Chapter 8 - Photosynthesis (PDF)

Chapter 8 pgs. 211-231 Big ideas and essential knowledge Organisms capture and store free energy for use in biological processes. Autotrophs capture free energy from physical sources in the environment. Photosynthetic organism capture free energy present in sunlig...

Chapter 8 pgs. 211-231 Big ideas and essential knowledge Organisms capture and store free energy for use in biological processes. Autotrophs capture free energy from physical sources in the environment. Photosynthetic organism capture free energy present in sunlight. Different energy-capturing processes use different types of electron acceptors. NADP+ in photosynthesis Oxygen in cellular respiration The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy present in light to yield ATP and NADPH, which power the production of organic molecules. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystem I and II. Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through an electron transport chain (ETC). Big ideas and essential knowledge cont. When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of hydrogen ions (protons) across the thylakoid membrane is established. The formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase. The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast. The electron transport chain captures free energy from electrons in a series of coupled reactions that establish an electrochemical gradient across membranes. Electron transport chain reactions occur in chloroplasts (photosynthesis), mitochondria (cellular respiration) and prokaryotic plasma membranes. In cellular respiration, electrons delivered by NADH and FADH2 are passed to a series of electron acceptors as they move toward the terminal electron acceptor, oxygen. In photosynthesis, the terminal electron acceptor is NADP+. Big ideas and essential knowledge cont. The passage of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial membrane or the thylakoid membrane of chloroplasts, with the membrane (s)separating a region of high proton concentration from a region of low proton concentration. In prokaryotes, the passage of electrons is accompanied by the outward movement of protons across the plasma membrane. The flow of protons back through membrane-bound ATP synthase by chemiosmosis generates ATP from ADP and inorganic phosphate. All living systems require constant input of free energy. Energy-related pathways in biological systems are sequential and may be entered at multiple points in the pathway. The structure and function of subcellular components, and their interactions, provide essential cellular species. Chloroplasts are specialized organelles fund in algae and higher plants that capture energy through photosynthesis. The structure and function relationship in the chloroplast allows cells to capture the energy available in sunlight and convert it to chemical bond energy via photosynthesis. Big ideas and essential knowledge cont. Chloroplasts contain chlorophylls, which are responsible for the green color of a plant and are the key light-trapping molecules in photosynthesis. There are several types of chlorophyll, but the predominant form in plants is chlorophyll a. Chloroplasts have a double outer membrane that creates compartmentalized structure, which supports its function. Within the chloroplasts are membrane-bound structures called thylakoids. Energy-capturing reactions housed in the thylakoids are organized in stacks, called “grana”, to produce ATP and NADPH, which fuel carbon-fixing reactions in the Calvin-Benson cycle. Carbon fixation occurs in the stroma, where molecules of CO2 are converted to carbohydrates. Variation in molecular units provides cells with a wider range of functions. Key concepts to know The summary equation of photosynthesis, including the source and fate of the reactants and products. How leaf and chloroplast anatomy relate to photosynthesis. How do the photosystems collect light energy and convert it to