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Chapter 10 Photosynthesis: Life from Light AP Biology Energy needs of life All life needs a constant input of energy Heterotrophs get their energy from “eating others” consumers of other organisms consume organic molecules Autotrophs get their energy from “self” get their energy from sunlight use light energy to synthesize organic molecules AP Biology How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules glucose + oxygen carbon + water + energy dioxide C6H12O6 + 6O2 6CO2 + 6H2O + ATP Autotrophs making energy & organic molecules from light energy carbon + water + energy glucose + oxygen dioxide 6CO2 + 6H2O + light C6H12O6 + 6O2 energy AP Biology Energy cycle sun Photosynthesis CO2 H 2O glucose Cellular Respiration ATP AP Biology O2 What does it mean to be a plant Need to… collect light energy transform it into chemical energy store light energy in a stable form to be moved around the plant & also saved for a rainy day need to get building block atoms from the environment C,H,O,N,P,S produce all organic molecules needed for growth carbohydrates, proteins, lipids, nucleic acids AP Biology Plant structure Obtaining raw materials sunlight leaves = solar collectors CO2 stomates = gas exchange H2O uptake from roots nutrients uptake from roots AP Biology AP Biology Plant structure Chloroplasts double membrane stroma thylakoid sacs grana stacks Chlorophyll & ETC in thylakoid membrane H+ gradient built up within thylakoid sac H+ AP Biology + + H + H H+ + H H + H+ H+ H+ + H H Pigments of photosynthesis chlorophyll & accessory pigments “photosystem” embedded in thylakoid membrane structure function AP Biology Why does this structure make sense? A Look at Light The spectrum of color AP Biology Light: absorption spectra Photosynthesis performs work only with absorbed wavelengths of light AP Biology chlorophyll a — the dominant pigment — absorbs best in red & blue wavelengths & least in green other pigments with different structures have different absorption spectra Chloroplasts Chloroplasts are green because they absorb light wavelengths in red & blue and reflect green back out structure function AP Biology Photosystems Photosystems collections of chlorophyll molecules 2 photosystems in thylakoid membrane act as light-gathering “antenna complex” Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light AP Biology Photosynthesis overview Light reactions convert solar energy to chemical energy ATP Calvin cycle AP Biology uses chemical energy (NADPH & ATP) to reduce CO2 to build C6H12O6 (sugars) Light reactions Similar to ETC in cellular respiration membrane-bound proteins in organelle electron acceptors NADPH proton (H+) gradient across inner membrane Where’s the double membrane? AP Biology ATP synthase enzyme The ATP that Jack built photosynthesis respiration sunlight breakdown of C6H12O6 moves the electrons runs the pump pumps the protons forms the gradient releases the free energy allows the Pi to attach to ADP forms the ATP AP Biology … that evolution built ETC of Respiration Mitochondria transfer chemical energy from food molecules into chemical energy of ATP use electron carrier NADH generate H2O AP Biology ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP AP Biology split H2O use electron carrier NADPH ETC of Photosynthesis AP Biology ETC of Photosynthesis AP Biology ETC of Photosynthesis AP Biology ETC of Photosynthesis ETC produces from light energy ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light AP Biology excited electron passes from chlorophyll to “primary electron acceptor” need to replace electron in chlorophyll enzyme extracts electrons from H2O & supplies them to chlorophyll splits H2O O combines with another O to form O2 O2 released to atmosphere and we breathe easier! Experimental evidence Where did the O2 come from? radioactive tracer = O18 Experiment 1 6CO2 + 6H2O + light C6H12O6 + 6O2 energy Experiment 2 6CO2 + 6H2O + light C6H12O6 + 6O2 energy Proved O2 came from H2O not CO2 = plants split H2O AP Biology 2 Photosystems Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH AP Biology Cyclic photophosphorylation If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH AP Biology coordinates light reactions to Calvin cycle Calvin cycle uses more ATP than NADPH Photophosphorylation cyclic photophosphorylation noncyclic photophosphorylation AP Biology Photosynthesis summary Where did the energy come from? Where did the H2O come from? Where did the electrons come from? Where did the O2 come from? Where did the H+ come from? Where did the ATP come from? Where did the O2 go? What will the ATP be used for? What will the NADPH be used for? …stay tuned for the Calvin cycle AP Biology Chapter 10. Photosynthesis: The Calvin Cycle Life from Air AP Biology Remember what it means to be a plant… Need to produce all organic molecules necessary for growth carbohydrates, lipids proteins, nucleic acids Need to store chemical energy in stable form can be moved around plant saved for a rainy day AP Biology Autotrophs Making energy & organic molecules from light energy photosynthesis carbon + water + energy glucose + oxygen dioxide 6CO2 + 6H2O + light C6H12O6 + 6O2 energy AP Biology How is that helpful? Want to make C6H12O6 synthesis How? From what? What raw materials are available? CO2 NADPH carbon fixation NADP C6H12O6 AP Biology reduce CO2 From CO2 C6H12O6 CO2 has very little chemical energy fully oxidized C6H12O6 contains a lot of chemical energy reduced endergonic Reduction of CO2 C6H12O6 proceeds in many small uphill steps each catalyzed by specific enzyme using energy stored in ATP & NADPH AP Biology From Light reactions to Calvin cycle Calvin cycle chloroplast stroma Need products of light reactions to drive synthesis reactions ATP NADPH AP Biology Calvin cycle (don’t count the carbons!) 1C ribulose bisphosphate 3. Regeneration RuBP 3 ATP PGAL to make glucose 5C 1. Carbon fixation Rubisco ribulose bisphosphate carboxylase 3 ADP PGAL sucrose cellulose etc. CO2 6C unstable intermediate 2x 3C 3C x2 PGA 2. Reduction 6 ATP 6 NADPH 6 NADP AP Biology 2x 3C 6 ADP Calvin cycle PGAL end product of Calvin cycle energy rich sugar 3 carbon compound “C3 photosynthesis” PGAL important intermediate PGAL AP Biology glucose carbohydrates lipids amino acids nucleic acids PGA PGA PGA RuBP RuBP RuBP PGAL PGAL AP Biology Rubisco Enzyme which fixes carbon from atmosphere ribulose bisphosphate carboxylase the most important enzyme in the world! it makes life out of air! AP Biology definitely the most abundant enzyme Accounting The accounting is complicated 3 turns of Calvin cycle = 1 PGAL 3 CO2 1 PGAL (3C) 6 turns of Calvin cycle = 1 C6H12O6 (6C) 6 CO2 1 C6H12O6 (6C) 18 ATP + 12 NADPH 1 C6H12O6 AP Biology 6 ATP = left over from light reactions for cell to use elsewhere Photosynthesis summary Light reactions produced ATP produced NADPH consumed H2O produced O2 as byproduct Calvin cycle consumed CO2 produced PGAL regenerated ADP regenerated NADP AP Biology Summary of photosynthesis 6CO2 + 6H2O + light C6H12O6 + 6O2 energy Where did the CO2 come from? Where did the CO2 go? Where did the H2O come from? Where did the H2O go? Where did the energy come from? What’s the energy used for? What will the C6H12O6 be used for? Where did the O2 come from? Where will the O2 go? What else is involved that is not listed in this equation? AP Biology Chapter 10. Photosynthesis: Variations on the Theme AP Biology Remember what plants need… Photosynthesis light reactions Calvin cycle light sun H2O ground CO2 air What structures have plants evolved to supply these needs? AP Biology A second look at stomates… Gas exchange CO2 in for Calvin cycle O2 out from light reactions H2O out for light reactions