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Photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-3
Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Thylakoid
Stroma Granum
space
Intermembrane
space
Inner
membrane
1 µm
Photosynthesis as a Redox Process
• A simplified equation of photosynthesis that
indicates only the net consumption of water is as
follows:
6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2
• Photosynthesis is a redox process in which water
is oxidized and carbon dioxide is reduced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the:
– light reactions (the photo
part) and
– Calvin cycle (the sugar
synthesis part)
• The light reactions split water,
release O2, produce ATP, and form
NADPH
• The Calvin cycle forms sugar
from CO2, using ATP and NADPH
made in the light reactions.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1- The light reactions (in the thylakoids)
split water, release O2, produce ATP, and
form NADPH
H2O
H2O → 2H+ + 2e- + ½ O2
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
2- The Calvin cycle (in the stroma) forms
sugar from CO2, using ATP and NADPH made in the
light reactions.
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
[CH2O]
(sugar)
The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
10–5 nm 10–3 nm
Gamma
rays
103 nm
1 nm
X-rays
106 nm
Infrared
UV
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light consists of colors we
can see, including wavelengths
that drive photosynthesis
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Comparing the Wavelengths of the Different
Colors of the Visible Light
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthetic Pigments
• The main photosynthetic pigment is called
– Chlorophyll a.
• The accessory pigments are:
– chlorophyll b
– carotenoids absorb excessive light that would damage
chlorophyll.
Types of carotenoids:
– Carotenes (are pure hydrocarbons, made solely from C &
H. For example: -carotene in carrots)
– Xanthophylls (are oxygenated hydrocarbons, made of C, H
& O. For example lycopene, the red pigment in tomatoes)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-12
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)
The Light Reactions
• During the light reactions, there are two
possible routes for electron flow:
1- the linear (noncyclic) electron flow.
2- the cyclic electron flow.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Linear Electron Flow
• The linear (noncyclic) electron flow:
– Is the primary pathway of the light reactions.
– Occurs in the thylakoid membranes.
– Involves both photosystems (PSI & PSII).
– Splits water (H2O → 2H+ + 2e- + ½ O2)
– Uses light energy to produces ATP and
NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-13_1
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)
LE 10-13_2
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)
LE 10-13_3
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)
LE 10-13_4
H2O
CO2
Light
The Linear Electron Flow
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)
LE 10-13_5
H2 O
The Linear Electron Flow
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)
A mechanical analogy for
the linear electron flow of
the light reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
The Cyclic Electron Flow
• The cyclic electron flow:
• Occurs in the thylakoid membranes.
• Uses only photosystem I
• Excited electrons leave P700 and return to it.
• Does not produce NADPH or O2
• 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
LE 10-15
The Cyclic Electron Flow
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
P700
Pc
Photosystem I
Photosystem II
ATP
NADPH
LE 10-17
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 Phases of the Calvin Cycle
• The Calvin cycle has three phases:
– Carbon fixation phase catalyzed by the
enzyme: ribulose bishosphate carboxylase
oxygenase (rubisco)
– Reduction: Making the sugar G3P.
– Regeneration of the CO2 acceptor
1,5-Ribulose Bisphosphate (RuBP)
http://www.science.smith.edu/departments/Biology/Bio
231/calvin.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-21
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)