Chapter 10Chapter 10
Photosynthesis
Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
• The structural organization of these cells allows for the chemical reactions of photosynthesis
I. Chloroplasts: Photosynthesis in Plants
• Leaves = major locations of photo
• Chlorophyll = green pigment w/in chloroplasts, absorbs light energy to make organic molecules in chloroplast
• CO2 enters and O2 exits leaf through microscopic pores = stomata
• Found mainly in cells of the mesophyll (interior tissue of leaf)
• Typical mesophyll cell has 30–40 chloroplasts
• Chlorophyll is in membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
• Stroma - dense fluid inside chloroplasts
Fig. 10-3a
5 µm
Mesophyll cell
StomataCO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section
Fig. 10-3b
1 µm
Thylakoidspace
Chloroplast
GranumIntermembranespace
Innermembrane
Outermembrane
Stroma
Thylakoid
II. Tracking Atoms Through Photosynthesis
• Photosynthesis summarized as:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
A. The Splitting of Water
• Chloroplasts split H2O into H and O, incorporating the electrons of H into sugar molecules
Reactants: 6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
III. Two Stages of Photosynthesis: A Preview
• Photosynthesis = light reactions (photo part) and Calvin cycle (synthesis part)
• Light reactions (in the thylakoids):
– Split H2O
– Release O2
– Reduce NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
• Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH. Begins with carbon fixation, incorporating CO2 into organic molecules
Light
Fig. 10-5-1
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
Light
Fig. 10-5-2
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
Light
Fig. 10-5-3
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
Light
Fig. 10-5-4
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)
Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
• Chloroplasts are solar-powered chemical factories
• Thylakoids transform light energy into chemical energy of ATP and NADPH
IV. Nature of Sunlight
• Light = form of electromagnetic energy, also called electromagnetic radiation
• Light travels in waves
• Wavelength = dist between crests of waves
• Wavelength determines the type of electromagnetic energy
• Electromagnetic spectrum = range of electromagnetic energy (radiation)
• Visible light = wavelengths (inc. those for photosynthesis) that produce colors we can see
• Light also acts as though it consists of small particles (photons)
UV
Fig. 10-6
Visible light
InfraredMicro-waves
RadiowavesX-raysGamma
rays
103 m1 m
(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energyHigher energy
Shorter wavelength
V. Photosynthetic Pigments
• Absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are reflected or transmitted
• Leaves appear green b/c chlorophyll reflects and transmits green light
Fig. 10-7
Reflectedlight
Absorbedlight
Light
Chloroplast
Transmittedlight
Granum
Galvanometer
Slit moves topass lightof selectedwavelength
Whitelight
Greenlight
Bluelight
The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
Refractingprism
Photoelectrictube
Chlorophyllsolution
1
2 3
4
• Spectrophotometer = measures a pigment’s ability to absorb various wavelengths
• Absorption spectrum = graph plotting a pigment’s light absorption vs wavelength
• Absorption spectrum of chlorophyll a suggests violet-blue and red light work best
• Action spectrum profiles the effectiveness of different wavelengths of radiation in a process
Fig. 10-9
Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’s experiment
Aerobic bacteria
RESULTS
Ra
te o
f p
ho
tos
yn
the
sis
(me
as
ure
d b
y O
2 re
lea
se
)A
bs
orp
tio
n o
f li
gh
t b
yc
hlo
rop
las
t p
igm
en
ts
Filamentof alga
Chloro- phyll a Chlorophyll b
Carotenoids
500400 600 700
700600500400
• Chlorophyll a = main photosynthetic pigment
• Accessory pigments
– chlorophyll b = broaden spectrum used for photosynthesis
– Carotenoids = absorb excessive light that would damage chlorophyll
• Overview Photosynthesis Structures
Fig. 10-10
Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center
in chlorophyll aCH3
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown
CHO in chlorophyll b
VI. Excitation of Chlorophyll by Light
• Pigment absorbs light, it goes from a ground state to an excited state (unstable)
• Excited e-s fall back to ground state, photons are given off, an afterglow called fluorescence
• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
Fig. 10-11
(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
Photon Groundstate
Photon(fluorescence)
En
erg
y o
f el
ectr
on
e–
Chlorophyllmolecule
VII. Photosystem
• Photosystem = a reaction-center complex (protein) surrounded by light-harvesting complexes
• Light-harvesting complexes (pigment molecules bound to proteins) funnel energy of photons to reaction center
• Primary electron acceptor in reaction center accepts an excited e- from chlorophyll a
• This solar-powered transfer of an e- from chlorophyll a to PEA is first step of light reactions
Fig. 10-12
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
STROMA
e–
Pigmentmolecules
Photon
Transferof energy
Special pair ofchlorophyll amolecules
Th
yla
koid
me
mb
ran
e
Photosystem
Primaryelectronacceptor
Reaction-centercomplex
Light-harvestingcomplexes
• Two types of photosystems in thylakoid membrane
– Photosystem II (PS II) functions first (# reflects order of discovery) and is best at absorbing a wavelength of 680 nm
• The reaction-center chlorophyll a of PS II is called P680
– Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
• The reaction-center chlorophyll a of PS I is called P700
• Photosynthesis Part 1 Bleier (15 min)
VIII. Linear Electron Flow
• During light reactions, there are two possible routes for e- flow: cyclic and linear
• Linear electron flow (primary pathway) involves both photosystems and produces ATP and NADPH using light energy
• Photon hits pigment and its energy is passed among pigment molecules until it excites P680
• An excited e- from P680 is transferred to the primary electron acceptor
Pigmentmolecules
Light
P680
2
1
Photosystem II(PS II)
Primaryacceptor
• P680+ (P680 missing an e-) is a very strong oxidizing agent
• H2O is split by enzymes, and e-s are transferred from H atoms to P680+, reducing it to P680
• O2 is released as a by-product
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
Fig. 10-13-2
Photosystem II(PS II)
• Each e- “falls” down an electron transport chain from primary electron acceptor of PS II to 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 (sound familiar???)
