Cellular Metabolism and Reproduction: Mitosis and Meiosis 87
- The main products of glycolysis are fructose diphos-
phate, which splits into two phosphoglyceraldehyde
molecules that oxidize to two phosphoglyceric acids,-
which convert to the final product of two pyruvic- acid
molecules.^
- When oxygen is present, two hydrogens from each
of the two phosphoglyceraldehydes go to the
electron- transport system beginning with the
cofactor- NAD. This produces six ATP.^
- When phosphoglyceric acid is decomposed to
pyruvic- acid four ATP are made, but we must pay
back the two that started the sequence. Thus, we
produce only a net gain of two ATP.^
- In glycolysis, we produce a total net gain of eight
ATP. In anaerobic glycolysis in muscle cells and in
fermentation, only two ATP are produced.
The Krebs Citric Acid Cycle
- The citric acid cycle is named after the British
biochemist Sir Hans Krebs who first proposed the
scheme in 1937.^
- The two pyruvic acid molecules produced in
glycolysis- are converted to acetic acid and then to
acetyl-CoA through the action of CoA enzyme.^
Acetyl--CoA now enters the cristae of the mitochon-
dria to go through the citric acid cycle.^
- The major chemical products in the cycle are citric
acid, alpha-ketogluteric acid, succinic acid, malic
acid, and oxaloacetic acid.^
- Most of the ATP is made by electron transport. For
each of the two pyruvic acid molecules broken down,
14 ATP are made via electron transport for a total
production of 28 ATP. In addition, 2 ATP or GTP are
produced in the citric acid cycle, for a total of 30 ATP
or 28 ATP and 2 GTP.^
- For each pyruvic acid molecule broken down, three
molecules CO 2 are given off as waste products for a
total of six CO 2 molecules produced.
The Electron Transport System
- The electron transport system functions as a series of
redox (reduction-oxidation) reactions.^
- There are several kinds of electron carriers in the
electron transport system: NAD (nicotinamide ad-
enine dinucleotide), FAD (flavin adenine dinucleo-
tide), quinone, and the cytochrome system.^
- If the system begins with NAD, then three ATP are
produced. If the system begins with FAD, then only
two ATP are produced.
4. Oxygen is necessary for respiration because oxygen is
the ultimate electron acceptor in the system.^
5.^ When oxygen accepts the electrons from the two^
hydrogen- atoms and the two hydrogen protons,
water (H 2 O) is produced as a waste product.
Anaerobic Production of ATP by Muscles.
1.^ Glycolysis produces a total net gain of eight ATP.^
2. The Krebs citric acid cycle produces for each of the
two pyruvic acid molecules 14 ATP via electron trans-
port and one ATP or GTP. Thus, the total ATP produc-
tion in the cycle is 28 ATP and two GTP or 30 ATP.^
3. The eight ATP from glycolysis and the 30 ATP from
the citric acid cycle yield a total of 38 ATP from each
glucose molecule.
Anaerobic Respiration- Fermentation is the process by which yeast cells
break down glucose in the absence of oxygen. This
process produces only two ATP and is much less
efficient- than glycolysis.^ - The other products of fermentation are carbon
dioxide- gas, which causes bread dough to rise, and
ethyl alcohol, which is used in the beer, wine, and
liquor industries.^ - When we overwork our muscles and cannot get
enough oxygen to the muscle cells, they begin to
break down glucose in the absence of oxygen. The
total net gain of ATP is only two ATP molecules and
pyruvic acid is converted to lactic acid.^ - The buildup of lactic acid in the muscle cells is
what causes the fatigue in overworked muscles.^
Our breathing and heartbeat rates accelerate to get
more O 2 to the cells. Eventually, the fatigue goes
away as lactic acid is converted back to pyruvic acid
when oxygen again becomes available.
Molecule. Summary of ATP Production from One Glucose
Food Compounds- The cellular furnace that “burns” food to produce ATP
consists of glycolysis, the Krebs citric acid cycle, and
electron transport.^ - Carbohydrates feed into the furnace at the level of
glucose in glycolysis.^ - Fats are digested into glycerol, which feeds into the
furnace at the phosphoglyceric acid stage of glycolysis,
and fatty acids, which feed into the citric acid cycle.