CHAPTER 8 PHOTOSYNTHESIS
ENERGY
Energy is the ability to do work.
 All living things depend on energy.
 Energy comes in the form of light, heat, electricity, or
sound.
 Energy can be stored in chemical compounds.
ATP
 ATP
(adenosine triphosphate) is the energy currency of
cells.
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Used to store and release energy in cells.
Consists of adenine (a nucleotide), ribose (a sugar or
carbohydrate), and three phosphate groups.
Release of energy – by breaking the chemical bonds to
convert ATP into ADP.
Storage of energy – by creating a chemical bond
between ADP and P to create ATP.
Energy used for active transport, protein synthesis,
homeostasis, etc.
Autotrophs and Heterotrophs
 Autotrophs
own food.
are organisms that make their
 Use
light energy from the sun to produce food
 Plants are examples of autotrophs.
 Heterotrophs
obtain energy from the food
they consume.
 Unable
 Must
to directly use the sun’s energy.
consume other organisms either by ingesting
(eating) them or decomposing them.
 Animals are examples of heterotrophs.
The Photosynthesis Equation
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Photosynthesis is the process whereby plants use the
energy of sunlight to convert water and carbon dioxide
into oxygen and high-energy carbohydrates.
Carbon dioxide +water  sugar and oxygen
In addition to water and carbon dioxide,
photosynthesis requires light and chlorophyll, a
molecule in chloroplasts.
The Photosynthesis Equation
3 Requirements for Photosynthesis:
1. Sunlight
2. Pigments
3. Energy storing compounds
1. LIGHT

Light travels to the Earth in the form of
sunlight
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We perceive sunlight as white light, but is
really a mixture of many different
wavelengths of light
Wavelengths of light that are visible to us
are known as the visible light spectrum
2. PIGMENTS
 Pigments
are light absorbing molecules
that help plants gather the sun’s energy

The main pigment found in plants is
chlorophyll
 Chlorophyll
absorbs red and blue
wavelengths of light, but it reflects green
making the plant appear green
 When the pigments absorb light they are also
absorbing the energy in that light, producing
more energy for the cell
INSIDE A CHLOROPLAST
 Photosynthesis
takes place in chloroplasts.
 Chloroplasts contain saclike
photosynthetic membranes called
thylakoids.
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Thylakoids contain clusters of chlorophyll and
other pigments and proteins known as
photosystems that are able to capture sunlight.
Light dependent reactions take place here.
 Thylakoids
are arranged in stacks known as
grana.
 The region outside the thylakoid
membrane is called the stroma.

Light independent (dark) reactions take place
here.
3. Energy Storing Compounds
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Used to trap high energy electrons into chemical
bonds.
Occurs in 2 ways:
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1. Electron carrier NADP+ accepts a pair of high
energy electrons and gets converted to NADPH.
2. AMP is converted to ADP which is then converted
to ATP.
NOTE: The energy stored in these molecules is
released by breaking chemical bonds to generate
things the cell needs, like glucose!
Light and Dark Rxns
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Light-dependent reactions produce oxygen
and make energy storing compounds (ATP
and NADPH).
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Occurs by converting ADP and NADP+ into ATP and
NADPH.
These reactions REQUIRE light.
Light-independent (dark) reactions use the
energy stored in NADPH and ATP to make
glucose.
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Glucose is more stable and can store up to 100 times
more energy than NADPH and ATP.
These reactions do not require light.
Light Dependent Rxns
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Photosynthetic membranes of chloroplast
(thylakoids) contain chlorophyll.
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This is where the light reactions occur.
The light reactions are divided into 4 processes:
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1. Light absorption
2. Electron transport
3. Oxygen production
4. ATP production
1. Light Rxns - Light absorption
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Green plants contain photosystems.
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Clusters of pigment molecules that absorb energy from
sunlight.
High energy electrons move through the photosystems
and are then released to electron carriers.
2. Light Rxns - electron
transport
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During electron transport, high energy
electrons are passed along electron
carriers in the photosynthetic membrane.
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These carriers are called the electron
transport chain.
At the end of the chain, high energy
electrons are passed to NADP+ converting
it to NADPH.
3. Light Rxns - Oxygen Production
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Electrons are getting used up by
chlorophyll and must be replaced!
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This occurs by taking electrons from water
to replace those used by chlorophyll.
Electrons are removed from water
molecules leaving H+ ions and oxygen.
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Oxygen is a by-product of the splitting of
water by is NEEDED by us!
4. Light Rxns - ATP production
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Because hydrogen (H+) ions were released
inside the thylakoid membrane as a product
of the splitting of water molecules, the inside
of the membrane becomes positively
charged, while the outside is negatively
charged.
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This difference in charges creates a gradient
that provides the energy to make ATP from ADP.
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ATP synthase – enzyme in thylakoid membrane
that binds a P to ADP to create ATP.
Light Rxns - SUMMARY
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The light reactions USE: water, light energy,
chlorophyll pigments
The light reactions PRODUCE: oxygen, NAPDH, ATP
Dark Rxns -The Calvin Cycle
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ATP and NADPH can hold large amounts
of chemical energy, but only for a few
minutes.
The Calvin Cycle uses CO2 as well as ATP
and NADPH from the light-dependent
reactions to produce glucose that can be
stored in the plant for long periods of time.
This process does not require light, but
often takes place while the sun is shining.
Calvin Cycle
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Carbon dioxide molecules enter the cycle
from the atmosphere.
The carbon dioxide molecules combine
with 5-carbon molecules.
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This reaction is catalyzed by the enzyme
rubisco.
The result is 3-carbon molecules.
Calvin Cycle
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The energy from breaking ATP into ADP
and NADPH into NADP+ is used to convert
the 3-carbon molecules into PGAL.
Most PGAL is recycled.
1 of 6 PGAL molecules formed is used to
make glucose.
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Plants use glucose for energy.
Organisms that eat plants indirectly also use
this energy from glucose.
Calvin Cycle - SUMMARY
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Calvin Cycle USES: NADPH, ATP ,CO2
Calvin cycle PRODUCES: glucose (C6H12O6)
3 factors affect photosynthesis
 1.
Temperature – enzymes that control
the reactions of photosynthesis work best
at 0-35 degrees C.
 2. Light – higher is better.
 3. Water – a raw material; shortage slows
or stops photosynthesis and damages
plant tissues.