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Chapter 08
Lecture and
Animation Outline
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2
8.1 Overview of Photosynthesis
• Photosynthesis converts solar energy into chemical energy of carbohydrates
• Organisms that carry on photosynthesis are called autotrophs
– Plants, algae, and cyanobacteria are organisms capable of photosynthesis
• Heterotrophs are organisms that feed on other organisms
3
8.1 Overview of Photosynthesis
• Autotrophs and heterotrophs use organic molecules produced by photosynthesis
• Pigments allow photosynthetic organisms to capture solar energy
• Most photosynthetic organisms contain the pigment chlorophyll
• Another common pigment group are carotenoids
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Flowering Plants as Photosynthesizers
• Photosynthesis occurs in the green parts of plants
– Particularly leaves, contain chlorophyll and other pigments
• Leaves contain mesophyll tissue specialized for photosynthesis
• Raw materials are water and CO2
5
8.1 Overview of Photosynthesis
– Water is taken up by roots and transported to leaves by veins
– Carbon dioxide enters through openings in the leaves called stomata
– Light energy is absorbed by chlorophyll and other pigments in thylakoids of chloroplasts
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8.1 Overview of Photosynthesis
• Chloroplast structure
– The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane
– Thylakoids are a different membrane system
within the stroma that form flattened sacs
– Thylakoids are stacked together to from grana
– Thylakoid space is formed by a continuous connection between individual thylakoids
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cuticle
CO2
O2
stomata
upperepidermis
mesophyll
lowerepidermis
Leaf vein
Leaf cross section
Figure 8.2
8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Grana
granum
stromastroma
inner membrane
outer membrane
Chloroplast
thylakoid space
thylakoid membrane
channel between
thylakoids
© Dr. George Chapman/Visuals Unlimited
Chloroplast, micrograph 37,000x
Figure 8.2
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Grana
cuticle
stomata
stromastroma
Leaf cross section
upperepidermis
mesophyll
lowerepidermis
leaf vein
CO2
O2
inner membrane
outer membrane
Chloroplast
thylakoid space
thylakoid membrane
granum
channel betweenthylakoids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Dr. George Chapman/Visuals Unlimited
Chloroplast, micrograph 37,000x
Figure 8.2
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Photosynthetic Reaction
• Glucose and oxygen are the products of
photosynthesis
• The oxygen given off comes from water
• CO2 gains hydrogen atoms and becomes
a carbohydrate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CO2
solar energy
+ 6 H2O C6H12O6 + 6 O2pigments
11
Two Sets of Reactions
• Photosynthesis consists of two sets of reactions
– Photo refers to capturing solar energy
– Synthesis refers to producing a carbohydrate
• The two sets of reactions are called the:
– Light Reactions (light-dependent)
– Calvin Cycle Reactions (light-independent)
• Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions
12
CH2O
H2O CO2
ADP + P
NADP+
ATP
O2
thylakoidmembrane
stroma
Calvincycle
reactionsLight
reactions
solarenergy
NADPH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 8.3
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8.2 Plants as Solar Energy
Converters• During the light reactions, different
pigments within the thylakoid membranes absorb energy
• Solar energy can be described in terms of its wavelength and energy content
14
8.2 Plants as Solar Energy
Converters• The electromagnetic
spectrum extends from
very short gamma rays to very long radio waves
• White or visible light is only a small portion of the
spectrum
• Visible light is further divided into wavelengths between 380 and 750 nm
Increasing wavelength
Increasing energy
X rays UV Infrared
visible light
500 600 750
Gammarays
Micro-waves
Radiowaves
Wavelengths (nm)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
380
Figure 8.4
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Visible Light
• Visible light contains various wavelengths
• The colors of visible light range from:
– Violet light
• Shortest wavelength but high energy
– Red light
• Longest wavelength but lowest energy
– Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range
– Higher wavelengths are screened by the ozone layer
