Photosynthesis Life from Light and Air

Obtaining the materials – sunlight leaves = solar collectors – CO 2 stomates = gas exchange –H2O–H2O uptake from roots – nutrients N, P, K, S, Mg, Fe… uptake from roots

Concurrent Reactions

Light-Dependent Reactions *Key to these rxns is light absorption Success lies in photosystems – 2 part complex 1.Antenna complex – site of absorption; excites electrons & passes them on 2.Reaction center – redox-rxns occur here; excited ele - transferred to electron acceptor

reaction center antenna pigments

PS 680 2H + + ½ O PS 700 NADP + + 2H + NADPH ADP + P ATP! - pheophytin - plastoquinone - Cytochrome complex - H + ion - ferredoxin - ele - - NADP + Reductase - ATP Synthase To Calvin Cycle!!!

Step 1: Light absorbed in PS II excites ele - (from H 2 O splitting) → passed to electron acceptor (pheophytin) Step 2: Plastoquinone(PQ) shuttles high energy ele - through series of redox rxns (ETC 1); passes through cytochrome complex, driving in H+, ↑ [H+] = ↓ pH in the ETC Step 3: De-energized ele - are passed off to protein plastocyanin (PC) → PS I re-excites ele - and ferredoxin shuttles through ETC 2 Step 4: a) Ele - reduce NADP+ to NADPH at NADP+ reductase b) H+ pumped out via ATP Synthase – activates chemiosmosis → production of ATP from ADP

Regulation of the Light Reaction – Calvin cycle uses more ATP than NADPH – If there is not enough electrons from PS1, cycles back to PS11 and NADPH production is turned off 18 ATP + 12 NADPH+ 1 C 6 H 12 O 6C 6 H 12 O 6  ATP

Review Animations Light dependent reactions

Light-INdependent Reactions Want to make C 6 H 12 O 6 → synthesis **How? From what? What raw materials are available? CO 2 C 6 H 12 O 6 NADPH NADP reduces CO 2 carbon fixation NADP

From CO 2  C 6 H 12 O 6 CO 2 has very little chemical energy – fully oxidized C 6 H 12 O 6 has a LOT of chemical energy – highly reduced Synthesis = endergonic process – put in a lot of energy Reduction of CO 2  C 6 H 12 O 6 proceeds in many small uphill steps – each catalyzed by specific enzyme – uses energy stored in ATP & NADPH ←from light rxns!

starch, sucrose, cellulose & more 1C CO 2 Calvin cycle 5C RuBP 3C RuBisCo 1. Carbon fixation 2. Reduction 3. Regeneration of RuBP ribulose bisphosphate ribulose bisphosphate carboxylase 6 NADP 6 NADPH 6 ADP 6 ATP 3 ADP 3 ATP used to make glucose 3C G3P aka PGAL glyceraldehyde-3-P CCCCCCCCCCCCCCC 6C CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC PGA phosphoglycerate CCCCCCCCCCCCCCCCCCCCC C C C CCC == | H | H | H | H | H | H CCC –– 5C

A little more about RuBisCo… Enzyme which fixes carbon from air ribulose bisphosphate carboxylase the most important enzyme in the world! **It makes life out of air!** most abundant enzyme

“Banking” Keeping the books straight… – 1 G3P = 3C  each turn of the cycle uses 1 CO 2 – 3 turns of Calvin cycle = 1 G3P – 3 CO 2  1 G3P (3C) – 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6C) – 6 CO 2  1 C 6 H 12 O 6 (6C) – 18 ATP + 12 NADPH  1 C 6 H 12 O 6 – Left over ATP from light reactions used elsewhere by cell

Sum it Up: Light Reactions O2O2 H2OH2O Energy Building Reactions ATP  produces ATP & NADPH  releases O 2 as waste sunlight H2OH2O ATP O2O2 light energy  +++ NADPH

Calvin Cycle sugars CO 2 Sugar Building Reactions ADP  builds sugars  uses ATP & NADPH  recycles ADP & NADP + ATP NADPH NADP CO 2 C 6 H 12 O 6  +++ NADPATP + NADPHADP

Supporting a biosphere Big Picture: photosynthesis is THE most important process for continuation of life on Earth Each year photosynthesis… captures 121 billion tons of CO 2 synthesizes 160 billion tons of carbohydrates heterotrophs depend on plants for food, fuel, & raw materials

Light Depedent Review: Where did the energy come from? Where did the electrons come from? Where did the H 2 O come from? Where did the O 2 come from? Where did the O 2 go? Where did the H + come from? Where did the ATP come from? What will the ATP be used for? Where did the NADPH come from? What will the NADPH be used for?

Light Independent Review II: Where did the CO 2 come from? Where did the CO 2 go? Where did the H 2 O come from? Where did the H 2 O go? Where did the energy come from? What’s the energy used for? What will the C 6 H 12 O 6 be used for? Where did the O 2 come from? Where will the O 2 go? What else is involved…not listed in this equation? 6CO 2 6H 2 O C 6 H 12 O 6 light energy  +++ 6O 2

Adaptations in Alternative Environments C4 Path CO 2 stored in mesophyll (enzyme has an affinity for CO2 and can fix carbon) & released in vascular tissue

Adaptations in Alternative Environments CAM Path Stomata open at night to preserve water– CO 2 stored in vacuoles for later use

Combating Photorespiration Photorespiration – rxn between RuBP & O2 O2 when [O 2 ] is high → uses ATP & produces CO 2 instead of making ATP Favored ONLY when [O 2 ] exceeds [CO 2 ] C4 PathCAM Plants Store CO 2 Evolution! Atmosphere had less O 2 than today. Rubisco retains some affinity for O 2