Biomolecules Chapter Notes | Biotechnology for Class 11 - NEET PDF Download

Biomolecules

• Biomolecules, also known as macromolecules, are essential components of cellular organelles, including carbohydrates, proteins, lipids, and nucleic acids.
• Cell membrane is composed of lipids and proteins, providing a barrier and facilitating transport.
• Cell wall, primarily made of carbohydrates, offers structural support in plants and bacteria.
• Chromosomes are predominantly composed of proteins and DNA, carrying genetic information.
• Ribosomes, made of proteins and RNA, are the sites of protein synthesis.
• These biomolecules serve as structural entities and play critical roles in cellular processes such as energy production, signaling, and genetic information transfer.

Carbohydrates

• Carbohydrates are among the most abundant biomolecules, widely distributed in all life forms.
• Chemically, they are aldehyde or ketone derivatives of polyhydric alcohols, containing carbon, hydrogen, and oxygen.
• Primary role is to function as a source of energy in living organisms.
• They serve as energy stores, metabolic intermediates, and major components of bacterial and plant cell walls.
• Carbohydrates are integral parts of DNA and RNA, contributing to genetic material.
• They act as informational molecules and are linked to proteins and lipids on cell surfaces, facilitating cell-cell interactions and interactions with the cellular environment.

Classification of Carbohydrates

• Carbohydrates range from simple sugars to complex polymers and are classified into monosaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides are simple sugars that cannot be hydrolyzed into simpler forms, containing free aldehyde (-CHO) or ketone (>C=O) groups, with a general formula of Cₓ(H₂O)ₓ.
• Monosaccharides are classified based on the number of carbon atoms (e.g., trioses, tetroses, pentoses, hexoses) and functional groups (aldoses or ketoses).
• Examples include glyceraldehyde (aldotriose), dihydroxyacetone (ketotriose), erythrose (aldotetrose), erythrulose (ketotetrose), ribose (aldopentose), ribulose (ketopentose), glucose (aldohexose), and fructose (ketohexose).
• Oligosaccharides consist of 2 to 10 monosaccharide units joined by glycosidic bonds, with common examples being disaccharides like maltose, lactose, and sucrose.
• Polysaccharides are polymers of more than 10 monosaccharide units linked by glycosidic bonds, classified by the type of repeating unit (homopolysaccharides or heteropolysaccharides), degree of branching, and type of glycosidic linkage.
• Examples of polysaccharides include starch, glycogen, cellulose, and chitin.
• Carbohydrates can conjugate with proteins and lipids to form glycoconjugates, including glycoproteins (protein-dominant), proteoglycans (carbohydrate-dominant), and glycolipids (carbohydrate-lipid conjugates).

Structure and Properties of Carbohydrates

• Monosaccharides, such as glucose, exist in both straight-chain and cyclic structures due to hemiacetal formation between the carbonyl group and a hydroxyl group.
• All monosaccharides, except dihydroxyacetone, contain one or more chiral carbon atoms, making them optically active with 2ⁿ stereoisomers (n = number of chiral centers).
• Glyceraldehyde, with one chiral center, has 2 stereoisomers; glucose, with four chiral centers, has 16 stereoisomers.
• The orientation of the -OH group farthest from the carbonyl carbon determines D (right) or L (left) isomers, with most biological sugars being D-isomers.
• Anomers are isomeric forms differing in configuration at the hemiacetal or hemiketal carbon (anomeric carbon); α-anomers have the -OH group opposite the CH₂OH group, while β-anomers have it on the same side.
• Mutarotation is the interconversion of α and β anomers in aqueous solution, where the ring opens to a linear form and recloses to form the other anomer.
• Epimers are isomers differing in configuration at only one carbon atom, e.g., glucose and mannose (epimers at C-2) or glucose and galactose (epimers at C-4).
• Oligosaccharides, such as maltose (two D-glucose units) and lactose (D-galactose and D-glucose), are formed by glycosidic linkages between the -OH group of one monosaccharide and the anomeric carbon of another.
• Disaccharides can be hydrolyzed into their constituent monosaccharides by boiling with dilute acid, e.g., sucrose yields glucose and fructose.
• Other oligosaccharides include trisaccharides (e.g., raffinose, composed of glucose, galactose, and fructose), tetrasaccharides, pentasaccharides, and hexasaccharides.
• Polysaccharides serve as energy storage (e.g., starch, glycogen) or structural components (e.g., cellulose, chitin).
• Starch, a plant storage polysaccharide, consists of amylose (15-20%, linear α(1→4) linked glucose) and amylopectin (80-85%, branched with α(1→4) and α(1→6) linkages).
• Amylose gives a characteristic blue color with iodine, while amylopectin gives a dull reddish-brown color.
• Salivary and pancreatic amylases hydrolyze α(1→4) glycosidic linkages in starch, digesting it into glucose monomers.
• Glycogen, an animal storage polysaccharide, is highly branched with α(1→4) and α(1→6) linkages, stored in muscle (1-2% dry weight) and liver (up to 10% dry weight).
• Cellulose, the most abundant biomolecule, is a linear polymer of up to 15,000 β(1→4) linked D-glucose units, forming the primary structural component of plant cell walls.
• Humans lack cellulase to digest β(1→4) linkages, but cattle and termites digest cellulose via symbiotic microorganisms producing cellulase.
• Chitin, a linear polysaccharide of β(1→4) linked N-acetyl-D-glucosamine, is a structural component in invertebrate exoskeletons and fungal cell walls, with extensive hydrogen bonding making it tough and insoluble.
• Peptidoglycan, a bacterial cell wall heteropolysaccharide, consists of alternating β(1→4) linked N-acetyl muramic acid (NAM) and N-acetyl-D-glucosamine (NAG), cross-linked by short peptides to form a strong sheath preventing osmotic rupture.
• Lysozyme in human tears hydrolyzes β(1→4) linkages in peptidoglycan, killing bacteria.
• Agar, a gelatinous heteropolysaccharide from red algae, is a mixture of sulfated polysaccharides (D-galactose and L-galactose derivatives), with agarose used in electrophoresis and agar for bacterial and plant tissue culture growth surfaces.

