4/12/15 Photosynthesis Reactions
Photosynthesis occurs in two stages linked by ATP and
NADPH
NADPH is similar to NADH seen in mitochondria; it is an
electron/hydrogen carrier
The complete process of photosynthesis consists of two linked
sets of reactions
The light reactions and the Calvin cycle.
Photosynthesis Reactions
The Light Reactions
•  Occur in the grana
•  Light energy is absorbed by the chlorophyll molecules to
split water and drive the transfer of electrons and H+ from
water to the electron acceptor NADP+ reducing it to
NADPH.
•  In this process, oxygen becomes released and ATP is
produced ( as well as NADPH)
1 4/12/15 Photosynthesis Reactions
The Calvin cycle
•  Occurs in the stroma and uses the energy generated
during the light reactions
•  Forms sugar from carbon dioxide. This is called carbon
fixation !
•  Since this is an endergonic (energy requiring, the making
of a larger molecule from a smaller one) reaction, it uses
ATP for energy and NADPH for reducing power
•  Sometimes called the “dark reaction” since these steps do
not require light energy
Figure 7.5_s3
H2O
CO2
Light
NADP+
ADP
P
Calvin
Cycle
(in stroma)
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
Sugar
2 4/12/15 Aspects of Light Energy
Visible light is only a small part of the electromagnetic
spectrum, the full range of electromagnetic wavelengths.
Electromagnetic energy travels in waves, and the wavelength is
the distance between the crests of two adjacent waves.
The smaller the wavelength, the more energy is packed in that
wave ( and the larger the wavelength, the less energy in that
wave).
Light behaves as discrete packets of energy called photons.
A photon is a fixed quantity of light energy.
Increasing energy
10-5 nm
10-3 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
1m
103 m
Radio
waves
Microwaves
Visible light
380 400
Which of these elements of the
electromagnetic spectrum are
dangerous for us ?
500
600
Wavelength (nm)
700
750
650
nm
3 4/12/15 Aspects of Light Energy
Many forms of Life on planet earth has evolved to use only a
certain part of the electromagnetic spectrum .
This is called the visible light spectrum and includes the region
of the electromagnetic spectrum between 380 and 750
nanometers.
•  This spectrum includes the colors of light we can see
•  It also includes the wavelengths that drive photosynthesis
Aspects of Light Energy
The way life reacts to the visible light spectrum is due to
molecules that absorb the energy in the parts of the visible
spectrum.
Pigments : Are substances that absorb visible light
The color of the pigment indicates what color is being absorbed
and what color is being reflected.
For example, a red apple look red because molecules (pigments)
in the apple skin absorb all colors except red, being reflected
back into our eyes.
4 4/12/15 Aspects of Light Energy
Light
Reflected
light
Chloroplast
Thylakoid
Why do leafs look green ?
Absorbed
light
Transmitted
light
Photosynthesis Pigments
Thus leaves look green because the color green is reflected. This
part of the electromagnetic spectrum is NOT absorbed and
NOT used for photosynthesis.
Those molecules in the leaves (chloroplasts) that absorb and use
light energy are called the Plant pigments
•  They absorb some wavelengths of light and
•  Reflect or transmit other wavelengths ( the colors
reflected are the colors we thus see).
5 4/12/15 Photosynthesis Pigments
The spectrophotometer can measure what is being absorbed and what
is being transmitted (not absorbed) at each wavelength.
Refracting Chlorophyll
prism
solution
White
light
Photoelectric
tube
Galvanometer
2
3
1
0
100
4
Slit moves to
pass light
of selected
wavelength
Green
light
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Blue
light
100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
Absorption Spectrum
With the use of simple spectrophotometers one can
generate an absorption spectrum.
An absorption spectrum of a solution of pigments
• 
Is a graph plotting light absorption (on Y-axis)
versus wavelength (on X-axis).
This will tell us what part of visible light is absorbed by
the pigments in solution and thus what part of visible
light may/is important for action of those molecules
involved.
6 4/12/15 Photosynthesis Pigments
Chloroplasts contain several different pigments in the
thylakoid membrane, which absorb light of different
wavelengths.
•  Chlorophyll a
•  Chlorophyll b
•  Carotenoids
The 3 different pigments can be isolated from greeny leaves and
analyzed for what spectrum of visible light they absorb.
Absorption Spectrum
The combined action spectrum experiments helped reveal
which wavelengths of light are photo-synthetically 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. Which pigment absorbs what colors ? What colors are NOT absorbed ?
7 4/12/15 Photosynthesis
3 Pigments
•  Chlorophyll a absorbs blue-violet and red light and
reflects green.
