Chapter 8 Energy-Releasing Pathways
I. The Killers Are Coming!
A. All active organisms use energy on a steady basis.
B. Carbon dioxide and water are metabolic byproducts produced by living cells.
C. At the biochemical level, there is unity among all forms of life.

II. ATP-Producing Pathways
A. ATP is the prime energy carrier for all cells.
B. ATP can be produced by different pathways.
1. Photosynthesis produces ATP to drive carbohydrate synthesis (see Chapter 7).
2. Aerobic respiration (with oxygen) is the main pathway for energy release from carbohydrate to ATP.
3. Fermentation and anaerobic electron transport (both without oxygen) release lesser amounts of energy for transfer to a small number of ATP.

III. Aerobic Respiration
A. Overview of the Reactions
1. Fermentation yields two ATP; aerobic respiration yields thirty-six ATP.
2. The aerobic route is summarized:
C6H12O6 + 6O2 Æ 6CO2 + 6H2O
3. Three series of reactions are required for aerobic respiration:
a. Glycolysis is the breakdown of glucose to pyruvate; small amounts of ATP are generated.
b. Krebs cycle degrades pyruvate to carbon dioxide, water, ATP, H+ ions, and electrons.
c. Electron transport phosphorylation processes the H+ ions and electrons to generate high yields of ATP; oxygen is the final electron acceptor.
B. Glycolysis: 1 glucose Æ 2 pyruvate molecules
1. Enzymes in the cytoplasm catalyze several steps in glucose breakdown.
a. Glucose is first phosphorylated in energy-requiring steps, then split to form two molecules of PGAL.
b. Enzymes remove H+ and electrons from PGAL to change NAD+ to NADH (which is used later in electron transport).
c. By substrate-level phosphorylation, four ATP are produced.
2. The end products of glycolysis are: two pyruvates, two ATP (net gain), and two NADH for each glucose molecule degraded.
C. Krebs Cycle
1. Pyruvate enters the mitochondria and is converted to acetyl-CoA, which then joins oxaloacetate already present from a previous “turn” of the cycle.
2. Krebs cycle serves three functions:
a. Most of the molecules are recycled to conserve oxaloacetate for continuous processing of acetyl-CoA.
b. Two molecules of ATP are produced by substrate-level phosphorylation.
c. H+ and e– are transferred to NAD+ and FAD.
3. Carbon dioxide is produced as a byproduct.
D. Electron Transport Phosphorylation
1. NADH and FADH2 give up their electrons to transport (enzyme) systems embedded in the mitochondrial inner membrane.
2. According to the chemiosmotic theory, energy is released in the passage of electrons through components of the transport series.
a. The energy is used to pump hydrogen ions out of the inner compartment.
b. When hydrogen ions flow back through the ATP synthase in the channels, the coupling of Pi to ADP yields ATP.
3. Oxygen joins with the “spent” electrons and H+ to yield water.
4. Electron transport yields thirty-two ATP; glycolysis yields two ATP; Krebs yields two ATP for a grand total of thirty-six ATP per glucose molecule.
E. Glucose Energy Yield
1. Normally, for every NADH produced within the mitochondria and processed by the electron transport system, three ATP are formed; FADH2 yields two ATP.
2. But NADH from the cytoplasm cannot enter the mitochondrion and must transfer its electrons!
a. In most cells (skeletal, brain) the electrons are transferred to FAD and thus yield two ATP (for a total yield of thirty-six).
b. But in liver, heart, and kidney cells, NAD+ accepts the electrons to yield three ATP; because two NADH are produced per glucose, this gives a total yield of thirty-eight ATP.

IV. Anaerobic Routes
A. Anaerobic pathways operate when oxygen is absent (or limited); pyruvate from glycolysis is metabolized to produce molecules other than acetyl-CoA.
B. Alcoholic Fermentation
1. Fermentation begins with glucose degradation to pyruvate.
2. Cellular enzymes convert pyruvate to acetaldehyde, which then accepts electrons from NADH to become alcohol.
3. Yeasts are valuable in the baking industry (carbon dioxide byproduct makes dough “rise”) and in alcoholic beverage production.
C. Lactate Fermentation
1. Certain bacteria (as in milk) and muscle cells have the enzymes capable of converting pyruvate to lactate.
2. As in alcoholic fermentation, no additional ATP is produced but NAD+ is regenerated.
D. Anaerobic Electron Transport
1. Some kinds of bacteria are able to strip electrons from organic compounds and send them through a special electron transport in their membranes to produce ATP.
2. Examples of such bacteria include those that reduce to H2S and those that convert NO o(3,–) to NO o(2,–) . 

V. Alternative Energy Sources in the Human Body
A. Carbohydrates are the body’s first source of energy.
1. Excess carbohydrate intake is stored as glycogen in liver and muscle for future use.
2. Free glucose is used until it runs low, then glycogen reserves are tapped.
B. Lipids are used when carbohydrate supplies run low.
1. Excess fats are stored away in cells of adipose tissue.
2. Fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Krebs cycle.
3. Because fatty acids have many more carbon and hydrogen atoms, they are degraded more slowly and yield greater amounts of ATP.
C. Proteins are used as the last resort for supplying energy for the body.
1. Amino acids are released by digestion and travel in the blood.
2. After the amino group is removed, the amino acid remnant is fed into the Krebs cycle.