8 photosynthesis
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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 © 2014 Pearson Education, Inc. 2TRANSCRIPT
8 Photosynthesis 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 2014 Pearson Education, Inc. 2
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 2014 Pearson Education, Inc. 3 Figure 8.1 Figure 8.1 How
can sunlight, seen here as a spectrum of colors in a rainbow, power
the synthesis of organic substances? 4 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 O2 2014 Pearson Education, Inc. 5 Photosynthesis occurs in
plants, algae, certain other protists, and some prokaryotes
These organisms feed not only themselves but also most of the
living world 2014 Pearson Education, Inc. 6 (c) Unicellular
eukaryotes
Figure 8.2 40 m (d) Cyanobacteria (a) Plants Figure 8.2
Photoautotrophs (b) Multicellular alga 1 m (e) Purple sulfur
bacteria 10 m (c) Unicellular eukaryotes 7 Figure 8.2a Figure 8.2a
Photoautotrophs (part 1: plants) (a) Plants 8 Concept 8.1:
Photosynthesis converts light energy to the chemical energy of
food
The structural organization of photosynthetic cells includes
enzymes and other molecules grouped together in a membrane This
organization allows for the chemical reactions of photosynthesis to
proceed efficiently Chloroplasts are structurally similar to and
likely evolved from photosynthetic bacteria 2014 Pearson Education,
Inc. 9 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
Chloroplasts are found mainly in cells of the mesophyll, the
interior tissue of the leaf Each mesophyll cell contains 3040
chloroplasts 2014 Pearson Education, Inc. 10 Chloroplasts also
contain stroma, a dense interior fluid
CO2 enters and O2 exits the leaf through microscopic pores called
stomata 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
interior fluid 2014 Pearson Education, Inc. 11 Leaf cross section
Chloroplasts Vein Mesophyll Stomata CO2 O2
Figure 8.3 Leaf cross section Chloroplasts Vein Mesophyll Stomata
CO2 O2 Chloroplast Mesophyll cell Figure 8.3 Zooming in on the
location of photosynthesis in a plant Outer membrane Thylakoid
Granum Thylakoid space Intermembrane space Stroma 20 m Inner
membrane 1 m 12 Tracking Atoms Through Photosynthesis: Scientific
Inquiry
Photosynthesis is a complex series of reactions that can be
summarized as the following equation 6 CO2 12 H2O Light energy
C6H12O6 6 O2 6 H2O 2014 Pearson Education, Inc. 13 The Splitting of
Water Chloroplasts split H2O into hydrogen and oxygen,
incorporating the electrons of hydrogen into sugar molecules and
releasing oxygen as a by-product 2014 Pearson Education, Inc. 14
Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 Figure
8.4
Figure 8.4 Tracking atoms through photosynthesis 15 Photosynthesis
as a Redox Process
Photosynthesis reverses the direction of electron flow compared to
respiration Photosynthesis is a redox process in which H2O is
oxidized and CO2 is reduced Photosynthesis is an endergonic
process; the energy boost is provided by light 2014 Pearson
Education, Inc. 16 becomes reduced becomes oxidized Figure
8.UN01
Figure 8.UN01 In-text figure, photosynthesis equation, p. 158 17
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 H2O Release O2 Reduce the electron acceptor,
NADP, to NADPH Generate ATP from ADP by adding a phosphate group,
photophosphorylation 2014 Pearson Education, Inc. 18 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 2014 Pearson Education, Inc. 19 Video:
Photosynthesis Calvin Cycle Light Reactions [CH2O] (sugar)
Figure 8.5 H2O CO2 Light NADP ADP P i Calvin Cycle Light Reactions
ATP Figure 8.5 An overview of photosynthesis: cooperation of the
light reactions and the Calvin cycle NADPH Chloroplast [CH2O]
(sugar) O2 21 H2O Light NADP ADP Light Reactions Chloroplast P i
