Glycolysis

Aerobic Respiration I: Glycolysis

We find the consumption of energy, as represented by the conversion of ATP into ADP and P in numerous situations:

Maintenance of sodium-potassium pump in neurons
Functioning of muscle cells (sarcomeres with actin & myosin fibers)
Formation of urea in liver cells by bonding of carbon dioxide & ammonia

The model for aerobic respiration is the oxidation of the glucose molecule:

(1) C₆H₁₂O₆ + 6 O₂ + 6 H₂O è 6 CO₂ + 12 H₂O + 686 Kcal

This equation has an oxidation component,

(2) C₆H₁₂O₆ è 6 CO₂

And a reduction component:

(3) 6 O_{2 è} 6 H₂O

Aerobic respiration involves the conversion of chemical energy stored in the chemical bonds of the glucose molecule, into chemical energy stored in the chemical bonds of ATP (Adenosine TriPhosphate) molecules, which are synthesized from ADP (Adenosine DiPhosphate) + P (Phosphate).

The conversion of the 686 Kcal to the energy of the chemical bonds in ATP is not 100% efficient.

Equation 1 can be changed to reflect the actual conversion of chemical energy:

(4) C₆H₁₂O₆ + 6 O₂ + 6 H₂O + 38 ADP + 38 P è 6 CO₂ + 12 H₂O + 38 ATP + 420 Kcal,

in which 266 Kcal of the total amount available (686) has been stored in the chemical bonds of the 38 ATP molecules. This means that the formation of each mole of ATP requires 7 kcal. The rest of the energy that was released by the complete oxidation of glucose is lost in the form of heat.

Aerobic respiration has four stages

Glycolysis - A six-carbon glucose molecule is converted to two, 3-carbon molecules of pyruvate. This process occurs in the cytoplasm. In order to initiate the process, 2 molecules of ATP are consumed. Four molecules of ATP and 2 molecules of NADH are produced;

Formation of acetyl coenzyme A - Each pyruvate molecules is oxidized to carbon dioxide and a 2-carbon acetyl group. The carbon dioxide is released as a waste product, and the 2-carbon acetyl group is bound to coenzyme A and brought into the mitochondrion;

The citric acid cycle - Each of the 2-carbon acetyl groups produced from the original glucose molecule is bonded to a pre-existing molecule of oxaloacetate to form citrate (i.e. citric acid). These two citric acid molecules are gradually oxidized, and the hydrogen ions are bound to NAD to form NADH and to FAD to form FADH₂. Oxaloacetate is produced when the last carbon atom is released in the form of carbon dioxide;

Electron transport chain and chemiosmosis - The electrons removed from the molecules in glycolysis and citric acid follow a series of cytochromes on the mitochondrial membrane, while the hydrogen ions (protons) are pumped across the inner membrane of the mitochondrion. These protons flow through ATP synthase enzyme molecules, and thereby release energy which drives the formation of ATP molecules.

In glycolysis, glucose (a 6-carbon compound) yields two pyruvate (3-C) molecules. All of these steps occur in the cytoplasm :

Reaction	Type of reaction
(5) Glucose + ATP è Glucose-6-phosphate + ADP	Phosphorylation
(6) Glucose-6-phosphate è Fructose-6-phosphate	Internal rearrangement
(7) Fructose-6-phosphate + ATP è Fructose-1,6-biphosphate + ADP	Phosphorylation
(8) Fructose-1,6-biphosphate è 2 G3P	Isomerization
(9) 2 G3P + 2 NAD è Two 1,3-biphosphoglycerate + 2 NADH	Oxidation of G3P, reduction of NAD
(10) Two 1,3-biphosphoglycerate + 2 ADP + P è Two 3-phosphoglycerate + 2 ATP	Phosphorylation
(11) Two 3-phosphoglycerate + 2 ADP + 2 P è Two pyruvate + 2 H₂O + 2 ATP	Dehydration + phosphorylation

In aerobic organisms, each pyruvate molecule is converted to a carbon dioxide molecule and an acetyl group, which is bonded to Coenzyme A, yielding acetyl Coenzyme A.

Coenzyme A will bring the acetyl group into the mitochondrion.
The acetyl group will be bonded to oxaloacetate (= oxaloacetic acid) to form citrate (citric acid)

In some cells exposed to an anaerobic environment, pyruvic acid molecules are directly converted into a waste product and excreted.

Yeast cells are facultative anaerobes. When they are exposed to oxygen, they can undergo aerobic respiration. When, however, they are in an environment which is lacking in oxygen, they will undergo ethanol, or alcohol fermentation, in which

(12) Pyruvic acid + NADH è CO₂ + C₂H₅OH + NAD

Some fungi and bacteria undergo lactate fermentation in which

(13) Pyruvic acid + NADH è Lactic acid + NAD

Skeletal muscle cells of mammals can also undergo lactic acid fermentation temporarily during anaerobic exercise, in which bursts of activity are required.