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The chloroplasts of plants use a process called photosynthesis to capture
light energy from the sun and convert it to chemical energy stored in
sugars and other organic molecules.
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Autotrophs produce
organic molecules
from CO2 and other
inorganic raw
materials obtained
from the
environment.
Heterotrophs live on
organic compounds
produced by other
organisms.
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Plant cells have
chloroplasts. The
structure of a leaf is
to the left.
Structures to note:
- Stoma (stomata)
- Cuticle
- Veins (xylem/
phloem)
- Guard Cells
- Epidermis
- Mesophyll (in this
pic it is called
parenchyma – that
is the type of cell)
Photosynthesis
mainly occurs in
mesophyll cells
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Chloroplasts are the organelles where
photosynthesis takes place. Know the
following structures: stroma, thylakoid,
grana, lumen (inside thylakoid)
Chlorophyll, the
green pigment in
chloroplasts, is
found in the
thylakoid
membranes.
Stomata  microscopic pores in the bottom of the leaf; used for
gas exchange
Veins  run throughout the plant to deliver water from the
roots and carry sugar from the leaves to the rest of the plant
IN THE CHLOROPLAST….
Stroma  fluid in the chloroplast; Calvin cycle happens here
Thylakoids  membrane sacs where the light reaction
happens
Thylakoid Space/ Lumen  inside the thylakoids
Grana  stacks of thylakoids
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Make sure you
know this picture!
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Process that converts sunlight (light energy)
into food (chemical energy)
6CO2 + 6 H2O + Light → C6H12O6 + 6O2
The oxygen given off by plants comes from
WATER, not CARBON DIOXIDE
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Cornelis B. Van Niel was a teacher and made great
contributions to the world of science as we know it.
He was a pioneer in the field of microbiology and
he also discovered some of the chemistry behind
photosynthesis.
He did research with plants and figured out that
water was split to give the ETC a source of
hydrogen (and electrons) and that oxygen was
given off as a byproduct. Thus, the oxygen that is
released during PS is from water, not the CO2 that
is consumed.
Twenty years later (in the 1950’s), this theory was
confirmed by doing research using a heavy isotope
of oxygen (18O) as a tracer to follow the path of the
oxygen. The radioactive label (isotope) was only
found in the released oxygen if the source of the
18O was originally in the water (not the CO ).
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Both photosynthesis and aerobic respiration involve redox
reactions.
Photosynthesis reverses the direction of electron flow.
Water is split and electrons are transferred with H+ from
water to CO2, reducing it to sugar.
Oxidized = water
Reduced = CO2
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The light reactions (photo) convert solar energy to chemical energy.
In the light reactions, water is split, providing a source of electrons and
protons (H+ ions) and giving off O2 as a by-product.
Light absorbed by chlorophyll drives the transfer of electrons and
hydrogen ions from water to NADP+ (nicotinamide adenine dinucleotide
phosphate), forming NADPH.
The light reactions also generate ATP using chemiosmosis, in a process
called photophosphorylation (comparable to oxidative phosphorylation
in cellular respiration.)
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The Calvin cycle (synthesis) uses energy from the light
reactions to incorporate CO2 from the atmosphere into
sugar (named for Melvin Calvin ~1940’s.)
The Calvin cycle begins with the incorporation of CO2
into organic molecules, a process known as carbon
fixation.
The fixed carbon is reduced with electrons provided by
NADPH.
ATP from the light reactions also powers parts of the
Calvin cycle.
Thus, it is the Calvin cycle that makes sugar (G3P), but
only with the help of ATP and NADPH from the light
reactions.
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Light is a form of
electromagnetic energy or
radiation.
 Light travels in waves and
the distance between crests
of electromagnetic waves is
called the wavelength.
 The entire range of
electromagnetic radiation is
the electromagnetic
spectrum.
 The most important
segment of the
electromagnetic spectrum
for life is a narrow band
between 380 and 750 nm,
the band of visible light
detected as colors by the
human eye. (ROY.G.BIV!)
 Although light travels as a
wave, many of its
properties are those of a
discrete particle, a photon.