Figure 8.5-1
Figure An overview of photosynthesis: cooperation of the light
reactions and the Calvin cycle (step 1) Chloroplast 22 H2O Light
NADP ADP Light Reactions Chloroplast O2 P i ATP NADPH
Figure 8.5-2 H2O Light NADP ADP P i Light Reactions ATP Figure An
overview of photosynthesis: cooperation of the light reactions and
the Calvin cycle (step 2) NADPH Chloroplast O2 23 Calvin Cycle
Light Reactions
Figure 8.5-3 H2O CO2 Light NADP ADP P i Calvin Cycle Light
Reactions ATP Figure An overview of photosynthesis: cooperation of
the light reactions and the Calvin cycle (step 3) NADPH Chloroplast
O2 24 Calvin Cycle Light Reactions [CH2O] (sugar)
Figure 8.5-4 H2O CO2 Light NADP ADP P i Calvin Cycle Light
Reactions ATP Figure An overview of photosynthesis: cooperation of
the light reactions and the Calvin cycle (step 4) NADPH Chloroplast
[CH2O] (sugar) O2 25 Concept 8.2: 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
2014 Pearson Education, Inc. 26 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 is the distance between crests of waves
Wavelength determines the type of electromagnetic energy 2014
Pearson Education, Inc. 27 The electromagnetic spectrum is the
entire range of electromagnetic energy, or radiation
Visible light consists of wavelengths (including those that drive
photosynthesis) that produce colors we can see Light also behaves
as though it consists of discrete particles, called photons 2014
Pearson Education, Inc. 28 1 m (109 nm) 105 nm 103 nm 1 nm 103 nm
106 nm 103 m Micro- waves
Figure 8.6 1 m (109 nm) 105 nm 103 nm 1 nm 103 nm 106 nm 103 m
Micro- waves Gamma rays Radio waves X-rays UV Infrared Visible
light Figure 8.6 The electromagnetic spectrum 380 450 500 550 600
650 700 750 nm Shorter wavelength Longer wavelength Higher energy
Lower energy 29 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 2014 Pearson
Education, Inc. 30 Animation: Light and Pigments
Right click slide / Select play Light Reflected light Chloroplast
Absorbed Granum light Transmitted
Figure 8.7 Light Reflected light Chloroplast Figure 8.7 Why leaves
are green: interaction of light with chloroplasts Absorbed light
Granum Transmitted light 32 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 2014 Pearson Education,
Inc. 33 The high transmittance (low absorption) reading
indicates
Figure 8.8 Technique White light Refracting prism Chlorophyll
solution Photoelectric tube Galvanometer 2 3 1 4 The high
transmittance (low absorption) reading indicates that chlorophyll
absorbs very little green light. Slit moves to pass light of
selected wavelength. Green light Figure 8.8 Research method:
determining an absorption spectrum The low transmittance (high
absorption) reading indicates that chlorophyll absorbs most blue
light. Blue light 34 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 Accessory pigments
include chlorophyll b and a group of pigments called carotenoids An
action spectrum profiles the relative effectiveness of different
wavelengths of radiation in driving a process For the Cell Biology
Video Space-Filling Model of Chlorophyll a, go to Animation and
Video Files. 2014 Pearson Education, Inc. 35 (c) Engelmanns
experiment
Figure 8.9c Aerobic bacteria Filament of alga 400 500 600 700
Figure 8.9c Inquiry: which wavelengths of light are most effective
in driving photosynthesis? (part 3: Engelmanns experiment) (c)
Engelmanns experiment 36 Chloro- phyll a Absorption of light by
chloroplast pigments
Figure 8.9a Chloro- phyll a Chlorophyll b Absorption of light by
chloroplast pigments Carotenoids 400 500 600 700 Figure 8.9a
Inquiry: which wavelengths of light are most effective in driving
photosynthesis? (part 1: absorption spectra) Wavelength of light
(nm) (a) Absorption spectra 37 (measured by O2 photosynthesis Rate
of release)
