1 Photosynthesis Chapter 10. 2 Outline Chloroplasts Light-Independent Reactions Absorption Spectra...

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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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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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