PHOTOSYNTHESIS
chapter 6
Capturing the energy in light
WHAT IS
PHOTOSYNTHESIS?
 The
process of transferring the energy
in light (electromagnetic radiation)
 into the energy of carbohydrates
(organic compounds).

Light -------> chemical energy
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sunlight + 6 CO2 + 6 H20 --> C6H12O6 + 6 O2
WHICH ORGANISMS DO
PHOTOSYNTHESIS?
WHICH ONES DON’T?
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AUTOTROPHS “self feeders” do. Ex. PLANTS
& kelp, sea weed, filamentous algae & some protists
(Eukaryotic cells with chloroplasts)
& also… CYANOBACTERIA (photosynthetic bacteria)
HETEROTROPHS “other feeders” don’t.
Ex. Animals, Decomposers (fungi), some bacteria &
protists.
(Cells without chlorophyll/chloroplasts)
WE COULDN’T EXIST WITHOUT PRODUCERS… WHY???
Plants transform energy in sunlight into chemical energy.
CONSUMERS
producer
carnivore
herbivore
saprophyte
carnivore
HEAT
SUN ---> PRODUCERS--> CONSUMERS ------------->
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There’s a LOT of photosynthesis going on in this rainforest!
Why are rainforests called “THE LUNGS OF THE EARTH?”
WHERE DOES PHOTOSYNTHESIS HAPPEN?
Organ?
Cells?
Organelle?
WHERE DOES PHOTOSYNTHESIS HAPPEN?
Organ:
LEAVES
Tissue:
MESOPHYLL cells
Organelle:
CHLOROPLASTS
DRAW & LABEL THE CROSS
SECTION (side view) OF A
LEAF…
Label the:
Waxy cuticle
Guard cells
Vein
xylem & phloem
Palisade Mesophyll layer
Spongy Mesophyll layer
waxy cuticle
palisade
spongy
LEAF
stoma/stomata
GUARD CELLS
chloroplast
DRAW AND LABEL THE PARTS
OF A CHLOROPLAST:
Outer membrane
Inner membrane/thylakoid membrane
Thylakoid
Granum
Grana
Stroma
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HOW DOES PHOTOSYNTHESIS WORK?
 BIOCHEMICAL

PATHWAY
A complex series of chemical reactions that are linked
together. The products of one reaction are consumed (are
reactants) in the next set of reactions.
 TWO
SETS OF REACTIONS:
1. The Light Reactions
occur on the thylakoid membrane… creates ATP & NADPH
2. The Calvin-Benson Cycle or C3 Cycle
aka: the light “independent” reactions, dark reactions
occur in the stroma…. Use ATP & NADPH to power the
endergonic process of creating glucose from CO2.
ENERGY MOLECULES
TRANSPORT ENERGY
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ATP transports energy in the form of
phosphate groups. When it transfers a
phosphate groups to another molecule
it becomes ADP.
NADPH transports energy in the form
of electrons and protons. When it transfers
electrons and protons to another molecules
it becomes NADP+.
PART ONE: THE LIGHT REACTIONS
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LIGHT REACTIONS USE “VISIBLE LIGHT” 750-350nm
HOW IS LIGHT ENERGY
ABSORBED AND USED?

PIGMENT PROTEINS in the thylakoid
membrane absorb photons of light and
use this energy to “excite” electrons.

Create electricity to do work!!!!

CHLOROPHYLL A absorbs red light
Accessory Pigments:
CHLOROPHYLL B absorbs blue light
CAROTENOIDS
PHYCOCYANINS
XANTHOPHYLL
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The Light Reactions of photosynthesis
light + water + chlorophyll --> oxygen gas + ATP + NADPH
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Energy transfers recap:
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1.Photons of light (radiant energy) move
2.Supercharged electrons from WATER through the ETC
proteins, which power the
3.Proton Pump (active transport/kinetic energy); which
pumps protons into the thylakoid space creating a
4.Proton Gradient (potential energy) which results in
5.Chemiosmosis of Protons (passive transport/ kinetic
energy) through ATP Synthase channels/enzyme which
results in the bonding of Pi to ADP
6.Synthesis of ATP
7.Electrons are passed to NADP+ (final electron acceptor)
to form NADPH (reduction).

LIGHT IS CONVERTED TO THE ENERGY MOLECULES:

ATP and NADPH which power the Calvin cycle.
THE LIGHT REACTIONS
are the first set of reactions in photosynthesis.
What must be embedded in the thylakoid membrane
for these reactions to occur????
e-
e-
Embedded in the phospholipid bilayer (in this order):
water splitting enzyme, photosystem 2, Electron transport chain
cytochrome proteins, proton pump, photosystem 1, Electron
transport chain cytochrome proteins, NADP+ reductase enzyme,
ATP synthase channel.
And the reactants/substrates: NADP+, H20, ADP and Pi.
e-
e-
1. Photons of light are absorbed by light absorbing pigments
within photosystems 2 &1 (combine the collected energy.)
2. Super charged electrons leave the reaction center w/in
Chlorophyll A and are passed down the Electron Transport
Chain of cytochrome proteins.
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ePhotosystem 2
ePhotosystem 1
3. Water provides electrons to the E.T.C
4. Moving electrons provide the energy needed to power the
proton pump, which pumps H+ into the thylakoid space.
THIS CREATES A CONCENTRATION GRADIENT.
5. NADP+ picks up the electrons at the end of the chain and
is reduced to NADPH (ELECTRON CARRIER MOLECULE).
ePhotosystem 2
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ePhotosystem 1
6. Protons diffuse out to the stroma through an ATPsynthase
ion channel = CHEMIOSMOSIS.
7. The moving protons provides the energy needed to add an
inorganic phosphate onto ADP to create ATP.
8. NADPH and ATP are the energy molecule products of the
light reactions. They are reactants (used) in the Calvin Cycle
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ePhotosystem 2
ePhotosystem 1
CHEMIOSMOSIS
diffusion of H+ through ATP synthase
High concentration
of protons inside the
thylakoid space.
Low concentration
out in the stroma.
Protons diffuse out
into the STROMA
where ATP is made.
LET’S SEE THE PROCESS STEP BY STEP…
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ePhotosystem 2
ePhotosystem 1
proton pump
NADP+
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PS 2
electron transport chain
water splitting
enzyme
PS1
ATP synthase
H+ channel
proton pump
NADP+
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PS 2
electron transport chain
water splitting
enzyme
PS1
ATP synthase
H+ channel
ATP
NADPH
proton pump
NADP+
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PS 2
electron transport chain
water splitting
enzyme
PS1
ATP synthase
H+ channel
LAB: PAPER
CHROMATOGRAPHY OF
PLANT PIGMENTS…

