A general overview of the major metabolic pathways

Metabolism is a series of chemical reactions occurring within a cell to make the cell alive, grow and divide. Metabolic processes are usually categorized as follows.

Assimilation - to create new cellular components by a process that demands normal energy and reduces the energy produced by the catabolism of nutrients

There are many metabolic pathways. In humans, the most important metabolic pathways are as follows.

Citric acid cycle (Krebs cycle) - Acetyl-CoA oxidation to obtain GTP and valuable intermediates

Oxidative phosphorylation - treatment of electrons emitted by glycolysis and citric acid cycles. Most of the energy released in this process can be stored as ATP

Pentose phosphate pathway - ability to synthesize pentose sugars and release anabolic reactions

Metabolic pathways interact in a complicated way for proper regulation. This interaction includes enzymatic control of each pathway, metabolic characteristics of each organ, and hormonal control.

Glycolysis - the metabolic pathway that converts glucose (sugar) to pyruvate - is the first major step in cell fermentation or respiration. This is an ancient metabolic path that could have been developed about 3.5 billion years ago when there was no oxygen in the environment. Glycolysis occurs not only in microorganisms but also in all living cells (Nelson & Cox 2008). Due to its importance, glycolysis is the first metabolic pathway resolved by biochemists. Scientists who are studying glycolysis are facing major challenges as they understand how many chemical reactions are involved and the order in which they occur. In glycolysis, a single glucose molecule (with 6 carbon atoms) is converted to two pyruvate molecules (each with 3 carbon atoms).

Each metabolic pathway consists of a series of biochemical reactions linked by its intermediates: the product of one reaction is the substrate for the subsequent reaction and so on. Metabolic pathways are generally thought to flow in one direction. Although all chemical reactions are technically reversible, the conditions within the cell generally make thermodynamically more advantageous that the flux proceeds in one direction of the reaction. For example, a pathway may be involved in the synthesis of a particular amino acid, but the degradation of that amino acid may occur through different distinct pathways. An example of such a "rule" exception is the metabolism of glucose. Although glycolysis causes glucose degradation, some reactions in the glycolytic pathway are reversible and are involved in the resynthesis of glucose (gluconeogenesis).