Figure 8.9b (measured by O2 photosynthesis Rate of release) 400 500
600 700 Figure 8.9b Inquiry: which wavelengths of light are most
effective in driving photosynthesis? (part 2: action spectrum) (b)
Action spectrum 38 Wavelength of light (nm) (a) Absorption
spectra
Figure 8.9 Results Chloro- phyll a Chlorophyll b Absorption of
light by chloroplast pigments Carotenoids 400 500 600 700
Wavelength of light (nm) (a) Absorption spectra (measured by O2
photosynthesis Rate of release) 400 500 600 700 Figure 8.9 Inquiry:
which wavelengths of light are most effective in driving
photosynthesis? (b) Action spectrum Aerobic bacteria Filament of
alga 400 500 600 700 (c) Engelmanns experiment 39 The action
spectrum of photosynthesis was first demonstrated in 1883 by
Theodor W. 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 the growth of aerobic
bacteria clustered along the alga as a measure of O2 production
2014 Pearson Education, Inc. 40 Chlorophyll a is the main
photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum
used for photosynthesis A slight structural difference between
chlorophyll a and chlorophyll b causes them to absorb slightly
different wavelengths Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll 2014 Pearson
Education, Inc. 41 Video: Chlorophyll Model interacts with
hydrophobic regions of proteins inside
Figure 8.10 CH3in chlorophyll a CH3 CHOin chlorophyll b Porphyrin
ring: light-absorbing head of molecule; note magnesium atom at
center Figure 8.10 Structure of chlorophyll molecules in
chloroplasts of plants Hydrocarbon tail: interacts with hydrophobic
regions of proteins inside thylakoid membranes of chloroplasts; H
atoms not shown 43 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 2014 Pearson Education,
Inc. 44 (a) Excitation of isolated chlorophyll molecule (b)
Fluorescence
Figure 8.11 Excited state e Heat Energy of electron Photon
(fluorescence) Photon Figure 8.11 Excitation of isolated
chlorophyll by light Ground state Chlorophyll molecule (a)
Excitation of isolated chlorophyll molecule (b) Fluorescence 45 A
Photosystem: A Reaction-Center Complex Associated with
Light-Harvesting Complexes
A photosystem consists of a reaction-center complex (a type of
protein complex) surrounded by light-harvesting complexes The
light-harvesting complexes (pigment molecules bound to proteins)
transfer the energy of photons to the reaction center 2014 Pearson
Education, Inc. 46 (INTERIOR OF THYLAKOID) Protein subunits
THYLAKOID SPACE
Figure 8.12 Photosystem STROMA Photon Light- harvesting complexes
Reaction- center complex Primary electron acceptor e Chlorophyll
STROMA Thylakoid membrane Thylakoid membrane Transfer of energy
Figure 8.12 The structure and function of a photosystem Special
pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE
(INTERIOR OF THYLAKOID) Protein subunits THYLAKOID SPACE (a) How a
photosystem harvests light (b) Structure of a photosystem 47
(INTERIOR OF THYLAKOID)
Figure 8.12a Photosystem STROMA Photon Light- harvesting complexes
Reaction- center complex Primary electron acceptor e Thylakoid
membrane Figure 8.12a The structure and function of a photosystem
(part 1) Pigment molecules Transfer of energy Special pair of
chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a)
How a photosystem harvests light 48 (b) Structure of a
photosystem
Figure 8.12b Chlorophyll STROMA Thylakoid membrane Protein subunits
THYLAKOID SPACE Figure 8.12b The structure and function of a
photosystem (part 2) (b) Structure of a photosystem 49 A primary
electron acceptor in the reaction center accepts excited electrons
and is reduced as a result Solar-powered transfer of an electron
from a chlorophyll a molecule to the primary electron acceptor is
the first step of the light reactions 2014 Pearson Education, Inc.
