Metabolic Pathways

metabolic pathways

Core Concepts

In this article, you will be able to understand how metabolic pathways work and the most important ones. You will also be able to understand its applications in science and molecular biology.

Topics Covered in Other Articles

> Citric Acid Cycle

> Glycolysis

> Proteins and Amino Acids

> What is ATP in biology?

> Carbohydrate Structure and Properties

What are Metabolic Pathways?

A Metabolic Pathway, in biochemistry, is a series of chemical reactions that occur in the cell to metabolize biomolecules. Enzymes modify metabolites, which are the reactants, products, and intermediates of these processes. There are countless metabolic pathways, and the majority of them have several stages, some of which can number in the hundreds. All metabolic pathways work together to form a complex network that is necessary for maintaining life. All cells go through a set of metabolic processes called metabolism, which essentially allow for cell development and division.

Metabolic Pathways Categories

Metabolic pathways can be classified onto 3 different categories:

  1. Catabolic pathways: Catabolism is the breakdown of complex substances. During catabolism, complex compounds are broken down to create glucose, amino acids, and fatty acids, which serve as substrates for metabolic activities. Three steps make up the entire procedure.
    • Step 1. Firstly, large organic substances like proteins, lipids, and polysaccharides are broken down into smaller components outside of cells. Starch, cellulose, and proteins are examples of molecules that must first be broken down into smaller bits in order for cells to use them for metabolism.
    • Step 2. Additionally, after being broken down, these molecules are absorbed by cells and transformed into even smaller molecules, which release some energy. Generally, this small molecule is acetyl-CoA.
    • Step 3. Finally, in the citric acid cycle and electron transport chain, the acetyl group on the CoA is oxidized to water and carbon dioxide, releasing the energy that has been stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.
  2. Anabolic pathways: Anabolism is the process by which the body first utilizes the energy released by catabolism and then goes on to create complex molecules. Little, fundamental precursors act as the building blocks for cellular structures, which are then constructed from these complex molecules. Three steps make up the entire procedure.
    • Step 1. Consists of the production of compounds such as nucleotides, isoprenoids, monosaccharides, and amino acids.
    • Step 2. Secondly, these precursors activate into their reactive form by using ATP.
    • Step 3. Thirdly, is the assembling of these building blocks into complex structures including proteins, lipids, nucleic acids, and polysaccharides.
  3. Amphibolic pathways: An amphibolic pathway is a type of biological process that incorporates both catabolism and anabolism. The citric acid cycle provides the greatest justification for the amphibolic route.
anabolic vs catabolic metabolism

Most important Metabolic Pathways

There are many vital metabolic routes that occur in our body all the time, however, the most important metabolic pathways are the following:


This metabolic pathway is a sequence of reactions that transform glucose into two pyruvate molecules, which have three carbons each. Glycolysis is the initial step in cellular respiration in species that carry it out. However many anaerobic species also have this route since glycolysis does not require oxygen. There are two primary stages to glycolysis, which take place in the cytosol of a cell: the phase during which energy is needed and the phase during which energy is released.

  1. The original glucose molecule undergoes a rearrangement and has two phosphate groups added to it at this stage. The modified fructose-1,6-bisphosphate molecule is unstable due to the two phosphate groups, which allows it to break into two halves and produce two three-carbon phosphate sugars. Two molecules of ATP must be required since the phosphates used in these stages are derived from ATP. Two separate three-carbon sugars are created as a result of the unstable sugar’s breakdown. Glyceraldehyde-3-phosphate may be the only one to go to the next round. DHAP is changed into the beneficial isomer, enabling both to eventually finish the pathway.
  2. Each three-carbon sugar undergoes a sequence of events in this step to become pyruvate, another three-carbon molecule. This process generates two ATP and one NADH molecule. There are a total of four molecules of ATP and two molecules of NADH since this phase happens twice, once for every pair of three-carbon sugars.

To fully understand how Glycolysis works check our Glycolysis article!


The term “glucogenesis” refers to the process of creating new glucose molecules in the body, as opposed to the production of glucose from the breakdown of the long-lasting store molecule known as glycogen. While it mostly affects the liver, it can also exist in trace amounts in the small intestine and kidney. It is the reverse process of glycolysis.

Citric Acid Cycle (Krebs Cycle)

While the citric acid cycle (TCA) occurs in the mitochondrial matrix of eukaryotes, it does so in the cytoplasm of prokaryotes. The citric acid cycle is a closed system; the last phase of the process reforms the initial molecule. The cycle consists of eight significant phases.

The citric acid cycle is a cycle in which acetyl CoA combines with a four-carbon acceptor molecule, oxaloacetate, to form a six-carbon molecule called citrate. This molecule releases two of its carbons as carbon dioxide molecules in a pair of similar reactions, producing a molecule of NADH. The remaining four-carbon molecule undergoes a series of additional reactions, first making an ATP molecule, then reducing the electron carrier FAD to FADH2 and finally generating another NADH. This cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates.

To fully understand how the Krebs Cycle works check our Citric Acid Cycle article!

Fatty Acid β-Oxidation

The fatty acid β-oxidation (FAO) occurs in the cytoplasm of prokaryotes and inside the mitochondria in eukaryotes. This process breaks down fatty acid molecules to produce Acetyl-CoA. It gets its name from the oxidation of the fatty acid’s beta-carbon into a carbonyl group.

Generally speaking, fatty acid protein transporters found on the cell surface are how fatty acids enter a cell. Once within, the fatty acid receives a CoA group from FACS. CPT1 then converts the long-chain acyl-CoA into long-chain acylcarnitine. The fatty acid moiety is carried by CAT when it passes through the inner mitochondrial membrane. CPT2 then converts the long-chain acylcarnitine back into long-chain acyl-CoA. One acetyl-CoA is produced from each cycle of β-oxidation as the long-chain acyl-CoA enters the fatty acid oxidation pathway.

beta-oxidation-metabolic pathways

Electron Chain Transport and Oxidative Phosphorylation

The electron transport chain contains four complexes, denoted by letters I to IV. The inner mitochondrial membrane of eukaryotes contains several copies of these molecules. The plasma membrane of prokaryotes contains the elements of the electron transport chain. In this stage, the energy of NADH and FADH2 will be covered by ATP.

The inner membrane of the mitochondria contains a number of chemical compounds and proteins that make up the electron transport chain. In a sequence of redox reactions, electrons are transferred from one component of the transport chain to another. In these processes, energy is released as a proton gradient, which is subsequently utilised to produce ATP through a process known as chemiosmosis. Oxidative phosphorylation is a process that involves both chemiosmosis and the electron transport chain.

Metabolic Pathways Applications

Metabolic pathways are very important for the creation of secondary metabolites. The metabolic engineering field has grown during the last years to generate these metabolites at low costs, and in the same way, to improve them synthetically. To be able to develop different techniques and to study metabolic engineering, it is crucial to understand how our metabolism works.

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