Chapter 07
Lecture and
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1
Chapter 7
Photosynthesis
Chapter Outline:

Overview of Photosynthesis

Reactions That Harness Light Energy

Molecular Features of Photosystems

Synthesizing Carbohydrates via the Calvin Cycle

Variations in Photosynthesis
2
Overview of Photosynthesis

Energy within light is captured and used to
synthesize carbohydrates
6 CO2 + 12 H2O + Light → C6H12O6 + 6 O2 + 6 H2O

CO2 is reduced
 H2O is oxidized
 Energy from light drives this endergonic reaction
3
Photosynthesis powers the biosphere

Biosphere – regions on the surface of the Earth
and atmosphere where living organisms exist

Largely driven by the photosynthetic power of
green plants

Energy cycle – cells use organic molecules for
energy and plants replenish those molecules
using photosynthesis
 In
the process, plants also produce oxygen
4
Trophic levels
Heterotroph
 Must
eat food (organic molecules from their
environment) to sustain life
Autotroph
 Makes
organic molecules from inorganic sources
 Most
are photoautotrophs that use light as a
source of energy

Green plants, algae, cyanobacteria
5
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Organic
molecules + O2
(C6H12O6)
Photosynthesis
Energy cycle
in the
biosphere
Cellular
respiration
Energy
intermediates
Light
CO2 + H2O
ATP
6
BIOLOGY PRINCIPLE
Living organisms use energy
Photosynthetic species capture light energy
and store it in organic molecules, which are
used by photosynthetic and nonphotosynthetic
organisms as energy sources.
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Chloroplast

Organelle in plants and algae that carries out
photosynthesis

Green pigment is chlorophyll

Majority of photosynthesis occurs internally in
leaves, in the mesophyll

Carbon dioxide enters and oxygen exits through
pores in leaf called stomata
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9
Chloroplast anatomy

Outer and inner membrane separated by
intermembrane space
 A third membrane, the thylakoid membrane
contains pigment molecules
 Membrane
 Enclose
forms thylakoids
thylakoid lumen
Granum – stack of thylakoids
 Fluid-filled region between thylakoid membrane
and inner membrane is the stroma

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11
Two stages of photosynthesis
Light reactions
 Use
light energy
 Take
place in thylakoid membranes
 Produce
ATP, NADPH and O2
Calvin cycle
 Occurs
in stroma
 Uses
ATP and NADPH to incorporate CO2 into
carbohydrate
12
13
Reactions That Harness
Light Energy

Light is a type of electromagnetic radiation

Travels as waves
 Short

to long wavelengths
Also behaves as particles called photons
 Shorter
wavelengths have more energy
14
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Increasing energy of photons
Increasing wavelength
Wavelength = Distance between 2 peaks
0.001 nm 10 nm
Gamma rays
X-rays UV
0.1 cm
0.1 m
1000 m
Infrared Microwaves Radio waves
Visible
380 nm 430 nm
500 nm 560 nm 600 nm 650 nm
Wavelength
740 nm
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Photosynthetic pigments

Pigments absorb some light energy and reflect others

Leaves are green because they absorb red and violet,
and reflect green wavelengths

Absorption boosts electrons to higher energy levels

Wavelength of light that a pigment absorbs depends
on the amount of energy needed to boost an
electron to a higher orbital

Having different pigments allows plants to absorb light
at many different wavelengths
16
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High-energy electron (photoexcited)
Photon
–
–
+
+
Electron
Nucleus
Ground state
Excited state

After an electron absorbs energy, it is an excited state
and usually unstable

Releases energy as heat or light

Excited electrons in pigments can be transferred to
another molecule or “captured”
17
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H2C
CHO in chlorophyll b
CH3 in chlorophyll a
CH
CH3
H3C
N
CH2CH3
N
Mg
N
H3C
N
CH2
CH2
C
COCH3
CH3
Porphyrin
ring
H3C
CH3
CH
CH3
O
CH3
O
O
H3C
O
H3C
CH2
H3C
CH3
CH3
CH
CH3
Phytol
tail
CH3
CH3
CH3
CH3
(a) Chlorophylls a and b
(b) -carotene
(a carotenoid)
Absorption vs. action spectrum
Absorption spectra

Wavelengths that are absorbed by different pigments
Action spectrum

Rate of photosynthesis by whole plant at specific wavelengths
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Chlorophyll a
8
Relative rate of photosynthesis
Relative absorption of light
at the wavelengths shown
on the x-axis
Chlorophyll b
β-carotene
350
400
Violet
450
500
Blue Green
(a) Absorption spectra
550
600
650
Yellow Red
Wavelength (nm)
700
750
7
6
5
4
3
2
1
0
350
400
450
500
550
600
650
700
Wavelength (nm)
(b) Action spectrum
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750
Photosystems I and II

Captured light energy can be transferred to
other molecules to produce energy
intermediate molecules for cellular work

Thylakoid membranes of chloroplast contain
two distinct complexes of molecules

 Photosystem
I (PSI) – discovered first
 Photosystem
II (PSII) – first step in photosynthesis
Light excites pigment molecules in both PSII
and PSI
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Photosystem II
 The
initial step in photosynthesis
 Excited
electrons travel from PSII to PSI
 Oxidizes
water, generating O2 and H+
 Releases
 Energy
energy in electron transport chain (ETC)
used to make H+ electrochemical gradient
Photosystem I
 Primary
role is to make NADPH
of H+ to NADP+ contributes to H+ gradient by
depleting H+ from the stroma
 Addition
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ATP synthesis in chloroplasts
 Achieved
by chemiosmotic mechanism called
photophosphorylation
by flow of H+ from thylakoid lumen into
stroma via ATP synthase
 Driven
H+ gradient generated three ways:
1.
↑H+ in thylakoid lumen by splitting of water
2.
↑H+ by ETC pumping H+ into lumen
3.
↓H + in stroma from formation of NADPH
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23
Three chemical products
1.
Oxygen, O2


2.
NADPH


3.
Produced in thylakoid lumen by oxidation of H2O by
PSII
Two electrons transferred to P680+ molecules
Produced in the stroma from high-energy electrons
that start in PSII and are boosted in PSI
NADP+ + 2 electrons + H + → NADPH
ATP

Produced in stroma by ATP synthase using the
H+ electrochemical gradient
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Noncyclic and cyclic electron flow

Noncyclic
 Electrons
begin at PSII and eventually transfer to
NADPH, a linear process
 Produces

both ATP and NADPH in equal amounts
Cyclic photophosphorylation (cyclic electron flow)
cycling releases energy to transport H+
into lumen driving ATP synthesis
 Electron
 Produces
only ATP
 PSI
electrons excited, release energy and eventually
return to PSI
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Molecular Features
of Photosystems
Photosystem II (PSII)
Two main components:
1.
2.
Light-harvesting complex (or antenna complex)

Directly absorbs photons

Energy transferred via resonance energy transfer
Reaction center

P680 →P680*

P680* is relatively unstable, so energy is transferred quickly

Electron transfers to primary electron acceptor and captured

Water is oxidized to replace the electron on P680+, producing
oxygen gas in the process
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29
Z scheme

Zigzag shape of energy curve

Photosynthesis involves increases and decreases in
the energy of an electron
as it moves from PSII through PSI to NADPH

Electron on a nonexcited pigment molecule
in PSII starts with the lowest energy

Light excites the electron in PSII

Photosystem I boosts the electron to an even higher
energy level
30
Primary electron
acceptor
Energy of electrons
e–
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e–
Primary
electron
acceptor
e–
e–
Fd
QA
Light
QB
NADP+
reductase
e–
Cytochrome
complex
NADPH
e–
+ H+
Pc
H2O
P680
H+
P700
Light
NADP+ + 2H+
2 e–
2 H+ + ½O2
Photosystem II
Photosystem I
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Synthesizing Carbohydrates
via the Calvin Cycle

Calvin Cycle (aka Calvin-Benson Cycle)
 CO2 incorporated into carbohydrates
 Precursors
 Energy

to other organic molecules
storage
Requires massive input of energy
 For
every 6 CO2 incorporated, 18 ATP and 12
NADPH must be used

Product is glyceraldehyde-3-phosphate (G3P)
 Glucose
is later made from G3P in separate process
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Phase 1 – Carbon fixation

CO2 incorporated into RuBP using rubisco

Reaction product is a six-carbon intermediate that splits into
two 3-phosphoglycerate molecules (3PG)
Phase 2 – Reduction and carbohydrate production

ATP is used to convert 3PG into
1,3-bisphosphoglycerate (1,3-BPG)

NADPH electrons reduce it to glyceraldehyde-3-phosphate (G3P)

6 CO2 → 12 G3P
 Only 2 G3P molecules used for carbohydrates
 10 G3P molecules must be used for regeneration of RuBP
Phase 3 – Regeneration of RuBP

10 G3P are converted into 6 RuBP using 6 ATP
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FEATURE INVESTIGATION
The Calvin Cycle was determined
by isotope-labeling methods
 14C-labeled
CO2 injected into cultures of green algae

Allowed to incubate different lengths of time

Separated newly made radiolabeled molecules using
two-dimensional paper chromatography

Autoradiography – radiation from 14C-labeled
molecules makes dark spots on the film

Identified 14C-labeled spots and the order they appeared

Melvin Calvin received Nobel Prize in 1961
FEATURE INVESTIGATION
FEATURE INVESTIGATION
Variations in Photosynthesis
Environmental conditions can influence both
the efficiency and way the Calvin cycle works
 Light
intensity
 Temperature
 Water
availability
38
Rubisco

Normally, rubisco adds CO2 to RuBP to make
3-phosphoglycerate
RuBP + CO2 → 2 3PG

Since 3PG has three carbons, plants that
require this form of carbon fixation are
called C3 plants

90% of plant species on earth are C3 plants
 example:
Oak trees
39
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Oak tree – a C3 plant
© David Sieren/Visuals Unlimited
40
Photorespiration

Rubisco can also add O2 to RuBP
RuBP + O2 → 3PG + Phosphoglycolate
Phosphoglycolate → Glycolate → →
Organic molecule + CO2

Affinity for CO2 is 10x better, so this occurs
only when CO2 is low and O2 is high
41

Phosphoglycolate is eventually released as
CO2 – so photorespiration uses O2 and
releases CO2

Releasing CO2 is wasteful since normally
used for growth
 So
photorespiration decreases the efficiency of
photosynthesis

More likely in hot and dry environments when
CO2 is low and O2 high

May protect plants from free radicals
42
EVOLUTIONARY CONNECTIONS
C4 and CAM plants have evolved a
mechanism to minimize photorespiration

C4 plants make oxaloacetate (4 carbon
molecule) in the first step of carbon fixation

Leaves have two-cell layer organization
 Mesophyll cells
 CO2 enters via stomata and 4 carbon compound formed
(PEP carboxylase does not promote photorespiration)
 Bundle-sheath cells
 4 carbon molecule transferred that releases steady supply of
CO2, minimizing photorespiration
EVOLUTIONARY CONNECTIONS
Which is better – C3 or C4?

It depends on the environment

In warm dry climates C4 plants conserve
water and prevent photorespiration

In cooler climates, C3 plants use less energy
to fix CO2

90% of plants are C3
44
EVOLUTIONARY CONNECTIONS
45
EVOLUTIONARY CONNECTIONS
CAM plants

Some C4 plants separate processes in time

Crassulacean Acid Metabolism

CAM plants open their stomata at night

CO2 enters and is converted to malate

Stomata close during the day to conserve water

Oxaloacetate converted to malate

Malate broken down into CO2 to drive Calvin
cycle during the day
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EVOLUTIONARY CONNECTIONS
47
BIOLOGY PRINCIPLE
Species evolve from
one generation to the next
C4 and CAM plant adaptations evolved
to help plants living in hot and dry environments
to conserve water and minimize photorespiration.
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