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Chapter 10: PHOTOSYNTHESIS 1. Overview of Photosynthesis 2. Light Absorption 3. The Light Reactions 4. The Calvin Cycle 1. Overview of Photosynthesis Chapter Reading – pp. 185-190, 206-207 What is Photosynthesis? The process of converting light energy (kinetic) into energy stored in the covalent bonds of glucose molecules (potential). 6 CO2 + 6 Carbon dioxide H2O Light energy Water C6H12O6 + 6 Glucose O2 Oxygen gas PHOTOSYNTHESIS • carried out by photoautotrophs • plants, phytoplankton, cyanobacteria (any photosynthetic organism) • the basis of almost all ecosystems • all “food energy” ultimately comes from the sun • source of all atmospheric oxygen (O2) Photosynthetic Organisms (a) Plants (c) Unicellular protist 10 µm (e) Purple sulfur bacteria (b) Multicellular alga (d) Cyanobacteria 40 µm 1.5 µm Photosynthesis occurs in Chloroplasts Leaf cross section Vein Mesophyll Stomata Chloroplast CO2 O2 Mesophyll cell 5 µm Chloroplast Chloroplast Structure Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm The Fate of Atoms Involved in Photosynthesis Reactants: Products: 6 CO2 C6H12O6 12 H2O 6 H 2O 6 O2 Revealed by experiments involving radioactive isotopes in key molecules: • 14C in CO2 and 18O in H2O and CO2 Two Stages of Photosynthesis CO2 H2O Light NADP+ ADP + P i Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 [CH2O] (sugar) 2. Light Absorption Chapter Reading – pp. 190-193 The Electromagnetic Spectrum 10–5 nm 10–3 nm 103 nm 106 nm 1 nm Gamma X-rays rays UV Infrared 1m (109 nm) 103 m Microwaves Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 650 700 750 nm Longer wavelength Lower energy Chlorophyll absorbs “non-green” light Light Reflected light Chloroplast Absorbed light Granum Transmitted light Green light passes on through or is reflected, causing the leaves to appear green • only wavelengths with exact amount of energy to excite an e- to a higher orbital are absorbed Spectrophotometry TECHNIQUE Refracting Chlorophyll Photoelectric prism solution tube White light Galvanometer 2 1 Slit moves to pass light of selected wavelength 3 4 Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Spectrophotometers measure the amount of light passing through a sample: • measures % absorbance OR Blue light The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. • measures % transmittance Light-Absorbing Pigments Chlorophyll a & b, and carotenoids CH3 in chlorophyll a CHO in chlorophyll b RESULTS Chlorophyll a Chlorophyll b Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center (b) Action spectrum Aerobic bacteria Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown Filament of alga (c) Engelmann’s experiment 400 500 600 700 Electrons absorb Photons • electrons excited to higher energy orbitals Energy of electron e– Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 3. The Light Reactions Chapter Reading – pp. 194-199 Photosystems Photosystem Photon Thylakoid membrane Light-harvesting Reaction-center complex complexes STROMA Primary electron acceptor • an array of lightabsorbing pigments e– Transfer of energy Special pair of chlorophyll a molecules Each photosystem in the thylakoid membrane consists of: Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) • a reaction center containing 2 molecules of chlorophyll a and a primary e- acceptor The Light Reactions Produces ATP (chemical energy) & NADPH (reducing power). 4 Primary acceptor 2 H+ + 1/ O 2 2 e– H2O 2 Primary acceptor e– Pq Cytochrome complex 7 Fd e– e– 8 NADP+ reductase 3 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) NADP+ + H+ Photosystem I (PS I) H 2O ½ O2 + 2 H+ + 2 *ePS I 1 PS II 2 e- transport chain (ETC) pumps H+ into thylakoid 4 PS II 2 e- to NADPH PS I 3 ATP Synthase uses H+ flow to make ATP 4 Stages of the Light Reactions 1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II H 2O sunlight O2 + H+ + *e- 2) Energy released by a series of *e- transfers is used to generate H+ gradient • H+ accumulates inside the thylakoid membrane 3) H+ gradient used to make ATP (chemiosmosis) 4) *e- “re-energized” in PS I, passed on to NADP+ • *e- ends up in NADPH (an electron carrier) Electron Energy Levels e– ATP e– e– NADPH e– e– e– Mill makes ATP e– Photosystem II Photosystem I ATP in Respiration vs Photosynthesis Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Stroma Matrix Key ADP + P [H+] Higher Lower [H+] i H+ ATP Summary of the Light Reactions STROMA (low H+ concentration) Cytochrome complex Photosystem II Light 4 H+ Light Photosystem I NADP+ reductase Fd NADP+ + H+ NADPH Pq H2O THYLAKOID SPACE (high H+ concentration) e– 1 e– 1/ Pc 2 2 3 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+ 4. The Calvin Cycle Chapter Reading – pp. 199-204 Overview of the Calvin Cycle A series of reactions called the Calvin cycle that synthesize glucose from CO2 and H2O: CO2 + H2O ATP, NADPH C6H12O6 (glucose) • uses energy stored in ATP and NADPH • produced by the light reactions • can occur in dark (doesn’t require light directly) • also occurs during daylight! • takes place in the stroma of chloroplasts • outside the thylakoids Input 3 The Calvin Cycle CO2 (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 6 P 3-Phosphoglycerate 3P P Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP 3 Calvin Cycle 6 P P 1,3-Bisphosphoglycerate ATP 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 Pi P 5 G3P 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Glucose and other organic compounds Phase 2: Reduction C4 Pathway helps retain H2O The C4 pathway C4 leaf anatomy Mesophyll cell Mesophyll cell Photosynthetic cells of C4 Bundleplant leaf sheath cell CO2 PEP carboxylase Oxaloacetate (4C) PEP (3C) ADP Vein (vascular tissue) ATP Malate (4C) Stoma Mechanism to store carbon from CO2 in soluble form, allowing stomata closure during day to conserve water Bundlesheath cell Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue C4 compared to CAM Crassulacean Acid Metabolism Sugarcane Pineapple C4 CAM CO2 Mesophyll cell Organic acid Bundlesheath cell CO2 1 CO2 incorporated into four-carbon organic acids (carbon fixation) CO2 Calvin Cycle Night Organic acid CO2 2 Organic acids Sugar (a) Spatial separation of steps release CO2 to Calvin cycle Day Calvin Cycle Sugar (b) Temporal separation of steps • a slightly different way to fix CO2 at night for use during the day without opening stomata Summary of Photosynthesis H2O CO2 Light NADP+ ADP + Pi Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain RuBP 3-Phosphoglycerate Calvin Cycle ATP NADPH G3P Starch (storage) Chloroplast O2 Sucrose (export) Key Terms for Chapter 10 • photoautotroph • chloroplast, thylakoid, stroma • chlorophyll, carotenoids • ATP, NADPH, photosystem, reaction center • electron transport chain (ETC) • ATP synthase • Light reactions, Calvin cycle • C4 and CAM carbon fixation Relevant Chapter Questions 1-8, 10, 12