50 There are two types of photosystems in the thylakoid
membrane
Photosystem II (PS II) functions first (the numbers reflect order
of discovery) and is best at absorbing a wavelength of 680 nm The
reaction-center chlorophyll a of PS II is called P680 2014 Pearson
Education, Inc. 51 Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
The reaction-center chlorophyll a of PS I is called P700 2014
Pearson Education, Inc. 52 Linear Electron Flow Linear electron
flow involves the flow of electrons through both photosystems to
produce ATP and NADPH using light energy 2014 Pearson Education,
Inc. 53 Linear electron flow can be broken down into a series of
steps
A photon hits a pigment and its energy is passed among pigment
molecules until it excites P680 An excited electron from P680 is
transferred to the primary electron acceptor (we now call it P680)
H2O is split by enzymes, and the electrons are transferred from the
hydrogen atoms to P680, thus reducing it to P680; O2 is released as
a by-product 2014 Pearson Education, Inc. 54 Calvin Cycle Light
Reactions
Figure 8.UN02 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions
ATP Figure 8.UN02 In-text figure, light reaction schematic, p. 164
NADPH O2 [CH2O] (sugar) 55 2 P680 1 Light Pigment molecules
Photosystem II (PS II) Primary
Figure Primary acceptor 2 e P680 1 Light Figure How linear electron
flow during the light reactions generates ATP and NADPH (steps 1-2)
Pigment molecules Photosystem II (PS II) 56 2 2 H 3 P680 1 Light
Pigment molecules Photosystem II (PS II) 1 2
Figure Primary acceptor 2 2 H H2O e 2 1 3 O2 e e P680 1 Light
Figure How linear electron flow during the light reactions
generates ATP and NADPH (step 3) Pigment molecules Photosystem II
(PS II) 57 4 Electron transport chain 2 2 H 3 P680 1 5 Light
Pigment
Figure 4 Electron transport chain Primary acceptor Pq 2 2 H H2O e
Cytochrome complex 2 1 3 O2 Pc e P680 1 e 5 Light ATP Figure How
linear electron flow during the light reactions generates ATP and
NADPH (steps 4-5) Pigment molecules Photosystem II (PS II) 58
Photosystem I (PS I) Photosystem II (PS II)
Figure 4 Electron transport chain Primary acceptor Primary acceptor
Pq e 2 2 H H2O e Cytochrome complex 2 1 3 O2 Pc e P700 e P680 1 5
Light Light 6 ATP Figure How linear electron flow during the light
reactions generates ATP and NADPH (step 6) Pigment molecules
Photosystem I (PS I) Photosystem II (PS II) 59 Photosystem I (PS I)
Photosystem II (PS II)
Figure 4 Electron transport chain 7 Electron transport chain
Primary acceptor Primary acceptor Fd Pq e 8 2 NADP e 2 H H2O e e
Cytochrome complex H NADP reductase 2 1 3 O2 NADPH Pc e P700 P680 1
e 5 Light Light 6 ATP Figure How linear electron flow during the
light reactions generates ATP and NADPH (steps 7-8) Pigment
molecules Photosystem I (PS I) Photosystem II (PS II) 60 Each
electron falls down an electron transport chain from the primary
electron acceptor of PS IIto PS I Energy released by the fall
drives the creation of a proton gradient across the thylakoid
membrane; diffusion of H (protons) across the membrane drives ATP
synthesis 2014 Pearson Education, Inc. 61 In PS I (like PS II),
transferred light energy excites P700, causing it to lose an
electron to an electron acceptor (we now call it P700) P700 accepts
an electron passed down from PS II via the electron transport chain
2014 Pearson Education, Inc. 62 Excited electrons fall down an
electron transport chain from the primary electron acceptor of PS I
to the protein ferredoxin (Fd) The electrons are transferred to
NADP, reducing it to NADPH, and become available for the reactions
of the Calvin cycle This process also removes an H from the stroma
2014 Pearson Education, Inc. 63 The energy changes of electrons
during linear flow can be represented in a mechanical analogy
2014 Pearson Education, Inc. 64 Mill makes ATP NADPH Photosystem II
Photosystem I Photon Photon
Figure 8.14 Mill makes ATP NADPH Photon Figure 8.14 A mechanical
analogy for linear electron flow during the light reactions Photon
