chapter 8: photosynthesis: capturing energy. photosynthesis: – absorb and convert light energy...
TRANSCRIPT
• Photosynthesis: – absorb and convert light energy into stored
chemical energy of organic molecules
Electromagnetic Spectrum
• Wavelength – all radiation travels in waves• Visible spectrum –
– 760 nm (red) - 380nm (purple)
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
2 ways excited electrons can behave (when absorb photon of light)
• 1st shifts to higher-energy orbital, THEN• 1) atom can return to ground state (e- are in
normal, lowest energy levels)– Energy lost as heat or light (fluorescence)
• 2) e- can leave atom and be accepted by e- acceptor molecule– photosynthesis
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
Photosynthesis in Chloroplasts
• Chlorophyll - green pigment, in chloroplasts, mesophyll
• Chloroplast – – Outer membrane – Inner membrane – encloses stroma
• Stroma (fluid-filled, enzymes to make carbs.)
• Thylakoids – – in stroma, 3rd sys. Of membranes – forms
interconnected flat, disclike sacs
• Thylakoid lumen – – fluid-filled space inside of thylakoid
• Grana = thylakoid stacks
Fig. 10-3Leaf cross section
Vein
Mesophyll
StomataCO2 O2
ChloroplastMesophyll cell
Outermembrane
Intermembranespace
5 µm
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
Fig. 10-3b
1 µm
Thylakoidspace
Chloroplast
GranumIntermembranespace
Innermembrane
Outermembrane
Stroma
Thylakoid
Chlorophyll
• Thylakoid membrane• Main pigment of photosynthesis• Absorbs mostly blue/red wavelengths• Green – green light is scattered/reflected
2 main parts of Chlorophyll
• 1) complex ring = porphyrin ring– Joined smaller rings of C and N– Absorbs light energy– Magnesium in center
• 2) long side chain – Hydrocarbons– Extremely nonpolar
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
Types of Chlorophylls• Chlorophyll a
– Most important– Bright green– Initiate light-dependent reactions
• Chlorophyll b– Accessory pigment– Yellow-green– Different functional group on porphyrin ring – shifts λ of
light that is absorbed/reflected
• Carotenoids – Accessory – yellow, orange
Spectrums
• Absorption spectrum – plot of a PIGMENT’S absorption of light of different λ
• Action spectrum – gives relative effectiveness of different λs of light in photosynthesis (PROCESS)– Rate of photosynthesis is measured at each λ for
leaf cells/tissues exposed to monochromatic light
Photosynthesis simplified:
• 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
• Redox– e- transferred from e- donor (reducing agent) to an e-
acceptor (oxidizing agent)
• Many complex steps• 2 parts:
– Light-dependent (photo) – thylakoids– Carbon fixation (synthesis) - stroma
Light
Fig. 10-5-4
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)
Overview of light-dependent reactions
• Chlorophyll captures light energy• 1 e- moves to higher state• e- transferred to acceptor molecule, replaced by
e- from water• Water is split• Oxygen released• Need some energy for
– ADPATP– NADP+ NADPH
Overview of Carbon fixation
• Fix C atoms from CO2 to existing C skeletons• No direct light needed
– “dark” reactions
• Depends on products of light-reactions
Photosystems I and II• Reaction center + many antenna complexes• Antenna complex (light-harvesting) =
– units of chlorophylls a + b and accessory pigments organized with pigment-binding proteins in thylakoid membranes
– Absorbs light energy and transfers it to reaction center
• Reaction center = – complex of chlorophyll molecules + proteins– Light energy chemical energy by series of e- transfers
• Photosystem I – chlorophyll a – 700 nm (P700)• Photosystem II – chlorophyll a 680 nm (P680)• Pigment absorbs light energy• Energy passed from 1 pigment molecule to
another until it reaches P700 or P680 at reaction center
• e- raised to higher energy level• e- donated to e- acceptor
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
Noncyclic electron transport
• Makes ATP and NADPH• Continuous linear process
– 1 way flow of e- from water to NADP+– Water photolysis e- to P680 ETC (e- lose
energy) P700 ETC NADP+
• See diagram
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• P680+ (P680 that is missing an electron) is a very strong oxidizing agent
• 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 of this reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Photolysis
• “light-splitting”• Catalyzed by manganese-containing enzyme;
breaks water into 2e-, 2p+ and O• Each e- donated to P680• p+ released into thylakoid lumen• 2 water must split to yield 1 O2 atmosphere