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Visible Light
• Most photosynthetic pigments in cells are chlorophylls a and band the carotenoids
• Can absorb specific various portions of visible light
• The absorption spectrum shown in figure on the right
Wavelengths (nm)
380 500 600 750
Chlorophyll a
Chlorophyll b
carotenoidsR
ela
tive
Ab
so
rpti
on
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 8.5
17
Visible Light
• Green light is reflected and only minimally
absorbed
– Leaves appear green
• Other plant pigments become noticeable in the fall when chlorophyll breaks down
and the other pigments are uncovered
18
Light Reactions
• Light Reactions
– Take place in thylakoid membrane
– Light reactions consist of two pathways:
• Noncyclic electron pathway
• Cyclic electron pathway
– Both pathways transform solar energy to chemical energy
– Both pathways produce ATP
– Only the noncyclic pathway produces NADPH
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Noncyclic Electron Pathway• Noncyclic electron pathway, named because
electron flow is traced from water to NADP+
– Uses two photosystems (Photosystems I and II)
– A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane
– The pigment complex can be described as a
“antenna” for gathering solar energy
20
Noncyclic Electron Pathway
• Noncyclic Electron Pathway begins with photosystem II (PSII)
– Pigment complex absorbs solar energy
– Energy passes from one pigment to another until it is concentrated in reaction center
• Chlorophyll a molecule
– Electrons in the reaction center chlorophyll become so energized
• Escape from the reaction center and move to a nearby electron acceptor
21
Noncyclic Electron Pathway
• Photosystem II would disintegrate without
replacement electrons
– Electrons provided by splitting water
– Releases oxygen (O2) to atmosphere which benefits all organisms that use O2
– Hydrogen ions (H+) stay in the thylakoid space
• Contribute to formation of hydrogen ion gradient
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Noncyclic Electron Pathway
• In PSII, an electron acceptor receives energized electrons from the reaction center
• It sends those electrons down an electron
transport chain, (series of carriers that pass electrons from one to the other)
• Energy is released to pump hydrogen ions (H+) into thylakoid space forming gradient
• When hydrogen ions flow through ATP synthase
it makes ATP
23
Noncyclic Electron Pathway
• PSI comes next in noncyclic electron pathway
– When the photosystem I complex absorbs solar
energy, energized electrons leave reaction center and are captured by a different electron acceptor
• Low energy PSII electrons used to replace those lost by PSI
– Electron acceptor in photosystem I passes its
electrons to NADP+and it becomes NADPH
24
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
NADP+
H+
reaction center reaction center
Calvin cyclereactions
en
erg
y level
sunsun
Photosystem II
Photosystem I
CO2 CH2O
e–
NADPH
electron
acceptorelectron
acceptor
pigmentcomplex
2H+ O212–
e–
e– e–
e–
e–
e–
pigmentcomplex
Figure 8.6
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
NADP+
H+
reaction center reaction center
Calvin cyclereactions
en
erg
y level
sunsun
Photosystem II
Photosystem I
CO2 CH2O
e–
NADPH
electron
acceptorelectron
acceptor
pigmentcomplex
2H+ O212–
e–
e– e–
e–
e–
e–
pigmentcomplex
CH2O
H2O CO2
solar
energy
ADP + P
NADP+
Light
reactions
Calv in
cycle
reactions
cycle
ATP
O2
NADPH
thylakoid
membrane
Figure 8.6
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Cyclic Electron Pathway
• Uses only photosystem I (PSI) and begins when PSI complex absorbs solar energy
– Energized electrons escape from the reaction center and travel down electron transport chain
– Released energy is stored in the form of a H+
gradient, which causes ATP production by ATP synthase
• Spent electrons return to PSI (cyclic)
• Pathway only results in ATP production
27
• Energized electrons leave the photsytem I
reaction center and return to photosystem
by an electron transport chain
• ATP from cyclic electron transport
used in Calvin cycle to make carbohydrates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Calvin cycle
reactions and
other enzymatic
reactions
Pigment
complex
reaction center
CO2 CH2O
sun
Photosystem I
electron
acceptor
en
erg
y le
ve
l
e–
e–
ATP
Figure 8.7
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The Organization of the Thylakoid Membrane
