The cells of a living organism are made of thousands of organic compounds. Besides these, many molecules are formed during metabolism. The compounds which are produced in or required for metabolism are called metabolites. All metabolites are biomolecules. Metabolites are biomolecules usually involved in many cellular functions such as energy, stimulation, inhibition, protection, etc.
Metabolites are divided into two types based on their role in cellular activities. A metabolite which is directly involved in physiological functions and helps in growth, reproduction, and maturity is called a primary metabolite. Amino acids, saccharides, nitrogen bases, cholesterol, etc., are a few examples of primary metabolites. The metabolite which is produced from the primary metabolite and is not directly involved in the physiological functions is known as a secondary metabolite, e.g. pigments, pheromones, antibiotics, alkaloids, etc.
Any carbon-containing molecule, i.e., organic compounds present in a living cell, is called a biomolecule. Based on the molecular weight, biomolecules are classified into two, namely micromolecules and macromolecules.
Micromolecules are those biomolecules which have a molecular weight of less than 10,000 Dalton (Da). This includes the primary metabolites and secondary metabolites. Micromolecules are acid soluble, and their molecular weights range from 18 to 800 Da.
Macromolecules are biomolecules which are characterized by their acid insoluble property. Proteins, carbohydrates, lipids, and nucleic acids are the macromolecules found in living organisms; they are also called biomacromolecules. They have a molecular weight of more than ten thousand daltons. However, lipids are an exception. Though lipids have a molecular weight of less than 800 Da, they are classified as biomacromolecules because of their acid insoluble property. Hence, lipids are not strictly macromolecules.
Lipids are an important component of cell membranes. Even if we try to crush and mix them with water or acid, they will always remain aloof from mixing. Despite their molecular weight, they are considered acid insoluble moieties. Thus, we could conclude that lipids are macromolecules for which the size doesn’t matter.
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