Transcript Photosynthesis

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Photosynthesis is carried out by
a number of different organisms
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All contain chlorophyll
o Absorbs light energy and
begins process of
photosynthesis
o Chlorophyll a (blue green)
o Chlorophyll b (yellow green)
o They absorb different
wavelengths of light
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Make up largest group of
photosynthesizing prokaryotes
First organisms to harness the
sun’s energy
Unicellular, but may grow in
colonies
Live in many different
environments
Grow rapidly in nutrient rich
water (known to cause algal
blooms)
o can be toxic to fish, birds,
humans, and other
mammals
 Unlike
cyanobacteria,
algae, some protests,
and plant cells contain
chlorophyll within the
photosynthetic
membranes of discrete
organelles called
chloroplasts
o gives leaves, stems and
un-ripened fruit their
characteristic green
colour!
 Colour
Code!
 Leaves
o The Factory!
 Transpiration
o Cooling and Transport
 Stomata
o The Gate Keepers
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Typical plant cell chloroplast approx. 3um to 8um in length and 2 um to
3 um in diameter
Have two limiting membranes (outer and inner) enclosing an interior
space filled with a liquid stroma
Within the stroma, a system of membrane bound sacs called thylakoids
stack on top of one another to form characteristic columns called grana
o typical chloroplast has 60 grana each consisting of 30-50 thylakoids
Adjacent grana connected to one another by unstacked thylakoids
called lamellae
Inside the thylakoids is a water filled space called the Lumen
Chloroplasts have there own DNA and ribosomes, reproduce via fission
http://www.youtube.com/watch?v=Oi2
_n2wbB9o&feature=related
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Converts light energy into the chemical bonds of glucose
(plants are Autotrophs).
It is an endergonic rxn that happens in two stages: The
Light Dependant Rxn and the Light-Independent Rxn.
The overall rxn is:
6CO2 + 6H2O + Light Energy  C6H12O6 + 6O2
Glucose
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Occurs in the Thylakoid Membrane
The chlorophyll captures light energy and uses it to break
down water molecules and create ATP and NADPH.
The oxygen is released as a product.
The ATP and NADPH are carried over to the Light
Independent Rxn.
 Occurs
in the stroma
 CO2 from the air is added to the H+ ions (carried by
NADPH) the glucose is formed
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http://www.youtube.com/watch?v=cX3Iev-JDgM
The sugars that result from photosynthesis are produced by adding
the hydrogen ions and electrons from water to carbon dioxide, NOT
by splitting CO2 and adding water!
H+ and e-
oxidized
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
reduced
 Involves
the splitting of water (photolysis)
 12 H2O + Energy  6 O2 + 24 H+ + 24e Requires light for the energy to split the water
molecule
 Also forms a highly energetic compound, adenosine
triphosphate (ATP)
 3 Parts:
o 1. PHOTOEXCITATION
o 2. ELECTRON TRANSPORT
o 3. CHEMIOSMOSIS
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http://vcell.ndsu.nodak.edu/animations/photosynthesis/m
ovie-flash.htm
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Chlorophyll A is the most important photosynthetic
pigment.
Other pigments called antenna or accessory pigments
are also present in the leaf.
o Chlorophyll B
o Carotenoids (orange / red)
o Xanthophylls (yellow / brown)
These pigments are embedded in the membranes of the
chloroplast in groups called photosystems.
Each pigment absorbs a particular wavelength of light in
the visible spectrum
What wavelengths of light do
you think plants use the least
in photosynthesis?
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Photon = Packets of electromagnetic radiation
Occurs in Photosystems (clusters of photosynthetic
pigments embedded in the thylakoid membranes
Photosystems absorb photons of particular wavelengths
Consist of an antenna complex and rxn centre
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Two types: 700, 680 refers to wavelength absorbed (nm)
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o PS I (P700)
o PS II (P680)
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Antenna complex absorbs a photon of light
Transfers the energy from pigment to pigment until it
reaches a chlorophyll a molecule in the centre of the rxn
centre
The electron of this chlorophyll moves from ground state to
a higher potential energy level (excitation)
ETC!
