An Introduction to Metabolism

1
Chapter 8

• An Introduction to Metabolism

2
Overview The Energy of Life

• The living cell is a miniature chemical factory
  where thousands of reactions occur
• The cell extracts energy and applies energy to
  perform work
• Some organisms even convert energy to light, as
  in bioluminescence

3
Concept 8.4 Enzymes speed up metabolic reactions
by lowering energy barriers

• A catalyst is a chemical agent that speeds up a
  reaction without being consumed by the reaction
• An enzyme is a catalytic protein
• Hydrolysis of sucrose by the enzyme sucrase is an
  example of an enzyme-catalyzed reaction

4
LE 8-13
Sucrose C12H22O11
Glucose C6H12O6
Fructose C6H12O6
5
The Activation Energy Barrier

• Every chemical reaction between molecules
  involves bond breaking and bond forming
• The initial energy needed to start a chemical
  reaction is called the free energy of activation,
  or activation energy (EA)
• Activation energy is often supplied in the form
  of heat from the surroundings

6
LE 8-14
A
B
C
D
Transition state
EA
A
B
Free energy
C
D
Reactants
A
B
DG lt O
C
D
Products
Progress of the reaction
7
How Enzymes Lower the EA Barrier

• Enzymes catalyze reactions by lowering the EA
  barrier
• Enzymes do not affect the change in free-energy
  (?G) instead, they hasten reactions that would
  occur eventually

Animation How Enzymes Work
8
LE 8-15
Course of reaction without enzyme
EA without enzyme
EA with enzyme is lower
Reactants
Free energy
Course of reaction with enzyme
DG is unaffected by enzyme
Products
Progress of the reaction
9
Substrate Specificity of Enzymes

• The reactant that an enzyme acts on is called the
  enzymes substrate
• The enzyme binds to its substrate, forming an
  enzyme-substrate complex
• The active site is the region on the enzyme where
  the substrate binds
• Induced fit of a substrate brings chemical groups
  of the active site into positions that enhance
  their ability to catalyze the reaction

10
LE 8-16
Substrate
Active site
Enzyme-substrate complex
Enzyme
11
Catalysis in the Enzymes Active Site

• In an enzymatic reaction, the substrate binds to
  the active site
• The active site can lower an EA barrier by
• Orienting substrates correctly
• Straining substrate bonds
• Providing a favorable microenvironment
• Covalently bonding to the substrate

12
LE 8-17
Substrates enter active site enzyme changes
shape so its active site embraces the substrates
(induced fit).
Substrates held in active site by
weak interactions, such as hydrogen bonds
and ionic bonds.

• Active site (and R groups of
• its amino acids) can lower EA
• and speed up a reaction by
• acting as a template for
• substrate orientation,
• stressing the substrates
• and stabilizing the
• transition state,
• providing a favorable
• microenvironment,
• participating directly in the
• catalytic reaction.

Substrates
Enzyme-substrate complex
Active site is available for two
new substrate molecules.
Enzyme
Products are released.
Substrates are converted into products.
Products
13
Effects of Local Conditions on Enzyme Activity

• An enzymes activity can be affected by
• General environmental factors, such as
  temperature and pH
• Chemicals that specifically influence the enzyme

14
Effects of Temperature and pH

• Each enzyme has an optimal temperature in which
  it can function
• Each enzyme has an optimal pH in which it can
  function

15
LE 8-18
Optimal temperature for typical human enzyme
Optimal temperature for enzyme of thermophilic
(heat-tolerant
bacteria)
Rate of reaction
0
20
40
60
80
100
Temperature (C)
Optimal temperature for two enzymes
Optimal pH for pepsin (stomach enzyme)
Optimal pH for trypsin (intestinal enzyme)
Rate of reaction
0
1
2
3
4
5
6
7
8
9
10
pH
Optimal pH for two enzymes
16
Cofactors

• Cofactors are nonprotein enzyme helpers
• Coenzymes are organic cofactors

17
Enzyme Inhibitors

• Competitive inhibitors bind to the active site of
  an enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another part of
  an enzyme, causing the enzyme to change shape and
  making the active site less effective

18
LE 8-19
Substrate
A substrate can bind normally to the active site
of an enzyme.
Active site
Enzyme
Normal binding
A competitive inhibitor mimics the substrate,
competing for the active site.
Competitive inhibitor
Competitive inhibition
A noncompetitive inhibitor binds to the enzyme
away from the active site, altering
the conformation of the enzyme so that its active
site no longer functions.
Noncompetitive inhibitor
Noncompetitive inhibition
19
Concept 8.5 Regulation of enzyme activity helps
control metabolism

