Chapter 10: Photosynthesis Life from Light
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Energy needs of life
 All life needs a constant input of energy

Heterotrophs
 get their energy from “eating others”
 consumers of other organisms
 consume organic molecules

Autotrophs
 get their energy from “self”
 get their energy from sunlight
 use light energy to synthesize organic
molecules

Chemoautotrophs
 Harvest energy from oxidizing inorganic
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substances such as sulfur and ammonia
 Unique to bacteria
How are they connected?
Heterotrophs
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
Autotrophs
making energy & organic molecules from light energy
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
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Energy cycle
sun
Photosynthesis
CO2
H2O
glucose
Cellular Respiration
The Great Circle
of Life!
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ATP
O2
What does it mean to be a plant
 Need to…

collect light energy
 transform it into chemical energy

store light energy
 in a stable form to be moved around the plant
& also saved for a rainy day

need to get building block atoms from
the environment
 C,H,O,N,P,S

produce all organic molecules needed for
growth
 carbohydrates, proteins, lipids, nucleic acids
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Plant structure
 Obtaining raw materials

sunlight
 leaves = solar collectors

CO2
 stomates = gas exchange
regulation
 Found under leaves

H2O
 uptake from roots

nutrients
 uptake from roots
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Structure of the Leaf
 Mesophyll

Tissue forming the
interior of the leaf; site
of most chlorophyll
 Stomata

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Microscopic pores in
the leaf; allows for gas
exchange
Each Mesophyll Cell:
 Has approx. 30-40

chloroplasts
Each chloroplast is
equipped to carry
out photosynthesis
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Plant structure
 Chloroplasts
double membrane
 stroma
 thylakoid sacs
 grana stacks

 Chlorophyll & ETC in
thylakoid membrane

H+ gradient built up
within thylakoid sac
H+
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+
+ H+ H H+
+
H
H
+ H+ H+ H+
+
H
H
Structure of the Chloroplast
 Chlorophyll

Photosynthetic pigment
found in the thylakoid
 Thylakoid


Membranous sacs filled
with fluid and chlorophyll
– Site of the LIGHT
REACTIONS
Granum = Stack of
Thylakoids
 Stroma

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Fluid portion of the
chloroplast; sight of the
LIGHT INDEPENDENT
REACTIONS (CalvinBenson Cycle)
Chloroplasts split water molecules
 Evidence
Discovery that the O2 given off by plants
comes from H2O not CO2
 Before the 1930’s the hypothesis was
that photosynthesis occurred in two
steps:

1. CO2 C + O2
 2. C+ H2O  CH2O

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Chloroplasts Split Water Molecules:
Changing the Hypothesis
 Studying bacteria, C.B. van Neil challenged
the hypothesis:

H2S was used, not water

Proposed the following Rxn

CO2 + 2H2S CH2O +2S

Applied the same principle to plants

CO2 + 2H20 + light CH2O +O2
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Pigments of photosynthesis
 chlorophyll & accessory
Why does this
structure
make sense?
pigments
“photosystem”
 embedded in thylakoid
membrane
 structure  function
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
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Pigments of Photosynthesis Continued
 Leaves look green because
they absorb red and blue light,
while transmitting and
reflecting green light
 Chlorophyll A

Dominant pigment – absorbs
red/blue
 Chlorophyll B

Directs photons to chlorophyll A

Funnel energy from other
wavelengths to Chlorophyll A
(mostly orange/yellow)
 Carotenoids
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Light: Absorption Spectra
 Photosynthesis performs work only with
absorbed wavelengths of light


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chlorophyll a — the dominant pigment —
absorbs best in red & blue wavelengths & least
in green
other pigments with different structures have
different absorption spectra
Photosynthesis overview
 Light reactions – Light Dependent Rxns

convert solar energy to chemical energy

ATP
 Calvin cycle – Light Independent Rxns

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uses chemical
energy (NADPH & ATP)
to reduce CO2 to
build C6H12O6 (sugars)
Photosystems
 Photosystems

collections of chlorophyll molecules
 2 photosystems in thylakoid membrane


act as light-gathering “antenna complex”
Photosystem II
 chlorophyll a
 P680 = absorbs 680nm
wavelength red light

