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3.3 Photosynthesis… The Details • Photosynthesis is divided into 2 sequential processes 1. Light reaction (stages 1 and 2) 2. Carbon Fixation (Calvin Cycle) (stage 3) Overview of Photosynthesis 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O • Photosynthesis, like cellular respiration, is a series of redox reactions! Tracking Atoms in Photosynthesis… Reactants 6 CO2 Products C6H12O6 12 H2O 6H2O 6O2 What does this suggest about photosynthesis? The sugars that result from photosynthesis are produced by adding the hydrogen ions and electrons from water to carbon dioxide, NOT by splitting CO2 and adding water! oxidized H+ and e- 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O reduced The stages of photosynthesis… Light Dependent Reactions • Occurs along the thylakoid membrane • this is where H2O is oxidized. • chemiosmotic production of ATP! • electron carrier (NADPH) is synthesized Light Reaction • Divided into 3 parts; 1. Photoexcitation-absorption of a photon by an electron of chlorophyll 2. Photosystems and the ETC 3. Chemiosmosis (sound familiar?) Photoexcitation • Before photons strike chlorophyll, their electrons are at ground state (lowest potential energy) • Once the photon strikes the electron it becomes “excited” and moves to a higher potential energy level • The electron eventually goes back to ground state (loss of potential energy appears as heat and light-fluorescence if it is not used in photosynthesis) Photoexcitation • Most chlorophyll do not fluoresce while in a thylakoid memebrane b/c the excited electron is captured by the primary electron acceptor • This is a redox rxn-the chlorphyll is oxidized and the primary acceptor is reduced Photosystems and the ETC ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP generates O2 use electron carrier NADPH Photosystem Photon PETC Photosynthetic Electron Transfer Chain (PETC) • series of electron carriers which take electrons from photosystem, and.. • ultimately carry electrons to NADP+ Photosystems • Experiments in the 1940’s suggested that light photons are absorbed at 2 different points along the same PETC.… • In fact, there are two Photosystems in operation chlorophyll a ETC of Photosynthesis Photosystem II chlorophyll b Photosystem I ETC of Photosynthesis sun 1 e e Photosystem II P680 chlorophyll a Inhale, baby! ETC of Photosynthesis thylakoid chloroplast +H+ H+ H+ + + + H+ H+H +H+ H H H H H+ ATP +H+ H+ H+ + H H + + H+H+ H+ HH SPLIT water! H H 1 O H e- e e Photosystem II P680 chlorophyll a fill the e– vacancy H+ e- +H OO e e H 2 ETC of Photosynthesis thylakoid chloroplast H+ +H+ H+ H+ + H H + + H+H+ H+ HH +H+ H+ H+ H+ H+H + + + + H+H H H H ATP 3 2 1 e e H+ 4 ATP H+ to Calvin Cycle H+ H+ H+ Photosystem II P680 chlorophyll a H+ H+ + H+ H ADP + Pi ATP H+ H+ energy to build carbohydrates ETC of Photosynthesis e e 5 e e Photosystem II P680 chlorophyll a Photosystem I P700 chlorophyll b sun ETC of Photosynthesis electron carrier 6 e e 5 sun Photosystem II P680 chlorophyll a Photosystem I P700 chlorophyll b $$ in the bank… reducing power! ETC of Photosynthesis sun sun + + + H H + + H+ H + H H H+H+ H+ H + H to Calvin Cycle O split H2O ATP Terms • Primary electron acceptor-pheophytin (PSII) • PQ-Plastoquinone (PSII) • Z protein-splits water (PSII) the 2 electrons are used to replace the ones lost in P680, oxygen leaves as “waste” and the protons remain adding to the gradient • B6-f cytochrome complex (PSII) • Pc-Plastocyanin (PSII) • Fd-ferredoxin (PSI) • NADP reductase (PSI) uses 2 electrons and H+ to reduce NADP+ to NADPH (b/c 2 electrons are needed this process must occur twice) • ATP is created by chemiosmosis due to the proton gradient in the thylakoid lumen-this drives ATP synthase. 4 protons are move into the lumen for each pair of electrons that move through the ETC ETC of Photosynthesis • ETC uses light energy to produce – ATP & NADPH (called noncyclic electron flow) • go to Calvin cycle • PS II absorbs light – excited electron passes from chlorophyll to “primary electron acceptor-pheophytin” – need to replace electron in chlorophyll – Enzyme (z protein) extracts electrons from H2O & supplies them to chlorophyll • splits H2O (h protons) • O combines with another O to form O2 • O2 released to atmosphere • and we breathe easier! Noncyclic Photophosphorylation • Light reactions elevate electrons in 2 steps (PS II & PS I) – PS II generates energy as ATP – PS I generates reducing power as NADPH ATP Cyclic photophosphorylation • If PS I can’t pass electron to NADP…it cycles back to PS II & makes more ATP, but no NADPH – coordinates light reactions to Calvin cycle – Calvin cycle uses more ATP than NADPH 18 ATP + 12 NADPH 1 C6H12O6 ATP Photophosphorylation cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP Light reactions • Electron Transport Chain • like in cellular respiration – proteins in organelle membrane – electron