Photosynthesis AGRI 6203

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

Z-Scheme Source: Kimball’s Biology Pages

Excitation of Electrons

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)

Electronegativity

Source: Lehninger p.657

“Downhill” flow of electrons Free energy is released -G

“Uphill” flow of electrons Input of free energy + G

 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

energy-rich electron has high reducing “pressure” 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

ATP Synthesis

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

Cyclic Phosphorylation

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?

CO2 Fixation

Calvin-Benson Cycle

Calvin Cycle

Synthesis of plant carbohydrates form glucose-6-phosphate

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

C4 Pathway

C4 Pathway

Photorespiration

Sources of Images Kimball’s Biology Pages Principles of Biochemistry, 1982, Lehninger,