Fatty Acids and Lipids

• Lipids are organic compounds in living organisms, characterized by their hydrophobic, non-polar nature and solubility in organic solvents.
• They are primarily composed of hydrocarbon chains linked to glycerol via ester bonds.
• Lipids are classified into simple lipids (e.g., triacylglycerols, waxes) and compound lipids (e.g., phospholipids, steroids).
• Fatty acids, obtained from fat hydrolysis, are synthesized from two-carbon units, typically containing an even number of carbon atoms.
• Fatty acids are either saturated (no double bonds) or unsaturated (one or more double bonds).
• Fatty acid nomenclature starts with the carboxyl carbon (C-1) and ends at the methyl carbon, denoted as X:Y(Δᵃ,ᵇ), where X is the number of carbons, Y is the number of double bonds, and a,b indicate double bond positions.
• Monounsaturated fatty acids have one double bond (e.g., oleic acid, 18:1(Δ⁹)).
• Polyunsaturated fatty acids have multiple double bonds (e.g., linoleic acid, 18:2(Δ⁹,¹²); linolenic acid, 18:3; arachidonic acid, 20:4(Δ⁵,⁸,¹¹,¹⁴)).
• Examples of saturated fatty acids include lauric acid (12:0), myristic acid (14:0), palmitic acid (16:0), stearic acid (18:0), and arachidic acid (20:0).
• Examples of unsaturated fatty acids include palmitoleic acid (16:1(Δ⁹)), oleic acid (18:1(Δ⁹)), linoleic acid (18:2(Δ⁹,¹²)), and arachidonic acid (20:4(Δ⁵,⁸,¹¹,¹⁴)).

Classification of Lipids

• Simple lipids are fatty acid esters with alcohols and no additional groups, including triacylglycerols and waxes.
• Triacylglycerols (triglycerides) are esters of glycerol and three fatty acids, either simple (same fatty acid in all positions) or mixed (different fatty acids).
• Triacylglycerols store energy, primarily in adipose tissue, serving as an energy vehicle.
• Waxes are esters of fatty acids with high molecular weight monohydric alcohols, serving as protective coatings on animal and plant surfaces and reducing water loss in tropical plants.
• Compound lipids are fatty acid esters with alcohols and additional groups, forming amphipathic molecules with hydrophobic tails and hydrophilic heads.
• Phospholipids, a type of compound lipid, are amphipathic, with two hydrophobic fatty acid tails and a hydrophilic phosphate head, primarily found in cell membranes.
• Phospholipids include glycerophospholipids (glycerol backbone) and sphingophospholipids (sphingoid base backbone).
• In glycerophospholipids, a modified phosphate group occupies the third carbon of glycerol, with common modifiers being choline (phosphatidylcholine) or serine (phosphatidylserine).
• In sphingolipids, a fatty acid is attached via an amide linkage; ceramide has only a hydrogen head group, while sphingomyelin has a phosphocholine head.
• Phospholipids form micelles in water due to their amphipathic nature.
• Steroids, another compound lipid, have a four-fused ring structure, are hydrophobic, and are insoluble in water.
• Steroids act as receptor ligands and control metabolism; cholesterol, synthesized by the liver, is a key precursor for steroid hormones (e.g., testosterone, estradiol) and is found in eukaryotic plasma membranes for rigidity.
• Plant steroids include phytosterols and stigmasterols, regulating membrane fluidity and permeability.
• Ergosterol, found in fungi, is a precursor to vitamin D.

