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

• 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,