Identify the correct statements about biomolecules: A. Lipids are generally wate

BiologyBiomolecules

Identify the correct statements about biomolecules: A. Lipids are generally water soluble. B. Proteins are polypeptides. C. Polysaccharides are long chains of sugars. D. Adenine and guanine are substituted pyrimidines. E. Almost all enzymes are proteins.

Options

C, D and E only

B, C and E only

B, D and E only

A, B and C only

Correct Answer

Option 2 : B, C and E only

Solution

A. Lipids are generally water soluble: ❌ WRONG. Lipids are HYDROPHOBIC (water-insoluble). They dissolve in organic solvents (chloroform, ether).

B. Proteins are polypeptides: ✅ CORRECT. Proteins are one or more polypeptide chains.

C. Polysaccharides are long chains of sugars: ✅ CORRECT. Starch, glycogen, cellulose = polymer of glucose.

D. Adenine and guanine are substituted pyrimidines: ❌ WRONG. Adenine and Guanine are PURINES (double ring). Pyrimidines = Cytosine, Thymine, Uracil (single ring).

E. Almost all enzymes are proteins: ✅ CORRECT. Ribozymes (RNA enzymes) are exception, but ALMOST all enzymes are proteins.

Correct: B, C and E only
A wrong (lipids = hydrophobic) | D wrong (Adenine/Guanine = PURINES, not pyrimidines)

Theory: Biomolecules

1. Purines vs Pyrimidines

Purines (double ring): Adenine (A) and Guanine (G). Found in both DNA and RNA. Remember: 'PURE AG' — Purines: Adenine, Guanine. Pyrimidines (single ring): Cytosine (C) — in DNA and RNA. Thymine (T) — only in DNA. Uracil (U) — only in RNA (replaces T). Remember: 'CUT' — Cytosine, Uracil, Thymine. Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds) in DNA. A=U in RNA. More G-C pairs → higher melting temperature (3 H-bonds > 2). Chargaff's rule: %A = %T, %G = %C in double-stranded DNA.

2. Proteins — Structure and Types

Proteins = polymers of amino acids linked by peptide bonds (−CO−NH−). 20 standard amino acids. Primary structure: sequence of amino acids (determined by gene). Secondary: α-helix (stabilised by H-bonds along backbone, 3.6 residues/turn) and β-pleated sheet. Tertiary: 3D folding — H-bonds, disulfide bonds, hydrophobic interactions, ionic bonds. Quaternary: association of multiple polypeptide chains (subunits). e.g., haemoglobin (2α + 2β subunits). Fibrous proteins: structural (keratin, collagen, elastin, silk). Globular proteins: functional (enzymes, antibodies, haemoglobin, insulin).

3. Carbohydrates — Classification

Monosaccharides: glucose, fructose, galactose, ribose (C₅), deoxyribose (C₅). Disaccharides: maltose (glucose+glucose), sucrose (glucose+fructose), lactose (glucose+galactose). Linked by glycosidic bonds. Polysaccharides: Starch (amylose + amylopectin) — storage in plants. Glycogen — storage in animals (liver, muscle). Cellulose — structural in plants (β-1,4 linkage — not digestible by humans). Chitin — structural in fungi and arthropod exoskeletons (β-1,4 N-acetylglucosamine). Inulin: fructose polymer in plants. Agar: from red algae — used in microbiology culture media.

4. Lipids — Properties

Lipids: diverse group — oils, fats, waxes, phospholipids, sterols. ALL hydrophobic (water-insoluble) — because they have long hydrocarbon chains. Dissolve in organic solvents: ether, chloroform, benzene. Simple lipids: triglycerides (glycerol + 3 fatty acids). Saturated FA (no C=C) → fats (solid at room temp). Unsaturated FA (C=C bonds) → oils (liquid at room temp). Complex lipids: phospholipids (glycerol + 2 FA + phosphate group + polar head). Form bilayer of cell membrane. Sterols: cholesterol (membrane fluidity, precursor to steroid hormones, bile salts, vitamin D). Waxes: water-repellent cuticle on leaves, beeswax.

5. Enzymes — Properties

Enzymes: biological catalysts. Almost all are globular proteins (exception: ribozymes = catalytic RNA). Properties: (1) Speed up reactions without being consumed. (2) Lower activation energy. (3) Highly specific (lock-and-key or induced fit model). (4) Sensitive to temperature (optimum ~37°C for human enzymes; denatured at high T). (5) Sensitive to pH (pepsin pH 2, trypsin pH 8, salivary amylase pH 7). (6) Required in small amounts. Cofactors: metal ions (Mg²⁺, Zn²⁺, Fe²⁺) required for enzyme activity. Coenzymes: organic cofactors (NAD⁺, FAD, CoA, vitamins). Prosthetic group: tightly bound coenzyme (e.g., heme in cytochrome).

6. Nucleotides and Nucleic Acids

Nucleotide = nitrogenous base + pentose sugar + phosphate. Nucleoside = base + sugar (no phosphate). DNA: deoxyribose sugar + A,T,G,C + double stranded helix. RNA: ribose sugar + A,U,G,C + usually single stranded. Types of RNA: mRNA (messenger — carries genetic code to ribosomes), tRNA (transfer — anticodon, carries amino acid), rRNA (ribosomal — part of ribosome structure). ATP: adenine + ribose + 3 phosphates = energy currency. NAD⁺, FAD: coenzymes in respiration. NADP⁺: coenzyme in photosynthesis. cAMP (cyclic AMP): second messenger in cell signalling.

7. Ribozymes — Enzymes That Are RNA

Thomas Cech and Sidney Altman discovered ribozymes (1980s) — Nobel Prize 1989. Examples: Self-splicing introns (Group I and II introns). RNase P: cleaves precursor tRNA — the RNA component is catalytic. Ribosomal peptidyl transferase activity: the 23S rRNA in the large ribosomal subunit catalyses peptide bond formation (the ribosome is fundamentally a ribozyme!). Significance: supports 'RNA World' hypothesis — early life used RNA as both genetic material and catalysts before DNA and proteins evolved. Ribozymes have potential therapeutic applications (targeting disease-related mRNA).

8. Hormones as Biomolecules

Protein/peptide hormones: insulin (51 aa), glucagon (29 aa), ADH (vasopressin, 9 aa), oxytocin (9 aa), GH (growth hormone, 191 aa), FSH, LH, TSH, ACTH, PTH. These are water-soluble → cannot cross cell membrane → act on surface receptors → second messenger cascade. Steroid hormones: from cholesterol. Examples: testosterone, estrogen, progesterone, cortisol, aldosterone. Lipid-soluble → cross cell membrane → act on intracellular/nuclear receptors → directly affect gene expression. Amine hormones: derived from amino acids. Adrenaline (epinephrine), noradrenaline (from tyrosine). Thyroxine (T₄), T₃ (from tyrosine + iodine).

Frequently Asked Questions

1. Why are adenine and guanine called purines and not pyrimidines? ⌄

Purines have a double ring structure: a pyrimidine ring fused with an imidazole ring = bicyclic system. Adenine and guanine are both purines. Pyrimidines have a single ring (6-membered ring with 2 N atoms). Cytosine, thymine, uracil are pyrimidines. Memory trick: 'Pure AG' (Purines: A and G). 'CUT' (Pyrimidines: C, U, T). In DNA: A pairs with T (both different ring sizes balance the helix width). G pairs with C. The helix has uniform width because each base pair consists of one purine + one pyrimidine.

