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Chapter 6
Photosynthesis


Overview: The Process That Feeds
the Biosphere
Photosynthesis

Is the process that converts solar
energy into chemical energy

Plants and other autotrophs

Are the producers of the biosphere

Plants are photoautotrophs

They use the energy of sunlight to
make organic molecules from water
and carbon dioxide

Photosynthesis
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
(a) Plants
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).

Occurs in plants, algae, certain other
protists, and some prokaryotes
(c) Unicellular protist 10 m
(e) Pruple sulfur
bacteria
(b) Multicellular algae
(d) Cyanobacteria
40 m
1.5 m

Heterotrophs


Obtain their organic material from
other organisms
Are the consumers of the biosphere

Concept 10.1: Photosynthesis
converts light energy to the
chemical energy of food
Chloroplasts: The Sites of
Photosynthesis in Plants

The leaves of plants

Are the major sites of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata CO2
O2

Chloroplasts


Are the organelles in which photosynthesis
occurs
Contain thylakoids and grana
Mesophyll
Chloroplast
5 µm
Outer
membrane
Thylakoid Thylakoid
StromaGranum
space
Intermembrane
space
Inner
membrane
1 µm
Tracking Atoms Through Photosynthesis:
Scientific Inquiry

Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
The Splitting of Water

Chloroplasts split water into

Hydrogen and oxygen, incorporating
the electrons of hydrogen into sugar
molecules
Reactants:
Products:
12
H2O
6 CO2
C6H12O
6
6
H2O
6
O2
Photosynthesis as a Redox
Process

Photosynthesis is a redox process

Water is oxidized, carbon dioxide is
reduced
The Two Stages of
Photosynthesis: A Preview

Photosynthesis consists of two
processes


The light reactions
The Calvin cycle

The light reactions


Occur in the grana
Split water, release oxygen, produce
ATP, and form NADPH

The Calvin cycle


Occurs in the stroma
Forms sugar from carbon dioxide, using
ATP for energy and NADPH for reducing
power

An overview of photosynthesis
H2O
CO2
Light
NADP 
ADP
+ P
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)

The light reactions convert solar
energy to the chemical energy of
ATP and NADPH
The Nature of Sunlight

Light

Is a form of electromagnetic energy,
which travels in waves

Wavelength


Is the distance between the crests of
waves
Determines the type of electromagnetic
energy

The electromagnetic spectrum

Is the entire range of electromagnetic energy,
or radiation
10–5 nm 10–3 nm
Gamma
X-rays
rays
UV
1m
106 nm
106 nm
103 nm
1 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy

The visible light spectrum


Includes the colors of light we can see
Includes the wavelengths that drive
photosynthesis
Photosynthetic Pigments: The
Light Receptors

Pigments

Are substances that absorb visible light

Reflect light, which include the colors
we see
Light
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light

The spectrophotometer

Is a machine that sends light through
pigments and measures the fraction of
light transmitted at each wavelength

An absorption spectrum

Is a graph plotting light absorption versus
wavelength
Refracting
prism
White
light
Chlorophyll
solution
Photoelectric
tube
Galvanometer
2
3
1
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
100
100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.

The absorption spectra of
chloroplast pigments

Provide clues to the relative
effectiveness of different wavelengths
for driving photosynthesis

The absorption spectra of three types
of pigments in chloroplasts
EXPERIME
NT
Three different experiments helped reveal which wavelengths of light
are photosynthetically important. The results are shown below.
Absorption of light by
chloroplast pigments
RESULTS
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.

The action spectrum of a pigment
Rate of photosynthesis
(measured by O2 release)

(b)
Profiles the relative effectiveness of
different wavelengths of radiation in
driving photosynthesis
Action spectrum. This graph plots the rate of photosynthesis versus wavelength.
The resulting action spectrum resembles the absorption spectrum for chlorophyll
a but does not match exactly (see part a). This is partly due to the absorption of light
by accessory pigments such as chlorophyll b and carotenoids.

The action spectrum for
photosynthesis

Was first demonstrated by Theodor W.
Engelmann
Aerobic bacteria
Filament
of alga
500
600
700
400
(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 O 2 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.

