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Photosynthesis: The
Reactions, Carbon Fixation
and the Environment
Lecture 9
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Photosynthesis: The Details
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Photosynthesis is divided into two processes:
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The light reactions begin when
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Carbon fixation occurs only when
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Light Reactions
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The light reactions of photosynthesis are
divided into three parts:
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1.
Photoexcitation:
2.
Electron transport:
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3.
Chemiosmosis:
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Photoexcitation
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Chlorophyll is a molecule
that will behave like any
other molecule
Its electrons are existing at
a potential energy level
know as its
When a photon of light hits
a chlorophyll molecule,
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Photoexcitation

The excitation of an
electron results in an
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Normally, the electron
will
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This process is known
as
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Photoexcitation
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In molecules of chlorophyll, the excited
electron is “captured” by a
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This results in a REDOX reaction,
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The primary electron acceptor
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Photosystems (Fig 3, p. 157)
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The photons that hit a
chloroplast are always
absorbed by a
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Photosystems have an

The antenna complex
transfers the energy of
a photon to the reaction
centre where it is
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Photosystems (Fig. 5 & 6 p. 160)
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Photosystem I and Photosystem II both occur
in a chloroplast
In both photosystems, a chlorophyll a
molecule
Photosystem I and II are both used to generate
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Cyclic and Non-cyclic Electron Flow

In NON-CYCLIC ELECTRON FLOW,
electrons flow from
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In CYCLIC ELECTRON FLOW, electrons
flow from
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The electron passes through a series of
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The Calvin Cycle (Fig. 9, p. 161)
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The CALVIN CYCLE is
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The Calvin Cycle has three phases
1. Carbon Fixation –

2.
Reduction Reactions
3.
Regeneration of RuBP
These reactions are catalyzed by
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Carbon Fixation (C3 Photosynthesis)

In carbon fixation, CO2
molecules bond with RuBP (5carbon molecule) to form
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This 6-carbon intermediate is
then split into 2 3-carbon
intermediates called
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These two 3-carbon molecules
are the basis of
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The Calvin Cycle of most plants
involves these 3-carbon
intermediates and is known as
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Reduction Reactions
The reduction reactions are as
follows:

1.
2.
3.
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A 3-carbon PGA molecule is
phosphorylated by ATP –
becomes 1,3bisphosphoglycerate (1,3BPG)
NADPH reduces BPG to G3P
Glyceraldehyde 3-phosphate
(G3P) is the final product of
the Calvin Cycle – 1 molecule
of G3P leaves
This is a description of what
happens to one molecule of
PGA…more than one molecule
enters this cycle
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Reduction Reactions – The Numbers
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Three molecules of CO2 will enter the Calvin
Cycle at once
Each molecule of CO2 will form 2 PGA
molecules (6 total)
Therefore, there are 6 PGA molecules being
reduced to G3P…Why?
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Regeneration of RuBP
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Remember that RuBP
is the molecule that
drives the Calvin Cycle
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The remaining 5 G3P
molecules that have
been formed are
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G3P – The Key Intermediate
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Plants will generally construct more sugars
than they need
G3P is needed to regenerate RuBP to drive
the Calvin Cycle and
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Typically, G3P will be used to make
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Alternative Mechanisms of Carbon Fixation:
C4 Photosynthesis and CAM Plants
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Rubisco is the enzyme that catalyzes the addition of
carbon to RuBP (carboxylation)
It can also add an
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If oxygen is more plentiful than CO2,
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PGA will not be produced and
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This process is known as
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Alternative Mechanisms of Carbon
Fixation: C4 Photosynthesis (Fig. 2, p. 169)
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In C4 plants, the lack of
PGA production has been
remedied by an enzyme
called
PEP carboxylase fixes CO2
to a molecule of
This creates a stable 4carbon molecule that can
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Alternative Mechanisms of Carbon
Fixation: CAM Plants
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Plants that store water
(cacti, pineapples –
succulents) are
members of the
These plants
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This action helps to
conserve water,
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Alternative Mechanisms of Carbon
Fixation: CAM Plants
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The CO2 that is taken
in is converted into a
The C4 molecule is
actually an organic acid
and the process is
called
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