Biochemical Classification of Lipids, Proteins, and Amino Acids

Posted on Nov 12, 2025 in Human Nutrition and Dietetics

Biochemical Classification of Lipids

Lipids serve as crucial energy reserves and structural components in living organisms. They are broadly classified based on their structure and ability to undergo saponification.

Saponifiable Lipids (Containing Fatty Acids)

Fats and Fatty Acids (Acylglycerols)

Fats (oil, lard, tallow) are energy reserves in animals, formed by glycerol and fatty acids. If the three fatty acids are identical, they are called simple fats; if they differ, they are mixed fats.

  • Solid Fats (Tallow): Melting point above 40 °C.
  • Liquid Fats (Oil): Melting point below 15 °C.
  • Semi-solid Fats (Butter): Melting point lies somewhere in between.

Fats generate the highest amount of energy per molecule. They also act as mechanical shock absorbers in some organs and provide thermal insulation.

Glycerolipids (Phospholipids)

These are triesters of glycerol, often combined with two fatty acids and a polar head group (like phosphate). They are characterized by their strong amphipathic behavior. They constitute the double lipid layer of cell membranes.

Sphingolipids

Sphingolipids are components of cell membranes and are particularly abundant in nervous tissue. They possess an amphipathic character. They are formed by the union of an alcohol and a fatty acid through an amide bond, resulting in ceramide. Ceramide binds to a specific polar molecule to form the final sphingolipid. They function as biomarkers and participate in cell growth and differentiation.

Waxes (Ceras)

Waxes correspond to monoesters of a long-chain fatty acid and a long-chain monoalcohol. They have both hydrophilic and apolar characteristics. Their primary function is protection, often found as coatings on fruits and stems.

Unsaponifiable Lipids

These lipids do not contain fatty acids and therefore cannot form soaps. There are three main types:

Terpenes

Terpenes are polymers of isoprene units, primarily found in plants. They are classified by the number of isoprene units:

  • Monoterpenes (2 Isoprene): Camphor, Limonene.
  • Diterpenes (4 Isoprene): Phytol (a component of chlorophyll).
  • Triterpenes (6 Isoprene): Squalene (a precursor of cholesterol).
  • Tetraterpenes (8 Isoprene): Carotenoids (plant pigments).
  • Polyterpenes (Many Isoprene): Such as natural rubber (latex).

Steroids

Steroids are lipid-derived molecules based on the cyclopentanoperhydrophenanthrene ring system.

Eicosanoids

Eicosanoids are derived from 20-carbon polyunsaturated fatty acids.

Proteins and Amino Acids

The Role and Composition of Proteins

Proteins are the most abundant molecules in living organisms. They perform diverse functions:

  • Transport of molecules.
  • Movement.
  • Hormonal regulation.
  • Catalysts in metabolic reactions (enzymes).

They can also be used to produce energy. Proteins are highly specific molecules; each organism has unique proteins that differentiate it. Along with nucleic acids, they are fundamental molecules of life. They are formed from Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S).

Amino Acids (AA)

Amino acids are released upon protein hydrolysis. They are composed of an amino group and a carboxyl group, and their union forms protein chains. The amino acids that constitute proteins are alpha-amino acids.

There are approximately 150 non-proteinogenic amino acids that do not play structural roles in proteins, such as neurotransmitters (e.g., gamma-aminobutyric acid) or vitamin precursors (e.g., beta-alanine, a precursor of pantothenic acid). There are 20 standard proteinogenic amino acids.

Amino Acid Classification by Metabolism

  • Nonessential AA: Can be synthesized by the living organism.
  • Essential AA: Must be obtained through feeding (e.g., Arginine or Lysine).

Chemical Classification of Amino Acids

Classification based on the nature of the side chain:

  • Neutral AA: At neutral pH, the net electrical charge is 0. They can be:
    • Nonpolar (e.g., Proline, Valine).
    • Polar (e.g., Serine, Tyrosine).
  • Acidic AA: Possess an extra carboxyl group. At pH 7, they typically carry a negative electric charge (e.g., Aspartic Acid).
  • Basic AA: Accept positively charged H+ ions, resulting in a net positive charge (e.g., Arginine, Lysine).

Properties of Amino Acids

Amphoteric Character and Buffering

Amino acids exhibit amphoteric character: they behave as an acid or a base depending on the pH of the medium. The carboxyl group can release H+, and the amino group is capable of accepting H+. This dual capacity gives them a buffer effect.

The net charge varies with pH:

  • At a more acidic pH, the net charge of the AA will be positive.
  • Conversely, at a more basic pH, the net charge will be negative.