Biomolecules & Its Types - Biology Class 11 - NEET PDF Download

A cell is composed of a variety of molecules (for example, carbon, hydrogen, oxygen) that perform specific structural and functional roles. In addition to these basic elements, certain metals and non‑metals occur as cellular materials. These elements and compounds combine to form diverse organic and inorganic substances collectively called biomolecules, which are present in the cells of all living organisms.

What are Biomolecules?

Biomolecules are organic and inorganic chemical compounds that participate in the structure and functioning of living organisms. Although the molecules themselves are non‑living, they carry out life processes. Major classes of organic biomolecules include carbohydrates, proteins, lipids and nucleic acids. Water is the most abundant chemical compound in living systems and is essential for life.

Analysis of Chemical Composition of Tissues

To determine the chemical composition of biological materials, tissues are subjected to chemical extraction and separation procedures. Knowledge of molecular formulae and structural features is obtained by a sequence of analytical steps and separation techniques.

Extraction and Fractionation

Tissue is typically treated with acid or other solvents to separate components into soluble and insoluble fractions. The usual result of such initial treatment is two fractions:

• Filtrate: the acid‑soluble fraction, containing many small organic and inorganic molecules.
• Retentate: the acid‑insoluble fraction, containing larger macromolecules and insoluble material.

Further separation and purification of components from the extracts are achieved using chromatographic, electrophoretic and other biochemical methods. These procedures allow estimation of molecular formulae and help propose probable structures for isolated compounds.

All carbon‑containing compounds obtained from living tissues are generally referred to as biomolecules.

Separation and Identification

Once fractions are obtained, specific tests and separation methods are used to identify and characterise individual biomolecules. Typical techniques include solvent extraction, column and paper chromatography, spectroscopic methods and chemical reactions specific to functional groups.

Analysis for Inorganic Compounds and Elements

Inorganic elements and compounds present in tissues are analysed by converting the tissue into ash and examining the residual inorganic material. Ash analysis reveals the presence and approximate quantity of metals and mineral ions that are essential for cellular functions.

1. A sample of tissue is oven‑dried to remove moisture; the weight after drying gives the dry weight.
2. The dried tissue is burnt completely; the remaining residue is called ash.
3. The ash contains inorganic elements such as potassium, sodium, calcium, magnesium and others. These may also appear in the acid‑soluble fraction during earlier extraction.

Cellular Pool

The cellular pool denotes the total collection of different biomolecules, ions and compounds present within a cell. A typical cell contains several thousand distinct chemical species; more than 5 000 different compounds have been reported in cellular extracts. Representative inorganic constituents and their approximate forms in tissues are often tabulated during ash analysis.

Representative Inorganic Constituents

Components - Formula

Introduction to Biomacromolecules

Biomolecules are commonly divided by size and function into two broad groups:

Micromolecules

• Micromolecules are small, low molecular weight compounds (typically 18–1800 Da). They are often highly soluble (if polar) and possess relatively simple conformations.
• Micromolecules include inorganic substances (water, mineral salts, gases) and small organic molecules (monosaccharides, amino acids, nucleotides).
• Most micromolecules are found in the soluble fraction of tissue extracts. Lipids are an exception: many are hydrophobic and occur as an insoluble pool associated with membranes or stored as vesicles.

Macromolecules

Macromolecules are large polymeric biomolecules such as polysaccharides, proteins and nucleic acids. They are typically insoluble or form colloidal dispersions and show complex three‑dimensional structures necessary for biological function.

Carbohydrates

Carbohydrates are organic compounds composed mainly of carbon, hydrogen and oxygen, often described as polyhydroxy aldehydes or ketones. They are synthesised by plants during photosynthesis and are commonly called saccharides (sugars).

Classification of Carbohydrates

Carbohydrates are classified by the number of simple sugar units:

• Monosaccharides - simplest form; cannot be hydrolysed further. Typically contain 3–7 carbon atoms per molecule. Examples: ribose, glucose, erythrose. Monosaccharides often exist in cyclic (furanose or pyranose) forms.
• Oligosaccharides - composed of 2–6 monosaccharide units joined by glycosidic bonds.

Monosaccharides are often called reducing sugars because they contain a free aldehyde (-CHO) or ketone (>C=O) group and can reduce Cu2+ in Benedict’s or Fehling’s solution to Cu+ (cuprous ion). Examples of reducing monosaccharides include glucose and ribose.

Oligosaccharide examples and subclasses:

• Disaccharides - two monosaccharide units (e.g., sucrose, maltose, lactose). Some disaccharides are non‑reducing (e.g., sucrose) when the glycosidic linkage involves the anomeric carbons of both monosaccharide units.
• Trisaccharides - three monosaccharide units (e.g., raffinose).
• Tetrasaccharides - four monosaccharide units (e.g., stachyose).

