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Chapter 8: Photosynthesis
Chapter 9: Respiration
Photosynthesis: 2 RXNS
Light-Dependent Reactions

Needs light to occur
(produces ATP and
NADPH)

Needs water (source of
electrons and hydrogen
ions)

Takes place in the
thylakoid membranes

Oxygen is released as a
by-product
Light-Independent Reactions
 Does NOT need light
 Takes place in stroma
 Needs carbon dioxide
 ATP and NADPH
(produced in light rxn)
are used to produce
high-energy sugars
Photosynthesis: 2 RXNS
Light-Dependent Reactions
Light-Independent Reactions
The Light-Dependent Reactions

Requires light

Uses energy from the sun to:

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Produce oxygen
Convert ADP to ATP
Convert NADP+ to NADPH
Occurs in the thylakoid
membrane which contain:


Clusters of chlorophyll
Photosystems – proteins
surrounded by accessory pigments


Absorb sunlight
Generate high-energy electrons
that are passed to a series of
electron carriers embedded in
the thylakoid membrane
Photosystem II  Electron Transport Chain  Photosystem I  Short ETC
NADPH
Chemiosmosis
The Light-Dependent Reactions
Photosystem II

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Light energy
absorbed by
pigments
Electrons’
energy levels
increase
e- passed to
ETC
Enzymes
break apart
water –
releasing O2,
H+, and 2 e2e- replace
electrons lost
The Light-Dependent Reactions
Electron Transport Chain
Series of electron carrier proteins that
shuttle high-energy electrons during ATPgenerating reactions

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
High energy
e- move down
the ETC
Energy from
the e- is used
by the
proteins in
the chain to
pump H+ ions
from the
stroma into
the thylakoid
e- then
passed to…
The Light-Dependent Reactions
Photosystem I


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
e- contain
less energy
because
some energy
has been
used to pump
H+ ions
Use pigments
to reenergize
eShort ETC
transfers eto NADP+
NADP+ picks
up e- and H+
to make
NADPH
The Light-Dependent Reactions
Hydrogen Ion Movement & ATP



Inside of
thylakoid is
positively
charged (lots of
H+) relative to
the outside
(=gradient)
H+ pass over
thylakoid
membrane
through ATP
synthase
ATP synthase
rotates and the
energy produced
is used to
convert ADP to
ATP
(chemiosmosis)
The Light-Independent Reactions

AKA Calvin Cycle

Does NOT require light


Uses the energy that ATP
and NADPH contain to build
stable high-energy
compounds that can be
stored for a long time
Uses 6CO2 to produce 1 6carbon sugar
The Light-Independent Reactions

Carbon dioxide enters the cycle
from the atmosphere
Energy from ATP and NADPH:
 Combine 6 carbon dioxide
molecules with six 5-carbon
compounds and then…
 Produce 12 3-carbon compounds
Midcycle
 2 3-carbon molecules are
removed
 These become the building
blocks of sugars, lipids, amino
acids, etc.
Finally
 10 3-carbon molecules are
converted back into 6 5-carbon
molecules – go on to repeat the
cycle
Factors Affecting Photosynthesis

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Temperature
Photosynthesis enzymes
function best between 0°C
and 35°C
Light Intensity
High light intensity
increases the rate of
photosynthesis
After a certain point the
maximum rate of
photosynthesis is reached
Availability of water
Questions for Review
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What is the first process in the lightdependent reaction?
What is the source of the e- in the lightdependent reaction?
What are 2 products of the lightdependent reaction that are used as a
source of energy in the Calvin cycle?
What are three factors that affect the
rate at which photosynthesis occurs?