• Chemical chaos would result if a cells metabolic
  pathways were not tightly regulated
• To regulate metabolic pathways, the cell switches
  on or off the genes that encode specific enzymes

20
Allosteric Regulation of Enzymes

• Allosteric regulation is the term used to
  describe cases where a proteins function at one
  site is affected by binding of a regulatory
  molecule at another site
• Allosteric regulation may either inhibit or
  stimulate an enzymes activity

21
Allosteric Activation and Inhibition

• Most allosterically regulated enzymes are made
  from polypeptide subunits
• Each enzyme has active and inactive forms
• The binding of an activator stabilizes the active
  form of the enzyme
• The binding of an inhibitor stabilizes the
  inactive form of the enzyme

22
LE 8-20a
Allosteric activator stabilizes active form.
Allosteric enzyme with four subunits
Active site (one of four)
Regulatory site (one of four)
Activator
Active form
Stabilized active form
Oscillation
Allosteric inhibitor stabilizes inactive form.
Non- functional active site
Inhibitor
Stabilized inactive form
Inactive form
Allosteric activators and inhibitors
23

• Cooperativity is a form of allosteric regulation
  that can amplify enzyme activity
• In cooperativity, binding by a substrate to one
  active site stabilizes favorable conformational
  changes at all other subunits

24
LE 8-20b
Binding of one substrate molecule to active site
of one subunit locks all subunits in active
conformation.
Substrate
Stabilized active form
Inactive form
Cooperativity another type of allosteric
activation
25
Feedback Inhibition

• In feedback inhibition, the end product of a
  metabolic pathway shuts down the pathway
• Feedback inhibition prevents a cell from wasting
  chemical resources by synthesizing more product
  than is needed

26
LE 8-21
Initial substrate (threonine)
Active site available
Threonine in active site
Enzyme 1 (threonine deaminase)
Isoleucine used up by cell
Intermediate A
Feedback inhibition
Enzyme 2
Active site of enzyme 1 cant bind theonine pathwa
y off
Intermediate B
Enzyme 3
Intermediate C
Isoleucine binds to allosteric site
Enzyme 4
Intermediate D
Enzyme 5
End product (isoleucine)
27
Specific Localization of Enzymes Within the Cell

• Structures within the cell help bring order to
  metabolic pathways
• Some enzymes act as structural components of
  membranes
• Some enzymes reside in specific organelles, such
  as enzymes for cellular respiration being located
  in mitochondria

28
LE 8-22
Mitochondria, sites of cellular respiration
1 µm
29
Chapter 9

• Cellular Respiration Harvesting Chemical Energy

30
Overview Life Is Work

• Living cells require energy from outside sources
• Some animals, such as the giant panda, obtain
  energy by eating plants others feed on organisms
  that eat plants

31
Concept 9.2 Glycolysis harvests energy by
oxidizing glucose to pyruvate

• Glycolysis (splitting of sugar) breaks down
  glucose into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two
  major phases
• Energy investment phase
• Energy payoff phase

Animation Glycolysis
32
LE 9-8
Energy investment phase
Glucose
2 ATP
2 ADP 2 P
used
Citric acid cycle
Glycolysis
Oxidative phosphorylation
Energy payoff phase
formed
4 ADP 4 P
4 ATP
ATP
ATP
ATP
2 NAD 4 e 4 H
2 H
2 NADH
2 Pyruvate 2 H2O
Net
2 Pyruvate 2 H2O
Glucose
2 ATP
4 ATP formed 2 ATP used
2 NADH 2 H
2 NAD 4 e 4 H
33
LE 9-9a_1
Glycolysis
Citric acid cycle
Oxidation phosphorylation
ATP
ATP
ATP
Glucose
ATP
Hexokinase
ADP
Glucose-6-phosphate
34
LE 9-9a_2
Citric acid cycle
Glycolysis
Oxidation phosphorylation
ATP
ATP
ATP
Glucose
ATP
Hexokinase
ADP
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6-phosphate
ATP
Phosphofructokinase
ADP
Fructose- 1, 6-bisphosphate
Aldolase
Isomerase
Dihydroxyacetone phosphate
Glyceraldehyde- 3-phosphate
35
LE 9-9b_1
2 NAD
Triose phosphate dehydrogenase
NADH
2
2 H
1, 3-Bisphosphoglycerate
2 ADP
Phosphoglycerokinase
2 ATP
3-Phosphoglycerate
Phosphoglyceromutase
2-Phosphoglycerate
36
LE 9-9b_2
2 NAD
Triose phosphate dehydrogenase
NADH
2
2 H
1, 3-Bisphosphoglycerate
2 ADP
Phosphoglycerokinase
2 ATP
3-Phosphoglycerate
Phosphoglyceromutase
2-Phosphoglycerate
Enolase
2 H2O
Phosphoenolpyruvate
2 ADP
Pyruvate kinase
2 ATP
Pyruvate
37
Concept 9.3 The citric acid cycle completes the
energy-yielding oxidation of organic molecules

