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Energy Metabolism: Photosynthesis (Energy and Reducing Powers (Other…
Energy Metabolism: Photosynthesis
Energy and Reducing Powers
Energy Carriers
Energy enters world through photosynthesis
allow pigments to enter every reaction, large pigments, not very mobile
Allow pigments to make several smaller intermediates, more mobile, easy to control
generates ATP from ADP
photophosphorylation, light energy in plants
substrate-level phosphorylation, compounds with high-energy phosphates
oxidative phosphorylation, last stage of respiration ADP-ATP
Other Electron Carriersr
Cytochromes
cofactor heme, holds iron atom,
iron atom carries electrons and cycles between +2, +3
Plastoquinones
like cytochromes, transport electrons
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Plastocyanin
like cytochromes, small protein that carries electrons on copper
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Photosynthesis
Light-Dependent Reaction
Nature of light
electromagnetic radiation spectrum: gamma rays, X-rays, ultraviolet light, infrared light, microwaves, radio and visible light.
Radiation: treated physically as set of particles called quanta, or as a set of waves
Short wavelengths: cosmic rays, gamma rays, ultraviolet light, large amounts of energy in wave lengths
Long wavelengths: inferred, microwave, radar, radio, little energy in waves
Nature of pigments
Any material that absorbs certain wavelengths therefore has distinct color
photosynthetic pigments transfer absorbed light energy to electrons causing chemical reaction
chlorophyll a: capture some red and blue pigments
because others have high energy quanta
long-wavelength not useful because its to weak to be used for chemical synthesis
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when quanta is absorbed by pigment, electron is activated
goes from ground state to excited state, can be used in chemical reactions
not used returns to stable orbital, emitting a quantum with less energy longer wavelength- release called fluorescence
action spectrum: shows which wave lengths are most effective at powering a photochemical process
chlorophyll a absorbs red 660 nm and blue 440 nm
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absorption spectrum: a graph which shows which wave length are most strongly absorbed
Accessory pigments: wavelengths not absorbed by chlorophyll a
resonance: chlorophyll b absorbs wavelengths chlorophyll a doesn't absorb
doesn't occurre at random, the pigments are held by protons called light-harvesting complex protons
embedded in the thylakoid membrane
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300 chlorophylls, carotenoids and protons are called antenna complex
light strikes the complex and the energy is transferred to a reaction center
only the reaction centers chlorophyll a molecule can donate an excited electron onto an electron acceptor molecule
light energy is unstable, fluoresces can occur, chlorophylls electron must react only with the proper molecule
photosynthetic unit: all pigments and carriers that work together are packed on this granule
Photosystems
photosystem I : pair of chlorophylls reaction center is given name P700, because of red light absorption
energy excites an electron of P700, gets absorbed by a membrane-bound electron
the Fx(transfer electron) absorbs an electron from P700, becomes a powerful reducing agent
transferred electron still extremely unstable, Fx passes it onto ferredoxin
in thylakoid membrane, small protein , electrons passed from ferredoxin to an enzyme ferredosin-NADP+ reductase
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photosystem II: reduces P700 so it can work repeatedly
molecule of plastocyanin donates an electron to chlorophyll a of the PSI reaction center
its oxidized, lacking an electron, must acquire one
receives new electron from cytochrome b6/f complex, which gets an electron from plastoquinone
receives electron from another Q, then receives electrons from phaeophytin
chlorophyll a that doesn't contain a magnesium atom, becomes oxidized when it donates an electron to Q
must obtain another electron from chlorophyll a when is absorbs light and is activated
different type of chlorophyll a , its the reaction center of PSII called P680
PSI and PSII work together to move electrons to the electron transport chain
electrons go through the noncyclic electron transport
goes from water to NADPH
electrons go through a cyclic electron transport
cycles the electrons to plastoquinones to create more ATP
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Stroma Reactions
conversion of carbon dioxide to carbohydrate also called the Calvin/Benson cycle or the C3 cycle
take place in the storma, mediated by enzyme that are not bound to the thylakoid membrane
an acceptor molecule( RuBP) reacts with a molecule of carbon dioxide
two identical molecules are formed each have 3-carbons: 3-phosphoglycerate (PGA), why its called the C3 cycle
enzyme that carries out the reaction is RuBP carboxylase (RUBISCO)
largest and most complex enzymes known, has a molecular weight of about 480,000 daltons
can constitute up to 30% of the protein in a leaf, making it the most abundant protein on Earth
ATP donates a high-energy phosphate to PGA converting it to 1,3-diphosphglycerate
gets reduced by NADPH to 3-phosphoglyceraldehyde (PGAL)
some PGAL cane be taken out of the chloroplast and used by the cell as sugars, fats, amino acids, etc.
remaining PGAL stays in the chloroplast to be used for multiple stoma reactions
goes through constructive metabolism called anabolism, and it consist of anabolic reactions, has to have multiple storage compounds
short term storage: ATP and NADPH can be used in the cell, last briefly
intermediate-term storage: simple sugar and disaccharide sucrose stable enough to go from cell to cell
long-term storage: has starch, lipids because they are stable and very large
