http://www.botany.com/index.16.htm
Chapter 6
Photosynthesis
Section 1 Vocabulary Pretest
 Autotroph
 Photosynthesis
 Heterotroph
 Light
Reactions
 Chloroplasts
 Thylakoid
 Stroma
A.
B.
C.
D.
E.
F.
G.
Cellular organelles where
photosynthesis occurs
The first stage of photosynthesis
An organism that can make its
own food
An organism that can not make
its own food
The process of converting
energy from the sun into
chemical energy of food
A membrane system found
inside chloroplasts
Solution surrounding the
thylakoids inside chloroplasts
 Granum
 Pigment
 Chlorophyll
 Carotenoid
 Photosystem
Electron
Acceptor
 Electron Transport
Chain
 Chemiosmosis
H.
I.
J.
K.
 Primary
L.
M.
N.
O.
Compounds that absorb light
A stack of thylakoids
Yellow, orange and brown
accessory pigments
A cluster of pigments that
harvest light energy for
photosynthesis
Green pigment in plants
Movement of protons down a
gradient to make ATP
Movement of electrons from
one molecule to another
Accepts electrons
Answer Key
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Autotroph
Photosynthesis
Heterotroph
Light Reactions
Chloroplasts
Thylakoid
Stroma
Granum
Pigment
Chlorophyll
Carotenoid
Photosystem
Primary Electron Acceptor
Electron Transport Chain
Chemiosmosis
C
E
D
B
A
F
G
I
H
L
J
K
O
N
M
Obtaining Energy
 The
sun is the direct or indirect source of
energy for most living things.
 Autotrophs —organisms that can make
their own food
 Heterotrophs —organisms that can not
make food. They obtain energy from
eating food.
http://image.wistatutor.com/content/environment/food-chain-system.jpeg
Photosynthesis
 Photosynthesis
is the
process used by
autotrophs to convert
light energy from sunlight
into chemical energy in
the form of organic
compounds.

Involves a complex series
of chemical reactions
known as a biochemical
pathway.
 Product
of one reaction is
consumed in the next
reaction
http://www.vtaide.com/png/images/photosyn.jpg
Overview
 Photosynthesis
is often summarized in the
following equation:
6CO2 + 6H2O
Light energy
C6H12O6 + 6O2
The Reactants are carbon dioxide and water
The Products are glucose and oxygen
The Stages of Photosynthesis
 There
are two stages to the
process
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Light Reactions —light energy is
converted to chemical energy,
which is temporarily stored in
ATP and the energy carrier
molecule NADPH
Dark Reactions (Calvin Cycle)—
organic compounds are
formed using CO2 and the
chemical energy stored in ATP
and NADPH
http://bioweb.uwlax.edu/bio203/s2009/schroeer_
paul/images/484pxSimple_photosynthesis_overview_svg.png
The Light Reactions
 Require
light to happen
 Take place in the chloroplasts
 Chloroplasts contain pigments that absorb
sunlight.

Pigment —a compound that absorbs light
http://www.quranandscience.com/images/stories/chloroplasts2.jpg
The Structure of a Chloroplast
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Surrounded by an outer and inner membrane
Thylakoids —membrane system arranged as
flattened sacs. (from the Greek meaning “pocket”)
Grana (pl.) Granum (singular)—stacks of thylakoid
membrane sacs
Stroma —solution that surrounds the grana
http://www.scool.co.uk/assets/learn_its/alevel/biolog
y/cells-andorganelles/organelles/chloroplast-b.gif
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Thylakoids contain the pigments known as
chlorophylls.
Chlorophylls —absorb colors other than
green. Therefore, green is reflected and is
visible. Two types:
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Chlorophyll a and Chlorophyll b
Chlorophyll a —directly involved in the light
reactions
Chlorophyll b —accessory pigment that assists in
photosynthesis
Carotenoids —accessory pigments responsible
for fall colors and also assist in photosynthesis
Converting Light Energy to Chemical Energy
 Chlorophylls
and carotenoids are
grouped in clusters embedded in proteins
in the thylakoid membrane.
 These clusters are called photosystems
 Two photosystems exist, each with its own
job to do:
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Photosystem I and Photosystem II
Plants have both photosystems. Prokaryotic
autotrophs only have photosystem II. It is
only numbered as II because it was the
second one discovered. However, it
probably evolved 1st.
How does it work?
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Light is absorbed by accessory pigments in
photosystem II.
When the absorbed energy from the light reaches
the chlorophyll a molecules of photosystem II, it
“excites” electrons to a higher energy level.
These excited electrons will leave the chlorophyll a
molecule (this is an oxidation reaction)
The electrons are accepted by the primary electron
acceptor (this is a reduction reaction) and begin to
move from molecule to molecule down an “electron
transport chain”
The energy they lose as they are transported is used
to pump H+ ions from the stroma into the thylakoid
space, creating a concentration gradient.
Stroma
II
Thylakoid Space
 At
the same time that light is absorbed by
photosystem II, it is also being absorbed
by photosystem I, again, exciting
electrons.
 These electrons move down a different
electron transport chain and are added
to NADP+ to form NADPH.
 The lost electrons from photosystem I are
replaced by the electrons moving down
the transport chain from photosystem II.
Stroma
I
II
Thylakoid Space
 Photosystem
II replaces its electrons by
splitting water, using a water-splitting
enzyme.
2H2O
 For
4H+ +
4e- + O2
every two molecules of water that are
split, four electrons become available to
replace those lost by the chlorophyll
molecules in photosystem II.
 The hydrogen ions and the oxygen
molecules are released into the thylakoid
space. This is where the oxygen gas given
off by photosynthesis comes from.
I
II
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The build up of H+ in the
thylakoid space stores potential
energy. This energy is harvested
by an enzyme called ATP
synthase.
As H+ ions diffuse through ATP
synthase down their
concentration gradient, the
enzyme uses the energy of the
moving ions to make ATP.
This is done by adding a
phosphate group to ADP in a
process called chemiosmosis.
ATP will then be used in the
second stage of photosynthesis
called the Calvin Cycle.
http://www.biojourney.org/modchemiosmosis.jpg
The Calvin Cycle
 Named
for Melvin Calvin
 Most common pathway for carbon fixation