chemical energy-linear electron flow. How photosystem II produces ATP by chemiosmosis in the light- dependent reactions. How the double membrane structure of the chloroplast (thylakoids) enable their function in chemiosmosis. How photosystem I produces NADPH in the light-dependent reactions. How the Calvin cycle (light-independent or carbon-fixing reactions) uses the products of the light-dependent reactions to synthesize sugar (G3P) The evolutionary origins of photosynthesis in prokaryotes, and the production of an oxygenated atmosphere and its consequences. The commonalities and distinctions between photosynthesis in chloroplasts and aerobic respiration in mitochondria. Variations on carbon fixation in other plants via alternative systems: photorespiration, C4 and CAM plants. Key terms Light-dependent reactions RuBP Calvin cycle= glyceraldehyde-3-phosphate Light-independent reactions C3 photosynthesis Photosystem: CAM plants Photosystem II photorespiration Photosystem I C4 photosynthesis Carbon fixation Absorption spectrum Pigments Chlorohyll Chlorohyll a Chlorophyll b Action spectrum Carotenoid NADPH Cyclic photophosphorylation Noncyclic photophosphorylation Rubisco  Photosynthesis is the process that converts solar energy into chemical energy.  Directly or indirectly, photosynthesis nourishes almost the entire living world.  Plants and other autotrophs are producers of biosphere  Photoautotrophs: use light E to make organic molecules  Heterotrophs: consume organic molecules from other organisms for E and carbon   All life needs a constant input of energy ◦ Heterotrophs (Animals)  get their energy from “eating others” consumers  eat food = other organisms = organic molecules  release energy through respiration ◦ Autotrophs (Plants)  produce their own food (from “self”)  convert energy of sunlight producers  build organic molecules (CHO) from CO2  produce energy & synthesize sugars through photosynthesis  organisms feed not only themselves but also most of the living world. Photoautotrophs (a) Plants Plants used as our model (c) Unicellular protist 10 µm (e) Purple sulfur 1.5 µm bacteria (b) Multicellular alga (d) Cyanobacteria 40 µm Heterotrophs releasing energy from ingesting organic molecules glucose + oxygen → carbon + water + energy dioxide C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP oxidation = exergonic Autotrophs Where’s making energy & organic molecules from light energy the ATP? carbon + water + energy → glucose + oxygen dioxide 6CO2 + 6H2O +light → C6H12O6 + 6O2 energy reduction = endergonic  Obtaining raw materials ◦ sunlight  leaves = solar collectors ◦ CO2  stomates = gas exchange ◦ H2O  uptake from roots ◦ nutrients  N, P, K, S, Mg, Fe…  uptake from roots  Thyl akoi d spac  Chloroplasts are structurally similar to and likely e evolved from photosynthetic bacteria. (ch. 25.1)- endosymbiotic theory ◦ Site of photosynthesis  Leaves are the major organs of photosynthesis ◦ Their green color is from chlorophyll, the green pigment located within chloroplasts.  Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast.  CO2 enters and O2 exits the leaf through microscopic pores called stomata located on the exterior lower epidermis of the leaf.  cross section of leaf absorb leaves sunlight & CO2 CO2 chloroplasts in plant cell chloroplasts chloroplast contain chlorophyll make energy & sugar  Cross section of a leaf stomate ✓transpiration ✓gas exchange Chloroplasts  Chloroplasts are organelles found mainly in cells of the mesophyll, the interior tissue of the leaf. ◦ About ½ million chloroplasts per square millimeter (mm2) of a leaf. ◦ A typical mesophyll cell has 30–40 chloroplasts  The chlorophyll is in the membranes of thylakoids =thylakoid membranes (connected sacs in the chloroplast). ◦ Thylakoids may be stacked in columns called grana  Chloroplasts also contain stroma, a dense fluid.  