Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of

NCERT Exemplar · Q11

Q. Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be _______.

(i) primary structure of proteins

(ii) secondary structure of proteins

(iii) tertiary structure of proteins

(iv) quaternary structure of proteins

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Concept understanding — Protein Structure Levels

Protein Structure Levels: From a String to a Working Machine

Imagine you have a long string of beads. Each bead is a different colour, and the order of colours is fixed. If you just lay that string on a table, it's a floppy, useless line. But if you could somehow make that string fold itself into a tiny, precise 3D shape — say, a key that fits a specific lock — you'd have something that actually does a job. That's exactly what a protein is.

A protein starts as a long chain of smaller units called amino acids. There are 20 different kinds, each with a unique side chain (the "colour" of the bead). The exact sequence of these amino acids is determined by your DNA. But a protein isn't just a chain — it's a chain that folds into a specific shape, and that shape determines what the protein does. If the shape is wrong, the protein can't work.

The folding happens in stages, and we call these stages the four levels of protein structure.

Level 1: Primary Structure — The Sequence

This is the simplest level: just the linear order of amino acids in the chain, linked by peptide bonds. Think of it as the sentence written in the language of proteins.

Primary structure = the sequence of amino acids from the N-terminus (start) to the C-terminus (end).

Why does this matter? Because the sequence determines everything else. Change one amino acid in a critical spot, and the entire protein can misfold. Example: sickle cell anaemia is caused by a single amino acid swap in haemoglobin — valine replaces glutamic acid at position 6. One bead out of hundreds changes colour, and the whole protein folds wrong.

Level 2: Secondary Structure — Local Folding Patterns

The chain doesn't stay straight. Hydrogen bonds form between the backbone atoms (not the side chains) of nearby amino acids. These bonds cause the chain to twist or fold into regular, repeating patterns.

Two common patterns:

Alpha helix ( $\alpha$ -helix): The chain coils like a spring or a spiral staircase. Hydrogen bonds form between every 4th amino acid, holding the coil tight.
Beta sheet ( $\beta$ -sheet): The chain folds back and forth like a pleated fan. Hydrogen bonds form between adjacent segments, creating a flat, sheet-like structure.

Note

Secondary structure is stabilised entirely by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another — both part of the peptide backbone. Side chains stick out and don't participate.

These patterns are local — they happen in short stretches of the chain. A single protein can have multiple $\alpha$ -helices and $\beta$ -sheets separated by loops.

Level 3: Tertiary Structure — The Global 3D Shape

Now the whole chain folds into its final, compact, three-dimensional shape. This is where the protein becomes functional. The tertiary structure is stabilised by interactions between the side chains of amino acids that may be far apart in the sequence but come close in space.

What holds it together?

Hydrophobic interactions: Nonpolar side chains cluster together in the protein's interior, away from water.
Hydrogen bonds: Between polar side chains.
Ionic bonds: Between positively and negatively charged side chains.
Disulfide bridges: Covalent bonds between the sulfur atoms of two cysteine amino acids — these are strong and lock parts of the chain together.
Van der Waals forces: Weak attractions between closely packed atoms.

Watch out

A common mistake: thinking tertiary structure is just "more secondary structure." It's not. Secondary structure is local folding; tertiary structure is the global arrangement of the entire chain, including how helices and sheets pack together.

Level 4: Quaternary Structure — Multiple Chains Working Together

Some proteins are made of more than one polypeptide chain. Each chain is a separate subunit, and the quaternary structure describes how these subunits assemble into a functional complex. …

Why this formula?

Protein Structure Levels: Understanding the "Why" Behind the Hierarchy

Protein structure is not defined by a single formula, but by a logical hierarchy of organization. Each level builds on the previous one, and the "formulas" here are really principles of molecular interaction that explain why proteins fold the way they do.

Let's break down each level and the reasoning behind its key features.

1. Primary Structure: The Sequence "Formula"

What it is: The linear sequence of amino acids linked by peptide bonds.

Key "formula":

\text{Protein} = \text{NH}_2\text{-[Amino Acid]}_1\text{-[AA]}_2\text{-...-[AA]}_n\text{-COOH}

Why this holds:

Peptide bond formation is a condensation reaction:

\text{-COOH} + \text{NH}_2\text{-} \rightarrow \text{-CO-NH-} + \text{H}_2\text{O}

This bond is rigid and planar due to resonance (partial double-bond character). This restricts rotation, which directly influences higher-order folding.
The sequence is determined by DNA (genetic code). Every change in sequence can alter the entire structure — this is why a single mutation (e.g., sickle cell anemia: Glu → Val at position 6) can cause disease.

Exam insight: The primary structure is the only level that is covalently determined. All higher levels are non-covalent interactions.

2. Secondary Structure: Local Folding Patterns

Key patterns: $\alpha$ -helix and $\beta$ -pleated sheet.

Why these form — the hydrogen bond "formula":

The $\alpha$ -helix

Hydrogen bonds form between the carbonyl oxygen ( $\text{C=O}$ ) of residue $n$ and the amide hydrogen ( $\text{N-H}$ ) of residue $n+4$ .
Why $n+4$ ? This spacing allows the backbone to coil into a right-handed helix with exactly 3.6 amino acids per turn.
Reasoning: The peptide bond's planar nature forces the backbone into a specific geometry. The $n+4$ pattern maximizes H-bonding while minimizing steric clashes.

