Photosynthesis AGRI 6203. Pathway of Electron Flow: the Zig Zag Scheme Raise energy level of...
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Transcript of Photosynthesis AGRI 6203. Pathway of Electron Flow: the Zig Zag Scheme Raise energy level of...
Pathway of Electron Flow: the Zig Zag Scheme
• Raise energy level of electrons derived from water to the energy level required to reduce NADP+ to NADPH.– Each electron must be boosted twice
• ATP production is coupled with electron flow
Redox Potentials (E)• Measure (volts) of a an affinity of a substance for
electrons - its electronegativity- compared with hydrogen (which is set a 0)
• positive redox potentials – capable of oxidizing– substance more electronegative than H (+E)
• negative redox potentials– capable of reducing
– substances less electronegative than H (-E)
E
• Greater the E between two substances, the greater the vigor with which electrons will flow spontaneously from the less positive to the more positive substance
Available free energy
G = -n(23.062 kcal) ( E)– n = number of electrons transferred– 23.0672 = amount of energy released when one
electron passes through a potential drop of 1 volt
Synthesis of glucose by Photosynthesis
• 24 electrons must be removed from water– water redox potential = +0.82v– must be pumped “uphill” to – carbon atoms which they partially reduce to
carbohydrate with a redox potential = -0.42v– difference is 1.24 v ( E), so
G = -24(23.062 kcal) (1.24) = +686 kcal
Light energy causes electrons to flow uphill
• Chlorophyll absorbs light and the electron in its structure is “boosted”
• excited chlorophyll migrates to the reaction center of the photosystem– causes an electron to acquire a large amount of
energy
• “hot” electron is expelled from the reaction center and accepted by the first electron carrier
• First electron carrier becomes reduced and the reaction center has become __________– oxidized reaction center now has an “electron
hole”
• energy-rich electron has high reducing “pressure”– passes to NADP+– must be sufficiently negative to reduce NADP+
Filling the electron holes
• PS I electron hole is filled by PSII
• PSII electron hole is filled by water– 2H2O -----> 4H+ 4e- = O2
• electron flow from water to NADP
• 2H2O + 2 _______ ---light---> O2 + _____ + 2H+
Energy Profile of Photosynthetic Electron Transport
• Transfer processes occurring the upward direction require energy
• downward flow of electrons proceed with a loss of free energy
• electrons flow from water (+0.82 V) to NADP (-0.32 V).
Photosynthetic Phosphorylation
• ADP to ATP is coupled with the energy released as high-energy electrons flow down the photosynthetic electron-transport chain from excited photosystem ____ to the electron holes in photosystem ___.
• One ATP is formed per pair of electrons
Cyclic Phosphorylation• Involves only PSI, cyclic because
– electron boosted to P430 by illumination of PSI, instead of passing to NADP+, flows back into the electron hole of PSI
• overall reaction– Pi + ADP + light energy ---> ATP + H20
• Why?– light reactions: 1:1 ATP:NADPH formed
– dark reactions: 3:2 ATP:NADPH consumed
Photosynthetic formation of glucose
• Generation of glucose and other carbs– from ____and _____,– at the expense of _______ and ________
produced in the light reactions
• overall equation
• What prevents animals from being able to carry out net conversion of CO2 to glucose?
C4 Plants
• Tropics
• all plant ultimately use the C3 pathway
• C3 pathway is preceded by preliminary fixation of CO2 into oxaloacetate
• phosphoenopyruvate caboxylase– OAA produced
Sources of Images
Kimball’s Biology Pages
Principles of Biochemistry, 1982, Lehninger,