Amino Acids

Amino acids are organic compounds that contain both an amino group (-NH2) and an acidic carboxyl group (-COOH) attached to the same carbon atom, called the α‑carbon. The α‑carbon also bears a hydrogen atom and a variable side chain designated as the R group. Proteinogenic amino acids are 20 in number and differ in their R groups (for example, glycine: R = H; alanine: R = -CH3; serine: R = -CH2-OH).

An amino acid can accept a proton at the amino group and donate a proton from the carboxyl group, so it behaves as both an acid and a base - it is amphoteric. Chemical and physical properties of amino acids are influenced by the nature of their side chains as well as the -NH2 and -COOH groups.

Classification of Amino Acids

• Acidic amino acids - contain two carboxyl groups and one amino group per molecule (for example, glutamic acid, aspartic acid).
• Basic amino acids - contain two amino groups and one carboxyl group (for example, arginine, lysine, histidine).
• Neutral amino acids - contain one amino and one carboxyl group (examples: methionine, isoleucine, serine, threonine, cysteine, glycine, alanine, valine, leucine, asparagine, glutamine, proline).
• Aromatic amino acids - contain aromatic rings in their side chains (for example, phenylalanine, tyrosine, tryptophan).

Zwitterion

In aqueous solution, amino acids commonly exist as zwitterions - molecules carrying both a positive and a negative charge but having no net charge at a particular pH (the isoelectric point). The ionisation states of -NH2 and -COOH vary with solution pH, altering the overall charge on the amino acid.

Lipids

Lipids are a heterogeneous group of hydrophobic or amphipathic organic compounds, often esters of fatty acids with alcohols. They are generally insoluble in water and soluble in organic solvents.

Fatty Acids

Fatty acids are carboxylic acids with long hydrocarbon chains ending in a carboxyl group (-COOH). The hydrocarbon chain (R group) may be a methyl, ethyl or a longer chain of -CH2 groups. For example, palmitic acid has 16 carbon atoms (including the carboxyl carbon); arachidonic acid contains 20 carbon atoms.

Based on the presence of C=C double bonds, fatty acids are of two types:

• Saturated fatty acids - no double bonds; generally solid at room temperature (for example many animal fats).
• Unsaturated fatty acids - contain one or more double bonds (C=C); generally liquid at room temperature (for example many plant oils).

Types of Lipids

Major categories of lipids include:

• Neutral or true fats - esters of fatty acids with glycerol (glycerides). Glycerol (trihydroxypropane) esterified with one, two or three fatty acid molecules gives mono‑, di‑ and triglycerides respectively. Triglycerides are the principal form of stored fat.

• Compound (conjugated) lipids - esters of fatty acids with alcohols that also contain additional groups, for example phospholipids (contain phosphorus and phosphorylated groups) and glycolipids. A common phospholipid is lecithin. Complex lipids are important components of membranes and specialised tissues (for example neural tissue).

• Derived lipids - lipid‑derived molecules such as steroids, prostaglandins and terpenes, obtained by hydrolysis or modification of parent lipids.

Fats are sometimes described by physical state at room temperature: hard fats (solid, long‑chain saturated fatty acids; e.g., many animal fats) and soft fats or oils (shorter chains or unsaturated fatty acids; e.g., plant oils such as groundnut, mustard, gingelly).

Nucleotides and Nucleic Acids

Nucleotides are the monomeric units of nucleic acids (DNA and RNA). A nucleotide consists of three components: a pentose sugar (ribose in RNA, deoxyribose in DNA), a nitrogenous base and one or more phosphate groups (mono‑, di‑ or triphosphate).

The pentose sugar commonly assumes a ring form (furanose or pyranose). Nucleotides are named after their bases: for example, adenylic acid (AMP), guanylic acid (GMP), thymidylic acid (TMP), uridylic acid (UMP), cytidylic acid (CMP).

Nitrogenous Bases

Nitrogenous bases are heterocyclic, planar compounds that contain nitrogen and carbon atoms. They are classified into two groups:

• Purines - larger, bicyclic bases: adenine (A) and guanine (G).
• Pyrimidines - smaller, monocyclic bases: cytosine (C), thymine (T) (in DNA) and uracil (U) (in RNA).

Phosphate Group

The phosphate (phosphoric acid) provides the acidic character of nucleotides. A nucleotide may contain one, two or three phosphate residues attached to the 5′ position of the pentose.

Nucleoside

A nucleoside is formed when a nitrogenous base is attached to the pentose sugar by a N‑glycosidic bond (for example, adenine + ribose → adenosine). When a phosphate group is esterified to the 5′‑position of a nucleoside, a nucleotide is formed.

Differences between Nucleoside and Nucleotide

Summary: Biomolecules include a wide range of organic and inorganic substances required for life. Understanding their structures, classifications and properties - from small micromolecules like monosaccharides and amino acids to large macromolecules like proteins and nucleic acids - is essential for studying cellular structure, metabolism and physiology.