Photosystem I
 chlorophyll b
 P700 = absorbs 700nm
wavelength red light
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Light reactions
 Similar to ETC in cellular respiration
membrane-bound proteins in organelle
 electron acceptors

 NADP+ (Oxygen in cellular respiration)
proton (H+)
gradient across
inner membrane
 ATP synthase
enzyme

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ETC of Photosynthesis
 ETC produces from light energy

ATP & NADPH
 NADPH (stored energy) goes to Calvin cycle
 PS II absorbs light

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excited electron passes from chlorophyll to
“primary electron acceptor” at the REACTION
CENTER.
 splits H2O (Photolysis!!)
 O2 released to atmosphere
 ATP is produced for later use
ETC of Photosynthesis
 Chloroplasts transform light
energy into chemical energy
of ATP

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use electron carrier NADPH
split H2O
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2 Photosystems
 Light reactions
elevate electrons in
2 steps (PS II & PS I)

PS II generates
energy as ATP

PS I generates
reducing power as
NADPH
This shows Noncyclic
photophosphorylation.
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ETC of Photosynthesis
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ETC of Photosynthesis
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Cyclic photophosphorylation
 If PS I can’t pass
electron to NADP,
it cycles back to
PS II & makes
more ATP, but no
NADPH


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coordinates light
reactions to Calvin
cycle
Calvin cycle uses
more ATP than
NADPH
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Photosynthesis summary so far…
Where did the energy come from?
Where did the H2O come from?
Where did the electrons come from?
Where did the O2 come from?
Where did the H+ come from?
Where did the ATP come from?
Where did the O2 go?
What will the ATP be used for?
What will the NADPH be used for?
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From Light reactions to Calvin cycle
 Calvin cycle

Chloroplast stroma
 Need products of light reactions to
drive synthesis reactions
ATP
 NADPH

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From CO2  C6H12O6
 CO2 has very little chemical energy

fully oxidized
 C6H12O6 contains a lot of chemical energy
reduced
 endergonic

 Reduction of CO2  C6H12O6 proceeds in
many small uphill steps
each catalyzed by specific enzyme
 using energy stored in ATP & NADPH

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Calvin cycle
1C
ribulose bisphosphate
3. Regeneration
RuBP
3 ATP
PGAL
to make
glucose
5C
1. Carbon fixation
Rubisco
ribulose
bisphosphate
carboxylase
3 ADP
PGAL
sucrose
cellulose
etc.
CO2
6C
unstable
intermediate
2x 3C
3C x2
PGA
2. Reduction
6 ATP
6 NADPH
6 NADP
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2x
3C
6 ADP
Rubisco
 Enzyme which fixes carbon from
atmosphere
ribulose bisphosphate carboxylase
 the most important enzyme in the world!

 it makes life out of air!

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definitely the most abundant enzyme
Calvin cycle
 PGAL
end product of Calvin cycle
 energy rich sugar
 3 carbon compound
 “C3 photosynthesis”

 PGAL   important intermediate
PGAL  



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glucose   carbohydrates
lipids
amino acids
nucleic acids
Photosynthesis summary
 Light reactions
produced ATP
 produced NADPH
 consumed H2O
 produced O2 as byproduct

 Calvin cycle
consumed CO2
 produced PGAL
 regenerated ADP
 regenerated NADP

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Summary of photosynthesis
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy










Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the H2O go?
Where did the energy come from?
What’s the energy used for?
What will the C6H12O6 be used for?
Where did the O2 come from?
Where will the O2 go?
What else is involved that is not listed in this
equation?
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Photosynthesis Drives Evolution
 Photosynthesis first evolved in


prokaryotic organisms
Scientific evidence supports that
prokaryotic (bacterial) photosynthesis 
responsible for production of an
oxygenated atmosphere
Prokaryotic photosynthetic pathways –
foundation of eukaryotic photosynthesis
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Plant Photosynthesis Adaptations
Due to Weather Conditions
 C3

 C4
Minimize the cost of photorespiration by
incorporating CO2 into four carbon compounds in
mesophyll cells
 4 carbon compounds exported to bundle sheath
cells, where they release CO2 used in the Calvin
cycle
CAM
 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


Most common type
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C4 Plants
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
Figure 10.19
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C4 and CAM Plants
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