A technique for separating and identifying pigments and
other molecules from cell extracts.
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The solvent moves up the paper by capillary action and
carries the mixture of photosynthetic pigments with it.

One by one, the heavy pigments fall from the solvent to the
paper. The lightest pigment gets carried the farthest
distance.
CHROMATOGRAPHY
•One strip per student.
•One BIG strip per group.
DIRECTIONS:
1) Use a dime or quarter to press
chloroplasts through the stomata
of a spinach leaf to make a thick
line of pigment mixture on your
chromtography paper.
2) Dip only the pointy tip of the paper in
the solvent.
3) Put the stopper on the tube.
4) Watch the solvent move up the
paper and separate the different
light absorbing pigments that allow
photosynthesis to occur!!!!
5) Take your paper out and measure
the distances from the original line
to the bottom of each band of color
and to the top of the “solvent
front”.
6) You should see 4 pigment/color
bands.
Olive-Green = chlorophyll B
Blue-Green = chlorophyll A
Yellow = xanthophyll
Orange = caroteen
Put your results in a Table.
Band #
1
2
3
4
solvent
Distance (mm)
Color
Calculate the Rf for each pigment
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Rf = Distance pigment migrated
Distance solvent migrated
Analysis:
1.How do you think your Rf values will compare with others? Should
they be similar or different? Why?
2.What factors are involved in the separation of pigments?
3. What is the main photosynthetic pigment of plants? What are the
accessory pigments?
4. What do these pigments do?
5. If leaves are usually green because of the mixture of these
pigment molecules, but turn red and yellow in the Autumn
before they drop their leaves what do you think is happening?
THE CALVIN CYCLE

Second set of reactions in photosynthesis
Uses the energy produced from the light
reactions (ATP & NADPH) to form glucose
 from atmospheric CO2.
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Named after MELVIN CALVIN 1911-1997
THREE STEPS OF THE CALVIN CYCLE
(this biochemical pathway reforms the first molecule RuBP so it is called a cycle)
1. CARBON FIXATION
The enzyme RUBISCO fixes (joins) 3 molecules of CO2 to 3
molecules of 5-carbon RuBP to make 3, 6-carbon
molecules which split in half to form 6 molecules of PGA
2. PGAL SYNTHESIS
Each molecule of PGA is converted into a molecule of PGAL
(uses 6 ATP & 6 NADPH, 1 of each per PGA)
3. RuBP REFORMATION
One ATP is used per RuBP reformed. Most of the PGAL is
converted back into RuBP but 2 molecules will be used to
make a sugar (glucose).
THE CYCLE OCCURS TWO TIMES TO MAKE 1 GLUCOSE
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Balance Sheet for Photosynthesis

TO make 1 glucose the Calvin Cycle
- fixes 6 CO2
- uses 18 ATP
- uses 12 NADPH
BALANCED EQUATION
Reactants
light + 6 CO2 + 6 H2O -->

Products
C6H12O6 + 6 O2
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GRAPH:
EFFECTS OF LIGHT INTENSITY ON PHOTOSYNTHESIS
BALANCED EQUATION
Reactants
light + 6 CO2 + 6 H2O -->

Products
C6H12O6 + 6 O2

Explain the graph. Why would the rate of photosynthesis
Be different for the fern and the corn plants?
GRAPH:
EFFECTS OF LIGHT INTENSITY ON PHOTOSYNTHESIS
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A C3 plant will close its
stomata on a hot day to
prevent water loss.
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Oxygen builds up, the
Calvin Cycle halts. No
more sugar is made.
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Corn and sugar cane
are adapted for dry heat
all the time.
C-4 PATHWAY

EX. Corn, sugar cane, crab grass

Alternative pathway fixes CO2
into a 4 carbon compound- during
the hottest part of the day the
stomata are partially closed.

Contain an enzyme that converts
low levels of CO2 into the 4
carbon compound even when O2
levels are high. Later the 4 C is
shuttled to bundle sheath cells &
is used as a source of CO2 for the
the Calvin cycle.

Loses about half as much water
as C 3 plants.
CAM PLANTS

EX. Cactuses & Pineapples
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Only open stomata at night.
They use the C4 pathway to
store CO2 as part of Malic Acid
in vacuoles during the night.
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light reactions occur during the
day and the Calvin Cycle but
since the stomata are CLOSED
during the day there is no CO2
getting in. The source of CO2
is breaking down the Malic
Acid… not atmospheric CO2.
The end.