All organisms require a continuous supply of energy for:

1) maintenance, including the repair and replacement of worn-out parts and the carrying out of specific cell functions,

2) growth

3) reproduction.

Coupled Reactions

Burning glucose in oxygen is a simple but dramatic event which releases large amount of energy all at once. Respiration on the other hand is a gradual process consisting of about 30 small steps, each of which is controlled by its own enzyme. Energy is released only a little at time, and much of it is transferred into the formation of chemical bonds.

In the living cell this can be achieved because chemical reactions era often coupled together so that the energy obtained from one energy-giving reaction (oxidation reaction) can be used to drive a parallel energy-consuming reaction. (reduction reaction)

The most frequently used compound is adenosine triphosphate, or ATP. Synthesis of the desired material and the breakdown of ATP are linked together so that the overall reaction possible. This linkage can take place by many mechanisms. Suppose, for example, the reaction of two substances, A and B, to form two products, C and D. If the products are at a higher energy level than the reactants, the reaction cannot go on spontaneously. If, however, D can react with a substance E to produce substances F and G at a much lower energy level--that is, a reaction that goes on readily--then, if the energy production by the second reaction exceeds the energy requirement of the first reaction, the two coupled reactions will proceed.

Glycolysis

This process is carried out by virtually all living organisms.

It happens in the cytoplasm of cells.

The steps from glucose to pyruvic acid may be called glycolysis, the anaerobic pathway (anaerobic meaning not requiring oxygen).

Summary:

For each molecule of glucose that undergoes glycolysis, two ATPs are split, but four ATPs are formed thus the net result is two ATPs, two NADH and two pyruvate molecules.

Oxidative decarboxilation

During oxidative decarboxilation a carbon is removed from the pyruvate molecule by an oxidation reaction. In the process Co-enzyme A is added.

Summary:

One NADH is produced for each molecule of pyruvate thus two NADH for each molecule of glucose.

Note that CO₂ is produced.

The Krebs cycle

The major source of energy in the cell is obtained from the oxidation of hydrogen obtained from food by respired oxygen to form water. The greatest single source of hydrogen molecules for oxidation is the Krebs cycle, named for its discoverer, Sir Hans Adolf KREBS. It is also called the tricarboxylic acid (TCA) cycle, or CITRIC ACID cycle.

In animal cells, the entire cycle is found within the mitochondrion, an organelle of the cytoplasm. The cycle consists of nine compounds, each convertible into the next; the last of the nine, a four carbon compound, is converted to the first, a six carbon compound, by the addition of an activated two-carbon compound, acetyl coenzyme A, completing one turn of the cycle and simultaneously beginning the next. Each of the steps in the cycle is catalyzed by a specific enzyme. On each turn of the cycle an acetyl molecule is oxidized, converting the carbons to carbon dioxide and removing the hydrogens, which are bound to a vitamin-derived carrier compound, nicotinamide adenine dinucleotide (NAD), involved in the electron transport system discussed below.

The Krebs cycle can be looked on as a machine for removing hydrogens from foodstuffs; the hydrogens are sent to the electron transport system, where they are combusted to water, and the free energy obtained is used to form the crucial high-energy compound, ATP.

Summary:

For each cycle one ATP, two NADH and one FADH are made. Thus for each molecule of glucose the net result is two ATP, four NADH and two FADH.

Also note that CO₂ is given off.

Oxidative phosphorilation: ATP production

Within and on the inner membrane of the mitochondrion are large molecules capable of rapidly alternating oxidation and reduction reactions. These make up the electron transport system (ETS). Each molecule of the hydrogen carrier mentioned above, NAD, delivers two electrons and one proton of a hydrogen molecule to the ETS. The system passes the electrons along the entire sequence of reactions; the protons are pumped into the inner side of the membrane. The electrons are then passed through a series of carrier molecules, the last of which catalyses the formation of water from the electrons, protons, and oxygen derived from respired air. Each member of this series has an increasingly greater affinity for electrons, so that the entire series runs "downhill" and energy is produced. If the energy is not used for chemical work, it is dissipated as heat.

Summary:

All NADH and FADH produced during the early stages of respiration (glycolysis, oxidative phosphorilation and the Krebs cycle) are oxidised, oxygen being the final electron acceptor.

Each NADH gives out enough energy to produce three ATP.

Each FADH gives out enough energy to produce two ATP.