• Before the citric acid cycle can begin, pyruvate
  must be converted to acetyl CoA, which links the
  cycle to glycolysis

38
LE 9-10
MITOCHONDRION
CYTOSOL
NAD
NADH
H
Acetyl Co A
Coenzyme A
CO2
Pyruvate
Transport protein
39

• The citric acid cycle, also called the Krebs
  cycle, takes place within the mitochondrial
  matrix
• The cycle oxidizes organic fuel derived from
  pyruvate, generating one ATP, 3 NADH, and 1 FADH2
  per turn

Animation Electron Transport
40
LE 9-11
Pyruvate (from glycolysis, 2 molecules per
glucose)
Citric acid cycle
Glycolysis
Oxidation phosphorylation
CO2
NAD
CoA
NADH
ATP
ATP
ATP
H
Acetyl CoA
CoA
CoA
Citric acid cycle
CO2
2
FADH2
3 NAD
NADH
3
FAD
3 H
ADP P
i
ATP
41

• The citric acid cycle has eight steps, each
  catalyzed by a specific enzyme
• The acetyl group of acetyl CoA joins the cycle by
  combining with oxaloacetate, forming citrate
• The next seven steps decompose the citrate back
  to oxaloacetate, making the process a cycle
• The NADH and FADH2 produced by the cycle relay
  electrons extracted from food to the electron
  transport chain

42
LE 9-12_1
Citric acid cycle
Glycolysis
Oxidation phosphorylation
ATP
ATP
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
Citric acid cycle
43
LE 9-12_2
Citric acid cycle
Glycolysis
Oxidation phosphorylation
ATP
ATP
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
CO2
Citric acid cycle
NAD
NADH
H
a-Ketoglutarate
CO2
NAD
NADH
Succinyl CoA
H
44
LE 9-12_3
Citric acid cycle
Glycolysis
Oxidation phosphorylation
ATP
ATP
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
CO2
Citric acid cycle
NAD
NADH
H
Fumarate
a-Ketoglutarate
FADH2
CO2
NAD
FAD
Succinate
NADH
P
i
Succinyl CoA
H
GTP
GDP
ADP
ATP
45
LE 9-12_4
Citric acid cycle
Glycolysis
Oxidation phosphorylation
ATP
ATP
ATP
Acetyl CoA
NADH
H2O
H
NAD
Oxaloacetate
Citrate
Malate
Isocitrate
CO2
Citric acid cycle
NAD
H2O
NADH
H
Fumarate
a-Ketoglutarate
FADH2
CO2
NAD
FAD
Succinate
NADH
P
i
Succinyl CoA
H
GTP
GDP
ADP
ATP
46
Concept 9.4 During oxidative phosphorylation,
chemiosmosis couples electron transport to ATP
synthesis

• Following glycolysis and the citric acid cycle,
  NADH and FADH2 account for most of the energy
  extracted from food
• These two electron carriers donate electrons to
  the electron transport chain, which powers ATP
  synthesis via oxidative phosphorylation

47
The Pathway of Electron Transport

• The electron transport chain is in the cristae of
  the mitochondrion
• Most of the chains components are proteins,
  which exist in multiprotein complexes
• The carriers alternate reduced and oxidized
  states as they accept and donate electrons
• Electrons drop in free energy as they go down the
  chain and are finally passed to O2, forming water

48
LE 9-13
NADH
50
FADH2
Multiprotein complexes
I
FAD
40
FMN
II
FeS
FeS
Q
III
Cyt b
Oxidative phosphorylation electron transport and
chemiosmosis
Citric acid cycle
Glycolysis
FeS
30
Cyt c1
IV
Free energy (G) relative to O2 (kcal/mol)
Cyt c
ATP
ATP
ATP
Cyt a
Cyt a3
20
10
O2
2 H 1/2
0
H2O
49

• The electron transport chain generates no ATP
• The chains function is to break the large
  free-energy drop from food to O2 into smaller
  steps that release energy in manageable amounts