• 2H2O O2 + 4H+
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 electron “falls” down an electron transport chain from the 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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)
• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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 electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
• The electrons are then transferred to NADP+ and reduce it to NADPH
• The electrons of NADPH are available for the reactions of the Calvin cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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–
Cyclic Electron Transport (simplest light-dependent reaction)
• Makes ATP, no NADPH• Only Photosystem I• Cyclic – energized e- that originate from P700
eventually return to P700• Light – continuous flow of e- through ETC in
thylakoid membrane
• e- passed from 1 acceptor to another, e- lose energy (some energy used to pump protons across thylakoid membranes)
• ATP synthase uses proton gradient to make ATP
• NADPH not made, water not split, O2 not made
Fig. 10-15
ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+
ATP synthesis
• By chemiosmosis• Photosystem II – as e- passed down ETC, some
energy releases (exergonic)• Some energy not released drives synthesis
of ATP (endergonic)• Synthesis of ATP (P +ADP) is coupled with e-
energized by light (photo), process = photophosphorylation
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
Carbon Fixation
• Requires ATP + NADPH – Energy used to form organic molecules from CO2
• Summary equation:
Calvin Cycle
• Most plants use – C3
• In stroma – 13 reactions• 3 phases:
– CO2 uptake
– Carbon reduction– RuBP regeneration
CO2 Uptake
• One reaction• CO2 + ribulose biphosphate (RuBP) [5-C]
• Enzyme = ribulose biphosphate carboxylase/oxygenase (Rubisco)
• Product = unstable 6-C intermediate• Immediately 2 phosphoglycerate (PGA) (3-
C each)• C3 pathway
Carbon Reduction phase• 2 steps• Energy from ATP and NADPH converts PGA
molecules to glyceraldehyde-3-phosphate (G3P)• For net synthesis of 1 G3P, the cycle must take
place three times, fixing 3 molecules of CO2
• 6C enter as CO2, 6C leave as 2 – G3P (can form glucose or fructose)
• 2 – G3P removed from cycle, 10 G3P remain = 30 C atoms total
RuBP regeneration phase
• 10 reactions• 30 C rearranged into 6 ribulose phosphate
(+P) RuBP (5-C where cycle started)
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
Summary of Carbon Fixation
• Inputs:– 6 CO2
– P from ATP– e- (as hydrogen) from NADPH
• End– 6C hexose molecule remaining G3P make RuBP
which combines with more CO2
C4 and CAM plants
• Initial carbon fixation step differs – precedes Calvin Cycle; does not replace it
C4 Pathway
• Fixes CO2 at low concentration
• 1st - fix CO2 into 4C oxaloacetate– in mesophyll cells (Calvin in bundle sheath cells)
• PEP carboxylase – catalyzes reaction– CO2 + phosphoenolpyruvate (PEP) (3C)
oxaloacetate
• Oxaloacetate +NADPH usually malate (into bundle sheath) decarboxylation pyruvate (3C) + CO2
• Malate + NADP+ Pyruvate + CO2 + NADPH
• CO2 combines with RuBP Calvin Cycle
• C3-C4 pathway – extra energy for pyruvate PEP ( 30 ATPs per hexose)– Increases CO2 conc. – stomate don’t need to be
open as much promotes rapid growth
• C3 alone (18 ATPs per hexose)
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
Fig. 10-19a
Stoma
C4 leaf anatomy
Photosyntheticcells of C4
plant leaf
Vein(vascular tissue)
Bundle-sheathcell
Mesophyll cell
Fig. 10-19b
Sugar
CO2
Bundle-sheathcell
ATP
ADP
Oxaloacetate (4C) PEP (3C)
PEP carboxylase
Malate (4C)
Mesophyllcell
CO2
CalvinCycle
Pyruvate (3C)
Vasculartissue
The C4
pathway
CAM plants
• Fix CO2 at night• Xeric plants• Crassulacean acid metabolism (CAM)• NIGHT = Use PEP carboxylase to fix CO2
oxaloacetate malate stored in vacuoles• DAY = CO2 removed from malate and ready
for Calvin cycle
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 structures such as roots, tubers, seeds, and fruits
• In addition to food production, photosynthesis produces the O2 in our atmosphere
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Describe the structure of a chloroplast2. Describe the relationship between an action
spectrum and an absorption spectrum3. Trace the movement of electrons in linear
electron flow4. Trace the movement of electrons in cyclic
electron flow5. Describe the role of ATP and NADPH in the
Calvin cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings