1. How Cells Release Stored Energy AKA: Cellular Respiration
Chapter 8
How Cells Release Stored Energy
AKA: Cellular Respiration

2. How do cells make ATP? ATP is the prime energy carrier for all cells
Aerobic Respiration (with oxygen) is the main pathway for energy release from carbohydrates to ATP

3. All energy-releasing pathways start with glycolysis
Glucose is split into two pyruvate molecules
Glycolysis reactions occur in the cytoplasm

4. Overview of Aerobic Respiration
Aerobic Respiration yields 36 ATP
Anaerobic Respiration (without oxygen) yields 2 ATP
Aerobic respiration route:
C6H12O6 + 6O2  6CO H2O
(Reverse equation to photosynthesis)

5. Three steps to aerobic respiration
1- Glycolysis: is the breakdown of glucose to pyruvate
Small amount of ATP are generate (2 ATP)
Takes place in the cytoplasm
2- Kreb Cycle: degrades pyruvate to carbon dioxide, water, ATP, H+ ions and electrons (accepted by NAD+ and FAD)
Takes place in the mitochondrian
Makes 2 ATP

7. Continue…
3- Electron Transfer Phosphorylation: processes the H+ ions and electrons to generate high yields of ATP; oxygen is the final electron acceptor
Takes place in the mitochondrion
Yields 32 ATP (this is the real takes place)

8. Glycolysis: First stage of energy-releasing pathways
2 ATP is required to start glycosis
Enzymes in the cytoplasm catalyze several steps in glucose breakdown
Glucose is first phosphorylated in energy-requiring steps, then the six-carbon intermediate is split to form two molecules of PGAL (which gives a phosphate to make ATP)
Enzymes remove H+ and electrons from PGAL and transfer them to NAD+ which becomes NADH (used later in the electron transfer)

9. The end product to glycolysis is:
By substrate-level phosphorylation, four ATP are produced
The end product to glycolysis is:
2 ATP (net gain)
2 pyruvates
2 NADH
For each glucose moleucule degraded

10. Continue…
The pyruvic acid diffuses into the inner compartment of the mitochondrion where a transition reaction occurs that serves to prepare pyruvic acid for entry into the next stage of respiration:
(a) pyruvic acid acetic acid + CO2 (a waste product of cell metabolism) + NADH +
(b) acetic acid + co-enzyme A -> acetyl CoA

11. Second Stage of the Aerobic Pathway: Kreb Cycle
Takes place in the inner mitochondria matrix
Pyruvate enters the mitochondria and is converted to acetyl-CoA, which then joins oxaloacetate already present from a previous “turn” of the cycle.
During each turn of the cycle, three carbon atoms enter (as pyruvate) and three leave as three carbon dioxide molecules

12. Functions of the second stage
H+ and e- are transferred to NAD+ and FAD (coenzymes)
Ten coenzymes are loaded with electrons and hydrogen
Two molecules of ATP are produced by substrate-level of phosphorlyation

13. Continue…
Most of the molecules are recycled to conserve oxaloacetate for continuous processing of acetyl-CoA
Carbon dioxide is produced as a by-product

15. Third Stage of the Aerobic Pathway
This is where the real work is done
NADH and FADH2 give up their electrons to transfer (enzyme) system embedded in the mitochondrial inner membrane

16. Electron Transfer Phosphorylation
According to the chemiosmotic model, energy is released in the passage of electrons through components of the transfer series
Oxygen joins with the “spent” electrons and H+ to yield water

17. Summary of the Energy Harvest
Net Gain
Electron transfer ATP
Glycolysis ATP
Kreb Cycle ATP
Total 36 ATP
(per glucose molecule)

18. Continue…
Normally, for every NADH produced within the mitochondria and processed by electrons transfer chain, three ATP are produced
FADH2 produced 2 ATP

19. Continue…
NADH from the cytoplasm cannot enter mitochondrian and must transfer its electrons!!
In most cells (skeletal and brain) the electrons are transferred to FAD and thus yield two ATP (for a total yield of 36)
But in the liver, heart, and kidney cells, NAD+ accepts the electrons to yield three ATP because two NADH are produced per glucose, this total yield of 38 ATP

20. Anaerobic Respiration
Cellular respiration without using oxygen (or very limited)
Pyruvate from glycolysis is metabolized to produce molecules other than acetyl-CoA
Example: Single Yeast Cells

21. Fermentation Pathways
With an energy yield of only 2 ATPs
Glycolysis serves the first stage (just like aerobic respiration)

22. Lactate Fermentation
Certain bacteria (as in bacteria) and muscles cells have the enzymes capable of converting pyruvate to lactate
Example: Muscle Cramps
No additional ATP beyond the net two from glycolysis is produced but NAD+ is regenerated

23. Alcoholic Fermentation
Fermentation begins with glucose degradation to pyruvate
Cellular enzymes convert pyruvate to acetaldehyde, which then accepts electrons from NADH to become alcohol.
Yeast are valuable in the baking industry (Carbon dioxide byproduct makes dough “rise”) and in alcoholic beverage production

24. Anaerobic Electron Transfer
Some kinds of bacteria are able to strip electrons from organic compounds and send them through a special electron transfer in their membranes to produce ATP
Example: Such bacteria include those that reduce sulfate to hydrogen sulfide (foul smelling gas) and those that convert nitrate to nitrite

26. Alternative Energy Sources in the Human Body
Excess carbohydrate intake is stored as glycogen in the liver and muscle for future use.
Free glucose is used until it runs low, then glycogen reserves are tapped

27. Energy from Fats
Excess fats (including those made from carbohydrates) are stored away in cells of adipose tissue
Fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Kreb Cycle
Fatty acids have more carbon and hydrogen atoms, they degraded more slowly and yield greater amounts of ATP

28. Energy from Proteins
Amino acids are released by digestion and travel in the blood
After the amino group is removed, the amino acid is removed, the amino acid remanant is fed into in the Kreb Cycle

29. Perspective on the Molecular Unity of LIfe
Photosynthesis and cellular respiration are intimately connected
Life is not some mysterious force, but a series of chemical reactions under highly integrated control.