Pathways of Metabolism

Cell chemistry does not rely on random reactions taking place in one container. The active presence of a specific enzyme in the right place at the right time contributes to the co-ordinated and essential sequencing of reactions. This orderly and controlled sequencing is called a metabolic pathway and normally ensures that reactions are elegantly balanced in favour of a positive outcome.

The laws of chemistry that apply to chemical reactions apply equally to those in biological chemistry. By analysing energy levels, concentration of reactants, action of enzymes, pH and other conditions, it is possible to determine whether the reaction can take place, the rate of reaction and the nature of the products.

Control of the reactions along a pathway is often brought about by a feedback mechanism. This prevents one series of reactions taking place until another is complete. In some pathways these are called ‘checkpoints’.
Cyclic metabolic pathways
Some metabolic pathways operate in a cyclic way. The scientists who elucidated two of the cycles have had their names attached to the pathways by the scientific community. The cycle concerned with the oxidation of fats and sugars to provide energy is called the Krebs cycle (or the tricarboxylic acid or citric acid cycle).

In the Calvin (or Calvin-Benson) cycle carbon dioxide fixation takes place as part of the process of photosynthesis to produce carbohydrate.

Order and sequencing is also brought about by reactions taking place on or in a membrane, within membrane bound organelles and within small membrane bound sacs called vesicles.

Specific pathways and ‘biological containers’ all help to keep opposing reactions separate from one another.

Errors in a metabolic pathway can cause disorders and disease.
Cystic fibrosis and phenylketonuria are disorders determined by genes but executed by abnormal metabolism. Carbon monoxide poisoning is an example of a chemical error being made. The result can be fatal.


  1. Although there are thousansds of chemicals in the cell, very few reactions are random; most follow an ordered sequence or pathway.
  2. Feedback mechanisms help to ensure the smooth operation of the reaction pathway. Sometimes special ‘checkpoints’ are built in to the system.
  3. Many reactions take place within special compartments in the cell. This keeps opposing and incompatible reactions separated. The breakdown of compartment and vesicle walls would create a chemical mess and cell death.