1. Photosynthesis
AGRI 6203

2. 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

3. Z-Scheme
Source: Kimball’s Biology Pages

4. Excitation of Electrons

5. 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)

6. Electronegativity

7. Source: Lehninger p.657

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

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

10.  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

11. Available free energy  G = -n(23.062 kcal) ( E)
n = number of electrons transferred
= amount of energy released when one electron passes through a potential drop of 1 volt

12. 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( kcal) (1.24) = +686 kcal

13. 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

14. 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+

15. 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+

16. 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).

17. 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

18. ATP Synthesis

19. 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

20. Cyclic Phosphorylation

21. 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?

22. CO2 Fixation

23. Calvin-Benson Cycle

24. Calvin Cycle

25. Synthesis of plant carbohydrates form glucose-6-phosphate

26. 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

27. C4 Pathway

28. C4 Pathway

29. Photorespiration

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