1. Energy Flow in the Life of a Cell6
Energy Flow in the Life of a Cell 1

2. Chapter 6 At a Glance 6.1 What Is Energy?
6.2 How Is Energy Transformed During Chemical Reactions?
6.3 How Is Energy Transported Within Cells?
6.4 How Do Enzymes Promote Biochemical Reactions?
6.5 How Are Enzymes Regulated?

3. 6.1 What Is Energy? Energy is the capacity to do work
Work is a transfer of energy to an object, which 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. 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. Figure 6-1 Converting potential energy to kinetic energy

7. 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

8. 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

9. 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

10. Figure 6-2 Energy conversions result in a loss of useful energy
Combustion
by engine
gas
100 units chemical
energy
80 units
heat energy 
20 units kinetic energy 10

11. 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

12. 6.2 How Is Energy Transformed During 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 either release energy or require a net input of energy
Exergonic reactions release energy
Endergonic reactions require an input of energy

13. Figure 6-3 An exergonic reaction
energy
reactants
products 13

14. Figure 6-4 An endergonic reaction
energy
products
reactants 14

15. 6.2 How Is Energy Transformed During 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

16. Figure 6-5 Reactants and products of burning glucose
energy
C6H12O6 
6 O2
(glucose)
(oxygen)
6 CO2 
6 H2O
(carbon
dioxide)
(water) 16

17. 6.2 How Is Energy Transformed During 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

18. Figure 6-6 Photosynthesis
energy
C6H12O6 
6 O2
(glucose)
(oxygen)
6 CO2 
6 H2O
(carbon
dioxide)
(water) 18

19. 6.2 How Is Energy Transformed During Chemical Reactions?
Endergonic reactions require a net input of 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

20. Figure 6-7 Activation energy in an exergonic reaction
Activation energy required
to start the reaction
high
energy level of reactants
reactants
energy
content of
molecules
energy level of products
products
low
progress of reaction
An exergonic reaction
Sparks ignite gas 20

21. 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
Energy-carrier molecules are high-energy, unstable molecules that are synthesized at the site of an exergonic reaction, capturing some of the released energy

22. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within 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
ATP is sometimes called the “energy currency” of cells

23. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within 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

24. Figure 6-8 The interconversion of ADP and ATP
energy
ATP
phosphate
ADP
ATP synthesis: Energy is stored in ATP
energy
ATP
phosphate
ADP
ATP breakdown: Energy is released 24

25. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within 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

26. 6.3 How Is Energy Transported Within Cells?
ATP and electron carriers transport energy within cells (continued)
ATP is not the only energy-carrier molecule in cells
Energy can be transferred to electrons in glucose metabolism and photosynthesis
Electron carriers such as 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

27. 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

28. Animation: Coupled Reactions

29. Figure 6-9 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  Pi
low-energy
reactants
(amino acids) 29

30. 6.4 How Do Enzymes Promote Biochemical Reactions?
Catalysts reduce the energy required to start a reaction
Enzymes, like all catalysts, lower activation energy
Enzymes are biological catalysts

31. 6.4 How Do Enzymes Promote Biochemical Reactions?
Catalysts reduce energy required to start a reaction (continued)
Catalysts are molecules that speed up the rate of a chemical reaction without themselves being used up or permanently altered
Catalytic converters in cars facilitate the conversion of carbon monoxide (CO) to carbon dioxide (CO2)
2 CO  O2  2 CO2  heat energy

32. 6.4 How Do Enzymes Promote Biochemical Reactions?
Catalysts reduce energy required to start a reaction (continued)
All catalysts have three important properties
1. They speed up reactions by lowering the activation energy required for the reaction to begin
2. They speed up only exergonic reactions
3. They are not consumed or changed by the reactions they promote

33. Figure 6-10 Catalysts such as enzymes lower activation energy
high
Activation energy
without catalyst
Activation energy
with catalyst
energy
content of
molecules
reactants
products
low
progress of reaction 33

