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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
e–
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule
Fluorescence
A Photosystem: A Reaction Center Associated with
Light-Harvesting Complexes
• A photosystem consists of a reaction center
surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment
molecules bound to proteins) funnel the energy
of photons to the reaction center
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A primary electron acceptor in the reaction
center accepts an excited electron from
chlorophyll a
• Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• There are two types of photosystems in the
thylakoid membrane
• Photosystem II functions first (the numbers reflect
order of discovery) and is best at absorbing a
wavelength of 680 nm
• Photosystem I is best at absorbing a wavelength
of 700 nm
• The two photosystems work together to use light
energy to generate ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Noncyclic Electron Flow
• During the light reactions, there are two possible
routes for electron flow: cyclic and noncyclic
• Noncyclic electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Light
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Cyclic Electron Flow
• Cyclic electron flow uses only photosystem I and
produces only ATP
• Cyclic electron flow generates surplus ATP,
satisfying the higher demand in the Calvin cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
• Chloroplasts and mitochondria generate ATP
by chemiosmosis (generation of ATP by the
movement of hydrogen ions across a
membrane), but use different sources of
energy
• Mitochondria transfer chemical energy from
food to ATP; chloroplasts transform light energy
into the chemical energy of ATP
• The spatial organization of chemiosmosis
differs in chloroplasts and mitochondria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mitochondrion
Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
H+
Intermembrane
space
Membrane
Lower [H+]
Thylakoid
space
Electron
transport
chain
ATP
synthase
Key
Higher [H+]
Diffusion
Stroma
Matrix
ADP + P i
ATP
H+
• The current model for the thylakoid membrane
is based on studies in several laboratories
• Water is split by photosystem II on the side of
the membrane facing the thylakoid space
• The diffusion of H+ from the thylakoid space
back to the stroma powers ATP synthase
• ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes
place
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2
Pc
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
The Calvin cycle uses ATP and NADPH to convert
CO2 to sugar
• The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
• The cycle builds sugar from smaller molecules by
using ATP and the reducing power of electrons
carried by NADPH
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
P
P
6
3-Phosphoglycerate
3 P
P
Ribulose bisphosphate
(RuBP)
6
6 ADP
CALVIN
CYCLE
ATP
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
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
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
CALVIN
CYCLE
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
Alternative mechanisms of carbon fixation have
evolved in hot, arid climates
• Dehydration is a problem for plants, sometimes
requiring tradeoffs with other metabolic processes,
especially photosynthesis
• On hot, dry days, plants close stomata, which
conserves water but also limits photosynthesis
• The closing of stomata reduces access to CO2 and
causes O2 to build up
• These conditions favor a seemingly wasteful
process called photorespiration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photorespiration: An Evolutionary Relic?
• In most plants (C3 plants), initial fixation of CO2,
via rubisco, forms a three-carbon compound
• In photorespiration, rubisco adds O2 to the Calvin
cycle instead of CO2
• Photorespiration consumes O2 and organic fuel
and releases CO2 without producing ATP or sugar
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when the
atmosphere had far less O2 and more CO2
• In many plants, 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C4 Plants
• C4 plants minimize the cost of photorespiration by
incorporating CO2 into four-carbon compounds in
mesophyll cells
• These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2 that
is then used in the Calvin cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthetic
cells of C4 plant
leaf
Mesophyll
cell
PEP carboxylase
Mesophyll cell
CO2
Bundlesheath
cell
The C4 pathway
Oxaloacetate (4 C) PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Stoma
Bundlesheath
cell
Pyruvate (3 C)
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue
CAM Plants
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CALVIN
CYCLE
Sugar
Temporal separation of steps
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
• In addition to food production, photosynthesis
produces the oxygen in our atmosphere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+ Pi
RuBP
Photosystem II
Electron transport
chain
Photosystem I
ATP
NADPH
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)