Chlorophyll a


Is the main photosynthetic pigment
Chlorophyll b

Is an accessory pigment
H3C
C
CH2
CH
C
C
H C
H3C C
C
C
H
CH2
CH2
C
O
CH2
C
N
CH3
CHO
H
C
CH3
C
N
C
C
C
C
N
N C
C
C
C
C
C
H
H C
C
O
O
O
O
CH3
Mg
CH2
H
CH3
CH3
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

Other accessory pigments

Absorb different wavelengths of light
and pass the energy to chlorophyll a
Excitation of Chlorophyll by Light

When a pigment absorbs light

It goes from a ground
state to an
Excited
excited state,e which state
is unstable
–
Heat
Photon
(fluorescence)
Photon
Figure 10.11 A
Chlorophyll
molecule
Ground
state

If an isolated solution of chlorophyll
is illuminated

Figure 10.11 B
It will fluoresce, giving off light and
heat
Associated with Light-Harvesting
Complexes
A photosystem

Is composed of a reaction center surrounded
by a number of light-harvesting complexes
Thylakoid
Photosystem
Photon
Light-harvesting
complexes
Thylakoid membrane

STROMA
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
Reaction
center
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)

The light-harvesting complexes


Consist of pigment molecules bound to
particular proteins
Funnel the energy of photons of light to
the reaction center

When a reaction-center chlorophyll
molecule absorbs energy

One of its electrons gets bumped up to
a primary electron acceptor

The thylakoid membrane

Is populated by two types of
photosystems, I and II
Noncyclic Electron Flow

Noncyclic electron flow

Is the primary pathway of energy
transformation in the light reactions
H2O
CO2
Light
NADP+
ADP

Produces NADPH, ATP, and oxygen
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Fd
2
e
H2O
2 H+
+
O2
Pq
e
NADP+
NADP+
+ 2 H+
reductase
3
NADPH
PC
e–
5
+ H+
P700
P680
Light
6
ATP
Figure 10.13
8
e–
Cytochrome
complex
e–
Light
1
7
4
Photosystem II
(PS II)
Photosystem-I
(PS I)

e–
A mechanical analogy for the light
e
reactions e
ATP
–
–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Figure 10.14
Photosystem II
Photosystem I
Cyclic Electron Flow

Under certain conditions

Photoexcited electrons take an
alternative path

In cyclic electron flow


Only photosystem I is used
Only ATP is producedPrimary
Primary
acceptor
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
NADP+
Photosystem I
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria

Chloroplasts and mitochondria


Generate ATP by the same basic
mechanism: chemiosmosis
But use different sources of energy to
accomplish this

The spatial organization of
Key
chemiosmosis
Higher [H ]
+
Lower

[H+]
Chloroplast
Differs Mitochondrion
in chloroplasts and
mitochondria
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
Figure 10.16
H+ Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
Thylakoid
space
Stroma
P
H+
ATP

In both organelles


Redox reactions of electron transport
chains generate a H+ gradient across a
membrane
ATP synthase

Uses this proton-motive force to make
ATP
H2O
CO2
The light reactions and
chemiosmosis: the organization of
the thylakoid membrane
LIGHT

NADP+
ADP
LIGHT
REACTOR
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
Photosystem II
complex
Photosystem I
NADP+
reductase
Light
2 H+
Fd
3
NADP+ + 2H+
NADPH + H+
Pq
Pc
2
H2O
THYLAKOID SPACE
1
(High H+ concentration)
1⁄
2
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
H+


Concept 10.3: The Calvin cycle uses
ATP and NADPH to convert CO2 to
sugar
The Calvin cycle


Is similar to the citric acid cycle
Occurs in the stroma

The Calvin cycle has three phases



Carbon fixation
Reduction
Regeneration of the CO2 acceptor
Light

H2 O
CO2
NADP+
ADP
The Calvin cycle
CALVIN
CYCLE
LIGHT
REACTION
Input
3 (Entering one
CO2 at a time)
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 P
6 NADPH
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
P
1
Figure 10.18
P
1,3-Bisphoglycerate
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction

Concept 10.4: Alternative
mechanisms of carbon fixation have
evolved in hot, arid climates

On hot, dry days, plants close their
stomata


Conserving water but limiting access to
CO2
Causing oxygen to build up
Photorespiration: An Evolutionary
Relic?

In photorespiration


O2 substitutes for CO2 in the active site
of the enzyme rubisco
The photosynthetic rate is reduced
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 used in the
Calvin cycle
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf

CO
CO
2 2
PEP carboxylase
Bundlesheath
cell
C4 leaf anatomy and the C4 pathway
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
Figure 10.19
CAM Plants

CAM plants

Open their stomata at night,
incorporating CO2 into organic acids

During the day, the stomata close

And the CO2 is released from the
organic acids for use in the Calvin cycle

The CAM pathway is similar to the
C4 pathway
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
Figure 10.20 types of cells.
CALVIN
CYCLE
Sugar
CAM
CO2
CO2
1 CO2 incorporated Organic acid
into four-carbon
organic acids
(carbon fixation)
2 Organic acids
release CO2 to
Calvin cycle
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.
The Importance of Photosynthesis:
A Review

Light reaction
Calvin cycle
H2O
CO2
ALightreview of photosynthesis
NADP
+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
Figure 10.21
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
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

Organic compounds produced by
photosynthesis

Provide the energy and building
material for ecosystems