Biology 102

Fall 2001

R. Brundage

A.Enzymes in the cytoplasm catalyze several steps in glucose breakdown.

1.Glucose is first phosphorylated in energy-requiring steps, then the

six-carbon intermediate is split to form two molecules of PGAL.

2.Enzymes remove H+ and electrons from PGAL and transfer them to

NAD+ which becomes NADH (used later in electron transport).

3.By substrate-level phosphorylation, four ATP are produced.

B.The end products of glycolysis are: two pyruvates, two ATP (net gain), and two

NADH for each glucose molecule degraded.

A.Preparatory Steps and the 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.During each turn of the cycle, three carbon atoms enter (as pyruvate) and

three leave as three carbon dioxide molecules.

B.Functions of the Second Stage

1.H+ and e– are transferred to NAD+ and FAD.

2.Two molecules of ATP are produced by substrate-level phosphorylation.

3.Most of the molecules are recycled to conserve oxaloacetate for

continuous processing of acetyl-CoA.

4.Carbon dioxide is produced as a by-product.

C.Third Stage of the Aerobic Pathway

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.

D.Summary of the Energy Harvest

1.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.

2.Normally, for every NADH produced within the mitochondria and

processed by the electron transport system, three ATP are formed;

FADH2 yields two ATP.

3.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.

A.Anaerobic pathways operate when oxygen is absent (or limited); pyruvate from

glycolysis is metabolized to produce molecules other than acetyl-CoA.

B.Fermentation Pathways

1.With an energy yield of only two ATPs, fermentation is restricted to

single-celled organisms and cells of multicelled organisms only at certain

limited times.

2.Glycolysis serves as the first stage, just as it does in aerobic respiration

3.Lactate Fermentation

a.Certain bacteria (as in milk) and muscle cells have the enzymes

capable of converting pyruvate to lactate.

b.No additional ATP beyond the net two from glycolysis is

produced but NAD+ is regenerated.

4.Alcoholic Fermentation

a.Fermentation begins with glucose degradation to pyruvate.

b.Cellular enzymes convert pyruvate to acetaldehyde, which then

accepts electrons from NADH to become alcohol.

c.Yeasts are valuable in the baking industry (carbon dioxide

byproduct makes dough "rise") and in alcoholic beverage

production.

C.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 sulfate to hydrogen

sulfide ( a foul-smelling gas indeed) and those that convert nitrate to

nitrite.

A.Carbohydrate Breakdown in Perspective.

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.Energy from Fats.

1.Excess fats (including those made from carbohydrates) 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.Energy from Proteins

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.