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LIGHT & DARK REACTIONS
OF PHOTOSYNTHESIS
How photosynthesis works
Light DependentReactions
Sugars
“DarkReactions”
Light IndependentReactions
Photosystem I
Photosystem II
Non-cyclic é flow
Cyclic é flow
H2O O2
CO2 Water SplittingReactions
Carbon Fixation
Step 1: Photoexcitation
• The structure of chlorophyll is very important to its function:– Notice the alternating double bonds– These é are said to be ‘delocalized’
• The polar chlorophyll head is found mixed with the phospholipids of the thylakoid membrane
• This is the where photosynthesis starts...
Step 1: Photoexcitation• Before a photon of light strikes chlorophyll, its é
are at their lowest energy level– ‘ground state’
• When a photon hits, an é gains energy – Becomes ‘excited’
• When an excited é returns to its original state it can:– Emit light ‘fluorescence’– Transfer é to another é carrier ‘primary é acceptor’
Cool Note
• If you separate a chlorophyll molecule from the thylakoid membrane...
• The excited é will fluoresce as its energy lowers back to its ground energy
The red fluorescence in the middle of the jellyfish comes from chlorophyll in the ingested algae
Step 2: Photosystems• Photosystems consist of:– Antenna complex– Reaction centre
• Antenna complex– composed of a # of chlorophyll molecules and accessory
pigments – Embedded in the thylakoid membrane– Photon is absorbed
and transfers energybetween pigmentsuntil it reacheschlorophyll a
Step 2: Photosystems
• Photosystems consist of:– Antenna complex– Reaction centre
• Reaction Centre– An é on chlorophyll a absorbs energy from the antenna
complex and becomes ‘excited’– A redox reaction transfers the excited é to the
primary é acceptor
*There are 2 Photosystems:Photosystem I
Photosystem II
Step 3: Non-cyclic é flowProcess:1.A photon strikes the antennae complex of photosystem II to excite an é
2.The excited é is captured by the primary é acceptor pheophytin
3.Through a series of redox reactions, é is transferred to plastoquinone (PQ) and then to the electron transport chain
4.The é powers a H+ pump allowing 4H+ to enter
Step 3: Non-cyclic é flow• At the same time,
A ‘Z’ protein splits water into oxygen, H+ ions (protons), and é
• The é from water are used to replenish the é lost in photosystem II
Step 3: Non-cyclic é flowPhotosystem I NADPH
1.Two photons excite 2 é from photosystem I
2.é from photosystem I pass through another ETC containing ferredoxin, Fd
3.Finally, Fd, gives its é to NADP reductase that uses H+ from the stroma to reduce NADP+ to NADPH
NADPH is used in the Calvin cycle
Step 3: Non-cyclic é flow
*** Meanwhile, there is a H+ gradient forming in the lumen that allows ATP to be produced as H+ ions pass though ATP synthase
We have produced NADPH and ATP from non-cyclic é flow!
Step 3: Non-cyclic é flow
Step 3: Non-cyclic é flow
• VIDEO
• http://www.youtube.com/watch?v=eY1ReqiYwYs
cyclic é flow In some cases, excited é can take a cyclic
pathway that stays within photosystem I
The excited é is picked up by Fd b6-f Pc
In the end, the excited é is returned to the chlorophyll it came from
This process adds to the H+gradient to help produce ATP
cyclic é flow In some cases, excited é can take a cyclic
pathway that stays within photosystem I
The excited é is picked up by Fd b6-f Pc
In the end, the excited é is returned to the chlorophyll it came from
This process adds to the H+gradient to help produce ATP
How photosynthesis works
Light DependentReactions
Sugars
“DarkReactions”
Light IndependentReactions
Photosystem I
Photosystem II
Non-cyclic é flow
Cyclic é flow
H2O O2
CO2 Water SplittingReactions
Carbon Fixation
Calvin Cycle (Dark Reactions)
Step 4: Carbon Fixation 3 CO2 molecules that are absorbed through the
stomata and spongy mesophyll cells are added to 3 molecules of RuBPRibulose – 1,6 – Bisphosphate (5-carbon molecule)
Together they form unstable 6-carbon compound Which, instantly breaks
down into 6 PGA molecules
Often called C3 photosynthesis as the first product contains 3-carbonC4 plants exist as well
Step 5: Reduction Reactions• Each of the 6 PGA molecules
is phosphorylated by ATP – Producing 1,3-BPG
• Then, NADPH gives each 1,3-BPG 2 é– REDUCING THEM to G-3-P
• ONE of the six G-3-P molecules exits the cycle to produce sugars
Step 6: RUBP REGENERATION• We have five 3-carbon G3P’s left• We need to regenerate RuBP so we
can continue the Calvin Cycle
• The five G3P molecules rearrange to REFROM the three 5-carbon RuBP
• Phosphorylation by 3 ATP molecules will finally regenerate the RuBP
Calvin Cycle TOtals ATP USED = 9 NADPH USED = 12
ATP/NADPH are produced in the light reactions
In the light reactions, 3 ATP and 2 NADPH
are produced Conveniently, the Calvin
cycle uses 3 ATP and 2 NADPHper CO2
Calvin Cycle TOtals ATP USED = 9 NADPH USED = 12
ATP/NADPH are produced in the light reactions
In the light reactions, 3 ATP and 2 NADPH
are produced Conveniently, the Calvin
cycle uses 3 ATP and 2 NADPHper CO2
How photosynthesis works
Light DependentReactions
Sugars
“DarkReactions”
Light IndependentReactions
Photosystem I
Photosystem II
Non-cyclic é flow
Cyclic é flow
H2O O2
CO2 Water SplittingReactions
Carbon Fixation