• The following molecular complexes are present in the thylakoid Membrane:
– PS II
• Pigment complex and electron acceptors
• Water is split to replace energized electrons
• Oxygen (O2) is released
– Electron transport chain
• Carries electrons from PS II to PS I
• Uses energy to pump H+ from the stroma into thylakoid space
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The Organization of the Thylakoid Membrane
– PS I
• Pigment complex and electron acceptors
• Adjacent to enzyme that reduces NADP+ to NADPH
– ATP synthase complex
• Has a channel for H+ flow
• Flow drives ATP synthase to join ADP and P
30
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
photosystem II
Stroma
Pq
H+
ATPsynthasecomplex
chemiosmosis
photosystem I
H2O +
ATP
+ ADPP
2
NADPH
O212
e–
electron transportchain
e–
e–
e– e–
thylakoidspace
NADPreductase
H+
H+
H+
H+H+
H+
H+
H+
NADP+
Figure 8.8
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31
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
photosystem II
Stroma
Pq
H+
ATPsynthasecomplex
chemiosmosis
photosystem I
granum
thylakoid membrane
thylakoid space
stroma
H2O +
ATP
+ ADPP
2
NADPH
O212
thylakoid
e–
electron transportchain
e–
e–
e– e–
thylakoidspace
NADPreductase
H+
H+
H+
H+H+
H+
H+
H+
NADP+
Figure 8.8
32
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
photosystem II
Stroma
Pq
H+
ATPsynthasecomplex
chemiosmosis
photosystem I
granum
thylakoid membrane
thylakoid space
stroma
H2O +
ATP
+ ADPP
2
NADPH
CH2O
H2O CO2
P
ATP
O2
O212
thylakoid
e–
thylakoid
membrane
electron transportchain
e–
e–
e– e–
thylakoidspace
NADPreductase
H+
H+
H+
H+H+
H+
solar
energy
Calvin
cyclereactionsLight
reactions
ADP +
NADP+
NADPH
H+
H+
NADP+
Figure 8.8
33
ATP Production
• ATP Production
– Thylakoid space acts as a reservoir for hydrogen ions (H+)
• H+ from water being split within thylakoid space
• Pumped in by electron transport chain
– More H+ in thylakoid space than stroma
• Electrochemical gradient
– H+ can only flow through ATP synthase
– Energy powers making ATP by chemiosmosis
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8.3 Plants as Carbon Dioxide
Fixers
• The Calvin Cycle (named after Melvin Calvin)
– Series of reactions that use CO2 from the atmosphere to produce carbohydrate
– Humans other most other organisms take in O2
and release CO2
– Includes
• Carbon dioxide fixation
• Carbon dioxide reduction
• Ribulose-1,5-bisphosphate (RuBP) regeneration
36
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3PG 3-phosphoglycerate
BPG 1,3-bisphosphoglycerate
Metabolites of the Calvin Cycle
P P
net gain of one G3P
Glucose
regeneration
of RuBP
intermediate
6 NADP+
6 BPG
C3
3 CO2
Other organic molecules
3 C6
x 2
3ADP + 3
These ATPmolecules wereproduced by thelight reactions.
3
ATP6 NADPH
6ADP + 6
6
ATP
RuBP ribulose-1,5-bisphosphate
G3P glyceraldehyde-3-phosphate
These ATP andNADPHmolecules wereproduced by thelight reactions.
6 3PG
C33 RuBP
C5
5 G3P
C36 G3P
C3
CO2
reduction
CO2
fixation
Calvin cycle
CH2O
stroma
O2
Lightreactions
NADP+
ATP
NADPH
Calvincycle
H2O CO2
solar
energy
ADP+ P
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Fixation of Carbon Dioxide
• Carbon dioxide fixation is the 1st step of
the Calvin cycle
– CO2 is attached to 5-carbon RuBP molecule
• This reaction occurs three times
• The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG)
– RuBP Carboxylase is the enzyme that makes
this happen
• Comparatively slow enzyme so there is a lot of it
38
Reduction of Carbon Dioxide
• Reduction of Carbon Dioxide
– Each 3PG molecules undergoes reduction to
G3P in two steps
– Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)
39
Reduction of Carbon DioxideCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
NADPH NADP+
3PG G3PBPG
ADP + P
ADP + P
ATP
As 3PG becomes G3P ATP becomes
, and NADPH becomes NADP+.
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Regeneration of RuBP
• Regeneration of RuBP
– It takes three turns of the Calvin cycle to allow
one G3P to exit
– For every three turns of Calvin Cycle, five G3P (3-carbon molecule) used
– This re-forms three RuBP (5-carbon molecule)
– 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)
41
Regeneration of RuBP
5 × 3 (carbons in G3P) = 3 × 5 (carbons in RuBP)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3 ATP
5 G3P 3 RuBP
3 ADP + P
As five molecules of G3P become three
molecules of RuBP, three molecules of ATP
become three molecules of ADP + P .