50
Chemiosmosis The Energy-Coupling Mechanism

• Electron transfer in the electron transport chain
  causes proteins to pump H from the mitochondrial
  matrix to the intermembrane space
• H then moves back across the membrane, passing
  through channels in ATP synthase
• ATP synthase uses the exergonic flow of H to
  drive phosphorylation of ATP
• This is an example of chemiosmosis, the use of
  energy in a H gradient to drive cellular work

51
LE 9-14
INTERMEMBRANE SPACE
A rotor within the membrane spins as shown when
H flows past it down the H gradient.
H
H
H
H
H
H
H
A stator anchored in the membrane holds the knob
stationary.
A rod (or stalk) extending into the knob also
spins, activating catalytic sites in the knob.
H
Three catalytic sites in the stationary knob join
inorganic phosphate to ADP to make ATP.
ADP

ATP
P
i
MITOCHONDRAL MATRIX
52

• The energy stored in a H gradient across a
  membrane couples the redox reactions of the
  electron transport chain to ATP synthesis
• The H gradient is referred to as a proton-motive
  force, emphasizing its capacity to do work

Animation Fermentation Overview
53
LE 9-15
Inner mitochondrial membrane
Oxidative phosphorylation electron transport and
chemiosmosis
Citric acid cycle
Glycolysis
ATP
ATP
ATP
H
H
H
H
Cyt c
Protein complex of electron carriers
Intermembrane space
Q
IV
III
I
ATP synthase
II
Inner mitochondrial membrane
H2O
2H 1/2 O2
FADH2
FAD
NAD
H
NADH
ADP
ATP
P
i
(carrying electrons from food)
H
Mitochondrial matrix
Electron transport chain Electron transport and
pumping of protons (H), Which create an H
gradient across the membrane
Chemiosmosis ATP synthesis powered by the flow of
H back across the membrane
Oxidative phosphorylation
54
An Accounting of ATP Production by Cellular
Respiration

• During cellular respiration, most energy flows in
  this sequence
• glucose ?NADH ??electron transport chain
  ?proton-motive force ?ATP
• About 40 of the energy in a glucose molecule is
  transferred to ATP during cellular respiration,
  making about 38 ATP

55
LE 9-16
Electron shuttles span membrane
MITOCHONDRION
CYTOSOL
2 NADH
or
2 FADH2
2 FADH2
2 NADH
6 NADH
2 NADH
Oxidative phosphorylation electron
transport and chemiosmosis
Glycolysis
2 Acetyl CoA
Citric acid cycle
2 Pyruvate
Glucose
2 ATP
2 ATP
about 32 or 34 ATP
by substrate-level phosphorylation
by substrate-level phosphorylation
by oxidation phosphorylation, depending on which
shuttle transports electrons form NADH in cytosol
About 36 or 38 ATP
Maximum per glucose
56
Concept 9.5 Fermentation enables some cells to
produce ATP without the use of oxygen

• Cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2 (in
  aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
  fermentation to produce ATP

57
Types of Fermentation

• Fermentation consists of glycolysis plus
  reactions that regenerate NAD, which can be
  reused by glycolysis
• Two common types are alcohol fermentation and
  lactic acid fermentation

58

• In alcohol fermentation, pyruvate is converted to
  ethanol in two steps, with the first releasing
  CO2
• Alcohol fermentation by yeast is used in brewing,
  winemaking, and baking

Play
59
LE 9-17a
P
2 ADP 2
2 ATP
i
Glucose
Glycolysis
2 Pyruvate
2 NADH
2 NAD
CO2
2
2 H
2 Acetaldehyde
2 Ethanol
Alcohol fermentation
60

• In lactic acid fermentation, pyruvate is reduced
  to NADH, forming lactate as an end product, with
  no release of CO2
• Lactic acid fermentation by some fungi and
  bacteria is used to make cheese and yogurt
• Human muscle cells use lactic acid fermentation
  to generate ATP when O2 is scarce

61
LE 9-17b
P
2 ADP 2
2 ATP
i
Glucose
Glycolysis
2 NADH
2 NAD
CO2
2
2 H
2 Pyruvate
2 Lactate
Lactic acid fermentation
62
Fermentation and Cellular Respiration Compared

• Both processes use glycolysis to oxidize glucose
  and other organic fuels to pyruvate
• The processes have different final electron
  acceptors an organic molecule (such as pyruvate)
  in fermentation and O2 in cellular respiration
• Cellular respiration produces much more ATP

63

• Yeast and many bacteria are facultative
  anaerobes, meaning that they can survive using
  either fermentation or cellular respiration
• In a facultative anaerobe, pyruvate is a fork in
  the metabolic road that leads to two alternative
  catabolic routes

64
LE 9-18
Glucose
CYTOSOL
Pyruvate
O2 present Cellular respiration
No O2 present Fermentation
MITOCHONDRION
Acetyl CoA
Ethanol or lactate
Citric acid cycle
65
Concept 9.6 Glycolysis and the citric acid cycle
connect to many other metabolic pathways

• Gycolysis and the citric acid cycle are major
  intersections to various catabolic and anabolic
  pathways

66
LE 9-19
Proteins
Carbohydrates
Fats
Amino acids
Sugars
Glycerol
Fatty acids
Glycolysis
Glucose
Glyceraldehyde-3-
P
NH3
Pyruvate
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation
67
Biosynthesis (Anabolic Pathways)

• The body uses small molecules to build other
  substances
• These small molecules may come directly from
  food, from glycolysis, or from the citric acid
  cycle

68
LE 9-20
Glucose
AMP
Glycolysis
Fructose-6-phosphate
Stimulates

Phosphofructokinase


Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation
69
Chapter 10

• Photosynthesis

70
Overview The Process That Feeds the Biosphere

• Photosynthesis is the process that converts solar
  energy into chemical energy
• Directly or indirectly, photosynthesis nourishes
  almost the entire living world

71

• Autotrophs sustain themselves without eating
  anything derived from other organisms
• Autotrophs are the producers of the biosphere,
  producing organic molecules from CO2 and other
  inorganic molecules
• Almost all plants are photoautotrophs, using the
  energy of sunlight to make organic molecules from
  water and carbon dioxide

72

• Heterotrophs obtain their organic material from
  other organisms
• Heterotrophs are the consumers of the biosphere
• Almost all heterotrophs, including humans, depend
  on photoautotrophs for food and oxygen

73
Concept 10.1 Photosynthesis converts light
energy to the chemical energy of food

• Chloroplasts are organelles that are responsible
  for feeding the vast majority of organisms
• Chloroplasts are present in a variety of
  photosynthesizing organisms

74
Chloroplasts The Sites of Photosynthesis in
Plants

• Leaves are the major locations of photosynthesis
• Their green color is from chlorophyll, the green
  pigment within chloroplasts
• Light energy absorbed by chlorophyll drives the
  synthesis of organic molecules in the chloroplast
• Through microscopic pores called stomata, CO2
  enters the leaf and O2 exits

75

• Chloroplasts are found mainly in cells of the
  mesophyll, the interior tissue of the leaf
• A typical mesophyll cell has 30-40 chloroplasts
• The chlorophyll is in the membranes of thylakoids
  (connected sacs in the chloroplast) thylakoids
  may be stacked in columns called grana
• Chloroplasts also contain stroma, a dense fluid

76
Tracking Atoms Through Photosynthesis Scientific
Inquiry

• Photosynthesis can be summarized as the following
  equation

6 CO2 12 H2O Light energy ? C6H12O6 6 O2
6 H2 O
77
LE 10-4
12 H2O
6 CO2
Reactants
C6H12O6
6 H2O
6 O2
Products
78
The Two Stages of Photosynthesis A Preview

• Photosynthesis consists of the light reactions
  (the photo part) and Calvin cycle (the synthesis
  part)
• The light reactions (in the thylakoids) split
  water, release O2, produce ATP, and form NADPH
• The Calvin cycle (in the stroma) forms sugar from
  CO2, using ATP and NADPH
• The Calvin cycle begins with carbon fixation,
  incorporating CO2 into organic molecules

79
Concept 10.2 The light reactions convert solar
energy to the chemical energy of ATP and NADPH

• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the
  chemical energy of ATP and NADPH

80
The Nature of Sunlight

• Light is a form of electromagnetic energy, also
  called electromagnetic radiation
• Like other electromagnetic energy, light travels
  in rhythmic waves
• Wavelength distance between crests of waves
• Wavelength determines the type of electromagnetic
  energy
• Light also behaves as though it consists of
  discrete particles, called photons

81

• The electromagnetic spectrum is the entire range
  of electromagnetic energy, or radiation
• Visible light consists of colors we can see,
  including wavelengths that drive photosynthesis

82
LE 10-6
1 m (109 nm)
103 nm
103 nm
106 nm
105 nm
1 nm
103 m
Gamma rays
Micro- waves
Radio waves
X-rays
UV
Infrared
Visible light
650
750 nm
450
500
550
600
700
380
Longer wavelength
Shorter wavelength
Lower energy
Higher energy
83
Photosynthetic Pigments The Light Receptors

• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are reflected
  or transmitted
• Leaves appear green because chlorophyll reflects
  and transmits green light

Animation Light and Pigments
84
LE 10-7
Light
Reflected light
Chloroplast
Absorbed light
Granum
Transmitted light
85

• A spectrophotometer measures a pigments ability
  to absorb various wavelengths
• This machine sends light through pigments and
  measures the fraction of light transmitted at
  each wavelength

86
LE 10-8a
White light
Refracting prism
Chlorophyll solution
Photoelectric tube
Galvanometer
0
100
The high transmittance (low absorption) reading
indicates that chlorophyll absorbs very little
green light.
Green light
Slit moves to pass light of selected wavelength
87
LE 10-8b
White light
Chlorophyll solution
Refracting prism
Photoelectric tube
0
100
The low transmittance (high absorption) reading
indicates that chlorophyll absorbs most blue
light.
Slit moves to pass light of selected wavelength
Blue light
88

• An absorption spectrum is a graph plotting a
  pigments light absorption versus wavelength
• The absorption spectrum of chlorophyll a
  suggests that violet-blue and red light work best
  for photosynthesis
• An action spectrum profiles the relative
  effectiveness of different wavelengths of
  radiation in driving a process

89
LE 10-9a
Chlorophyll a
Chlorophyll b
Carotenoids
Absorption of light by chloroplast pigments
400
500
600
700
Wavelength of light (nm)
Absorption spectra
90
LE 10-9b
Rate of photo- synthesis (measured by O2 release)
Action spectrum
91

• The action spectrum of photosynthesis was first
  demonstrated in 1883 by Thomas Engelmann
• In his experiment, he exposed different segments
  of a filamentous alga to different wavelengths
• Areas receiving wavelengths favorable to
  photosynthesis produced excess O2
• He used aerobic bacteria clustered along the alga
  as a measure of O2 production

92
LE 10-9c
Aerobic bacteria
Filament of algae
500
600
700
400
Engelmanns experiment
93

• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments, such as chlorophyll b,
  broaden the spectrum used for photosynthesis
• Accessory pigments called carotenoids absorb
  excessive light that would damage chlorophyll

94
LE 10-10
in chlorophyll a
CH3
in chlorophyll b
CHO
Porphyrin ring light-absorbing head of
molecule note magnesium atom at center
Hydrocarbon tail interacts with
hydrophobic regions of proteins inside thylakoid
membranes of chloroplasts H atoms not shown
95
Excitation of Chlorophyll by Light

• When a pigment absorbs light, it goes from a
  ground state to an excited state, which is
  unstable
• When excited electrons fall back to the ground
  state, photons are given off, an afterglow called
  fluorescence
• If illuminated, an isolated solution of
  chlorophyll will fluoresce, giving off light and
  heat

96
LE 10-11
Excited state
e
Heat
Energy of electron
Photon (fluorescence)
Photon
Ground state
Chlorophyll molecule
Fluorescence
Excitation of isolated chlorophyll molecule
97
A Photosystem A Reaction Center Associated with
Light-Harvesting Complexes

• A photosystem consists of a reaction center
  surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment molecules
  bound to proteins) funnel the energy of photons
  to the reaction center

98

• A primary electron acceptor in the reaction
  center accepts an excited electron from
  chlorophyll a
• Solar-powered transfer of an electron from a
  chlorophyll a molecule to the primary electron
  acceptor is the first step of the light reactions

99
LE 10-12
Thylakoid
Photosystem
STROMA
Photon
Light-harvesting complexes
Reaction center
Primary electron acceptor
e
Thylakoid membrane
Special chlorophyll a molecules
Pigment molecules
Transfer of energy
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
100

• There are two types of photosystems in the
  thylakoid membrane
• Photosystem II functions first (the numbers
  reflect order of discovery) and is best at
  absorbing a wavelength of 680 nm
• Photosystem I is best at absorbing a wavelength
  of 700 nm
• The two photosystems work together to use light
  energy to generate ATP and NADPH

101
Noncyclic Electron Flow

• During the light reactions, there are two
  possible routes for electron flow cyclic and
  noncyclic
• Noncyclic electron flow, the primary pathway,
  involves both photosystems and produces ATP and
  NADPH

102
LE 10-13_1
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
CH2O (sugar)
O2
Primary acceptor
e
Energy of electrons
Light
P680
Photosystem II (PS II)
103
LE 10-13_2
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
CH2O (sugar)
O2
Primary acceptor
e
H2O
2 H

