Transcript Chapter 8: Photosynthesis: Capturing Energy

Chapter 8: Photosynthesis:
Capturing Energy
• 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)
Fig. 10-6
10–5 nm 10–3 nm
103 nm
1 nm
Gamma
X-rays
rays
UV
106 nm
Infrared
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Light
• Behaves as waves and particles
• Photons = particles/packets of energy
• E= hc/λ
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 eacceptor molecule
– photosynthesis
Fig. 10-11
Energy of electron
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
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. 10-3
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
5 µm
Fig. 10-3a
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
Fig. 10-3b
Chloroplast
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
Chlorophyll
•
•
•
•
Thylakoid membrane
Main pigment of photosynthesis
Absorbs mostly blue/red wavelengths
Green – green light is scattered/reflected
Fig. 10-7
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
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
CH3
CHO
in chlorophyll a
in chlorophyll b
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
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 eacceptor (oxidizing agent)
• Many complex steps
• 2 parts:
– Light-dependent (photo) – thylakoids
– Carbon fixation (synthesis) - stroma
Fig. 10-4
Reactants:
Products:
6 CO2
C6H12O6
12 H2O
6 H2 O
6 O2
Redox
Fig. 10-5-1
H2O
Light
NADP+
ADP
+ P
Light
Reactions
Chloroplast
i
Fig. 10-5-2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
ATP
NADPH
Chloroplast
O2
Fig. 10-5-3
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
ATP
NADPH
Chloroplast
O2
Calvin
Cycle
Fig. 10-5-4
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[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
– ADPATP
– 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
Light-Dependent Reactions Details
• Light energy  chemical energy
• Summary equation:
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
Photosystem
STROMA
Light-harvesting Reaction-center
complex
complexes
Primary
electron
acceptor
Thylakoid membrane
Photon
e–
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
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
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Fig. 10-13-1
Primary
acceptor
e–
2
P680
1 Light
Pigment
molecules
Photosystem II
(PS II)
• 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
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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+
Fig. 10-13-2
Primary
acceptor
2 H+
+
1/ O
2
2
H2O
e–
2
3
e–
e–
P680
1 Light
Pigment
molecules
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
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Fig. 10-13-3
4
Primary
acceptor
1/
2
H+
2
+
O2
H2O
e–
2
Pq
Cytochrome
complex
3
Pc
e–
e–
5
P680
1 Light
ATP
Pigment
molecules
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
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Fig. 10-13-4
4
Primary
acceptor
1/
2
H+
2
+
O2
H2O
e–
2
Primary
acceptor
e–
Pq
Cytochrome
complex
3
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
• 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
Fig. 10-13-5
4
Primary
acceptor
2
H+
+
1/ O
2
2
H2O
e–
2
Primary
acceptor
e–
Pq
Cytochrome
complex
7
Fd
e–
e–
8
NADP+
reductase
3
NADPH
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
NADP+
+ H+
Photosystem I
(PS I)
Fig. 10-14
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Cyclic Electron Transport (simplest lightdependent 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
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
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 eenergized by light (photo), process =
photophosphorylation
Fig. 10-17
STROMA
(low H+ concentration)
Cytochrome
Photosystem I
complex
Light
Photosystem II
4 H+
Light
Fd
NADP+
reductase
NADP+ + H+
NADPH
Pq
H2O
THYLAKOID SPACE
(high H+ concentration)
e–
1
e–
1/
Pc
2
2
3
O2
+2 H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
Light Reactions
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
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
3P
Ribulose bisphosphate
(RuBP)
P
6
P
3-Phosphoglycerate
Fig. 10-18-2
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
3P
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
Fig. 10-18-3
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
3P
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
ATP
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
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
Calvin Cycle
Photosynthesis Summary Video
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
The C4 pathway
C4 leaf anatomy
Mesophyll
cell
Mesophyll cell
CO2
PEP carboxylase
Photosynthetic
cells of C4
Bundleplant leaf
sheath
cell
Oxaloacetate (4C)
Vein
(vascular tissue)
PEP (3C)
ADP
Malate (4C)
Stoma
Bundlesheath
cell
ATP
Pyruvate (3C)
CO2
Calvin
Cycle
Sugar
Vascular
tissue
Fig. 10-19a
C4 leaf anatomy
Mesophyll cell
Photosynthetic
cells of C4
Bundleplant leaf
sheath
cell
Vein
(vascular tissue)
Stoma
Fig. 10-19b
The C4
pathway
Mesophyll
cell
PEP carboxylase
Oxaloacetate (4C)
PEP (3C)
ADP
Malate (4C)
Bundlesheath
cell
CO2
ATP
Pyruvate (3C)
CO2
Calvin
Cycle
Sugar
Vascular
tissue
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
• C3 and C4 – different location
• C3 and CAM – different times, same cell
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 chloroplast
2. Describe the relationship between an action
spectrum and an absorption spectrum
3. Trace the movement of electrons in linear
electron flow
4. Trace the movement of electrons in cyclic
electron flow
5. Describe the role of ATP and NADPH in the
Calvin cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings