1. How Cells Obtain Energy
Chapter 4
Part B

2. Energy-releasing Pathways
Consumers (heterotrophs) break down glucose to convert its chemical energy to ATP using either
Aerobic respiration
With O2 (molecular oxygen)
C6H12O6 + 6O2  6CO2 + 6H2O + ATP
Uses Glycolysis, Krebs Cycle, and Oxidative Phosphorylation
Takes place in the cytoplasm and mitochondria
Fermentation and anaerobic respiration
Without O2
Uses Glycolysis and various fermentation pathways
Takes place in the cytoplasm only

3. Aerobic Respiration Overview
Glycolysis
Glucose is broken down to 2 pyruvate molecules
2 ATP (high energy molecule)
2 NADH (carries high energy electrons)
Link Reaction (Sometimes depicted with the Krebs Cycle)
Each pyruvate molecule is converted to Acetyl-CoA
2 CO2
2 NADH
Krebs Cycle/Citric Acid Cycle
Both acetyl-CoA molecules are broken down to C02
2 ATP
6 NADH
2 FADH2
Oxidative Phosphorylation
High energy electrons are used to create a H+ concentration gradient resulting in the production of ATP
34 ATP (varies)
6 H20
Cytoplasm
Mitochondrion

4. Questions What is another term for consumer?
Which type of respiration occurs in the presence of oxygen?
What metabolic pathway is used by both aerobic and anaerobic respiration?
How many ATP (net) is produced by glycolysis?
What molecule is produced that carries high energy electrons?
Which pathway/process produces the most ATP?

5. Aerobic Respiration Glycolysis
A metabolic pathway that occurs in the cytoplasm
Glucose is broken down through a series of intermediates into two pyruvate molecules
Also called pyruvic acid
How many carbons are in glucose?
How many carbons are in each pyruvate?
In both pyruvate molecules together?

6. Aerobic Respiration Glycolysis
Two ATP molecules are used to energize the rearrangement of glucose into two 3-carbon molecules called Glyceraldehyde 3-phosphate (PGAL)
For glycolysis the cell has to spend some energy to earn some energy
Both PGAL molecules are rearranged through several intermediates
High energy electrons are stripped from PGAL
NAD+ + 2e- + H+  NADH
Phosphate groups are transferred to ADP forming ATP
ADP + P (from intermediate molecules)  ATP
Phosphorylation
The final product is two pyruvate molecules
Step Art

8. Aerobic Respiration Link Reaction Occurs in the mitochondria
Sometimes depicted with the Krebs Cycle
Occurs in the mitochondria
Pyruvate is transported from the cytoplasm into the mitochondria
Pyruvate is converted to Acetyl CoA
CO2 is removed and Coenzyme A is attached
High energy electrons are stripped from pyruvate producing NADH

10. Aerobic Respiration Krebs Cycle (Also called the Citric Acid Cycle)
Takes place in mitochondria
Acetyl-CoA combines with oxaloacetate to form citrate
Several rearrangements and intermediates result in
Release of two more CO2 molecules
Three more NADH and one FADH2 (another electron carrying co-enzyme)
NAD+ + 2e- + H+  NADH
One molecule of ATP is formed
Ultimately oxaloacetate is reformed to start the cycle again
Step Art

13. Questions
During the link reaction pyruvate is changed to what molecule?
What carbon containing molecule is released during the link reaction?
Where does the Krebs cycle occur?
At the beginning of the Krebs cycle acetyl-CoA is bound to ____ to form ___.
During the Krebs cycle electrons and hydrogen atoms are stripped and carried by what two co-enzymes?
How many ATP molecules are formed (per one glucose)?

14. Aerobic Respiration Oxidative Phosphorylation
Occurs across the inner mitochondrial membrane
Two step process
Electron Transport Chain (ETC)
Chemiosmosis

15. Aerobic Respiration Oxidative Phosphorylation
Electron Transport Chain (ETC)
NADH and FADH2 carry electrons and H+ to the electron transport chain
Inner mitochondrial membrane

16. Aerobic Respiration Oxidative Phosphorylation
Electron Transport Chain (ETC)
As the electrons move through the chain they release small amounts of energy allowing the transfer chain to shuttle H+ over the membrane
Requires energy to push H+ up the concentration gradient
Inner mitochondrial membrane

17. Aerobic Respiration Oxidative Phosphorylation
Electron Transport Chain (ETC)
A H+ gradient forms in the outer mitochondrial compartment
In between the two membranes
Outer mitochondrial compartment
Inner mitochondrial membrane

18. Aerobic Respiration Oxidative Phosphorylation
Electron Transport Chain (ETC)
Once the electrons have moved through the electron transfer chain they are accepted by O2 which is the terminal electron acceptor
O2 combines the electrons and H+ to form water (O2 + H+ + electrons  H2O)
Outer mitochondrial compartment
Inner mitochondrial membrane

19. Aerobic Respiration Oxidative Phosphorylation Chemiosmosis
The concentration gradient created by the ETC propels H+ across the membrane through ATP Synthase
H+ can not pass through the phospholipid membrane so a transmembrane protein is required
ATP Synthase is a transmembrane protein molecule
The flow of H+ has enough force to turn ATP Synthase which results in the generation of ATP by phosphorylation of ADP
ADP + Pi  ATP

21. Aerobic Respiration Oxidative Phosphorylation
Results per glucose molecule
Depending on the needs of the cell the amounts can fluctuate
Shifting concentrations of reactant, intermediates and products
Shuttling mechanisms for moving NADH may use some ATP
Up to 34 molecules of ATP could be generated

22. Questions Where is the electron transfer chain?
What carries the electrons to the chain?
What happens when energy is released by electrons moving through the electron transfer chain molecules?
The build up of a H+ concentration gradient provides the force to cause what important event?
For aerobic respiration what is the terminal electron acceptor?
How many molecules of ATP can be formed by the electron transfer chain (per glucose molecule)?

23. Fermentation
Anaerobic respiration uses glycolysis, but due to the lack of oxygen the Krebs and electron transfer are not used
The cell then needs a different final electron acceptor to be able to re-oxidize NADH to NAD+ for reuse in glycolysis
An organic molecule is used as the final electron acceptor

24. Fermentation Lactic acid fermentation
Pyruvate is the final electron acceptor
Pyruvate accepts electrons from NADH producing lactic acid (also called lactate)
Muscle cells and red blood cells use this pathway when they are not receiving enough O2
Lactic acid build up in muscles causes muscle stiffness and fatigue

25. Fermentation Alcohol fermentation
Pyruvate is converted to acetaldehyde which becomes the final electron acceptor
Acetaldehyde accepts electrons from NADH producing ethanol and CO2
Used by yeast to make alcoholic beverages and bread

26. Connections to Other Metabolic Pathways
All of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the Krebs cycle pathways

27. Fig. 7-12, p.119

28. Questions Anaerobic respiration can also be called _____.
During anaerobic respiration what type of molecule becomes the electron accepter for NADH?
What pathway is used by yeast in bread making?
What pathway can be used by some muscle fibers?
What other molecules can be used for energy?
What are alternate energy sources generally broken down to?