•  Chlorophyll b absorbs blue and orange and reflects
yellow-green.
•  Carotenoids
•  broaden the spectrum of colors that can drive photosynthesis
(such as blue to violet) but reflect in the orange/red region
•  They also appear to provide photo-protection by absorbing
and dissipating excessive light energy that would otherwise
damage chlorophyll or interact with oxygen to form reactive
oxidative molecules.
Action Spectrum
How effective each wavelength of light is in stimulating
photosynthesis can be deduced by measuring how much
oxygen is made over a certain period of time
Light
energy
6 CO2
+
Carbon dioxide
6
H2O
Water
C6H12O6
PHOTOSYNTHESIS
Glucose
+
6
O2
Oxygen gas
Graphing the rate of oxygen production against the
wavelength used creates an action spectrum ; it profiles
the relative effectiveness of different wavelengths of
radiation in driving photosynthesis in a leaf.
8 4/12/15 Rate of photosynthesis
(measured by O2 release)
Action Spectrum
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. This is partly due to the absorption of light
by accessory pigments such as chlorophyll b and carotenoids.
Action vs Absorption Spectra
Absorption Rate
Photosynthesis
Rate
•  Comparison between Action spectrum and Absorption
spectrum
Wavelength of light (nm)
9 4/12/15 Action Spectrum
•  The action spectrum for photosynthesis was first demonstrated by
Theodor W. Engelmann
Aerobic bacteria
Filament
of alga
400
500
600
700
In 1883, Theodor W. Engelmann exposed 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 in previous slide.
Photosynthesis Pigments
So what happens when photosynthetic pigments absorb
light energy ?
When Pigments in
chloroplasts absorb photons
(capturing solar power), it
•  increases the potential energy
of the pigments’ electrons
and
•  sends the electrons into an
excited (higher level) but
unstable state.
10 4/12/15 Photosynthesis Pigments
Generally, when isolated pigment molecules absorb light, their
excited electrons drop back down to the ground state and release their
excess energy (emission energy) as heat and some form of photon
energy with less energy as initially absorbed (the basis of most
fluorescence principles).
Excited state
Photon
of light
Heat
Photon
(fluorescence)
Ground state
Chlorophyll molecule
Photosynthesis Pigments
Solution of Chlorophyll illuminated
with UV light.
In normal white light, it looks green. The UV
light excited electrons which now fall back to
ground state, emitting photons with less energy
More energy
Less energy
11 4/12/15 The Light Reaction
In the thylakoid membranes, chlorophyll molecules are
organized along with other pigments and proteins into
photosystems.
When light is absorbed, the excited electrons are passed on to
other electron acceptor molecules.
The solar-powered transfer of an electron from the reaction-center
chlorophyll a pair to the primary electron acceptor is the first step in the
transformation of light energy to chemical energy in the light reactions.
Photosynthesis Pigments
There are two photosystems, called PS II and PS I, that work
in concert.
When PS II becomes excited by light energy, the result is
•  splitting of water into Oxygen, H+ and electrons
•  the H+ are released in the inner space of the thylakoids
•  the electrons are funneled into an electron transport
chain
The electron chain passes the electrons from photosystem II
to photosystem I and the energy released funnels more
protons into the lumen of the thylakoids
12 4/12/15 Chloroplast stroma
Thylakoid membrane
Thylakoid inner space
Photosynthesis Pigments
When PS I becomes excited by light energy, the result is
•  Reduction of NADP into NADPH by accepting electrons
and H+
The buildup of hydrogen high inside the thylakoid (and thus
low outside) will now drive the synthesis of ATP via an
ATPsynthase by the movement of hydrogen from inside to
outside.
This NADPH and ATP is required for the Calvin cycle that
happens in the stroma…. The process that results in the
making of sugars.
13 4/12/15 A “construction” analogy of PS II and PSI
ATP
Photon
Electron
transport
chain ramp
Photosystem II
n
Photo
NADPH
Photosystem I
Mitochondria/ Chloroplasts
So, when we compare
mitochondria and chloroplasts,
similar mechanisms are at work.
The buildup of hydrogen drives
the production of ATP.
The ATP made by mitochondria
is used to fuel all kinds of
cellular activities
The ATP made by chloroplasts
is only used to fuel the Calvin
Cycle in order to make sugars.
14 4/12/15 Mitochondria/ Chloroplasts
In mitochondria, the production of ATP
is referred to as oxidative
phosphorylation via the chemiosmosis
mechanisms ( the making of ATP via
the use of an ETC, hydrogen gradient
and the use of oxygen).