Chloroplast Structure of a chloroplast Outer membrane Thylakoid Intermembrane Stroma Granum Thylakoid space space Inner membrane 1 µm  chloroplast +H+ H H+ + H+ H+H H + H+H+ H+  Chloroplasts ATP H + thylakoid ◦ double membrane ◦ stroma  fluid-filled interior outer membrane inner membrane ◦ thylakoid sacs ◦ grana stacks stroma  Thylakoid membrane contains ◦ chlorophyll molecules thylakoid granum ◦ electron transport chain ◦ ATP synthase  H+ gradient built up within thylakoid sac.  ATP generated in the stroma  Photosynthesis consists of the light reactions (the photo part) and the Calvin cycle (the synthesis part): Stage 1.The light reactions (in the thylakoids): light-dependent reactions energy conversion reactions ◦ Split H2O ◦ Release O2 ◦ Reduce NADP+ to NADPH Generate ATP from ADP and phosphate by photophosphorylation Stage 2.The Calvin cycle (in the stroma): Light-independent reactions- occurs 24/7 sugar building reactions forms sugar from CO2, using ATP and NADPH and regenerates NADP+, ADP. begins with carbon fixation, incorporating CO2 into organic molecules. Overview of H2O CO2 photosynthesis “”photo” “synthesis” Light NADP+ ADP + P i Calvin Light Cycle Reactions ATP NADPH Chloroplast [CH2O] O2 (sugar) To summarize:  Chloroplasts are solar-powered chemical factories.  Their thylakoids transform light energy into the chemical energy of ATP and the reducing power of NADPH, which are used in the Calvin cycle to synthesize sugar 1. What is the main function of the Light Reactions? 2. What are the reactants of the Light Reactions? What are the products? 3. Where do the Light Reactions occur? 4. What is the major photosynthetic pigment? Nature of sunlight  Light = Energy = electromagnetic radiation  Shorter wavelength (λ): higher E  Visible light - detected by human eye  Light: reflected, transmitted or absorbed  Electromagnetic spectrum  Light is a form of energy known as electromagnetic energy or electromagnetic radiation.  The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation. ◦ Propagated in waves ◦ Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see.  380nm to 700 nm----most important to life. ◦ Behaves as discrete particles of energy called photons  Photons have fixed quantities of energy. -amount of energy is inversely related to the wavelength of light. -the shorter the wavelength the greater the energy for each photon.  Electromagnetic spectrum 1m 10–5 nm 10–3 nm 1 nm 103 nm 106 nm (109 nm) 103 m Gamma Micro- Radio X-rays UV Infrared waves waves rays Visible light V B G Y O R 380 450 500 550 Y 600 O 650 R700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy Figure 8.12 The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA)  Pigments are substances that absorb visible light. ◦ Different pigments absorb light of different wavelengths. eg.  chlorophyll a (blue-green color) –absorbs best in red and violet-blue wavelengths, least green.  chlorophyll b (olive-green color) –absorbs best in violet and red wavelengths, least green  carotenoids (orange-yellow color) absorb violet and blue-green light  Wavelengths that are absorbed disappear.  Wavelengths that are not absorbed are reflected or transmitted.  Leaves appear green because chlorophyll reflects and transmits green light.  **Whether a pigment absorbs the photon’s energy, depends on: ◦ the chemical nature of the pigment. ◦ how much energy the photon carries (defined by its wavelength).  The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and the excited state. This energy difference varies from one molecule to another. Spectrophotometer  A spectrophotometer used to measure a pigment’s ability to absorb or transmit various wavelengths of light.  