LIGHT & DARK REACTIONS OF PHOTOSYNTHESIS

How photosynthesis works “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem I Photosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions 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 composed of a # of chlorophyll molecules and accessory pigments Embedded in the thylakoid membrane Photon is absorbed and transfers energy between pigments until it reaches chlorophyll a

Step 2: Photosystems Photosystems consist of: Reaction Centre 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 flow Process: A photon strikes the antennae complex of photosystem II to excite an é The excited é is captured by the primary é acceptor pheophytin Through a series of redox reactions, é is transferred to plastoquinone (PQ) and then to the electron transport chain 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 flow Photosystem I  NADPH Two photons excite 2 é from photosystem I é from photosystem I pass through another ETC containing ferredoxin, Fd 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 “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem I Photosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions 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 RuBP Ribulose – 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-carbon C4 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 NADPH per 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 NADPH per CO2

How photosynthesis works “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem I Photosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions Carbon Fixation