Chapter 10 Photosynthesis Photosynthesis The process that converts solar energy into chemical energy Nourishes the living world Autotrophs Self-feeders Organisms that are able to produce their own chemical energy Do NOT need to consume other organisms Most plants and some bacteria and algae Producers Photoautotrophs These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (b) Multicellular algae (c) Unicellular protist 10 m 40 m (d) Cyanobacteria 1.5 m (e) Pruple sulfur bacteria Figure 10.2 Photoautotrophs Use energy from the sun to convert CO2 and H2O into organic molecules Figure 10.1 Heterotrophs Get energy (organic materials) from other organism Consumers All animals Most fungi Many prokaryotes Chloroplasts Organelle in all photosynthetic cells Site of photosynthesis Present in all green parts of plants Most are found in mesophyll (interior of leaf) Color comes from chlorophyll (pigment) Chloroplasts: The Sites of Photosynthesis in Plants Stomata pores that allow transfer of gases Veins transport water taken up by roots and to export sugar Vein Leaf cross section Figure 10.3 Mesophyll CO 2 O2O2 Stomata Chloroplasts Thylakoid membrane bound sacs; interconnected Grana stacked column of thylakoids Stroma fluid inside of the chloroplast Chlorophyll found in thylakoid membrane Chloroplast Mesophyll 5 m Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 m Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is summarized as 6 CO H 2 O + Light energy C 6 H 12 O O H 2 O The Splitting of Water Chloroplasts split water into H and O; H gets incorporated into sugar 6 CO 2 12 H 2 O Reactants: Products: C 6 H 12 O 6 6H2O6H2O 6O26O2 Figure 10.4 Use of isotopes Oxygen-18 used as a tracer O2 produced by plants only contained oxygen-18 if the water contained the tracer Did not contain oxygen-18 if the tracer was in CO2 instead. Experiment 1: CO2 + H2O [CH2O] + H2O + O2 Experiment 2: CO2 + H2O [CH2O] + H2O + O2 Photosynthesis Photosynthesis is a redox process What is oxidized? What is reduced? What is the oxidizing agent? What is the reducing agent? Are electrons increasing in potential energy or decreasing in potential energy? Is this reaction spontaneous or nonspontaneous? Photosynthesis has two stages Light Reactions Conversion of light energy to chemical energy Calvin Cycle Production of sugar from CO2 The Two Stages of Photosynthesis: A Preview Light reactions (in thylakoids) split water, release O2, make ATP, & form NADPH Calvin cycle (in stroma) forms sugar from CO2, using ATP & NADPH Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules Light Reactions Thylakoid Membrane Splits water Releases oxygen Produces ATP and NADPH NADPH = NADH with an extra phosphate group Calvin Cycle Stroma Breaks down CO2; forms sugar using carbon Uses ATP produced by light reactions for energy Uses NADPH as reducing agent Overview of photosynthesis H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP ADP + P O2O2 Figure 10.5 ATP Light Reactions Light energy ATP and NADPH The Nature of Sunlight Wave-like behavior Wavelength () = distance between crests of wave Electromagnetic spectrum range of radiation Visible light = 380 nm nm Particle-like behavior Photons = unit of light; fixed quantity of energy Amount of energy inversely related to wavelength Electromagnetic spectrum Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 5 nm 10 3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy Figure 10.6 The visible light spectrum Includes the colors of light we can see Includes the s that drive photosynthesis Photosynthetic Pigments: The Light Receptors Pigments substances that absorb visible light Appear the color that is transmitted, not absorbed Chlorophyll Absorbs mostly violet-blue light Reflected light, includes colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 Spectrophotometer machine that sends light through pigments & measures the fraction of light transmitted at each Absorption Spectra of Chloroplasts Provide clues to the relative effectiveness of different s for driving photosynthesis for each type of chloroplast Three types of chlorophyll Chlorophyll a Chlorophyll b Carotenoids Absorption Spectrum Graph that plots light absorption vs Action Spectrum Rate of photosynthesis (measured by O 2 release) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. (b) The action spectrum for photosynthesis Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga Engelmanns experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. (c) Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION Types of Chlorophyll Chlorophyll a main photosynthetic pigment Chlorophyll b accessory pigment C CH CH 2 C C C C C CN N C H3CH3C C C C C C C C C N C C C C N Mg H H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing head of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10 Other accessory pigments Absorb diff s of light & pass the energy to chlorophyll a Excitation of Chlorophyll by Light When a pigment absorbs light it goes from ground state to excited state. Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon ee Figure A If an isolated solution of chlorophyll is illuminated It will fluoresce, giving off light and heat Figure B Photosystems A photosystem is a reaction center (protein complex) associated with light-harvesting complexes Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure ee Light-harvesting complexes Pigment molecules bound to proteins Funnel the energy of photons to the reaction center When a reaction-center chlorophyll molecule absorbs energy, an electron bumps up to the primary electron acceptor Thylakoid Membrane Photosystems I discovered first, occurs second Photosystems II discovered second, occurs first Each has specific reaction center Example: P680 (II) and P700 (I); both chlorophyll a, different proteins Noncyclic Electron Flow Noncyclic electron flow primary pathway of energy transformation in the light reactions 8-step process Produces NADPH, ATP, & O2 Figure Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP + ADP CALVIN CYCLE CO 2 H2OH2O O2O2 [CH 2 O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 ee ee O2O2 + H2OH2O 2 H + Light ATP Primary acceptor Fd e ee NADP + reductase Electron Transport chain Electron transport chain P700 Light NADPH NADP H + + H Mechanical analogy for light reactions Mill makes ATP ee ee ee ee ee Photon Photosystem II Photosystem I ee ee NADPH Photon Figure 10.14 Cyclic Electron Flow Under certain conditions Photoexcited e-s take an alternative path Photosystems I but NOT photosystems II ATP is produced, NADPH is NOT produced Calvin cycle consumes ATP; concentration of NADPH regulates electron flow. What kind of feedback loop is this? Primary acceptor Pq Fd Cytochrome complex Pc Primary acceptor Fd NADP + reductase NADPH ATP Figure Photosystem II Photosystem I NADP + A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts & mitochondria Generate ATP by: chemiosmosis use different sources of energy to do this (food vs. light) Mitochondria vs. Chloroplasts In both organelles Redox reactions of ETCs generate a H+ gradient across a membrane ATP synthase Uses this proton-motive force to make ATP LIGHT REACTOR NADP + ADP ATP NADPH CALVIN CYCLE [CH 2 O] (sugar) STROMA (Low H + concentration) Photosystem II LIGHT H2OH2O CO 2 Cytochrome complex O2O2 H2OH2O O2O2 1 1212 2 Photosystem I Light THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase Pq Pc Fd NADP + reductase NADPH + H + NADP + + 2H + To Calvin cycle ADP P ATP 3 H+H+ 2 H + +2 H + 2 H + Figure 10.17 Calvin Cycle Uses ATP & NADPH to convert CO2 to sugar Similar to citric acid cycle Occurs in the stroma Calvin Cycle 3 phases Carbon fixation Reduction Regeneration of the CO2 acceptor (G3P) Input (Entering one at a time) CO 2 3 Rubisco Short-lived intermediate 3 PP P Ribulose bisphosphate (RuBP) P 3-Phosphoglycerate P6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH + 6 P P 6 Glyceraldehyde-3-phosphate (G3P) 6 ATP 3 ATP 3 ADP CALVIN CYCLE P 5 P 1 G3P (a sugar) Output Light H2OH2O CO 2 LIGHT REACTION ATP NADPH NADP + ADP [CH 2 O] (sugar) CALVIN CYCLE Figure O2O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 2: Reduction Phase 3: Regeneration of the CO 2 acceptor (RuBP) 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates On hot, dry days, plants close their stomata Conserving water but limiting access to CO2 Causes O2 to build up Photorespiration: An Evolutionary Relic? Photorespiration- O2 substitutes for CO2 in the active site of the enzyme rubisco photosynthetic rate is reduced C4 Plants C4 plants minimize the cost of photorespiration By incorporating CO2 into 4 C compounds in mesophyll cells These 4C compounds Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle C 4 leaf anatomy and the C 4 pathway CO 2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C 4 plant leaf Stoma Mesophyll cell C 4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Bundle- Sheath cell CO 2 Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure CO 2 CAM Plants CAM plants Open their stomata at night, incorporating CO2 into organic acids During the day, stomata close & CO2 is released from organic acids for use in the Calvin cycle CAM pathway is similar to the C 4 pathway Spatial separation of steps. In C 4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO 2 Organic acid CALVIN CYCLE Sugar C4C4 CAM CO 2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO 2 to Calvin cycle Figure 10.20 The Importance of Photosynthesis: A Review Light reactions: Are carried out by molecules in the thylakoid membranes Convert light energy to the chemical energy of ATP and NADPH Split H 2 O and release O 2 to the atmosphere Calvin cycle reactions: Take place in the stroma Use ATP and NADPH to convert CO 2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions O2O2 CO 2 H2OH2O Light Light reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21