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Photosynthesis Chapter 10
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Outline Overview of photosynthesis Leaf Structure and Chloroplasts Absorption Spectra Pigments Light-Dependent Reactions Light-Independent Reactions Photosystems C3 Photosynthesis C4 Photosynthesis CAM Photosynthesis
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Photosynthesis The energy used by most living cells ultimately comes from the sun, and is captured by plants, algae, or bacteria via photosynthesis. Light dependent reactions capture energy from sunlight use energy to produce ATP and NADPH Light independent reactions :Calvin cycle formation of organic molecules
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Overall reaction for photosynthesis
6H2O + 6 CO2 6 O2 + C6H12O6 Enzymes Light ATP Look familiar???!!??
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Where does photosynthesis occur?
Leaves only Not stem (support) Not roots (absorption)
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Leaf Structure Photosynthetic layer Palisade Spongy parenchyma
Guard cells Trans-section Cross-section
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The Chloroplast Chloroplasts Chloroplasts Inner membrane
Outer membrane Nucleus Granum Vacuole Stroma Cell wall Thylakoid
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Chloroplasts Internal membranes, thylakoids, are organized into grana. Thylakoid membranes house pigments for capturing light and the machinery to produce ATP. Pigments are clustered together to form a photosystem that acts as an antenna, gathering light energy harvested by multiple pigment molecules
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Fig. 10.2ca (TEArt) Inner membrane Outer membrane Granum Stroma
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inner membrane Outer membrane Granum Stroma Thylakoid
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The importance of Light
Light is required to drive photosynthesis. What is light? * Light is energy measured in photons. * photons= a unit of energy dependent on wavelength
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Energy in Photons Energy content of a photon is inversely proportional to the wavelength of light. Highest intensity photons, at the short-wavelength end of the electromagnetic spectrum, are gamma rays. Ultraviolet light possesses considerably more energy than visible light. potent force in disrupting DNA
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Electromagnetic Spectrum
The photons of light required to drive the light dependent reactions of photosynthesis are within visible light range.
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Pigments Pigments are molecules that absorb light in the visible range. green plant photosynthesis Carotenoids (beta carotene and xathophyll) Chlorophyll chlorophyll a - main pigment chlorophyll b - accessory pigment *The exact wavelength absorbed can be determined in the lab
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Absorption Spectra Photon absorption depends on its wavelength, and the chemical nature of the molecule it hits. Each molecule has a characteristic absorption spectrum. range and efficiency of photons the molecule is capable of absorbing Determined by placing the molecule or pigment into a spectrophotometer.
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The Spectrophotometer
A spectrophotometer can be used to measure several values by emitting light at various wavelengths through a sample: * Concentration of a substance wavelength of light is absorbed Density of particles
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An absorption spectrum is determined when a
substance is exposed to various wavelengths and absorbance of photons is determined in a unit for absorption called optimal density(OD). By graphing the wavelength vs OD, wavelength of photon absorption can be determined.
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Absorption Spectra
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Why lowest at 500nm? Plants appear green because the wavelength of green light (500nm) is reflected! Color we perceive is wavelengths of light reflecting from objects to our retina. Ex: A red shirt….red is reflected and all others absorbed. What is happening with green plants?
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The Pigment Chlorophyll
The main photosynthetic pigment in Green plants in chlorophyll a. Chlorophyll is located in the membrane of the thylacoids. Chlorophyll is a lipid…similar structure to the phospholipids of membranes.
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Fig. 10.6a (TEArt) Chlorophyll molecules embedded in a protein complex
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chlorophyll molecules embedded in a protein complex in the thylakoid membrane Thylakoid membrane
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Fig. 10.6c (TEArt) H2C CH H R CH3 CH2CH3 Porphyrin head N N H Mg H N N
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2C CH H R CH3 CH2CH3 Porphyrin head N N H Mg H N N CH3 CH3 H H CH2H CH2 O O C CO2CH3 O CH2 CH CCH3 CH2 CH2 CH2 CHCH Hydrocarbon tail CH2 CH2 CH2 CHCH3 CH2 CH2 CH CHCH3 CH3 Chlorophyll a : R — CH3 Chlorophyll b : R — CHO
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Chlorophyll Chlorophylls absorb photons by means of an excitation process. Photons excite electrons in the pigment’s ring structure, and are channeled away through alternating carbon-bond system. Wavelengths absorbed depend on the available energy levels to which excited electrons can be boosted.