Photosystem II Photosystem I 65 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 Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but also shows similarities
2014 Pearson Education, Inc. 66 In mitochondria, protons are pumped
to the intermembrane space and drive ATP synthesis as they diffuse
back into the mitochondrial matrix In chloroplasts, protons are
pumped into the thylakoid space and drive ATP synthesis as they
diffuse back into the stroma 2014 Pearson Education, Inc. 67
Electron transport chain ATP synthase
Figure 8.15 MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter-
membrane space H Diffusion Thylakoid space Electron transport chain
Inner membrane Thylakoid membrane Figure 8.15 Comparison of
chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix
Stroma Key ADP P i ATP Higher [H] H Lower [H] 68 ATP and NADPH are
produced on the side facing the stroma, where the Calvin cycle
takes place
In summary, light reactions generate ATP and increase the potential
energy of electrons by moving them from H2O to NADPH 2014 Pearson
Education, Inc. 69 Calvin Cycle Light Reactions
Figure 8.UN02 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions
ATP Figure 8.UN02 In-text figure, light reaction schematic, p. 164
NADPH O2 [CH2O] (sugar) 70 Cytochrome complex NADP reductase To
Calvin Cycle ATP synthase
Figure 8.16 Cytochrome complex NADP reductase Photosystem II
Photosystem I Light 4 H 3 Light NADP H Fd Pq NADPH e Pc e 2 H2O 1 2
1 O2 THYLAKOID SPACE (high Hconcentration) 2 H 4 H To Calvin Cycle
Figure 8.16 The light reactions and chemiosmosis: the organization
of the thylakoid membrane Thylakoid membrane ATP synthase STROMA
(low Hconcentration) ADP ATP P i H 71 Cytochrome complex ATP
synthase
Figure 8.16a Cytochrome complex Photosystem II Photosystem I Light
4 H Light Fd Pq Pc e 2 e H2O 1 2 1 O2 THYLAKOID SPACE (high
Hconcentration) 2 H 4 H Figure 8.16a The light reactions and
chemiosmosis: the organization of the thylakoid membrane (part 1)
Thylakoid membrane ATP synthase STROMA (low Hconcentration) ADP ATP
P i H 72 Cytochrome complex NADP reductase To Calvin Cycle ATP
synthase
Figure 8.16b Cytochrome complex NADP reductase Photosystem I Light
3 NADP H Fd NADPH Pc 2 THYLAKOID SPACE (high Hconcentration) 4 H To
Calvin Cycle Figure 8.16b The light reactions and chemiosmosis: the
organization of the thylakoid membrane (part 2) ATP synthase STROMA
(low Hconcentration) ADP ATP P i H 73 Concept 8.3: The Calvin cycle
uses the chemical energy of ATP and NADPH to reduce CO2 to
sugar
The Calvin cycle, like the citric acid cycle, regenerates its
starting material after molecules enter and leave the cycle Unlike
the citric acid cycle, the Calvin cycle is anabolic It builds sugar
from smaller molecules by using ATP and the reducing power of
electrons carried by NADPH 2014 Pearson Education, Inc. 74 The
Calvin cycle has three phases
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
The Calvin cycle has three phases Carbon fixation Reduction
Regeneration of the CO2 acceptor 2014 Pearson Education, Inc. 75
Phase 1, carbon fixation, involves the incorporation of the CO2
molecules into ribulose bisphosphate (RuBP) using the enzyme
rubisco 2014 Pearson Education, Inc. 76 Calvin Cycle Light
Reactions
Figure 8.UN03 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions
ATP Figure 8.UN03 In-text figure, Calvin cycle schematic, p. 168
NADPH O2 [CH2O] (sugar) 77 Phase 1: Carbon fixation Rubisco
Figure Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P
P 6 P RuBP 3-Phosphoglycerate Calvin Cycle Figure The Calvin cycle
(step 1) 78 Phase 1: Carbon fixation Rubisco
Figure Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P
P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 6 P P
1,3-Bisphosphoglycerate 6 NADPH 6 NADP 6P i Figure The Calvin cycle
(step 2) 6 P G3P Phase 2: Reduction Glucose and other organic
compounds 1 P G3P Output 79 Phase 1: Carbon fixation Rubisco
Figure Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P