This machine sends light (particular wavelengths) through pigments and measures the fraction of light transmitted at each wavelength which can be computed to light absorbed.. TECHNIQUE White Refracting Chlorophyll Photoelectric light prism solution tube Galvanometer 2 3 1 4 The high transmittance Slit moves to Green (low absorption) pass light light reading indicates that of selected chlorophyll absorbs wavelength very little green light. The low transmittance Blue (high absorption) light reading indicates that chlorophyll absorbs most blue light. Absorption Spectrum: representation of how well a pigment absorbs different wavelengths of visible light. Photosynthesis gets energy by absorbing different wavelengths of light -chlorophyll a absorbs best in red & violet-blue wavelengths & least in green -accessory pigments with different structures absorb light of different wavelengths chlorophyll b, carotenoids (xanthophylls)  Engelmann’s experiments- fig. 8.9  The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann  In his experiment, he exposed different segments of a filamentous alga to different wavelengths of visible light.  Areas receiving wavelengths favorable to photosynthesis produced excess O2  He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production Action Spectrum: plots rate of photosynthesis vs. wavelength (absorption of chlorophylls a, b, & carotenoids combined) Engelmann: used bacteria to measure rate of photosynthesis in algae; established action spectrum Which wavelengths of light are most effective in driving photosynthesis? Engelmann’s experiments Photosynthetic pigments  Chlorophyll a is the main photosynthetic pigment-only pigment that converts light energy into chemical energy.  Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis.  Accessory pigments called carotenoids absorb excessive light =photoprotection, that would damage chlorophyll. ◦ Scavenge free radicals=antioxidants  Chlorophylls & other pigments ◦ embedded in thylakoid membrane ◦ arranged in a “photosystem”  collection of molecules ◦ structure-function relationship  A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes. ◦ The light-harvesting complexes (pigment molecules 300-400 chlorophylls and other pigments, bound to proteins) funnel the energy of photons to the reaction center. ◦ The reaction-center complex –pair of special chlorophyll a molecules within a unique microenvironment of proteins. A primary Photosystem STROMA Photon electron acceptor in Light-harvesting Reaction-center Primary complexes electron the reaction complex acceptor center accepts an excited pair of electrons from Thylakoid membrane chlorophyll a e– Transfer Special pair of Pigment of energy chlorophyll a molecules molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)  Photosystems I and II  There are two types of photosystems in the thylakoid membrane that work sequentially:  Photosystem II (PS II=P680) functions first (the numbers reflect order of discovery). ◦ The reaction-center chlorophyll a of PS II is called P680 and is best at absorbing light at wavelength of 680 nm  Photosystem I (PS I=P700): ◦ The reaction-center chlorophyll a of PS I is called P700 is best at absorbing light at wavelength of 700 nm.  During the light reactions, there are two possible routes for electron flow: ◦ cyclic electron flow ◦ linear (noncyclic) electron flow-primary pathway.  Linear electron flow/noncyclic the primary pathway, involves both photosystems and produces ATP and NADPH using light energy. ◦ Light energy drives the synthesis of ATP and NADPH by energizing both photosystems through the flow of electrons.  Linear electron flow A photon hits a pigment and Primary acceptor its energy is passed among e– 2 pigment molecules until it excites P680. P680 An excited electron from P680 is transferred to the 1 Light primary electron acceptor. Pigment molecules Photosystem II (PS II) Linear electron flow P680+ (P680 that is missing an electron) is the strongest Primary acceptor oxidizing agent known. 