1. Photon strikes PS II, excites electron
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o Usually, excited electron is captured by the primary electron
acceptor (REDOX rxn)
o Electron then transferred to Plastoquinone (PQ) aka Q cycle
o Then to ETC similar to Cell Resp
o Process occurs twice, so 2 electrons
2. A Z protein associated with PS II splits water into oxygen,
protons, electrons
o Oxygen leaves chloroplast as a by-product
o 2 electrons used to replace those that have left PS II
o Protons remain in Thylakoid space, contributing to H+ gradient that
drives chemosmosis
3. Electrons pass through the Q cycle which transports protons
into lumen from stroma, adding to H+ gradient (4 protons
for every 2 electrons), then through the b6 –f complex
o Electrons then move through Plastocyanin (Pc), eventually replacing
two electrons lost from PS I when it was struck by photons.
4. Electrons from PS I pass through another ETC containing
ferredoxin (Fd)
o They then move to the enzyme NADP reductase that uses the two
electrons and protons from the stroma to reduce NADP+ to NADPH
3. Chemiosmosis – Protons that have accumulated in the
lumen cause electrochemical gradient that drives
phosphorylation of ADP to ATP
o Called photophosphorylation, since light is required
o Page 166 #2,3,4,6
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Calvin Cycle
Also called C3 photosynthesis (since first compound formed
is a 3C molecule
Occurs in stroma of chloroplasts
Cyclic series of reactions
Three phases: carbon fixation, REDOX reactions, RuBP
(ribulose 1,5-bisphosphate) regeneration
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Phase 1: Carbon Fixation
o CO2 are added to RuBP, forming 3 unstable 6C intermediate
molecules
o Each of these instantly splits into two 3C molecules called PGA (3
phosphoglycerate) for a total of six
o The enzyme that catalyses these reactions is called Rubisco, a very
large enzyme that works very slowly, most abundant protein on
earth
Phase 2: REDOX
o Each of the six PGA is phosphorylated by 6 ATP to form six
1,3bisphosphoglycerate
o 6 NADPH molecules each use a pair of electrons to produce six G3P
(glyceraldehyde 3 – phosphate
o One molecule of G3P exits as a final product
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Phase 3: RuBP regeneration
o The five G3P are rearranged to regenerate 3 RuBP
o 3 ATP are used!
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The Bottom Line
o For the net synthesis of one G3P, 9 ATP and 6 NADPH are used!
o Page 167, #9,10,11
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Is the primary end product of photosynthesis
It may be converted into glucose and polymerized into
starch w/in the stroma
Or it may be transported to the cytoplasm and used to
produce glucose and sucrose.
Sucrose is the main carb transported from mesophyll cells
of the leaf to other cells of the plant
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Rubisco – the double agent!
Normally rubisco adds CO2 to RuBP but when O2 is very
plentiful it adds it to RuBP
This is called Photorespiration – it occurs in light!
It removes PGA molecules from the Calvin cycle
Produces phosphoglycolate
Decrease CO2 fixation and less sugar formed
Carbon fixation 4X that of oxygen fixation (20% loss of C)
Rubisco is an evolutionary remnant (from a time in earth’s
history when O2 was not a prevalent
Some plants have adapted strategies to work around this!
HELPFUL!
ANNOYING!!
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Hot, dry environments
C4 photosynthesis – a pathway of carbon fixation that
reduces the amount of photorespiration that takes place by
continually pumping CO2 molecules from mesophyll cells
into bundle sheath cells, where rubisco brings them into the
C3 Calvin cycle
C4 refers to the four carbon oxaloacetate that is formed in
the mesophyll
CO2 level in leaf is increased, no photoresp.
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Crassulacean Acid Metabolism
Plants in dry, desert environments
Stomata open at night to take in CO2 and incorporate it into
organic acids, and close during the day to allow the acids to
release CO2 into the Calvin Cycle.
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Light Intensity (P173)
Temperature (P174)
O2 concentration (P175)
Efficiency (P175)
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