Biomolecules: Essential Components of Life for Class 11 Biology

Chemical Composition of Living Organisms

Living organisms, despite their diversity, share common chemical elements. The major elements include carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), which form the backbone of biomolecules. Compared to non-living matter like the earth's crust, living tissues have a much higher percentage of carbon and hydrogen, essential for organic compound formation.

This elemental composition forms the basis for the complex biomolecules found in cells. Chemical analysis of tissues involves grinding samples in trichloroacetic acid to separate acid-soluble and acid-insoluble fractions, helping identify organic and inorganic compounds.

Classification and Types of Biomolecules

Biomolecules are broadly classified into four major categories:

• Carbohydrates: Composed of carbon, hydrogen, and oxygen, carbohydrates provide energy and structural support. Examples include glucose and starch.
• Proteins: Made of amino acids linked by peptide bonds, proteins perform structural, enzymatic, and regulatory roles.
• Lipids: Water-insoluble molecules like fats, oils, and phospholipids, lipids store energy and form cell membranes.
• Nucleic Acids: DNA and RNA, polymers of nucleotides, carry genetic information.

Each biomolecule type has unique chemical properties and biological functions. For example, amino acids have an amino group (-NH2) and a carboxyl group (-COOH) attached to the alpha-carbon, with variable side chains (R groups) that determine their characteristics.

Amino Acids and Proteins: Building Blocks of Life

Amino acids are the monomers of proteins. There are 20 standard amino acids, each with a specific R group that influences its polarity and function. They are classified as acidic, basic, neutral, or aromatic based on their side chains.

Proteins form when amino acids join via peptide bonds, creating polypeptide chains that fold into functional shapes. Proteins serve as enzymes, structural components, transporters, and signaling molecules.

Example: The formation of a dipeptide from two amino acids involves a condensation reaction:

$$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$$

This reaction links the carboxyl group of one amino acid to the amino group of another.

Lipids: Structure and Biological Roles

Lipids are hydrophobic molecules primarily made of fatty acids and glycerol. Fatty acids vary in chain length and saturation:

• Saturated fatty acids: No double bonds (e.g., palmitic acid, 16 carbons)
• Unsaturated fatty acids: One or more double bonds (e.g., arachidonic acid, 20 carbons)

Glycerol combines with fatty acids to form mono-, di-, and triglycerides, which store energy. Phospholipids, containing a phosphate group, are essential for forming cell membranes due to their amphipathic nature.

Worked example: Calculate the number of water molecules released when forming a triglyceride from glycerol and three fatty acids.

Each ester bond formation releases one water molecule. Since three fatty acids attach to glycerol, three water molecules are released:

$$3 \times H_2O = 3 \text{ water molecules}$$

Nucleotides and Nucleic Acids: Genetic Material

Nucleotides consist of three parts:

• A nitrogenous base (adenine, guanine, cytosine, thymine, or uracil)
• A pentose sugar (deoxyribose in DNA, ribose in RNA)
• One or more phosphate groups

Nucleotides polymerize to form nucleic acids:

• DNA: Stores genetic information
• RNA: Involved in protein synthesis and gene regulation

Nucleosides are nitrogenous bases attached to sugars without phosphate groups. Adding phosphate forms nucleotides.

These molecules carry hereditary information and are vital for cell function and reproduction.

Inorganic Constituents in Living Organisms

Besides organic biomolecules, living organisms contain inorganic elements and compounds that support biological functions. Common inorganic ions include:

These elements help maintain osmotic balance, nerve impulses, muscle contraction, and enzyme activities. For example, calcium ions are crucial for bone structure and blood clotting.

The ash residue after burning tissues contains these inorganic elements, highlighting their presence and importance.