 It
Carbon fixation —changing CO2 into organic
compounds (carbohydrates)
is the second set of reactions in
photosynthesis and does not require light.
 It uses the energy that was stored in ATP and
NADPH during the light reactions to produce
organic compounds in the form of sugars.
 The Calvin Cycle occurs in the stroma of the
chloroplasts and requires CO2
The Calvin Cycle Step by Step

Step 1: Create 6 molecules of 3-PGA
 Three molecules of CO2 diffuse into the stroma
 An enzyme combines each CO2 molecule with a 5-carbon
molecule called RuBP (ribulose bisphosphate) to make 3
very unstable 6-carbon molecules. Each immediately
breaks down into two 3-carbon molecules called 3-PGA
(3-phosphoglycerate). This results in 6 molecules of 3-PGA.
3 molecules of CO2
3 molecules
of RuBP
6 molecules of 3-PGA
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Step 2: Convert 3-PGA to G3P
 Each of the 6 molecules of 3-PGA is
converted into a molecule of G3P
(glyceraldehyde 3-phosphate)
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This is a two-step process
 First: 6 ATP molecules (from the light
reactions) donate a phosphate group
to the 3-PGA. (Changing ATP to ADP)
 Second: 6 NADPH molecules (from
the light reactions) donate a H+
(Changing NADPH to NADP+) and the
phosphate group is released.
 The result is 6 molecules of G3P. The
ADP, NADP+ and phosphates that are
released can be used again in the
light reactions to make more ATP and
NADPH
6- 3PGA
6 ATP
6ADP
6 molecules
of G3P
6NADPH
6NADP+
6P
 Step
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3: Make organic compounds
One of the G3P molecules leaves the
Calvin cycle and is used to make organic
compounds (carbohydrates) in which
energy is stored for later use.
glucose
1 molecule
of G3P
starch
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Step 4: Convert G3P to RuBP
 The remaining G3P molecules are converted back into
RuBP by adding phosphate groups from ATP molecules.
The RuBP is used again in the cycle.
3 ADP
3 CO2
3 ATP
3 RuBP
6- 3PGA
6 ATP
6 ADP
glucose
5 G3P
1 G3P
6 G3P
6 NADPH
starch
6NADP+
6P
http://bioap.wikispaces.com/file/view/Carbon_Fixation.gif/120055293/Carbon_Fixation.gif
 Plant
species that fix carbon using the
Calvin Cycle only are known as C3 plants
because of the three-carbon compound
that is initially formed in the process. They
include most plants.
http://stjoseph.iaswcd.org/23rd%20Annual%20Tree%20Sale.htm
Alternative Pathways
 Plants
living in hot, dry climates have
trouble using the Calvin Cycle to fix
carbon.
 This is because they must partially
close their stomata to conserve
water.
 This allows less CO2 to enter and an
excess of O2 to build up, both of
which inhibit the Calvin Cycle
 Two alternate pathways have
evolved for these plants—both
allow the plants to conserve water.
 They are the C4 pathway and the
CAM pathway
The C4 Pathway
 C4
plants include: corn,
sugar cane and crab grass
 Cells called mesophyll cells
in C4 plants use an enzyme
to fix CO2 into a four carbon
compound
 This compound travels to
other cells where CO2 can
be released and enter the
Calvin Cycle
 These plants lose about ½
as much water as C3 plants
when producing the same
amount of carbohydrates.
The CAM Pathway
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CAM plants include:
cactuses, pineapples, and
jade plants.
These plants open their
stomata at night and close
them during the day
(opposite of most plants).
CO2 absorbed at night can
enter the Calvin Cycle during
the day, allowing the
stomata to stay closed and
conserve water.
These plants lose less water
than any other plants