42
Importance of the Calvin Cycle
• G3P (glyceraldehyde-3-phosphate) can be
converted to many other molecules
– These molecules meet the plant needs
• The hydrocarbon skeleton of G3P can form:
– Fatty acids and glycerol to make plant oil
– Glucose phosphate (simple sugar)
– Fructose (+ glucose = sucrose)
– Starch and cellulose
– Amino acids
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8.4 Alternate Pathways for
Photosynthesis
• C3 Photosynthesis
– The leaves of C3 plants have a different structure
and means of fixing CO2 than C4 plants
– C3 plants such as wheat, rice, oats have
mesophyll cells of leaves in parallel layers
– Bundle sheath cells around the plant veins do not
contain chloroplasts
– As a result, cells using Calvin cycle exposed to CO2
44
C3 Photosynthesis
• RuBP carboxylase binds O2 as well as CO2
– When bound to O2, the enzyme undergoes
photorespiration
– Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle
– O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata
closed to conserve water
45
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
vein
stoma
mesophyllcells
bundle sheathcell
a. C3 Plant
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Calvincycle
CO2
G3P
RuBP
mesophyll cell
3PG(C3)
a. CO2 fixation in a C3 plant, tuplip
© The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer
Figure 8.10Figure 8.11
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C4 Photosynthesis
• C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells
– They also contain chloroplasts
– In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule
– The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2
– CO2 enters the Calvin cycle
47
C4 Photosynthesis
• C4 Pathway
– C4 plants tend to be found in hot, dry climates
– In these climates, stomata tend to close to conserve water
– Oxygen then builds-up in the leaves
– But, RuBP carboxylase is not exposed to this O2 in C4
plants and photorespiration does not occur
– Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces
48
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
vein
stoma
mesophyllcells
bundle sheathcell
b. C4 Plant
CO2
CO2
C4
G3P
Calvincycle
b. CO2 fixation in a C4 plant, corn
mesophyllcell
bundlesheathcell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Corbis RF
Figure 8.11 Figure 8.10
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C4 Photosynthesis
• When the weather is moderate, C3 plants ordinarily have the advantage.
• When the weather is hot and dry, C4 plants have
the advantage, and can be expected to predominate.
• In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts
of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over.
50
CAM Photosynthesis
• CAM Pathway
– This pathway is prevalent among most succulent
plants that grow in deserts, including the cacti.
– CAM plants partition carbon fixation according to time.
• During the night, CAM plants fix CO2, forming C4
molecules.
• The C4 molecules are stored in large vacuoles.
• During daylight, C4 molecules release CO2 to the Calvin cycle.
51
CO2
CO2
C4
G3P
night
day
c. CO2 fixation in a CAM plant, pineapple
Calvin
cycle
© S. Alden/PhotoLink/Getty RF Figure 8.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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8.5 Photosynthesis Versus Cellular Respiration
• Both plant and animal cells carry out cellular respiration.
– Occurs in mitochondria
– Breaks glucose down
– Utilizes O2 and gives off CO2
• Plant cells photosynthesize, but animal cells do not.
– Occurs in chloroplasts
– Builds glucose
– Utilizes CO2 and gives off O2
• Both processes utilize an electron transport chain and chemiosmosis for ATP production.
53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thylakoidmembrane H2O O2
ADP ATP
solarenergy
H2O CO2
Lightreactions
ADP + P
Calvincycle
reactions
NADP+
NADPH
thylakoidmembrane
O2 CH2O
stroma
Stroma NADPH NADP+
CO2CH2O
Photosynthesis
ATP
Figure 8.12
54
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cristae O2 H2O
ADP ATP
NADH+H+
e–
e–
NADH+H+
e–
e–
e–
e–
Preparatory reaction Citric acidcycle
NADH+H+
and FADH2
Electron transport
chain
2 ATP2 ADP
4 ADP 4 ATP total
2 ATP net gain 2 ADP 2 32 ADP 32
or 34 or 34
ATP ATP
MatrixNAD+
CH2O CO2
Cellular Respiration
Glycolysis
glucose pyruvate
NADH
Figure 8.12
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55
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thylakoidmembrane H2O O2 Cristae O2 H2O
ADP ATPADP ATP
solar
energy
H2O CO2
NADH+H+
e–
e–
NADH+H+e–
e–
e–
e–
Preparatory reaction Citric acid
cycle
NADH + H+
and FADH2
Electron transportchain
2 ATP2 ADP
4 ADP 4 ATP total
2 ATP net gain 2 ADP 2 32 ADP 32or 34 or 34
ATP ATP
MatrixNAD+
CH2O CO2
Cellular Respiration
Lightreactions
ADP + P
Calvincycle
reactions
NADP+
NADPH
thylakoid
membrane O2 CH2O
stroma
StromaNADPH NADP+
CO2 CH2O
Photosynthesis
Glycolysis
glucose pyruvate
ATP
NADH
Figure 8.12