CUET UG Biology Biomolecules Carbohydrates Proteins Lipids Nucleic Acids Structure and Function

Must-Know (15–20 detailed bullets)

• Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed further; glucose (C₆H₁₂O₆) is the most common example.
• Disaccharides consist of two monosaccharide units linked by a glycosidic bond; sucrose = glucose + fructose (verify from NCERT).
• Starch is a storage polysaccharide in plants made of amylose (linear, α-1,4 linkages) and amylopectin (branched, α-1,6 linkages at branch points).
• Cellulose is a structural polysaccharide in plant cell walls with β-1,4-glycosidic bonds; humans lack cellulase to digest it.
• Glycogen is the storage form of glucose in animals, highly branched, stored in liver and muscles (verify from NCERT).
• Amino acids are building blocks of proteins; each has an amino group (–NH₂), carboxyl group (–COOH), and a variable R group attached to α-carbon.
• Peptide bond forms between the carboxyl group of one amino acid and the amino group of another with the loss of a water molecule (dehydration synthesis).
• The primary structure of a protein is the linear sequence of amino acids; even a single change can alter function (e.g., sickle cell hemoglobin).
• Secondary structure includes α-helix and β-pleated sheets stabilized by hydrogen bonds between backbone –C=O and –N–H groups.
• Tertiary structure is the 3D folding of a polypeptide chain stabilized by R-group interactions: disulfide bridges, hydrogen bonds, hydrophobic interactions, ionic bonds.
• Quaternary structure involves two or more polypeptide chains assembled into a functional protein; hemoglobin has 4 subunits (2α + 2β).
• Enzymes are proteins that act as biological catalysts; most are globular proteins (e.g., amylase, pepsin).
• Lipids are insoluble in water, soluble in organic solvents; include fats, phospholipids, steroids.
• Triglycerides consist of one glycerol and three fatty acids; energy-rich molecules yielding ~9 kcal/g upon oxidation.
• Saturated fatty acids have no double bonds (e.g., palmitic acid); unsaturated have one or more (e.g., oleic acid).
• Phospholipids are amphipathic, with hydrophilic head (phosphate group) and hydrophobic tails (fatty acids); form the lipid bilayer of cell membranes.
• Steroids have a four-ring structure; cholesterol is a key animal cell membrane component and precursor for steroid hormones.
• Nucleotides consist of a nitrogenous base, pentose sugar (ribose or deoxyribose), and phosphate group; building blocks of nucleic acids.
• DNA has deoxyribose sugar, is double-stranded, and contains bases: adenine, thymine, guanine, cytosine (A=T, G≡C pairing).
• RNA has ribose sugar, is usually single-stranded, and contains uracil instead of thymine; involved in protein synthesis (mRNA, tRNA, rRNA).

Difficulty Level

Intermediate — requires understanding of molecular structures, bonding types, and functional roles across biomolecules; diagrams frequently tested.

Common CUET Traps

• Trap: Assuming all carbohydrates are reducing sugars.
  Avoid: Know that sucrose is a non-reducing sugar because its anomeric carbons are involved in glycosidic linkage.

• Trap: Believing tertiary structure is only stabilized by disulfide bonds.
  Avoid: Tertiary structure is stabilized by multiple interactions: hydrophobic, hydrogen bonds, ionic, van der Waals, and disulfide bridges.

• Trap: Confusing DNA and RNA sugar types or base composition.
  Avoid: DNA = deoxyribose + thymine; RNA = ribose + uracil — this distinction is frequently tested.

Practice MCQs (5 questions)

Q1. Which of the following is a non-reducing disaccharide?
A. Lactose
B. Maltose
C. Sucrose
D. Cellulose
Answer: C
Explanation: Sucrose lacks a free aldehyde or ketone group due to glycosidic bonding between anomeric carbons of glucose and fructose.
Why others fail: Lactose and maltose have free anomeric carbons, making them reducing sugars.

Q2. The bond formed between two amino acids during protein synthesis is:
A. Glycosidic bond
B. Ester bond
C. Peptide bond
D. Phosphodiester bond
Answer: C
Explanation: A peptide bond (–CO–NH–) forms between the carboxyl group of one amino acid and the amino group of another.
Why others fail: Glycosidic bonds are in carbohydrates; phosphodiester bonds link nucleotides.

Q3. Which biomolecule is the primary component of the cell membrane bilayer?
A. Triglyceride
B. Cholesterol
C. Phospholipid
D. Glycogen
Answer: C
Explanation: Phospholipids are amphipathic and spontaneously form bilayers in aqueous environments, forming the structural basis of membranes.
Why others fail: Triglycerides store energy but do not form membranes; glycogen is a carbohydrate.

Q4. In a nucleotide, the nitrogenous base is attached to the pentose sugar at which carbon position?
A. 1'
B. 3'
C. 5'
D. 2'
Answer: A
Explanation: The nitrogenous base attaches to the 1' carbon of the pentose sugar in both DNA and RNA nucleotides.
Why others fail: The 3' and 5' carbons are involved in phosphodiester bonds between nucleotides.

Q5. Which level of protein structure is determined by the sequence of amino acids?
A. Secondary
B. Tertiary
C. Quaternary
D. Primary
Answer: D
Explanation: The primary structure is the linear sequence of amino acids linked by peptide bonds, encoded by DNA.
Why others fail: Secondary and higher structures depend on folding of the primary chain.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ Glucose is a hexose sugar with formula C₆H₁₂O₆ — testable in numerical questions.
• ⚠️ Sucrose = glucose + fructose; non-reducing due to α1→β2 glycosidic bond.
• ⚠️ Maltose = glucose + glucose (α-1,4); reducing sugar.
• ⚠️ Lactose = glucose + galactose; found in milk.
• ⚠️ Cellulose has β-1,4 linkages — indigestible by humans.
• ⚠️ Glycogen is stored in liver and muscles — highly branched polymer.
• ⚠️ Peptide bond = amide linkage formed by dehydration.
• ⚠️ Primary structure → amino acid sequence; determines all higher structures.
• ⚠️ α-helix: right-handed coil stabilized by H-bonds (every 4th residue).
• ⚠️ β-pleated sheet: H-bonds between adjacent polypeptide chains (parallel/antiparallel).
• ⚠️ Hemoglobin has quaternary structure — 2α + 2β chains.
• ⚠️ Enzymes are mostly globular proteins — catalyze biochemical reactions.
• ⚠️ Triglyceride = glycerol + 3 fatty acids — energy reserve.
• ⚠️ Saturated fatty acids: no double bonds (e.g., palmitic acid, 16:0).
• ⚠️ Phospholipid bilayer: hydrophilic heads face water, tails inward — basis of membrane.
• ⚠️ Cholesterol: steroid with OH group — modulates membrane fluidity.
• ⚠️ Nucleotide = sugar + base + phosphate; nucleoside = sugar + base.
• ⚠️ DNA: deoxyribose, double helix, A=T (2 H-bonds), G≡C (3 H-bonds).
• ⚠️ RNA: ribose, single-stranded, uracil replaces thymine.
• ⚠️ Mnemonic: "Proteins: Primary is sequence, Secondary is shape (α/β), Tertiary is 3D fold, Quaternary is multiple chains."