2 H2O e– 2 H+ + 1/ O2 3 ▪ H2O is split by an enzyme, and produces 2 2 e– e– P680 electrons, 2 protons (H+) 1 Light and an oxygen atom. ▪ Two electrons one by one Pigment are transferred to P680+, thus reducing it to P680. molecules Photosystem II (PS II) ▪ O2 is released as a by- product of this reaction  Each pair of electrons “falls” down an electron transport chain from the primary Primary 4 electron acceptor of PS II to PS acceptor I. Pq 2 H2O e– Cytochrome 2 + H+ complex Energy released by the fall of 1/ O2 2 3 Pc electrons drives the creation e– e– of a proton gradient across P680 5 the thylakoid membrane into 1 Light the thylakoid space (lumen). ATP H+ moves from stroma to thylakoid space (lumen). With or against the concentration Pigment molecules gradient? Photosystem II (PS II) Diffusion of H+ (protons) back across the membrane through ATP synthase drives ATP synthesis.  In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor. P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain. Primary Primary 4 acceptor acceptor Pq e– 2 H2O e– Cytochrome 2 H+ complex + 1/ O2 3 2 Pc e– e– P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd). The 2 electrons are then transferred to NADP + and reduce it to NADPH. H+ is removed from the stroma. The electrons of NADPH are available for the reactions of the Calvin cycle. Primary Primary 4 acceptor 7 acceptor Fd Pq e– 2 e– 8 H2O e– e– NADP+ Cytochrome 2 H+ NADP+ + H+ complex + reductase 1/ O2 3 NADPH 2 Pc e– e– P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) What is the ultimate source of electrons?  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 electron flow uses only photosystem I and produces ATP, but not NADPH. ◦ eg. Found in bundle sheath cells of C4 plants  Cyclic electron flow generates surplus ATP (when ATP supplies run low), satisfying the higher demand in the Calvin cycle.  Why does cyclic electron flow exist? ◦ switch to this when the ratio of NADPH to NADP+ is too high. ◦ Common in photosynthetic cell types with especially high ATP needs.  eg. Bundle sheath cells of C4 plants ◦ May play a photoprotective role, preventing excess light from damaging photosystem proteins and promoting repair of light-induced damage.  Cyclic electron flow Primary Primary acceptor Fd acceptor Fd NADP+ Pq NADP+ + H+ reductase Cytochrome NADPH complex Pc Photosystem I Photosystem II ATP  If PS I can’t pass electron to NADP+ it cycles back from ferrodoxin (Fd) to the cytochrome complex & 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 cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP  ETC of Photosynthesis sun 1 e e Photosystem II P680 chlorophyll a  ETC of Photosynthesis chloroplast thylakoid +H+ H H+ + H+ H+H H+H H H H + + + + +H+ H H+ + H+ H+H H+H H H H + + + ATP + O2 Plants SPLIT water! 1 H H 2 O e e H O H O H+ +H e- e- e e fill the e– vacancy Photosystem II P680 chlorophyll a  ETC of Photosynthesis chloroplast thylakoid +H+ H H+ + H+ H+H H+H H H H + + + + +H+ H H+ + H+ H+H H+H H H H + + + ATP + 3 1 2 e e H+ 4 ATP H+ to Calvin Cycle H+ H+ H+ energy to build H+ H+ H + H+ carbohydrates Photosystem II P680 ATP ADP + P i chlorophyll a H+  ETC of Photosynthesis e e sun 5 e e Photosystem I P700 Photosystem II chlorophyll a P680 chlorophyll a  ETC of Photosynthesis electron carrier 6 e e 5 sun Photosystem I P700 Photosystem II chlorophyll b P680 $$ in the bank… chlorophyll a reducing power!  ETC of Photosynthesis Photosystem II Photosystem I sun sun H+ H+ +H+ H+H H+ + H + +H O + H + H H to Calvin Cycle H + split H2O ATP   Where did the O2 come from? ◦ radioactive tracer = O18 Experiment 1 6CO2 + 6H2O +light light → C6CH6H OO + 6O2 energy energy 1212 6 6 Experiment 2 6CO2 + 6H2O + light → C6H12O6 + 6O2 energy Proved O2 came from H2O not CO2 = plants split H2O! 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  Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy---oxidative phosphorylation and photophosphorylation respectively. ◦ Mitochondria transfer chemical energy from food to ATP. ◦ chloroplasts transform light energy into chemical energy in ATP.  Mitochondria- ◦ source of electrons is organic molecules=sugar  Chloroplasts- ◦ source of electrons is water.  Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria VERY IMPORTANT  In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix via ATP synthase.  In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma via ATP synthase.  