Biomolecules

9.1 How to Analyse Chemical Composition?

Living organisms exhibit a vast diversity, but a fundamental question arises: Are all living organisms composed of the same chemical elements and compounds? Elemental analysis of living tissues such as plant, animal, or microbial samples reveals the presence of elements like carbon, hydrogen, oxygen, nitrogen, and others. When compared with non-living matter, such as the earth's crust, the elemental composition appears qualitatively similar, but the relative abundance of certain elements differs. Notably, living tissues have a higher relative abundance of carbon and hydrogen compared to the earth's crust, as shown in Table 9.1. This indicates that while the elemental constituents are common, their proportions vary significantly between living and non-living matter. To analyze the organic compounds present in living organisms, chemical analysis is performed. A common method involves grinding living tissue in trichloroacetic acid (Cl3CCOOH) using a mortar and pestle, producing a thick slurry. This slurry is filtered to separate two fractions: the acid-soluble pool (filtrate) and the acid-insoluble fraction (retentate). Thousands of organic compounds are found in the acid-soluble pool, including amino acids, sugars, nucleotides, and others. Further analysis involves isolating and purifying compounds from the extract using various separation techniques, followed by structural elucidation through analytical methods. All carbon compounds extracted from living tissues are termed biomolecules. Besides organic compounds, living organisms also contain inorganic elements and compounds. To identify these, a destructive experiment is conducted where the wet weight of tissue is measured, dried to remove water, and then burnt to oxidize carbon compounds into gaseous forms like CO2 and water vapor. The residue, called ash, contains inorganic elements such as calcium and magnesium. These inorganic compounds, including sulphates and phosphates, are also found in the acid-soluble fraction. From a biological perspective, functional groups such as aldehydes, ketones, and aromatic compounds are classified into categories like amino acids, nucleotide bases, and fatty acids. Amino acids, for example, are organic compounds with an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon (the alpha-carbon), making them alpha-amino acids. There are twenty proteinaceous amino acids distinguished by their variable R groups, such as hydrogen in glycine or methyl in alanine. Their chemical and physical properties depend on the amino, carboxyl, and R groups, and they can be acidic, basic, neutral, or aromatic. Lipids, generally water-insoluble, include simple fatty acids with carboxyl groups attached to hydrocarbon chains (R groups). Fatty acids vary in length and saturation; for example, palmitic acid has 16 carbons, and arachidonic acid has 20. Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds). Glycerol, a trihydroxy propane, combines with fatty acids to form mono-, di-, and triglycerides (fats and oils). Phospholipids, containing phosphorus and phosphorylated organic compounds, are critical components of cell membranes. Additionally, living organisms contain heterocyclic nitrogen bases such as adenine, guanine, cytosine, uracil, and thymine. When attached to sugars, these form nucleosides; with an added phosphate group, they form nucleotides. Nucleic acids like DNA and RNA are polymers of nucleotides and serve as genetic material. **Table on page 2 (12×3)** | Element | % Weight of | | | --- | --- | --- | | | Earth's crust | Human body | | Hydrogen (H) | 0.14 | 9.5 | | Carbon (C) | 0.03 | 18.5 | | Oxygen (O) | 46.6 | 65.0 | | Nitrogen (N) | very little | 3.3 | | Sulphur (S) | 0.03 | 0.3 | | Sodium (Na) | 2.8 | 0.2 | | Calcium (Ca) | 3.6 | 1.5 | | Magnesium (Mg) | 2.1 | 0.1 | | Silicon (Si) | 27.7 | negligible | | * Adapted from CNR Rao, Understanding Chemistry, Universities Press, Hyderabad. | | | **Table on page 2 (7×2)** | Component | Formula | | --- | --- | | Sodium | Na+ | | Potassium | K+ | | Calcium | Ca++ | | Magnesium | Mg++ | | Water | H2O | | Compounds | NaCl, CaCO3, PO43-, SO42- |

• Elemental analysis shows living tissues and earth's crust share elements but differ in relative abundance, especially carbon and hydrogen.
• Chemical analysis separates living tissue into acid-soluble and acid-insoluble fractions.
• Acid-soluble pool contains thousands of organic compounds including amino acids and nucleotides.
• Inorganic elements are identified by burning tissue to obtain ash containing minerals like calcium and magnesium.
• Amino acids are alpha-amino acids with amino and carboxyl groups on the same carbon; 20 types occur in proteins.
• Lipids include fatty acids (saturated and unsaturated), glycerol, triglycerides, and phospholipids important for membranes.
• 📌 Biomolecules: Carbon compounds extracted from living tissues.
• 📌 Alpha-amino acids: Amino acids with amino and carboxyl groups attached to the alpha-carbon.
• 📌 Acid-soluble pool: Fraction of tissue extract soluble in acid containing small organic molecules.

9.2 Primary and Secondary Metabolites

The chemistry of living organisms reveals thousands of organic compounds, which can be broadly classified as metabolites—products of metabolism. Primary metabolites are compounds directly involved in normal growth, development, and reproduction. These include amino acids, sugars, nucleotides, and other small molecules essential for cellular function. They are universally present across different organisms and have well-defined physiological roles. In contrast, secondary metabolites are compounds not directly involved in those primary physiological processes but often have ecological functions such as defense, attraction, or competition. Secondary metabolites are abundant in plants, fungi, and microbes and include alkaloids, flavonoids, terpenoids, essential oils, pigments, toxins, gums, and rubber. Their roles in the host organism are not always fully understood, but many have significant applications for humans, including medicinal drugs, spices, and industrial materials. Table 9.3 lists representative secondary metabolites such as carotenoids and anthocyanins (pigments), morphine and codeine (alkaloids), monoterpenes and diterpenes (terpenoids), lemon grass oil (essential oils), abrin and ricin (toxins), concanavalin A (lectins), vinblastin and curcumin (drugs), and polymeric substances like rubber and gums. While primary metabolites are essential for survival, secondary metabolites often provide adaptive advantages in specific environments, such as deterring herbivores or inhibiting microbial growth. Many secondary metabolites have been harnessed by humans for pharmaceuticals, dyes, flavorings, and other applications. Understanding these metabolites bridges biochemistry with ecology and biotechnology. **Table on page 5 (8×2)** | Pigments | Carotenoids, Anthocyanins, etc. | | --- | --- | | Alkaloids | Morphine, Codeine, etc. | | Terpenoides | Monoterpenes, Diterpenes etc. | | Essential oils | Lemon grass oil, etc. | | Toxins | Abrin, Ricin | | Lectins | Concanavalin A | | Drugs | Vinblastin, curcumin, etc. | | Polymeric substances | Rubber, gums, cellulose |

• Primary metabolites are essential compounds involved in basic physiological functions.
• Secondary metabolites are diverse compounds with ecological roles, often unique to certain species.
• Secondary metabolites include alkaloids, pigments, terpenoids, essential oils, toxins, and gums.
• Many secondary metabolites have important applications in medicine, industry, and agriculture.
• The exact biological roles of many secondary metabolites in host organisms remain unclear.
• Primary metabolites are common to all organisms, while secondary metabolites vary widely.
• 📌 Primary metabolites: Compounds directly involved in growth and metabolism.
• 📌 Secondary metabolites: Compounds not essential for basic metabolism but important ecologically.
• 📌 Alkaloids: Nitrogen-containing secondary metabolites with pharmacological effects.