34. 6.4 How Do Enzymes Promote Biochemical Reactions?
Enzymes are biological catalysts
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

35. 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
1. Enzymes are very specific for the reactions they catalyze
2. Enzyme activity is regulated

36. 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

37. 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(s) into the active site and diffuse(s) away
The enzyme reverts back to its original configuration and is ready to accept more substrates

38. Animation: Enzymes and Substrates

39. Figure 6-11 The cycle of enzyme–substrate interactions
substrates
active site
of enzyme
enzyme
product
Substrates enter
the active site in a
specific orientation
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 39

40. 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

41. 6.5 How Are Enzymes Regulated?
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

42. Figure 6-12 Simplified metabolic pathways
Initial reactant
Intermediates
End products
PATHWAY 1
enzyme 1
enzyme 2
enzyme 3
enzyme 4
PATHWAY 2
enzyme 5
enzyme 6 42

43. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity
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
Metabolic pathways are controlled in several ways
Control of enzyme synthesis, which regulates availability
Control of enzyme activity

44. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
Genes that code for enzymes may be turned on or off
Genes that code for specific proteins are turned on and off according to the cell’s changing need
An increase in substrate can trigger increased enzyme production, leading to decreased substrate levels

45. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
Some enzymes are regulated by being synthesized only during specific stages in the life of an organism
This regulation can be altered by mutation

46. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
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)

47. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
Enzyme activity may be controlled by competitive or noncompetitive inhibition
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

48. Figure 6-13a A substrate binding to an enzyme
active site
enzyme
noncompetitive
inhibitor site
A substrate binding to an enzyme 48

49. Figure 6-13b Competitive inhibition
A competitive inhibitor
molecule occupies the
active site and blocks
entry of the substrate
Competitive inhibition 49

50. Figure 6-13c Noncompetitive inhibition
The active site changes
shape so the substrate
no longer fits when a
noncompetitive inhibitor
molecule binds the
enzyme
noncompetitive
inhibitor molecule
Noncompetitive inhibition 50

51. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
Some enzymes are controlled by allosteric regulation
Allosteric enzymes are enzymes that participate in metabolic pathways.
Small regulator molecules can bind to enzymes and enhance or inhibit activity by allosteric regulation, which can either activate or inhibit the enzyme

52. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued)
Some enzymes are controlled 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

53. 6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and 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 allosterically inhibited by the end product of the pathway

54. Figure 6-14 Allosteric regulation of an enzyme by feedback inhibition
intermediates
enzyme 1
enzyme 2
enzyme 3
enzyme 4
enzyme 5
As levels of isoleucine rise,
isoleucine binds to the regulatory
site on enzyme 1, inhibiting it
threonine
(initial
reactant)
enzyme 1
isoleucine
(end product)
isoleucine 54

55. 6.5 How Are Enzymes Regulated?
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
Environmental conditions can denature enzymes, distorting the three-dimensional structure crucial for their function

56. 6.5 How Are Enzymes Regulated?
Poisons, drugs, and environmental conditions influence enzyme activity (continued)
Some poisons and drugs are competitive or noncompetitive inhibitors of enzymes
Competitive inhibitors of enzymes, including some nerve gases and insecticides, permanently block the active site of acetylcholinesterase
Arsenic, mercury, and lead bind permanently to the nonactive sites of various enzymes, inactivating them

57. 6.5 How Are Enzymes Regulated?
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

58. 6.5 How Are Enzymes Regulated?
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

59. 6.5 How Are Enzymes Regulated?
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

60. Figure 6-15a Effect of pH on enzyme activity
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 1 2 3 4 5 6 7 8 9 10 pH
Effect of pH on enzyme activity 60

61. Figure 6-15b Effect of temperature on enzyme activity
fast
For most human enzymes,
maximum activity occurs
at about 98.6F (37C)
rate of
reaction
slow 32 68
104
140 (F) 20 40
60 (C)
temperature
Effect of temperature on enzyme activity 61