Amino Acids

• Amino acids are the building blocks of proteins, with a general formula of NH₂-C(R)-COOH, where the central α-carbon is linked to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R side chain.
• The R side chain varies among the 20 standard amino acids, ranging from a hydrogen atom (glycine) to a methyl group (alanine).
• The α-carbon is chiral (except in glycine), leading to L and D isomers; only L isomers are found in proteins, with D isomers being rare in biological systems.
• The 20 standard amino acids are categorized by their side chains: polar uncharged (e.g., serine, threonine, cysteine, asparagine, glutamine), aromatic (e.g., phenylalanine, tryptophan, tyrosine), non-polar aliphatic (e.g., glycine, valine, alanine, proline, leucine, isoleucine, methionine), positively charged (e.g., lysine, arginine, histidine), and negatively charged (e.g., aspartate, glutamate).
• Non-standard amino acids (e.g., 4-hydroxyproline, 5-hydroxylysine) and non-protein amino acids (e.g., L-ornithine, L-citrulline) also exist.

Protein Structure

• Proteins are polypeptides formed by covalently linking amino acids via peptide bonds in a linear chain.
• Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.
• Primary structure is the linear sequence of amino acids linked by peptide bonds.
• Secondary structure refers to the three-dimensional arrangement of the polypeptide chain, primarily forming α-helices and β-sheets, stabilized by hydrogen bonds.
• Tertiary structure is the overall three-dimensional folding of a single polypeptide, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges, with non-polar R groups typically in the interior.
• Techniques like X-ray crystallography and nuclear magnetic resonance (NMR) determine the tertiary structure.
• Quaternary structure involves the spatial arrangement of multiple polypeptide subunits (identical or different), stabilized by hydrogen bonds, electrostatic interactions, ionic bonds, and disulfide bridges.
• Quaternary structures are classified as dimers, trimers, etc., with identical subunits forming homodimers/homotrimers and non-identical subunits forming heterodimers/heterotrimers.

Nucleic Acids

• Nucleic acids are polymers of nucleotides found in the nucleus, mitochondria, and chloroplasts, associated with histone proteins in the nucleus to form chromatin.
• Two types of nucleic acids exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
• DNA is the genetic material in most organisms, inheriting information across generations, while RNA serves as genetic material in some viruses.
• Nucleotides consist of a nitrogenous base, a pentose sugar, and a phosphate group.
• DNA contains 2'-deoxy-D-ribose, while RNA contains D-ribose, both as closed five-membered rings, with carbon atoms numbered with a prime (') to distinguish from base atoms.
• Nitrogenous bases include purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA, uracil in RNA).
• Purine and pyrimidine bases have aromatic ring structures, absorbing light near 260 nm.

Polynucleotide Chain

• Nucleotides in DNA and RNA are linked by phosphodiester bonds, where the 3' hydroxyl group of one nucleotide’s sugar is esterified to the phosphate attached to the 5' carbon of the next nucleotide.
• Polynucleotide chains have polarity, with a 5' end (phosphate group) and a 3' end (free -OH group), and base sequences are written in the 5'→3' direction.

Structure of DNA

• In 1953, James Watson and Francis Crick proposed the double helical structure of DNA, consisting of two antiparallel polynucleotide chains wound around a common axis to form a right-handed double helix.
• The sugar-phosphate backbones are exposed to the polar environment, while the bases are stacked inside the hydrophobic core.
• Base pairing occurs via hydrogen bonds: adenine (A) forms two hydrogen bonds with thymine (T) (A=T), and guanine (G) forms three hydrogen bonds with cytosine (C) (G=C).
• B-DNA, the most stable form, is a right-handed double helix; A-DNA is a wider right-handed helix with 11 base pairs per turn, and Z-DNA is a left-handed helix with 12 base pairs per turn.
• Heating disrupts hydrogen bonds, causing denaturation (melting) of the double helix, with the melting temperature (Tₘ) being the point where half the DNA is denatured.
• Acid or alkali can also cause melting, and separated strands can reassociate (anneal) below Tₘ to reform the double helix.

Types of RNA

• Messenger RNA (mRNA) is a single-stranded linear polyribonucleotide carrying genetic information from DNA to ribosomes for protein synthesis.
• mRNA structure includes a 5' untranslated region (UTR), initiation codon, coding region, stop codon, and 3' UTR; eukaryotic mRNA undergoes 5' capping (methylated guanylate) and 3' polyadenylation (addition of adenylate residues).
• Ribosomal RNA (rRNA) forms structural components of ribosomes; prokaryotic 70S ribosomes have 16S rRNA (30S subunit) and 23S/5S rRNAs (50S subunit), while eukaryotic 80S ribosomes have 18S rRNA (40S subunit) and 28S/5.8S/5S rRNAs (60S subunit).
• Transfer RNA (tRNA) is a small RNA molecule that transfers amino acids to ribosomes during protein synthesis, with a clover-leaf secondary structure featuring four arms.
• The tRNA acceptor arm has a CCA sequence at the 3' OH end for amino acid binding, and the anticodon arm contains a three-base anticodon that recognizes a specific mRNA codon during translation.