O2
1/2
e
e
Energy of electrons
Light
P680
Photosystem II (PS II)
104
LE 10-13_3
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
CH2O (sugar)
O2
Primary acceptor
Electron transport chain
Pq
e
H2O
Cytochrome complex
2 H

O2
1/2
Pc
e
e
Energy of electrons
Light
P680
ATP
Photosystem II (PS II)
105
LE 10-13_4
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
CH2O (sugar)
O2
Primary acceptor
Primary acceptor
Electron transport chain
Pq
e
e
H2O
Cytochrome complex
2 H

O2
1/2
Pc
e
e
P700
Energy of electrons
Light
P680
Light
ATP
Photosystem I (PS I)
Photosystem II (PS II)
106
LE 10-13_5
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
Electron Transport chain
O2
CH2O (sugar)
Primary acceptor
Primary acceptor
Electron transport chain
Fd
e
Pq
e
e
e
NADP
H2O
Cytochrome complex
2 H
2 H
NADP reductase

O2
NADPH
1/2
Pc
e
H
P700
Energy of electrons
e
Light
P680
Light
ATP
Photosystem I (PS I)
Photosystem II (PS II)
107
LE 10-14
e
ATP
e
e
NADPH
e
e
e
Mill makes ATP
Photon
e
Photon
Photosystem II
Photosystem I
108
Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and
  produces only ATP
• Cyclic electron flow generates surplus ATP,
  satisfying the higher demand in the Calvin cycle

109
LE 10-15
Primary acceptor
Primary acceptor
Fd
Fd
NADP
Pq
NADP reductase
Cytochrome complex
NADPH
Pc
Photosystem I
ATP
Photosystem II
110
A Comparison of Chemiosmosis in Chloroplasts and
Mitochondria

• Chloroplasts and mitochondria generate ATP by
  chemiosmosis, but use different sources of energy
• Mitochondria transfer chemical energy from food
  to ATP chloroplasts transform light energy into
  the chemical energy of ATP
• The spatial organization of chemiosmosis differs
  in chloroplasts and mitochondria

111
LE 10-16
Mitochondrion
Chloroplast
MITOCHONDRION STRUCTURE
CHLOROPLAST STRUCTURE
H
Diffusion
Intermembrane space
Thylakoid space
Electron transport chain
Membrane
ATP synthase
Key
Stroma
Matrix
Higher H
ADP
P
Lower H
i
ATP
H
112

• The current model for the thylakoid membrane is
  based on studies in several laboratories
• Water is split by photosystem II on the side of
  the membrane facing the thylakoid space
• The diffusion of H from the thylakoid space back
  to the stroma powers ATP synthase
• ATP and NADPH are produced on the side facing the
  stroma, where the Calvin cycle takes place

Animation Calvin Cycle
113
LE 10-17
H2O
CO2
Light
NADP
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
NADPH
O2
CH2O (sugar)
STROMA (Low H concentration)
Cytochrome complex
Photosystem I
Photosystem II
Light
NADP reductase
Light
2 H
NADP 2H
Fd
NADPH
H
Pq
Pc
H2O
O2
1/2
THYLAKOID SPACE (High H concentration)
2 H
2 H
To Calvin cycle
Thylakoid membrane
ATP synthase
STROMA (Low H concentration)
ADP

ATP
P
i
H
114
Concept 10.3 The Calvin cycle uses ATP and NADPH
to convert CO2 to sugar

• The Calvin cycle, like the citric acid cycle,
  regenerates its starting material after molecules
  enter and leave the cycle
• The cycle builds sugar from smaller molecules by
  using ATP and the reducing power of electrons
  carried by NADPH
• Carbon enters the cycle as CO2 and leaves as a
  sugar named glyceraldehyde-3-phospate (G3P)
• For net synthesis of one G3P, the cycle must take
  place three times, fixing three molecules of CO2

115

• The Calvin cycle has three phases
• Carbon fixation (catalyzed by rubisco)
• Reduction
• Regeneration of the CO2 acceptor (RuBP)