Mitochondrion Chloroplast MITOCHONDRION CHLOROPLAST STRUCTURE STRUCTURE H+ Diffusion Intermembrane Thylakoid space space Electron Inner Thylakoid transport membrane chain membrane ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]  ETC of Respiration Mitochondria transfer chemical energy from food molecules into chemical energy of ATP ◆ use electron carrier NADH and FADH2 ◆ Oxygen is the final electron acceptor generates H2O ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP ◆ use electron carrier NADPH generates O2 Summary of light reactions  ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place.  In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O (low state of potential energy) to NADPH (stored at a high state of potential energy).  Convert solar energy to chemical energy ATP ◦ ATP → energy ◦ NADPH → reducing power  What can we do now? → → build stuff !! photosynthesis  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  Calvin cycle ◦ chloroplast stroma  Need products of light reactions to drive synthesis reactions ◦ ATP ◦ NADPH stroma ATP thylakoid  The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle.  The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. Important Differences:  Citric acid cycle-catabolic, oxidizes acetyl CoA, makes ATP  Calvin cycle-anabolic, reduce CO2 to sugar, use ATP The Calvin cycle  Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phosphate (G3P)=PGAL (phosphoglyceraldehyde)  The cycle spends ATP as an energy source and NADPH as reducing power.  For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2  The Calvin cycle (C3 photosynthesis, light independent reactions) has three phases: ◦ Carbon fixation (catalyzed by the enzyme rubisco) ◦ Reduction ◦ Regeneration of the CO2 acceptor (RuBP)  Input 3 (Entering one at a time) CO2 Phase 1:Carbon fixation Phase 1: Carbon fixation Rubisco 3Short-lived P P intermediate 3P P 6 P C3 photosynthesis Ribulose bisphosphate 3-Phosphoglycerate (RuBP) 5 carbon sugar =ribulose 1,5 bisphosphate= RuBP Rubisco=enzyme also called ribulose bisphosphate carboxylase/oxygenase (most abundant enzyme in chloroplasts and on earth---fixes CO2 from air. 6 carbon intermediate-short lived, unstable, splits immediately into 2-3 phosphoglycerates (PGA) (3-carbon compound) for every CO2=Total 6 for three CO2 molecules.  Input 3 (Entering one at a time) CO2 Phase 2:Reduction Phase 1: Carbon fixation Rubisco 3Short-lived P P intermediate 3 P Ribulose bisphosphate P 6 P 3-Phosphoglycerate (RuBP) 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 molecules of G3P produced 6 NADPH 6 NADP+ 6 Pi 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction Output 1 G3P P Glucose and other organic (a sugar) compounds  Input 3 (Entering one at a time) CO2 Phase 3: Regeneration of the RuBP Phase 1: Carbon fixation Rubisco 3Short-lived P P intermediate 3 P Ribulose bisphosphate P 6 P 3-Phosphoglycerate (RuBP) 6 ATP 6 ADP 3 ADP Calvin Cycle 3 ATP 6 P P 1,3-Bisphosphoglycerate 6 NADPH Phase 3: 6 NADP+ Regeneration of 6 Pi the(RuBP) 5 G3P P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction Output 1 G3P P Glucose and other organic (a sugar) compounds  Calvin cycle C3 photosynthesis 85% of plants= C3 plants eg wheat, rice, soybean, all trees C C C C C C C C 1C CO2 C C C C C 1. Carbon fixation 3. Regeneration C C C C C of RuBP C C C C C C RuBP 5C RuBisCo C C C C C C ribulose bisphosphate starch, sucrose, 3 ATP ribulose 6C C C C C C C cellulose bisphosphate & more carboxylase/ 3 ADP oxygenase C C C used C C C to make glyceraldehyde-3-P C C C glucose G3P PGA 3C C C C C C C phosphoglycerate C C C C C C C= C= C C C C 3C C C C C C C H H | H C C C 2.Reduction 6 ATP | | C– C– C C C C 6 NADPH 6 ADP 3C | | C C C | H H H 6 NADP 1, 3 bisphosphoglycerate Products: Per 3 turns of the cycle  Net ONE G3P molecule-starting material for synthesis of glucose and other organic molecules.  Consumes 9 ATP molecules  Consumes 6 NADPH molecules  How are NADPH and ATP regenerated?  