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Photosynthesis
Chapter 10
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Outline
• Chloroplasts• Light-Independent Reactions• Absorption Spectra
– Pigments• Light-Dependent 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
– Calvin cycle formation of organic molecules
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Leaf Structure
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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.
clustered together to form a photosystem
acts as an antenna, gathering light energy harvested by multiple pigment molecules
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Chloroplasts
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Light and Reducing Power
• Light-dependent reactions of photosynthesis use the energy of light to reduce NADP to NADPH and to manufacture ATP.
– Reducing power generated by splitting water is used to convert CO2 into organic matter during carbon fixation.
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Light-Independent Reactions
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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
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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
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Pigments
• Pigments are molecules that absorb light in the visible range.
– green plant photosynthesis carotenoids chlorophyll
chlorophyll a - main pigmentchlorophyll b - accessory pigment
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Absorption Spectra
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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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Chlorophyll
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Light-Dependent Reaction Stages
• Primary photoevent• Charge separation• Electron transport• Chemiosmosis
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Photosystems
• Photosynthesis output increases linearly at low light intensities but lessens at higher intensities.
– saturation point• 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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Light to Chemical Energy
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Photosystem Function
• Bacteria use a single photosystem.– electron is joined with a proton to make
hydrogen– electron is recycled to chlorophyll
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Photophosphorylation
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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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Photosystems I and II
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bio13.exe
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Calvin Cycle
• Also referred to as C3 photosynthesis
– C3 plants - ribulose 1,5-bisphosphate is carboxylated to form a three-carbon compound via rubisco activity
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Calvin Cycle
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Photorespiration
• In photorespiration, O2 is incorporated into RuBP, which undergoes additional reactions that release CO2.
– decreased yields of photosynthesis
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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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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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Carbon Fixation
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Summary
• Chloroplasts• Light-Independent Reactions• Absorption Spectra
– Pigments• Light-Dependent Reactions• Photosystems• C3 Photosynthesis
• C4 Photosynthesis• CAM Photosynthesis
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