acceptors • NADPH – proton (H+) gradient across inner membrane • find the double membrane! – ATP synthase enzyme thylakoid chloroplast +H+ H+ H+ + + + H+ H+H +H+ H H H H ATP +H+ H+ H+ H+ H+H + + + + H+H H H H Photosynthesis summary Where did the energy come from? Where did the electrons come from? Where did the H2O come from? Where did the O2 come from? Where did the O2 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? …stay tuned for the Calvin cycle Practice Questions • Q 1-8 p.166 The stages of photosynthesis… Light Independent Reactions-Calvin Cycle • Occurs in the stroma of the chloroplast. • Where CO2 is “fixed” into sugars! Whoops! Wrong Calvin… The Calvin Cycle 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 (ATP) produced from light reactions – in a more stable form – that can be moved around plant – saved for a rainy day carbon + water + energy glucose + oxygen dioxide 6CO2 + 6H2O + light C6H12O6 + 6O2 energy Light reactions • Convert solar energy to chemical energy ATP – ATP energy – NADPH reducing power • What can we do now? build stuff !! photosynthesis How is that helpful? • Want to make C6H12O6 – synthesis – How? From what? CO2 are available? What raw materials NADPH carbon fixation reduces CO2 NADP C6H12O6 NADP From CO2 C6H12O6 • CO2 has very little chemical energy – fully oxidized • C6H12O6 contains a lot of chemical energy – highly reduced • Synthesis = endergonic process – put in a lot of energy • Reduction of CO2 C6H12O6 proceeds in many small uphill steps – each catalyzed by a specific enzyme – using energy stored in ATP & NADPH From Light reactions to Calvin cycle • Calvin cycle – chloroplast stroma • Need products of light reactions to drive stroma synthesis reactions – ATP ATP – NADPH thylakoid C C Calvin cycle C C C C C 1C C C C C C 3. Regeneration C C C C C of RuBP RuBP starch, sucrose, cellulose & more ribulose bisphosphate 3 ATP H H H | | | C–C–C C C C C C C C C C C C C C C C C C C CO2 1. Carbon fixation C C C C C C RuBisCo ribulose bisphosphate carboxylase 3 ADP used to make glucose C=C=C 5C C C C C C C C 6C C C C C C C 5C glyceraldehyde-3-P G3P C C C PGA phosphoglycerate 3C 6 NADP C C C C C C 6 ATP 2. Reduction 6 NADPH 3C C C C C C C 3C 6 ADP C C C C C C H | H | H | Remember G3P? glycolysis glucose C-C-C-C-C-C 2 ATP 2 ADP fructose-1,6bP P-C-C-C-C-C-C-P DHAP P-C-C-C G3P glyceraldehyde 3-phosphate C-C-C-P 2 NAD+ 2 4 ADP Photosynthesis pyruvate C-C-C 4 ATP To G3P and Beyond! • Glyceraldehyde-3-P – end product of Calvin cycle – energy rich 3 carbon sugar – “C3 photosynthesis” • G3P is an important intermediate • G3P glucose carbohydrates lipids phospholipids, fats, waxes amino acids proteins nucleic acids DNA, RNA RuBisCo • Enzyme which fixes carbon from air – ribulose bisphosphate carboxylase – the most important enzyme in the world! • it makes life out of air! – definitely the most abundant enzyme Accounting • The accounting is complicated – 3 turns of Calvin cycle = 1 G3P – 3 CO2 1 G3P (3C) – 6 turns of Calvin cycle = 1 C6H12O6 (6C) – 6 CO2 1 C6H12O6 (6C) – 18 ATP + 12 NADPH 1 C6H12O6 – any ATP left over from light reactions will be used elsewhere by the cell Photosynthesis summary • Light reactions – produced ATP – produced NADPH – consumed H2O – produced O2 as byproduct • Calvin cycle – consumed CO2 – produced G3P (sugar) – regenerated ADP – regenerated NADP ADP NADP Light Reactions light ATP + NADPH + O 2 energy H 2O + H2O sunlight Energy Building Reactions NADPH ATP O2 produces ATP produces NADPH releases O2 as a waste product Calvin Cycle CO2 + ATP + NADPH C6H12O6 + ADP + NADP CO2 ADP NADP Sugar Building Reactions NADPH ATP sugars builds sugars uses ATP & NADPH recycles ADP & NADP back to make more ATP & NADPH Putting it all together light CO2 + H2O + energy C6H12O6 + O2 H2O CO2 sunlight ADP Energy NADP Building Reactions Sugar Building Reactions NADPH ATP O2 sugars Plants make both: energy ATP & NADPH sugars even though this equation is a bit of a lie… it makes a better story sun Energy cycle Photosynthesis light CO2 + H2O + energy C6H12O6 + O2 plants CO2 glucose H2O animals, plants ATP C6H12O6 + O2 energy + CO2 + H2O Cellular Respiration The Great Circle of Life,Mufasa! ATP O2 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…not listed in this equation? Supporting a biosphere • On global scale, photosynthesis is the most important process for the continuation of life on Earth – each year photosynthesis… • captures 121 billion tons of CO2 • synthesizes 160 billion tons of carbohydrate – heterotrophs are dependent on plants as food source for fuel & raw materials The poetic perspective… • All the solid material of every plant was built by sunlight out of thin air • All the solid material of every animal was built from plant material air Then all the plants, cats, dogs, elephants & people … are really particles of air woven together by strands of sunlight! sun Practice Questions • Q 9-13 p.166