What happens to the other 5 G3P ◦ 3 turns of Calvin cycle = 1 G3P ◦ 3 CO2 → 1 G3P (3C) ◦ 6 NADPHs and 9ATPs ◦ 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  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 CO2 + H2O + energy → C6H12O6 + O2 H2 O CO2 Plants make both: sunlight ▪ ATP & NADPH ADP ▪ sugars Energy NADP Sugar Building Building Reactions Reactions NADPH ATP O2 sugars  sun Photosynthesis light CO2 + H2O +energy → C6H12O6 +O2 plants CO2 H2O glucose O2 animals, plants ATP C6H12O6+O2 → energy +CO2 +H2O Cellular Respiration The Great Circle ATP of Life,Mufasa!  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?  Hot or dry days ◦ stomates close to conserve water  The closing of stomata reduces access to CO2 and causes O2 to build up.  conditions favor a seemingly wasteful process called photorespiration. ◦ guard cells  gain H2O = stomates open  lose H2O = stomates close ◦ adaptation to living on land, but…creates PROBLEMS!.  RuBisCo in Calvin cycle ◦ carbon fixation enzyme  normally bonds CO2 to RuBP  CO2 is the optimal substrate  reduction of RuBP  building sugars photosynthesis ◦ when O2 concentration is high in the air spaces  RuBisCo bonds O2 to RuBP  O2 is a competitive substrate photorespiration  oxidation of RuBP  breakdown sugars  Process of photorespiration affected by two factors:  A. temperature: rate increases in the range of 25-30 C.  Why??  B. concentration of oxygen: higher the 02 concentration, higher the rate of photorespiration  Calvin cycle when CO2 is abundant 1C CO2 RuBP ATP 5C RuBisCo unstable ADP 6C intermediate G3P to make 5C glucose C3 plants-85% PGA Eg. Rice, wheat, soybean and all trees 3C G3P 3C NADPH ATP NADP ADP 3C Calvin cycle when O2 is high to mitochondria Photorespiration consumes ATP Additional and does not make sugar reactions that O2 release CO2 without RuBP making ATP 5C RuBisCo Hey Dude, 2C are you high 3C on oxygen! It’s so sad to see a good enzyme, go BAD! Photorespiration Occurs in the light and consumes oxygen.  Oxidation of RuBP-oxygenase reaction ◦ short circuit of Calvin cycle ◦ loss of carbons to production of CO 2  can lose 50% of carbons fixed by Calvin cycle ◦ reduces production of photosynthesis  no ATP (energy) produced  no C6H12O6 (food) produced ◦ if photorespiration could be reduced, plant would become 50% more efficient  strong selection pressure to evolve alternative carbon fixation systems  We’ve all got baggage!  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-problem ◦ strong selection pressure to evolve better way to fix carbon & minimize photorespiration Problem with C3 Plants: ◦ CO2 fixed to 3-C compound in Calvin cycle ◦ Ex. Rice, wheat, soybeans ◦ Hot, dry days:  partially close stomata, ↓CO 2  Photorespiration  ↓ photosynthetic output (no sugars made)  In some plant species-alternate forms of carbon fixation have evolved to minimize photorespiration and optimize the Calvin cycle. ◦ By separating CO2 uptake from CO2 fixation in the Calvin cycle thereby limiting exposure of Rubisco to O2.  TWO most important metabolic adaptations: C4 photosynthesis CAM-crassulacean acid metabolism  The key point is how CO2 is grabbed out of the air and delivered to the Calvin cycle.  Separate carbon fixation from Calvin cycle ◦ C4 plants stomata partially close during the day.  PHYSICALLY separate carbon fixation from Calvin cycle  different cells to fix carbon vs. where Calvin cycle occurs  store carbon in 4C compounds  different enzyme to capture CO2 (fix carbon)  PEP carboxylase  different leaf structure ◦ CAM plants stomata close during the day, open at night.  separate carbon fixation from Calvin cycle by TIME OF DAY  fix carbon during night  store carbon in 4C compounds  perform Calvin cycle during day  A better way to capture CO2 ◦ 1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase  store as 4C compound corn ◦ adaptation to hot, dry climates  have to close stomates a lot  unique leaf anatomy ◦ eg. sugar cane, corn, other grasses (eg. crabgrass)… sugar cane  Eg. sugarcane, corn 3% of vascular plants  TWO types of photosynthetic cells:  Mesophyll cells-carbon fixation.  