2. Why are lipids called hydrophobic? ⌄

Lipids have long hydrocarbon chains (−CH₂−CH₂−...) which are nonpolar. Like dissolves like principle: nonpolar chains cannot interact with polar water molecules. Water molecules prefer to hydrogen bond with each other rather than interact with nonpolar lipid chains. When lipids are forced into water: water molecules form ordered shells around them (hydrophobic effect) → entropic cost → lipids aggregate together to minimise water contact. This hydrophobicity is essential for membrane function: phospholipid bilayer is a hydrophobic barrier → controls what enters/exits cells. Only fat-soluble vitamins (A, D, E, K) and steroid hormones can cross membrane by simple diffusion.

3. What are the exceptions to 'all enzymes are proteins'? ⌄

Ribozymes: RNA molecules with catalytic activity. Discovered by Thomas Cech (self-splicing Tetrahymena rRNA intron) and Sidney Altman (RNase P). Examples: RNA-based enzyme in ribosome (peptidyl transferase activity = 23S rRNA). Self-splicing introns. RNase P (RNA component is the catalyst; protein is just structural). Abzymes: antibodies with enzyme-like catalytic activity (rare, physiological relevance unclear). Metallic catalysts: not biomolecules per se. For NEET: the statement 'almost all enzymes are proteins' is CORRECT — it acknowledges the exception of ribozymes while stating the general rule.

4. What is the difference between starch and cellulose? ⌄

Both are polysaccharides made of glucose units. Starch: α-glucose + α-1,4 glycosidic bonds (amylose = linear) and α-1,6 bonds (amylopectin = branched). Helical structure → can be digested by amylase → energy storage in plants. Cellulose: β-glucose + β-1,4 glycosidic bonds. Linear chains hydrogen bond to each other → microfibrils → high tensile strength. Humans CANNOT digest cellulose (no β-glucosidase/cellulase). Cellulose is dietary fibre. Termites and ruminants digest cellulose via gut microbes (cellulase-producing bacteria). Glycogen: animal storage carbohydrate — like amylopectin but more branched (α-1,4 and α-1,6, more frequent branching).

5. What is the role of phospholipids in membranes? ⌄

Phospholipids: glycerol + 2 fatty acids (hydrophobic tails) + phosphate group + polar head (hydrophilic). Amphipathic: both hydrophilic and hydrophobic regions. In water: spontaneously form bilayer (tails face inward, heads face outward). Cell membrane = fluid mosaic model (Singer and Nicolson, 1972). Phospholipid bilayer is: selective barrier, fluid (proteins and lipids can move laterally), asymmetric (inner and outer leaflets differ). Cholesterol: inserted between phospholipids → reduces fluidity at high T, prevents freezing at low T → modulates membrane fluidity. Unsaturated fatty acids increase fluidity (don't pack as tightly).

6. What are the functions of carbohydrates in living organisms? ⌄

(1) Energy source: glucose is primary fuel. Starch (plants) and glycogen (animals, fungi) = energy storage. (2) Structural: cellulose (plant cell walls), chitin (fungal cell walls, insect exoskeleton). (3) Cell recognition: glycoproteins and glycolipids on cell surface → cell signalling, blood group antigens (ABO), immune recognition. (4) Nucleic acid backbone: ribose (RNA), deoxyribose (DNA). (5) Coenzyme components: NAD⁺ contains ribose, ATP contains ribose. (6) Anticoagulant: heparin (glycosaminoglycan). (7) Lubrication: hyaluronic acid in synovial fluid.

7. What are essential amino acids? ⌄

Essential amino acids: cannot be synthesised by the body → must be obtained from diet. In humans (adults): 8 essential AAs: valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan. Mnemonic: 'PVT TIM HaLL' (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine) — but histidine and arginine are semi-essential. Non-essential: synthesised in body (alanine, glycine, serine, etc.). Complete protein: contains all essential AAs (eggs, meat, fish, milk). Incomplete protein: lacking one or more essential AAs (most plant proteins). Kwashiorkor: protein deficiency disease in children — protein-calorie malnutrition.

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