinesterase
Arsenic, mercury, and lead bind permanently to the non-active sites of various enzymes, inactivating them

63. 6.5 How Do Cells Regulate Their Metabolic Reactions?
Poisons, drugs, and environmental conditions influence enzyme activity (continued)
The activity of an enzyme is influenced by the environment
The three-dimensional structure of an enzyme is sensitive to pH, salts, temperature, and the presence of coenzymes

64. 6.5 How Do Cells Regulate Their Metabolic Reactions?
The activity of an enzyme is influenced by the environment (continued)
Enzyme structure is distorted (denatured) and function is destroyed when pH is too high or low
Salts in an enzyme’s environment can also destroy function by altering structure
Salt ions can bind with key amino acids in enzymes, influencing three-dimensional structure and destroying function

65. 6.5 How Do Cells Regulate Their Metabolic Reactions?
The activity of an enzyme is influenced by the environment (continued)
Temperature also affects enzyme activity
Low temperatures slow down molecular movement
High temperatures cause enzyme shape to be altered, destroying function
Most enzymes function optimally only within a very narrow range of these conditions

66. Human Enzymes Function Best Within Narrow Ranges of pH and Temperature
For pepsin, maximum
activity occurs at about
pH 2
For trypsin, maximum
activity occurs at about
pH 8
fast
For most cellular
enzymes, maximum
activity occurs
at about pH 7.4
rate of
reaction
slow
Fig. 6-15a
(a) Effect of pH on enzyme activity

67. Human Enzymes Function Best Within Narrow Ranges of pH and Temperature
fast
For most human enzymes,
maximum activity occurs
at about 98.6°F (37°C)
rate of
reaction
slow
(b) Effect of temperature on enzyme activity
Fig. 6-15b