Metabolic Pathways (how they work)

This follows the general description of a metabolic pathway which includes definitions of terms used to describe metabolic pathways and the roles played by certain chemicals within them e.g. reactants, enzymes (acting as catalysts), intermediates, common intermediates, metabolites, branching points, metabolic cycles, etc..

The following outlines how metabolic pathways work in general.

These characteristics of metabolic pathways can be applied to furtherd understanding of specific metabolic pathways such as, e.g.

  • Glycolysis (oxidation of glucose to produce ATP)
  • Krebs' Cycle, also known as the Citric Acid Cycle
  • Urea Cycle

Aspects of how metabolic pathways work:

Metabolic pathways generally modify biochemicals via a series of small steps (stages in the process) rather than a single chemical reaction.


Minor adjustments can be made to the structure of the molecules.
Energy is released / used in manageable quantities.

Each step in a metabolic pathway is a chemical reaction catalyzed a specific enzyme.
Each enzyme required for a step in a metabolic pathway is a point of control of the overall metabolic pathway when each step in the overall process (pathway) is essential.


Many balances - i.e. requirements for specific enzymes - keep the rate of metabolic processes in check.

Chemical equilibrium may never be reached because the products of reactions do not continue to exist in that form but change due to participation in the next step along the metabolic pathway.


The biochemical reactions at each stage of a metabolic pathway are more likely to proceed (that is, continue to happen) because the products of that particular reaction do not accumulate because they go on to participate in the next step in the metabolic pathway ... which changes them into other biochemical(s).

[This is in contrast to an isolated chemical reaction, i.e. one that is not part of a metabolic pathway but is considered entirely on its own - the meaning of "isolated". In that case only a certain proportion of the reactants may be converted into the products of the reaction due to a natural "equilibrium" situation at which a certain proportion of the total matter concerned is in the form of the reactants and a certain (other) proportion in the form of the products of the reaction. In that case the proportions of reactants/products at the "equilibrium" situation is determined by physical conditions such as e.g. temperature and pressure.]

Steps in a metabolic pathway tend to be arranged in physical space such that the product of one reaction is in the right place to become a reactant (or "precursor") in the next step in the sequence.


Accumulation of concentrations of biomolecules that are the reactants of a step in a metabolic pathway can help to increase the rate (i.e. the "speed") of the process. This is related to the above considerations of "equilibrium".

See also what is metabolism?, metabolic rate, BMR, introduction to metabolic pathways.

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