photosynthesis xylem (water) O2 CO2 phloem (sugars) gas exchange water loss AP Biology H2O O2 CO2 Controlling water loss from leaves Hot or dry days stomates close to conserve water guard cells gain H2O = stomates open lose H2O = stomates close adaptation to living on land, but… creates PROBLEMS! AP Biology Closed stomates closed stomates lead to… O2 builds up (from light reactions) CO2 is depleted (in Calvin cycle) causes problems in Calvin Cycle AP Biology Inefficiency of Rubisco: CO2 vs O2 Rubisco in Calvin cycle carbon fixation enzyme normally bonds C to RuBP reduction of RuBP photosynthesis building sugars when O2 concentration is high Rubisco bonds O to RuBP O2 is alternative substrate oxidation of RuBP breakdown sugars AP Biology photorespiration Calvin cycle review 1C RuBP 5C Rubisco 6C ATP PGAL to make glucose unstable intermediate ADP PGAL 2x 3C 3C x2 NADP PGA ATP NADPH AP Biology CO2 2x 3C ADP to mitochondria ----------lost as CO2 without making ATP Calvin cycle with O2 O2 RuBP 5C Rubisco 2C 3C photorespiration AP Biology Impact of Photorespiration Oxidation of RuBP short circuit of Calvin cycle loss of carbons to CO2 can lose 50% of carbons fixed by Calvin cycle decreases photosynthetic output by siphoning off carbons no ATP (energy) produced no C6H12O6 (food) produced AP Biology if photorespiration could be reduced, plant would become 50% more efficient strong selection pressure Reducing photorespiration Separate carbon fixation from Calvin cycle C4 plants physically separate carbon fixation from Calvin cycle different enzyme to capture CO2 PEP carboxylase stores carbon in 4C compounds different leaf structure CAM plants separate carbon fixation from Calvin cycle by time of day fix carbon (capture CO2) during night store carbon in organic acids AP Biology perform Calvin cycle during day C4 plants A better way to capture CO2 before Calvin cycle, fix carbon with enzyme PEP carboxylase store as 4-C compound adaptation to hot, dry climates have to close stomates a lot different leaf anatomy AP Biology sugar cane, corn, other grasses… C4 Plants AP Biology corn sugar cane O2 PEP carboxylase light reactions PEP carboxylase enzyme higher affinity for CO2 than O2 (better than Rubisco) fixes CO2 in 4C compounds regenerates CO2 in inner cells for Rubisco AP Biology phosphoenolpyruvate (3C) + CO2 oxaloacetate (4C) Comparative anatomy Separate reactions in different cells light reactions carbon fixation Calvin cycle C3 AP Biology C4 C4 photosynthesis Physically separated carbon fixation from Calvin cycle Outer cells light reaction & carbon fixation pumps CO2 to inner cells keeps O2 away from inner cells away from Rubisco Inner cells CO2 O2 AP Biology O2 CO2 Calvin cycle glucose to veins CAM (Crassulacean Acid Metabolism) plants Different adaptation to hot, dry climates succulents, some cacti, pineapple separate carbon fixation from Calvin cycle by time close stomates during day open stomates during night at night, open stomates & fix carbon in “storage” compounds organic acids: malic acid, isocitric acid AP Biology in day, close stomates & release CO2 from “storage” compounds to Calvin cycle increases concentration of CO2 in cells CAM plants AP Biology C4 vs CAM Summary solves CO2 / O2 gas exchange vs. H2O loss challenge C4 plants CAM plants separate 2 steps of C fixation anatomically in 2 different cells separate 2 steps of C fixation temporally at 2 different times AP Biology Why the C3 problem? Possibly evolutionary baggage Rubisco evolved in high CO2 atmosphere there wasn’t strong selection against active site of Rubisco accepting both CO2 & O2 Today it makes a difference 21% O2 vs. 0.03% CO2 photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day strong selection pressure to evolve better way to fix carbon & minimize photorespiration AP Biology Sunshine is good! Any Questions?? Any Questions?? AP Biology