Play
116
LE 10-18_1
H2O
CO2
Input
Light
3
(Entering one at a time)
NADP
CO2
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
Phase 1 Carbon fixation
NADPH
Rubisco
CH2O (sugar)
O2
3
P
P
Short-lived intermediate
6
P
3
P
P
3-Phosphoglycerate
Ribulose bisphosphate (RuBP)
6
ATP
6 ADP
CALVIN CYCLE
117
LE 10-18_2
H2O
CO2
Input
Light
(Entering one at a time)
3
NADP
CO2
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
Phase 1 Carbon fixation
NADPH
Rubisco
CH2O (sugar)
O2
3
P
P
Short-lived intermediate
6
P
P
P
3
3-Phosphoglycerate
Ribulose bisphosphate (RuBP)
6
ATP
6 ADP
CALVIN CYCLE
6
P
P
1,3-Bisphosphoglycerate
6
NADPH
6 NADP
6
P
i
6
P
Glyceraldehyde-3-phosphate (G3P)
Phase 2 Reduction
P
1
G3P (a sugar)
Glucose and other organic compounds
Output
118
LE 10-18_3
H2O
CO2
Input
Light
(Entering one at a time)
3
NADP
CO2
ADP
CALVIN CYCLE
LIGHT REACTIONS
ATP
Phase 1 Carbon fixation
NADPH
Rubisco
CH2O (sugar)
O2
3
P
P
Short-lived intermediate
6
P
P
P
3
3-Phosphoglycerate
Ribulose bisphosphate (RuBP)
6
ATP
6 ADP
3 ADP
CALVIN CYCLE
6
P
P
3
ATP
1,3-Bisphosphoglycerate
6
NADPH
Phase 3 Regeneration of the CO2 acceptor (RuBP)
6 NADP
6
P
i
P
5
G3P
6
P
Glyceraldehyde-3-phosphate (G3P)
Phase 2 Reduction
P
1
G3P (a sugar)
Glucose and other organic compounds
Output
119
Concept 10.4 Alternative mechanisms of carbon
fixation have evolved in hot, arid climates

• Dehydration is a problem for plants, sometimes
  requiring tradeoffs with other metabolic
  processes, especially photosynthesis
• On hot, dry days, plants close stomata, which
  conserves water but also limits photosynthesis
• The closing of stomata reduces access to CO2 and
  causes O2 to build up
• These conditions favor a seemingly wasteful
  process called photorespiration

120
Photorespiration An Evolutionary Relic?

• In most plants (C3 plants), initial fixation of
  CO2, via rubisco, forms a three-carbon compound
• In photorespiration, rubisco adds O2 to the
  Calvin cycle instead of CO2
• Photorespiration consumes O2 and organic fuel and
  releases CO2 without producing ATP or sugar

121

• Photorespiration may be an evolutionary relic
  because rubisco first evolved at a time when the
  atmosphere had far less O2 and more CO2
• In many plants, photorespiration is a problem
  because on a hot, dry day it can drain as much as
  50 of the carbon fixed by the Calvin cycle

122
C4 Plants

• C4 plants minimize the cost of photorespiration
  by incorporating CO2 into four-carbon compounds
  in mesophyll cells
• These four-carbon compounds are exported to
  bundle-sheath cells, where they release CO2 that
  is then used in the Calvin cycle

123
LE 10-19
Mesophyll cell
Mesophyll cell
CO2
PEP carboxylase
Photosynthetic cells of C4 plant leaf
Bundle- sheath cell
The C4 pathway
PEP (3 C)
Oxaloacetate (4 C)
Vein (vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Pyruvate (3 C)
Bundle-sheath cell
CO2
Stoma
CALVIN CYCLE
Sugar
Vascular tissue
124
CAM Plants

• CAM plants open their stomata at night,
  incorporating CO2 into organic acids
• Stomata close during the day, and CO2 is released
  from organic acids and used in the Calvin cycle

125
LE 10-20
Sugarcane
Pineapple
CAM
C4
CO2
CO2
Mesophyll cell
Night
CO2 incorporated into four-carbon organic
acids (carbon fixation)
Organic acid
Organic acid
Bundle- sheath cell
Day
CO2
CO2
Organic acids release CO2 to Calvin cycle
CALVIN CYCLE
CALVIN CYCLE
Sugar
Sugar
Spatial separation of steps
Temporal separation of steps
126
The Importance of Photosynthesis A Review

• The energy entering chloroplasts as sunlight gets
  stored as chemical energy in organic compounds
• Sugar made in the chloroplasts supplies chemical
  energy and carbon skeletons to synthesize the
  organic molecules of cells
• In addition to food production, photosynthesis
  produces the oxygen in our atmosphere

127
LE 10-21
Light reactions
Calvin cycle
H2O
CO2
Light
NADP
ADP

P
i
RuBP
3-Phosphoglycerate
Photosystem II Electron transport chain Photosyste
m I
ATP
G3P
Starch (storage)
NADPH
Amino acids Fatty acids
Chloroplast
O2
Sucrose (export)