KHADIJAH HANIM BT ABDUL RAHMAN WEEK 15: 17/12/2012
[email protected] PHOTOSYNTHESIS Slide 2 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. 12H 2 O + 6CO 2 ----- light
-----> 6O 2 + C 6 H 12 O 6 + 6H 2 0 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: 2H 2 O ----light---> O 2 + 4[H] Slide 3 The electrons
thereby obtained subsequently reduce CO2: 4[H] + CO 2 (CH 2 O) + H
2 O These 2 stages reaction normally referred as light reaction and
dark reaction. Slide 4 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 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. Slide 5 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.
Slide 6 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. Slide 7 Slide 8 Chloroplast Structure
1. Outer membrane 2. Inner membrane systems 3. Thylakoid membranes
4. Thylakoid space (within the thylakoids) 5. Granum(a) (stack(s)
of thylakoidsmembranes) 6. Stroma (the liquid area outside the
thylakoid membranes) Slide 9 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 Slide 10 Photosystem I (PSI) Energizes and transfers
electrons that are donated to 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. Slide 11 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
D 1 and D 2, cytochrome b 559, and special pair chlorophyll a
molecules that absorb light at 680nm (P680). Slide 12 Cytochrome b
6 f 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. Slide 13 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. Slide 14 LIGHT REACTIONS Mechanism by
which electrons are energized ans subsequently 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. Slide 15 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. Slide 16 Slide 17 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 CO 2 + 6 NADPH + 9 ATP ----->
Glyceraldehyde-3- phosphate + 6 NADP + + 9 ADP + 8 P i Slide 18 The
reaction can be divided into 3 phases Carbon fixation - The
mechanism, by which inorganic CO 2 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). Slide 19 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. Slide 20 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. Slide 21 The Calvin Cycle Slide 22
Summary of photosynthesis process