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Photosystems Photosystem - network of pigments that channels excitation energy gathered by any of the molecules to the reaction center reaction center allows photon excitation to move away from chlorophylls and is the key conversion of light to chemical energy
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Overview of a Photosystem
Electron acceptor Reaction center chlorophyll Photon e- Electron e- donor Chlorophyll molecules Photosystem Membrane of a thylacoid
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Photosystem Function Plants use two photosystems photosystem I and II generate power to reduce NADP+ to NADPH with enough left over to make ATP two stage process: photosystem II – I. noncyclic photophosphorylation ejected electrons end up in NADPH
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Photosynthesis: The reactions
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Light-Dependent Reaction Stages
Absorption of photons of light by chlorophyll at Photosystems and electrons are released, reducing NADP to NADPH and forming ATP. Photosystem II (p680) * the photons absorbed excite electrons in the thylacoid moving them to a higher energy level. * These “lost” electrons are replaced by splitting water. Oxygen released. *Excited electrons are picked up by a series of proteins (electron transport chain) - at the b6f complex, H+ gradient formed forcing H+ to move through ATPase complex, ATP formed
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Fig. 10.14 (TEArt) Electron transport chain (in thylacoid membrane)
Proton gradient formed for ATP synthesis Water- splitting enzyme Photon Ferredoxin Plastoquinone Photosystem II NADP+ + H+ b6-f complex NADP reductase Photosystem I NADP reductase H O 2H O2 + pC Q P 700 P680 NADPH Fd Excited reaction center Reaction e– Plastocyanin 1 2 Energy of electrons __ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electron transport chain (in thylacoid membrane) Plastocyanin
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Photosystem I (p700) * The electrons reach the next photosystem where they are again excited to a higher energy level by light energy. *These electrons reduce NADP to NADPH
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Fig. 10.14 (TEArt) Proton gradient formed for ATP synthesis Water-
splitting enzyme Photon Ferredoxin Plastoquinone Photosystem II NADP+ + H+ b6-f complex NADP reductase b6-f complex Photosystem I NADP reductase H O 2H O2 + pC Q P 700 P680 NADPH Fd Excited reaction center Reaction e– Plastocyanin 1 2 Energy of electrons __ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Fig. 10.14 (TEArt) Excited reaction center Ferredoxin Excited reaction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Excited reaction center Ferredoxin Excited reaction center e– Fd NADP reductase Plastoquinone e– e– NADPH NADP+ + H+ Q b6-f complex Plastocyanin Reaction center pC Photon Energy of electrons e– H + Water- P Photon 700 splitting Proton Reaction center enzyme gradient formed for ATP synthesis P680 H O __ 1 2H O2 2 Photosystem II b6-f complex Photosystem I NADP reductase
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Fig. 10.15b (TEArt) This occurs in the thylacoid membrane. H+ + NADP+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. This occurs in the thylacoid membrane. Photon Photon Antenna complex H+ + NADP+ NADPH Thylakoid membrane Fd e- e- Q e- e- pC H2O Proton gradient Plastoquinone Plastocyanin Ferredoxin Water-splitting enzyme __ 1 H+ 2H+ Thylakoid space O2 2 Photosystem II b6-f complex Photosystem I NADP reductase
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Light Independent Reaction/ Dark Reaction
Calvin Cycle Also referred to as C3 photosynthesis ATP , NADPH, and CO2 are used to form organic molecules with a series of intermediate steps. This occurs in the stroma of the chloroplast Also called carbon fixation because this is the stage when carbon from CO2 in incorporated (fixed) into organic molecules.
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Fig. 10.17a (TEArt) THE CALVIN CYCLE 1 3 CO2 P 3 6 3-phosphoglycerate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. THE CALVIN CYCLE 1 3 CO2 P 3 6 3-phosphoglycerate RuBP (Starting material) The Calvin cycle begins when a carbon atom from a CO2 molecule is added to a five-carbon molecule (the starting material). The resulting six-carbon molecule is unstable and immediately splits into three-carbon molecules.
