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,