An electron transport chain also
produces a gradient of H+ across the
thylakoid membrane, which drives H+
through ATP synthase, producing ATP.
Because the initial energy input is light
(“photo”), this chemiosmotic production
of ATP is called photophosphorylation.
The Importance of Photosynthesis: A Review
•  A review of photosynthesis
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
Chloroplast
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
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
15 4/12/15 The Importance of Photosynthesis
H2O
Light
Chloroplast
CO2
NADP+
Light
Reactions
ADP
P
RuBP
Photosystem II
Electron
transport chain
Thylakoids
Photosystem I
ATP
NADPH
O2
Calvin
Cycle 3-PGA
(in stroma)
Stroma
G3P
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  The greenhouse effect operates on a global
scale.
–  Solar radiation includes visible light that penetrates the
Earth’s atmosphere and warms the planet’s surface.
–  Heat radiating from the warmed planet is absorbed by
gases in the atmosphere, which then reflects some of
the heat back to Earth.
–  Without the warming of the greenhouse effect, the Earth
would be much colder and most life as we know it could
not exist.
© 2012 Pearson Education, Inc.
16 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
Some heat
energy escapes
into space
Sunlight
ATMOSPHERE
Radiant heat
trapped by CO2
and other gases
7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  The gases in the atmosphere that absorb heat
radiation are called greenhouse gases. These
include
–  water vapor,
–  carbon dioxide, and
–  methane.
Methane source
© 2012 Pearson Education, Inc.
17 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  Remember that photosynthesis captures CO2 into
carbohydrates
!  The carbohydrates of plants become cellulose,
wood,….
!  When plants, trees, animals died million of years
ago, they became submerged into sediments of
swamps, rivers, oceans and eventually became
covered with sand, mud,…
!  Over time the carbohydrates, woods became
converted to what we know as fossil fuels.
© 2012 Pearson Education, Inc.
7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  The history of the earth includes the movement of
CO2 from above the air into the deeper regions of
the earth
!  The industrial revolution
has been the beginning
of increased exploration
and use of fossil fuels
such as coal, oils,
petroleums,…
© 2012 Pearson Education, Inc.
18 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  Since 1850, there has been an increased and
progressive acceleration in the usage and
combustion of fossil fuels
!  And this implies, a faster release of the hidden CO2
captured by those fossilized organisms resulting in
a 40% increase in atmospheric CO2 concentrations
since 1850
© 2012 Pearson Education, Inc.
7.13 CONNECTION: Photosynthesis may
moderate global climate change
These Increasing concentrations of greenhouse gases have
been linked to global climate change (global warming).
There has been a slow
but steady rise in
Earth’s surface
temperature and an
overall change in
climatic conditions in
different regions of the
world.
Some heat
energy escapes
into space
Sunlight
ATMOSPHERE
Radiant heat
trapped by CO2
and other gases
© 2012 Pearson Education, Inc.
19 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  The presence of large
amount of oceans and algae
that perform photosynthesis
has slowed down the effect.
!  CO2 also dissolves easier in
water providing a buffer for
CO2 changes
!  But… too much CO2 can
make oceans acidic…
© 2012 Pearson Education, Inc.
GreenHouse Effect Data
20 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  The predicted consequences of continued CO2
increase in the atmosphere and global climate
change include
–  melting of polar ice,
–  rising sea levels, acidification of oceans
–  extreme weather patterns,
–  droughts,
–  increased extinction rates, and
–  the spread of tropical diseases.
© 2012 Pearson Education, Inc.
7.13 CONNECTION: Photosynthesis may
moderate global climate change
Effects of increasing carbon dioxide and temperature on coral reefs.
© 2012 Pearson Education, Inc.
21 4/12/15 7.13 CONNECTION: Photosynthesis may
moderate global climate change
!  Widespread deforestation has aggravated the
global warming problem by reducing an effective
CO2 sink.
!  Global warming caused by increasing CO2 levels
may be reduced by
–  limiting deforestation,
–  reducing fossil fuel consumption, and
–  growing biofuel crops that remove CO2 from the
atmosphere.
© 2012 Pearson Education, Inc.
7.13 CONNECTION: Photosynthesis may
moderate global climate change
© 2012 Pearson Education, Inc.
22 4/12/15 Global Warming
Bad Quotes
"Trees cause more pollution than automobiles.”
Quote by Ronald Reagan, 1981
“The dangers of carbon dioxide? Tell that to a plant, how
dangerous carbon dioxide is”
Quote by Santorum, presidential candidate (2012)
Why are these examples of politician who obviously do
not understand the importance of plants and ecology ?
23