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