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Fig. 10-13-3
Photosystem II(PS II)
• PS I (like PS II), transferred light energy excites P700, which loses an e- to an electron acceptor
• P700+ (P700 missing an e-) accepts an electron passed down from PS II via the electron transport chain
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fig. 10-13-4
Photosystem II(PS II)
• Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
• e-s are then transferred to NADP+ and reduce it to NADPH
• e-s of NADPH are available for the Calvin cycle
• Photo ETC and ATP Production
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fd
Electron transport chain
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
Fig. 10-13-5
Photosystem II(PS II)
Fig. 10-14
MillmakesATP
e–
NADPH
Ph
oto
n
e–
e–
e–
e–
e–
Ph
oto
n
ATP
Photosystem II Photosystem I
e–
IX. Cyclic Electron Flow
• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
• Generates surplus ATP, satisfying the higher demand in the Calvin cycle
Fig. 10-15
ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+
• Some organisms (purple sulfur bacteria) have PS I but not PS II
• Cyclic electron flow is thought to have evolved before linear electron flow and may protect cells from light-induced damage
• Cyclic vs Linear E- Flow
X. Comparison of Chloroplasts and Mitochondria
• Both generate ATP by chemiosmosis, but use different sources of energy
• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into chemical energy of ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In mitochondria, protons are pumped OUT to intermembrane space and drive ATP synthesis as they diffuse back INTO mitochondrial matrix
• In chloroplasts, protons are pumped INTO thylakoid space and drive ATP synthesis as they diffuse back OUT to the stroma
Fig. 10-16
Key
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
MITOCHONDRIONSTRUCTURE
Intermembranespace
Innermembrane
Electrontransport
chain
H+ Diffusion
Matrix
Higher [H+]Lower [H+]
Stroma
ATPsynthase
ADP + P i
H+ATP
Thylakoidspace
Thylakoidmembrane
• ATP and NADPH are produced on the side facing the stroma, where Calvin cycle takes place
• In summary, light reactions generate ATP and increase the potential energy of e-s by moving them from H2O to NADPH
Fig. 10-17
Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
XI. Calvin Cycle
• The Calvin cycle, like citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
• Builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
• Calvin Cycle
• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
Fig. 10-18-1
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
Fig. 10-18-2
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
Fig. 10-18-3
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
• Photosynthesis Part 2 Bleier (15 min)
XII. Alternative carbon fixation
• Dehydration is a problem for plants
• Hot, dry days, plants close stomata, which conserves H2O but limits photosynthesis
– This reduces access to CO2 and causes O2 to build up
– Favor a wasteful process called photorespiration
XIII. Photorespiration
• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (thus the C3)
• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle
• Photoresp consumes O2 and organic fuel and releases CO2 w/o producing ATP or sugar
• May be an evolutionary relic because rubisco first evolved at a time when atmosphere had far less O2 and more CO2
• Limits damaging products of light reactions that build up in the absence of the Calvin cycle
• 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
XIV. C4 Plants
• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
• Requires enzyme PEP carboxylase
• PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 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
Fig. 10-19
C4 leaf anatomy
Mesophyll cellPhotosyntheticcells of C4
plant leafBundle-sheathcell
Vein(vascular tissue)
Stoma
The C4 pathway
Mesophyllcell CO2PEP carboxylase
Oxaloacetate (4C)
Malate (4C)
PEP (3C)ADP
ATP
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
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
Fig. 10-20
CO2
Sugarcane
Mesophyllcell
CO2
C4
Bundle-sheathcell
Organic acidsrelease CO2 to Calvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Pineapple
Night
Day
CAM
SugarSugar
CalvinCycle
CalvinCycle
Organic acid Organic acid
(a) Spatial separation of steps (b) Temporal separation of steps
CO2 CO2
1
2
XV. Photosynthesis: A Review
• Sunlight gets stored as chemical energy in organic compounds
• Sugar supplies chemical energy and carbon skeletons to synthesize molecules of cells
• Plants store excess sugar as starch in roots, tubers, seeds, and fruits
• Produces the O2 in our atmosphere
• Overview - Anderson (13 min)
Fig. 10-21
LightReactions:
Photosystem II Electron transport chain
Photosystem I Electron transport chain
CO2
NADP+
ADP
P i+
RuBP 3-Phosphoglycerate
CalvinCycle
G3PATP
NADPHStarch(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2
Fig. 10-UN1
CO2
NADP+
reductase
Photosystem II
H2O
O2
ATP
Pc
Cytochromecomplex
Primaryacceptor
Primaryacceptor
Photosystem I
NADP+
+ H+
Fd
NADPH
Electron transport
chain
Electron transport
chain
O2
H2O Pq
Fig. 10-UN2
Regeneration ofCO2 acceptor
1 G3P (3C)
Reduction
Carbon fixation
3 CO2
CalvinCycle
6 3C
5 3C
3 5C