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Light can be transmitted, reflected, or
absorbed. Substances that absorb visible light
are called PIGMENTS. Whatever color
wavelength is reflected is the color that the
pigment appears to be. This color wavelength
is NOT helpful in providing energy because
none of it is absorbed (therefore green is NOT
helpful for photosynthesis because
chlorophyll reflects green light).
This is the visible light
spectrum. The shorter the
wavelength (violet), the
more energy it has.
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The spectrophotometer is a
machine that measures a
pigments ability to absorb at
different wavelengths of light.
The more light that is
absorbed, the less that gets
through, so the lower the
transmittance.
Low Transmittance = High
Absorption
An absorption spectrum plots
a pigment’s light absorption
versus wavelength.
An overall action spectrum for
photosynthesis profiles the
relative effectiveness of
different wavelengths of light
in driving the process.
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Engelmann’s Experiment →
Engelmann shone light of different colors
(wavelengths) of light onto filaments of
Spirogyra (through a prism). Spirogyra is an
alga made up of filaments (chains) of cells. He
added motile bacteria (which could swim)
and which needed oxygen and observed
where they went. He found that the bacteria
clustered around the blue and red colors of
the spectrum, where the Spirogyra were
producing the most oxygen. So he concluded
that these two colors were the most important
for photosynthesis.
RESULT: Red and Blue are the most effective
colors for photosynthesis!
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Accessory Pigments:
-Chlorophyll b →
similar to chlorophyll
a, but traps a slightly
different wavelength
of energy
-Carotenoids →
yellow/orange color;
absorb and pass on
energy; function in
photoprotection
Only Chlorophyll a can participate
directly in the light reactions.
However, it gets energy from other
pigments called accessory
pigments. These pigments absorb
wavelengths of light that
chlorophyll cannot, and then pass
along that energy to the
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chlorophyll a.
Part 1 – Light Reactions
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Light Reaction makes ATP and NADPH and sends it to
the Calvin Cycle
It occurs in the thylakoid
Makes ATP by Photophosphorylation (similar to oxidative
phosphorylation with the ETC and chemiosmosis)
Energy carrier = NADPH (reduced from NADP+)
Gets electrons from H2O and gives off O2
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Photosystems  composed of a reaction center
complex and several light harvesting complexs
that contain pigments that absorb light energy
Reaction Center → made up of the special
chlorophyll a molecule and a primary eacceptor
Photosystem II → comes first in the ETC, but
was discovered second; called P680 because has
an absorption peak at 680nm
Photosystem I → comes second in the ETC, but
was discovered first; gives e- to NADP+; called
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P700 because has an absorption peak at 700nm
There are two ways electrons
can pass through the ETC:
Cyclic e- Flow → this is when
the e- go down PS II to the next
photosystem, but then get
passed from ferredoxin BACK
to the cytochrome complex; this
makes more ATP and makes
NO NADPH
Linear (Non-Cyclic) e- Flow →
this passes through both
electrons transport chains and
makes both ATP and also
NADPH
Because the Calvin uses more
ATP than NADPH, there is a
way to make more ATP (and
less NADPH) → cyclic eflow!
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Makes NADPH
and ATP
Electron Flow:
Water  PSII 
Discuss:
primary e-Water → donates e- to PSII, releases O2 as waste, contributes H+ to acceptor 
concentration gradient
plastoquinone
-Electrons get energized by light and then get trapped by a primary (Pq) 
e- acceptor and passed down the ETC (energy released pumps H+
cytochromes 
INTO the lumen)
plastocyanin (Pc)
-First ETC→ made up of Plastoquinone, Cytochromes, and
 PSI  primary
Plastocyanin
e- acceptor 
st
-Electrons from 1 ETC supply e- to PSI where they get reFerredoxin (Fd) 
energzizd by light and passed down the second ETC (ferredoxin
NADP+ reductase
and NADP+ reductase) to NADP+ to make NADPH
 NADPH 19
-Makes ATP only
-Ferredoxin returns the electrons back to the cytochrome complex in the first
ETC rather than passing them to NADP+ reductase; no NADPH is made here
-Both cyclic and non-cyclic are used because the Calvin cycle uses more ATP
than NADPH……so we need more ATP than NADPH
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Makes both ATP (via
ATP Synthase) and
NADPH (via NADP+
reductase)
Uses the proton gradient
and ATP synthase to
make ATP via
photophosphorylation
During the Light Reactions, protons (H+) are pumped from the stroma into the
lumen (thylakoid space) against their concentration gradient. The want to diffuse
back out and the only way they can get out of the thylakoid is to go through ATP
SYNTHASE. This is an enzyme that works like a water wheel and converts ADP to
ATP. This is utilized in cellular respiration as well.