P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P
3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of RuBP
6 NADP 6P i 5 P Figure The Calvin cycle (step 3) G3P 6 P G3P Phase
2: Reduction Glucose and other organic compounds 1 P G3P Output 80
Phase 2, reduction, involves the reduction and phosphorylation of
3-phosphoglycerate to G3P
2014 Pearson Education, Inc. 81 Phase 3, regeneration, involves the
rearrangement of G3P to regenerate the initial CO2 receptor, RuBP
2014 Pearson Education, Inc. 82 Calvin Cycle Regeneration of CO2
acceptor
Figure 8.UN06 3 CO2 Carbon fixation 3 5C 6 3C Calvin Cycle
Regeneration of CO2 acceptor 5 3C Figure 8.UN06 Summary of key
concepts: the Calvin cycle Reduction 1 G3P (3C) 83 Evolution of
Alternative Mechanisms of Carbon Fixation in Hot, Arid
Climates
Adaptation to dehydration is a problem for landplants, sometimes
requiring trade-offs with othermetabolic processes, especially
photosynthesis On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis The closing of stomata
reduces access to CO2 and causes O2to build up These conditions
favor an apparently wasteful process called photorespiration 2014
Pearson Education, Inc. 84 In most plants (C3 plants), initial
fixation of CO2,via rubisco, forms a three-carbon compound
(3-phosphoglycerate) In photorespiration, rubisco adds O2 instead
ofCO2 in the Calvin cycle, producing a two-carbon compound
Photorespiration decreases photosynthetic output by consuming ATP,
O2, and organic fuel and releasing CO2 without producing any ATP or
sugar 2014 Pearson Education, Inc. 85 Photorespiration may be an
evolutionary relic because rubisco first evolved at a time when the
atmosphere had far less O2 and more CO2 Photorespiration limits
damaging products of light reactions that build up in the absence
of the Calvin cycle 2014 Pearson Education, Inc. 86 C4 Plants C4
plants minimize the cost of photorespiration by incorporating CO2
into a four-carbon compound An enzyme in the mesophyll cells has a
high affinity for CO2 and can fix carbon even when CO2
concentrations are low These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2 that is then used in
the Calvin cycle 2014 Pearson Education, Inc. 87 Figure 8.18a
Figure 8.18a C4 and CAM photosynthesis compared (part 1: C4,
sugarcane) Sugarcane 88 CAM Plants Some plants, including
succulents, use crassulacean acid metabolism (CAM) to fix carbon
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 2014 Pearson
Education, Inc. 89 Figure 8.18b Figure 8.18b C4 and CAM
photosynthesis compared (part 2: CAM, pineapple) Pineapple 90
Calvin Cycle Calvin Cycle
Figure 8.18 Sugarcane Pineapple 1 1 CO2 CO2 C4 CAM Mesophyll cell
Organic acid Organic acid Night CO2 2 CO2 2 Figure 8.18 C4 and CAM
photosynthesis compared Bundle- sheath cell Day Calvin Cycle Calvin
Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal
separation of steps 91 Figure 8.UN04 Figure 8.UN04 Skills exercise:
making scatter plots with regression lines 92 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 Plants store excess sugar as starch in
the chloroplasts and in structures such as roots, tubers, seeds,
and fruits In addition to food production, photosynthesis produces
the O2 in our atmosphere 2014 Pearson Education, Inc. 93 Electron
transport chain Electron transport chain
Figure 8.19 H2O CO2 Light NADP ADP P i Light Reactions: RuBP
3-Phosphpglycerate Photosystem II Electron transport chain Calvin
Cycle Photosystem I Electron transport chain ATP G3P Figure 8.19 A
review of photosynthesis Starch (storage) NADPH Chloroplast O2
Sucrose (export) 94 Primary acceptor Electron transport chain
Primary acceptor
Figure 8.UN05 Primary acceptor Electron transport chain Primary
acceptor Electron transport chain Fd NADP H2O Pq NADP reductase H
O2 NADPH Cytochrome complex Pc Figure 8.UN05 Summary of key
concepts: the light reactions Photosystem I ATP Photosystem II 95
Figure 8.UN07 pH 4 pH 7 pH 4 pH 8 Figure 8.UN07 Test your
understanding, question 9 (thylakoid experiment) 96