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PHOTOSYNTHESIS
KHADIJAH HANIM BT ABDUL RAHMAN
WEEK 15: 17/12/2012
[email protected]
What is photosynthesis?
 Photosynthesis is a process where plants through specialized
pigment molecules absorb light energy, consume CO2 and
H20 to produce O2 and carbohydrate.
12H2O + 6CO2 ----- light -----> 6O2+ C6H12O6 +
6H20
 It involves oxidation-reduction reaction (reverse of
carbohydrate metabolism) which produces carbohydrate.
 In plant cell- the main site where this reaction occurs is in
chloroplast.
 This reaction happen in 2 stages:
2H2O ----light---> O2 + 4[H]
 The electrons thereby obtained subsequently reduce CO2:
4[H] + CO2  (CH2O) + H2O
 These 2 stages reaction normally referred as light reaction
and dark reaction.
PHOTOSYNTHETIC PIGMENTS
 The essential features of photosynthesis is the absorption of
light energy by specialized pigment molecules.
 Pigment is a substance that absorbs visible light that behave as
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packets of energy called photons.
There are 3 types of primary photosynthetic pigments.
Chlorophylls- the green pigments that absorb blue-violet and red
wavelengths of light
Carotenoids- the orange pigments molecules which serves as
antenna pigments and protect from ROS (reacting oxygen species)
Xanthophylls- oxygenated derivatives of the carotenes which also
serves as antenna pigments.
 Two types of chlorophyll in plants.
 Chlorophyll a plays the principle role in eukaryotes
photosynthesis because its absorption of light energy directly
drives photochemical events.
 Chlorophyll b- light harvesting pigments by absorbing light
energy and pass on to chlorophyll a.
Photosynthesis occurs in chloroplasts
Chloroplasts have two parts:
1. A double membrane encloses a fluid-filled space called
the stroma or ground substance
2. Thylakoids = flattened sacs organized into stacks
called grana
3. Chlorophylls and other pigments involved in absorption
of solar energy are embedded within thylakoid
membranes; these pigments absorb solar energy
responsible for light-dependent reaction.
Chloroplast Structure
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6.
Outer membrane
Inner membrane systems
Thylakoid membranes
Thylakoid space (within the thylakoids)
Granum(a) (stack(s) of thylakoidsmembranes)
Stroma (the liquid area outside the thylakoid membranes)
REACTIONS IN PHOTOSYNTHESIS
Photosynthesis has two sets of reactions
1. Light-dependent reactions = the energy from the sun is
captured in energy carrying molecules
2. Light-independent reactions = energy carrying
molecules from the light-dependent reactions are used to
make carbohydrates
Photosystem I (PSI)
 Energizes and transfers electrons that are donated to
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NADP+
PSI is a protein-pigment complex that composed of several
polypeptides.
It possess over 200 molecules of chlorophyll a
Their essential role is performed by 2 special chlorophyll a
molecules that reside within the reaction centre.
These molecules referred as a special pair located in the core
complex of PSI, AB dimer.
AB dimer absorb light at 700nm, the special pair referred as
P700.
PHOTOSYSTEM II (PSII)
 PSII oxidizes water molecules and donates energized
electrons to electron carriers that eventually reduce PSI
 PSII is a large membrane-spanning protein-pigment complex
believed to possess at least 23 components
 The most prominent- reaction centre- protein-pigment
complex composed of 2 polypeptide subunits known as D1
and D2, cytochrome b559, and special pair chlorophyll a
molecules that absorb light at 680nm (P680).
Cytochrome b f
6
 Found throughout thylakoid membrane- similar structure
and fuction to cytochrome bc1 complex in mitochondrial
inner membrane.
 Plays important role in the transfer of electrons from PSII to
PSI.
 An iron-sulfur side on the complex accepts electrons from
the membrane-soluble electron carrier plastoquinone and
donates them to a small water-soluble copper-containing
protein called plastocyanin.
 The mechanism of transport appears to be similar to the Q
cycle in mitochondria.
ATP synthase
 Structurally similar to ATP synthase of mitochondria
 The CF0 component is a membrane-spanning protein
complex that contains a protein-conducting channel
 The CF1 head piece, which project into the stroma- possess
an ATP synthesizing activity
 A transmembrane proton gradient produced during light
driven electron transport drives ADP phosphorylation.
LIGHT REACTIONS
 Mechanism by which electrons are energized ans subsequently
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used in ATP and NADPH synthesis.
In O2 evolving species- PSI and PSII are required
The process of light-driven photosynthesis begins with the
excitation of PSII by light energy.
One electron at a time is transferred to a chain of electron carriers
that connects the 2 photosystems.
As electrons are transferred from PSII to PSI, protons are pumped
across the thylakoid membrane from the stroma to thylakoid
space.
ATP is synthesized as protons flow back into the stroma through
the ATP synthase.
 When P700 absorbs additional photon it releases an
energized electron.
 The newly energized electron is passed through a series of
iron-sulfur proteins and falovoprotein to NADP+, the final
electron acceptor.
 Illustrated as the Z scheme.
LIGHT-INDEPENDENT REACTION
(CALVIN CYCLE)
 The process of CO2 incorporation into carbohydrate which
occur within chloroplast stroma- Calvin cycle.
 This process can occur without light if sufficient ATP and
NADPH are supplied and can occur only when the plant is
illuminated.
 The net equation for Calvin cycle:
3 CO2 + 6 NADPH + 9 ATP -----> Glyceraldehyde-3phosphate + 6 NADP+ + 9 ADP + 8 Pi
The reaction can be divided into 3
phases
 Carbon fixation
- The mechanism, by which inorganic CO2 is incorporated
into organic molecules, consists of a single reaction.
- Ribulose-1,5-bisphosphate (RuBP) carboxylase catalyzes the
carboxylation of ribulose-1,5-bisphosphate to form two
molecules of glycerate-3-phosphate.
- Plants that produce glycerate-3-phosphate (G3P) as the first
stable product of photosynthesis are referred to as C3 plants
(e.g. soya beans and oats).
 Reduction
- Next, six molecules of glycerate-3-phosphate are
phosphorylated at the expense of six ATP molecules to form
glycerate-1,3-bisphosphate.
- The latter molecules are then reduced by NADP+glyceraldehyde-3-phosphate dehydrogenase to form six
molecules of glyceraldehyde-3-phosphate.
 Regeneration
- The net production of fixed carbon in the Calvin cycle is one
molecule of glyceraldehyde-3-phosphate.
- The other five glyceraldehyde-3-phosphate molecules are
processed in the remainder of the Calvin cycle reactions to
regenerate three molecules of ribulose-1,5-bisphosphate.
The Calvin Cycle
Summary of photosynthesis process