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Fig. 10.17b (TEArt) THE CALVIN CYCLE 2 P 6 3-phosphoglycerate 6 ATP 6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. THE CALVIN CYCLE 2 P 6 3-phosphoglycerate 6 ATP 6 NADPH P Glyceraldehyde 3-phosphate 6 P Glyceraldehyde 3-phosphate 1 Glucose Then, through a series of reactions, energy from ATP and hydrogens from NADPH (the products of the light-dependent reactions) are added to the three-carbon molecules. The now-reduced three-carbon molecules either combine to make glucose or are used to make other molecules.
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Fig. 10.17c (TEArt) 3 3 RuBP (Starting material) 3 ATP P 5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 3 RuBP (Starting material) 3 ATP P 5 Glyceraldehyde 3-phosphate Most of the reduced three-carbon molecules are used to regenerate the five-carbon starting material, thus completing the cycle.
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Fig. 10.18a (TEArt) Calvin cycle ATP NADPH Light-dependent reactions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Calvin cycle ATP NADPH Light-dependent reactions Thylakoid space
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Fig. 10.18b (TEArt) 3 molecules of Stroma of chloroplast Carbon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 molecules of Stroma of chloroplast Carbon dioxide (CO2) 3 molecules of Rubisco 6 molecules of Ribulose 1,5-bisphosphate (RuBP) (5C) 3-phosphoglycerate (3C) (PGA) 6 ATP Carbon fixation PGA kinase 3 ADP 6 ADP 6 molecules of Reforming RuBP 1,3-bisphosphoglycerate (3C) 3 ATP 6 NADPH 2Pi G3P dehydrogenase Reverse of glycolysis 6 NADP+ 5 molecules of 6Pi 6 molecules of Glyceraldehyde 3-phosphate (3C) Glyceraldehyde 3-phosphate (3C) (G3P) 1 molecule of Glyceraldehyde 3-phosphate (3C) (G3P) Glucose and other sugars
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Fig. 10.2db (TEArt) Sunlight Photosystem H20 O2 Thylakoid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sunlight Photosystem H20 O2 Thylakoid Light-dependent reactions ADP ATP NADPH NADP+ Calvin cycle Organic molecules CO2 Stroma
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C4 Pathway Plants adapted to warmer environments deal with the loss of CO2 in two ways: C4 conducted in mesophyll cells, Calvin cycle in bundle sheath cells creates high local levels of CO2 to favor carboxylation reaction of rubisco isolates CO2 production spatially
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Fig. 10.20a (TEArt) Leaf epidermis Heat Under hot, arid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Leaf epidermis Heat Under hot, arid conditions, leaves lose water by evaporation through openings in the leaves called stomata. H2O H2O Stoma
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Fig. 10.21a (TEArt) CO2 Phosphoenol- pyruvate (PEP) Oxalo- acetate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 Phosphoenol- pyruvate (PEP) Oxalo- acetate PPi + AMP Pi Mesophyll cell + ATP Pyruvate Malate Pyruvate Malate Bundle- sheath cell CO2 Calvin cycle Glucose C4 pathways
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Crassulacean Acid Metabolism
CAM plants open stomata during the night, and close them during the day to cut-down the loss of water vapor. isolates CO2 production temporally
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Fig. 10.20b (TEArt) The stomata close to conserve water
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The stomata close to conserve water but as a result, O2 builds up inside the leaves, and CO2 cannot enter the leaves. O2 O2 CO2 CO2
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Fig. 10.21b (TEArt) CO2 CO2 Mesophyll cell C4 pathway C4 pathway Night
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 CO2 Mesophyll cell C4 pathway C4 pathway Night CO2 CO2 Mesophyll cell Bundle- sheath cell Calvin cycle Calvin cycle Day Glucose Glucose C4 plants CAM plants C4 versus CAM pathways
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Fig. 10.19 (TEArt) Putting it all together Sunlight Heat O2 Photo-
system II Photo- system I Electron transport system ATP H2O NADP+ ADP NADPH ATP NAD+ NADH Calvin cycle CO2 Krebs cycle ATP Glucose Pyruvate Mitochondrion Chloroplast ATP
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Light Reaction Animation
Calvin cycle Animaltion
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Animation of light reaction
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