1. Pathways that Harvest and Store Chemical Energy

2. Chemical energy available to do work is termed free energy (G).
Energy is stored in chemical bonds and can be released and transformed by metabolic pathways.
Chemical energy available to do work is termed free energy (G).
Exergonic reaction = -∆G
Endergonic reactions = +∆G

3. Five Principles Governing Metabolic Pathways
Chemical transformation occurs in a series of intermediate reactions that form a metabolic pathway.
Each reaction is catalyzed by a specific enzyme.
Most metabolic pathways are similar in all organisms.
In eukaryotes, many metabolic pathways occur inside specific organelles.
Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

4. Role of ATP in Energy Transfer
Energy released by exergonic reactions is stored in the bonds of ATP
When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

5. Role of Oxidation and Reduction
Energy can also be transferred by the transfer of electrons in oxidation–reduction, or redox reactions.
Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e-)
When a molecule loses a hydrogen atom, it becomes oxidized.

6. Role of Coenzymes
Coenzyme NAD+ is a key electron carrier in redox reactions.
Reduction of NAD+ is highly endergonic:
Oxidation of NADH is highly exergonic:

7. Oxidative Phosphorylation
Oxidative phosphorylation couples the oxidation of NADH
with the production of ATP:
This coupling is achieved by the process of chemiosmosis.

8. Chemiosmosis
Chemiosmosis is the diffusion of protons across a membrane, which drives the synthesis of ATP.

9. Cellular Respiration and Photosynthesis
Cellular respiration is a major catabolic pathway:
Photosynthesis is a major anabolic pathway:

10. Oxidation occurs in a series of small steps in three pathways:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy
A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2.
Oxidation occurs in a series of small steps in three pathways:
1. glycolysis
2. pyruvate oxidation
3. citric acid cycle

11. Oxidation of Carbohydrates
Glycolysis
10 reactions
Occurs in the cytosol
Final Products
2 molecules of pyruvate
2 molecules of ATP (substrate-level phosphorylation)
2 molecules of NADH

12. Oxidation of Carbohydrates
Pyruvate Oxidation
Final Products
2 molecules of CO2
2 molecules of Acetate- which is bound to coenzyme A (Acetyl CoA)

13. Oxidation of Carbohydrates
Citric Acid Cycle
8 reactions- turns twice for each glucose molecule
Final Products
4 molecules of CO2 (2 per turn)
2 molecules of ATP (by substrate-level phosphorylation)
6 molecules of NADH
2 molecules of FADH2

14. Oxidation of Carbohydrates
Electron Transport and ATP Synthesis
NADH is reoxidized to NAD+ and O2 is reduced to H2O in a series of steps.
Final Products
32 molecules of ATP
6 molecules of H2O

15. Electron Transport/ ATP Synthesis
Respiratory chain- series of redox carrier proteins imbedded in the cristae of the mitochondria oxidize NADH and FADH2 by passing the electrons from one carrier to the next.
Energy is used to transport H+ ions out of the mitochondrial matrix so ATP synthase can synthesize ATP by chemiosmosis.

16. Under anaerobic conditions, NADH is reoxidized by fermentation.
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy
Under anaerobic conditions, NADH is reoxidized by fermentation.
The overall yield of ATP is only two—the ATP made in glycolysis.
Lactic acid fermentation
NADH reduces pyruvate to lactic acid.
Alcoholic fermentation
NADH reduces acetaldehyde to ethanol

17. Concept 6.4 Catabolic and Anabolic Pathways are Integrated
Metabolic pathways are linked.
Catabolism
Anabolism
Amounts maintain a constant level- the metabolic pool.

18. Photosynthesis involves two pathways:
Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy
Photosynthesis involves two pathways:
Light reactions convert light energy into chemical energy.
Carbon-fixation reactions use the ATP and NADPH, along with CO2, to produce carbohydrates.

19. Pigments Pigment molecules absorb wavelengths in the visible spectrum.
Chlorophylls a and b provide most of the photosynthesis in plants
Absorption spectrum
Action spectrum

20. Light-Harvesting Reaction
Pigments are arranged into light-harvesting complexes, or antenna systems.
A photosystem, consisting of antenna systems and a reaction center, spans the thylakoid membrane in the chloroplast

21. Chlorophyll
When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron.
Final electron acceptor is NADP+, which gets reduced:
ATP is produced chemiosmotically during electron transport (photophosphorylation).

22. Noncyclic Electron Pathway
(noncyclic photophosphorylation)

23. Cyclic Photophosphorylation
Cyclic electron transport uses only photosystem I and produces ATP; an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll.

24. Each reaction is catalyzed by a specific enzyme.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates
The Carbon-fixation reactions occur in the stroma in the metabolic pathway known as the Calvin Cycle.
Each reaction is catalyzed by a specific enzyme.

25. The Calvin Cycle 1. Fixation of CO2:
CO2 is added to ribulose 1,5-bisphosphate (RuBP).
Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction.

26. The Calvin Cycle
2. 3PG is reduced to form glyceraldehyde 3-phosphate (G3P).

27. The Calvin Cycle 3. The CO2 acceptor, RuBP, is regenerated from G3P.
Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose).

29. REFERENCES
Campbell, Neil A. and Jane B. Reece; Biology, 6th Ed. Benjamin Cummings, 2002
Campbell, Neil A. and Jane B. Reece; Biology, 8th Ed. Benjamin Cummings, 2008
Hillis, David M. et. al., Principles of Life, 1st Ed. W.H. Freeman and Company, 2012