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Chapter 10:
Photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Melvin Calvin - American biochemist who his elucidated the mechanism by
which carbon dioxide is incorporated into green plants, for which he received
the 1961 Nobel Prize for Chemistry. In the Calvin Cycle, he described the
"dark reactions" of photosynthesis occuring through the night turning carbon
dioxide into sugar.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.1 Sunlight consists of a spectrum of colors,
visible here in a rainbow
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.2 Photoautotrophs
These organisms use light energy to drive the
synthesis of organic molecules from carbon dioxide
and (in most cases) water. They feed not only
themselves, but the entire living world. (a) On
land, plants are the predominant producers of
food. In aquatic environments, photosynthetic
organisms include (b) multicellular algae, such
as this kelp; (c) some unicellular protists, such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other photosynthetic
prokaryotes, such as these purple sulfur
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(a) Plants
(c) Unicellular protist
10 m
(d) Pruple sulfur
bacteria
(b) Multicellular algae
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(c) Cyanobacteria
40 m
1.5 m
Figure 10.3 Focusing in on the location of
photosynthesis in a plant
Leaf cross section
Vein
Mesophyll
CO2 O2
Mesophyll cell
Stomata
Chloroplast
5 µm
Outer
membrane
Granum
Storma
Thylakoid Thylakoid
Space
Intermembrane
space
Inner membrane
1 µm
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Tracking atoms through photosynthesis
Reactants:
Products:
12 H2O
6 CO2
C6H12O6
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
6
H2O
6
O2
An overview of photosynthesis: cooperation of the light
reactions and the Calvin cycle
H2O
Light
LIGHT
REACTIONS
Chloroplast
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An overview of photosynthesis: cooperation of the light
reactions and the Calvin cycle
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An overview of photosynthesis: cooperation of the light
reactions and the Calvin cycle
H2O
CO2
Light
NADP
ADP
+ Pi
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
[CH2O]
(sugar)
The electromagnetic spectrum
10–5
nm
10–3
Gamma
rays
103
1 nm
nm
X-rays
UV
106
nm
Infrared
1m
106 nm
nm
Microwaves
103 m
Radio
waves
Visible light
380
450
500
550
Shorter wavelength
Higher energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
600
650
700
750 nm
Longer wavelength
Lower energy
Research Method Determining an Absorption
Spectrum
APPLICATION An absorption spectrum is a visual
representation of how well a particular pigment absorbs different
wavelengths of visible light. Absorption spectra of various chloroplast
pigments help scientists decipher each pigment’s role in a plant.
TECNIQUE A spectrophotometer measures the relative
amounts of light of different wavelengths absorbed and
transmitted by a pigment solution.
1
White light is separated into colors (wavelengths) by a prism.
2
One by one, the different colors of light are passed through the
sample (chlorophyll in this example). Green light and blue light
are shown here.
3
The transmitted light strikes a photoelectric tube, which
converts the light energy to electricity.
4
The electrical current is measured by a galvanometer. The meter
indicates the fraction of light transmitted through the sample,
from which we can determine the amount of light absorbed.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Refracting Chlorophyll
solution
prism
White
light
Photoelectric
tube
Galvanometer
2
1
3
4
Slit moves to
pass light
of selected
wavelength
Green
light
0
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Blue
light
Result
100
100
The low transmittance
(high absorption) reading indicates that
chlorophyll absorbs most blue light.
See Figure 10.9a for absorption spectra of three types of chloroplast pigments.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Inquiry Which wavelengths of light are most
effective in driving photosynthesis?
EXPERIMENT Three different experiments helped reveal which wavelengths of light are photosynthetically
important. The results are shown below.
RESULTS
Chlorophyll a
Absorption of light by
chloroplast pigments
Chlorophyll b
Carotenoids
400
500
600
700
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.
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Aerobic bacteria
Filament
of alga
400
500
600
700
(c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with
light that had been passed through a prism, exposing different segments of the alga to different
wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine
which segments of the alga were releasing the most O2 and thus photosynthesizing most.
Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue
or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.
Light in the violet-blue and red portions of the spectrum are most effective in
driving photosynthesis.
CONCLUSION
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A mechanical analogy for the light reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
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Photosystem I
The Calvin cycle
CO2
H2 O
Input
3 (Entering one
CO2 at a time)
Light
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
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 P
5
i
P
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
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Glucose and
other organic
compounds
Phase 2:
Reduction
C4 carbon fixation is one of three biochemical
mechanisms, along with C3 and CAM
photosynthesis, used in carbon fixation.
C4 carbon fixation is an elaboration of C3 pathways
that allows more efficient use of scarce CO2.
CAM Photosynthesis (also called crassulacean acid
metabolism) is a carbon fixation pathway that
evolved in some plants as an adaptation to arid
conditions.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C4 leaf anatomy and the C4 pathway
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf
CO
CO
2 2
PEP carboxylase
Bundlesheath
cell
PEP (3 C)
ADP
Oxaloacetate (4 C)
Vein
(vascular tissue)
Malate (4 C)
ATP
C4 leaf anatomy
BundleSheath
cell
Pyruate (3 C)
CO2
Stoma
CALVIN
CYCLE
Sugar
Vascular
tissue
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C4 and CAM photosynthesis compared
Pineapple
Sugarcane
C4
Mesophyll Cell
Organic acid
Bundlesheath
cell
(a) Spatial separation
of steps. In C4
plants, carbon fixation
and the Calvin cycle
occur in different
types of cells.
CAM
CO2
CO2
CALVIN
CYCLE
CO2
1 CO2 incorporated Organic acid
into four-carbon
organic acids
(carbon fixation)
2 Organic acids
release CO2 to
Calvin cycle
Sugar
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
CALVIN
CYCLE
Sugar
Night
Day
(b) Temporal separation
of steps. In CAM
plants, carbon fixation
and the Calvin cycle
occur in the same cells
at different times.
A review of photosynthesis
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Light reactions:
• Are carried out by molecules in the
thylakoid membranes
• Convert light energy to the chemical
energy of ATP and NADPH
• Split H2O and release O2 to the
atmosphere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sucrose (export)
Calvin cycle reactions:
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate, and
NADP+ to the light reactions