Metabolism is the sum of chemical and physical reactions that occur within the body, and more specifically, is an energy producing activity that occurs within the cells of all living organisms. Metabolic activities convert chemical energy from ingested food into mechanical energy or heat, otherwise known as calories. Metabolic activity is required for the sustenance of life, growth, and development. All metabolic reactions are initiated by enzymatic activity. Enzymes are specific proteins, with catalytic properties, capable of acting independently. These catalysts induce chemical reactions within the body while maintaining their original properties. The basis of metabolism involves two very specific processes: anabolism and catabolism.  Anabolism is the conversion and synthesis of smaller molecules and constructing them into larger compounds. Catabolism is the breakdown of larger substances into smaller, more usable molecules. Metabolic activities help to regulate and maintain the body and keep it functioning properly.

The metabolic pathway begins with a series of individual reactions that result in the formation of an end “product”. Catabolism is the first, of the two metabolic processes, that occurs within the body. This involves the breakdown of nutrients, to produce energy within the body. Most food components are macromolecules. Food macromolecules are biological chains of ‘repeating’ smaller molecules linked together called polymers. Polymers are comprised of proteins, carbohydrates, nucleic acids, and fats. The repeating units, forming macromolecules, are called monomers (building blocks). Proteins, carbohydrates, and lipids constitute the main substrates for digestive enzymes. These substrates are catabolized and released as free energy known as ATP (adenosine triphosphate). Upon the breakdown of nutrients, the body uses and stores the energy converted during the catabolic process. This energy can now be used to form larger molecules by the covalent bonding of electrons, protons and free energy. Energy used during anabolic activity comes at the expense of free energy converted through catabolic activity.


Carbohydrates, fats and proteins are broken down and used by the cells in the formation of ATP. 

Carbohydrates are broken down into glucose, proteins are broken down into amino acids, and fats are broken down into fatty acids. The body can immediately use simple carbohydrates, in the form of glucose. Complex carbohydrates are not immediately released into the bloodstream, like glucose molecules, because they are broken down at a slower rate. However, glycogen can be stored in the liver and muscle, enabling it to be later metabolized and converted into energy when required.

Amino acids are the building blocks of proteins and are the intermediaries essential for metabolism. Unlike carbohydrates, amino acids cannot be stored, requiring a constant replenishment of protein. 

Lipids are digested and broken down at a much slower rate than both carbohydrates and proteins, and therefore enter the bloodstream (and are metabolized), at a much slower rate. Excess fatty acids, not immediately utilized for cellular function, will eventually be stored throughout the body. However, when needed, these fatty acids can be metabolized for energy. 

Regardless of the function occurring within the body, the energy required to initiate and complete all cellular processes must begin with metabolism.

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