Bundle sheath cells-Calvin cycle ▪ PEP carboxylase enzyme (found only in mesophyll cells) ◆ higher affinity for CO2 than RuBisCO ◆ no affinity for O2 ◆ fixes CO2 in 4C compounds ◆ regenerates CO2 in inner bundle sheath cells for RuBisCo ▪ keeping O2 away from RuBisCo Can you tell the story? cacti succulents pineapple ▪ Adaptation to hot, dry climates=deserts ◆ separate carbon fixation or uptake of CO2 from Calvin cycle by TIME- both occur in the mesophyll cells ▪ close stomates during day-conserve water loss ▪ open stomates during night ◆ at night: open stomates & fix carbon as 4C compounds stored in vacuoles in mesophyll cells. ◆ CO2 + PEP ⇨ oxaloacetate ⇨ malate (other organic acids) ◆ in day: release CO2 from 4C acids It’s all in (eg malic acid) to Calvin cycle the timing! ▪ increases concentration of CO2 in cells ◆ succulents, some cacti, pineapple ❑ The CAM pathway requires ATP at multiple steps (not shown above), so like C4 photosynthesis, it is not an energetic "freebie." ❑ However, plant species that use CAM photosynthesis not only avoid photorespiration, but are also very water- efficient. ❑ Their stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce water loss from leaves. ❑ CAM plants are typically dominant in very hot, dry areas, like deserts.  solves CO2 / O2 gas exchange vs. H2O loss challenge Both incorporate CO 2 into organic acids and then release CO2 for entry into the Calvin cycle. ↓photorespiration, ↑sugar production C4 plants CAM plants separate 2 steps separate 2 steps of C fixation of C fixation anatomically in 2 temporally = different cells 2 different times night vs. day  Can you differentiate between C3, C4 and CAM plants? How are they alike? C3 plant Different? Which plants will be more affected by increasing CO2 levels? Or temperature? CAM plant C4 plant  The energy entering chloroplasts as sunlight, gets stored as chemical energy in organic compounds.  Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells. ◦ Approximately 50% is consumed as fuel for respiration. ◦ Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits  In addition to food production, photosynthesis produces the O2 in our atmosphere.  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 150 billion tons of carbohydrate/year =60 trillion copies of this textbook. ◦ heterotrophs are dependent on plants as food source for fuel & raw materials ◦ 1000 kg =1.1 tons ◦ No chemical process is more important than photosynthesis to the welfare of life on earth.  H2O Can you tell the story CO2 Light NADP+ ADP + P i Light RuBP Reactions: 3-Phosphoglycerate Photosystem II Calvin Electron transport chain Cycle Photosystem I Electron transport chain ATP G3P Starch NADPH (storage) Chloroplast O2 Sucrose (export) 1. Which of the following is NOT directly associated with Photosystem II? a. P680 b. harvesting light energy by chlorophyll c. release of oxygen d. splitting of water e. production of NADPH 2. Cyclic photophosphorylation results in the formation of a. ATP only b. ATP and NADPH c. NADPH only d. ATP, NADPH and sugar e. sugar only 3. CAM plants keep their stomates closed during the daytime to reduce excess water loss. They can do this because? a. can fix CO2 into sugars in the mesophyll cells b. can use photosystems I and II at night c. modify rubisco so it does not bind with oxygen d. can incorporate CO2 into organic acids at night e. have lenticels instead of stomates 4. A scientist claims that Elysia chlorotica, a species of sea slug, is capable of photosynthesis. Which of the following observations provides the best evidence to support the claim? a. Elysia chlorotica will die if not exposed to light. b. Elysia chlorotica grows when exposed to light in the absence of other food sources. c. Elysia chlorotica grows faster when exposed to light than when placed in the dark. d. Elysia chlorotica grows in the dark when food sources are available.