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-Occurs in the stroma of the chloroplast
-Carbon enters as CO2 and leaves as
sugar
-Produces a sugar called
glyceraldehyde-3-phosphate (G3P)
-Each turn “fixes” one molecule of
carbon, so one G3P takes 3 turns of the
Calvin
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Phase 1: Carbon fixing 
Each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).
This reaction is catalyzed by RuBP carboxylase-oxygenase, or rubisco. The six-carbon
intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate
for each CO2 fixed.
Phase 2: Reduction 
During reduction, each 3phosphoglycerate receives another
phosphate group from ATP to form 1,3bisphosphoglycerate.
A pair of electrons from NADPH reduces
each 1,3-bisphosphoglycerate to G3P.
For every three molecules of CO2 that enter
the cycle, there are six molecules of G3P
formed…but 5 have to be recycled so we
only get net gain of one G3P.
This molecule exits the cycle to be used by
the plant cell, while the other five
molecules are recycled to regenerate the
three molecules of RuBP
Phase 3: Regeneration of RuBP 
In a complex series of reactions, the carbon skeletons of five molecules of G3P are
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rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.
In hot and dry environments, plants close their stomata so that they don’t
lose too much water (dehydrate). However, closed stomata lead to no gas
exchange, which inhibits photosynthesis. This leads to…..
Photorespiration → makes no ATP, makes no food, wastes the plants
materials (takes away from the Calvin Cycle.
In C3 plants, with the higher levels of O2 (byproduct it can’t get rid of) and
low levels of CO2, rubisco (enz) actually adds OXYGEN to the Calvin instead
of CO2…..it wastes all the raw materials.
Evolutionary baggage  early environment didn’t have a lot of O2 so the
inability of rubisco to exclude O2 wouldn’t have made a difference.
In order to minimize photorespiration, plants have adapted a C4 pathway or
a CAM pathway
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The key enzyme,
phosphoenolpyruvate
carboxylase, adds CO2 to
phosphoenolpyruvate (PEP)
to form the four-carbon
product oxaloacetate.
PEP carboxylase has a very
high affinity for CO2 and no
affinity for O2. Therefore,
PEP carboxylase can fix CO2
efficiently when rubisco
cannot (that is, on hot, dry
days when the stomata are
closed). In effect, the
mesophyll cells pump CO2
into the bundle-sheath cells,
keeping CO2 levels high
enough for rubisco to accept
CO2 and not O2. C4
photosynthesis minimizes
photorespiration and
enhances sugar production.
C4 plants thrive in hot regions
with intense sunlight.
C4 plants → the first product of
carbon fixing is a 4C molecule
(instead of a 3C molecule); it
occurs in the mesophyll cells and
sends the product to the bundle
sheath cells and the calvin cycle
happens there (Calvin cycle is
confined to bundle sheath cells!)
PEP Carboxylase
(enzyme) has a high
affinity for CO2 so it
can work when
rubisco can’t
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CAM (crassulacean acid
metabolism) Plants → They open
their stomata at night and store the
CO2 in organic acids; then during
the day they break those acids down
to use the CO2
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Both C4 and CAM plants add CO2 to organic
intermediates before it enters the Calvin cycle.
In C4 plants, carbon fixation and the Calvin
cycle are structurally separated (bundle sheath
cells vs. mesophyll cells)
In CAM plants, carbon fixation and the Calvin
cycle are temporally separated (day vs. night)
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