The $\beta$ -sheet

Hydrogen bonds form between adjacent strands (either parallel or antiparallel).
Why not $n+4$ ? The backbone is extended (pleated), so H-bonds occur between different segments, not within the same chain.

Key formula (Ramachandran plot):

Only certain backbone dihedral angles ( $\phi, \psi$ ) are allowed:

$\alpha$ -helix: $\phi \approx -57^\circ$ , $\psi \approx -47^\circ$
$\beta$ -sheet: $\phi \approx -130^\circ$ , $\psi \approx +130^\circ$

Why these angles? Steric hindrance — atoms cannot overlap. The Ramachandran plot shows the only regions where no two atoms clash.

3. Tertiary Structure: The 3D Fold

Key "formula": The hydrophobic effect drives folding.

Why this holds:

Water molecules form a cage-like structure around nonpolar (hydrophobic) side chains. This is entropically unfavorable (water loses freedom).
To minimize this, hydrophobic side chains cluster together in the protein's core, away from water.
Result: The protein collapses into a compact globule, with polar/charged residues on the surface.

Supporting interactions (the "glue"):

Interaction	Why it matters
Hydrogen bonds	Between side chains (e.g., Ser–Glu)
Ionic bonds	Between charged groups (e.g., Lys–Asp)
Van der Waals forces	Close packing of atoms
Disulfide bridges	Covalent S–S bonds (only in oxidizing environments)

Why not just one formula? Tertiary structure is unique to each protein — it's the sum of all these interactions, not a single equation.

4. Quaternary Structure: Multiple Subunits

Key "formula":

\text{Functional protein} = \sum_{i=1}^{n} \text{Subunit}_i

Why this holds:

Some proteins need multiple polypeptide chains to function (e.g., hemoglobin: $\alpha_2\beta_2$ ). …

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The specific sequence of amino acids in a polypeptide chain defines the primary structure of a protein. The correct option is (i).

The key to this question lies in understanding the hierarchy of protein structure. Proteins are built from four distinct levels of organisation, each adding a layer of complexity. The question asks about the sequence of amino acids — that is the most fundamental, linear description of a protein.

Let’s break down each level so you can see why only one fits.

Primary structure is exactly what the question describes: the linear order of amino acids in a polypeptide chain, held together by covalent peptide bonds. It’s like the letters in a word — change the sequence, and you change the entire protein’s identity and function. This is the only level that is purely about sequence.
Secondary structure refers to local, regular folding patterns within the chain, such as alpha-helices and beta-pleated sheets. These are stabilised by hydrogen bonds between the backbone atoms (not the side chains). The sequence determines where these folds occur, but the folds themselves are not the sequence.
Tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It results from interactions between side chains (hydrophobic forces, ionic bonds, disulfide bridges, etc.). Again, the sequence dictates this shape, but the shape is not the sequence. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

COMEDK 2026Set 2026-A1 markMCQ
$Q. A small segment of a polypeptide gave on complete hydrolysis 3 molecules of alanine, 2 molecules of glycine and 3 molecules of cysteine. What is the number of peptide linkages in the segment of the polypeptide? (A) 7 (B) 8 (C) 6 (D) 5$
›Reveal solutionSolution
The number of peptide bonds in a polypeptide is always one less than the number of amino acid residues. Here, total residues = 3 + 2 + 3 = 8, so peptide bonds = 8 − 1 = 7. The correct option is (A).

Concept & Intuition

A polypeptide is a chain of amino acids linked by peptide bonds. Each peptide bond forms between the carboxyl group of one amino acid and the amino group of the next. If you have $n$ amino acids in a chain, you need exactly $n-1$ peptide bonds to connect them — like linking $n$ beads on a string requires $n-1$ knots. Hydrolysis breaks all these bonds, releasing the individual amino acids. So counting the total number of amino acids released tells you the original chain length, and subtracting one gives the number of peptide bonds.

Step-by-step reasoning

Identify the total number of amino acid residues.

Complete hydrolysis of the polypeptide yields:

3 molecules of alanine

2 molecules of glycine

3 molecules of cysteine Total = $3 + 2 + 3 = 8$ amino acid molecules.

Relate residues to peptide bonds. …
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COMEDK 2026Set 2026-M1 markMCQ
$\alpha$
›Reveal solutionSolution
The secondary structure of a protein refers to the local, regular folding patterns of the polypeptide backbone, specifically the α-helix and β-pleated sheet, not the sequence, overall shape, or subunit assembly. The correct option is (C).

The key here is to recall the hierarchy of protein structure: primary, secondary, tertiary, and quaternary. Each level describes a different aspect of the protein’s architecture. The secondary structure is the local spatial arrangement of the polypeptide backbone, stabilized mainly by hydrogen bonds between the carbonyl oxygen and amide hydrogen of amino acids that are close in sequence. It does not involve the side chains (R-groups) or the overall 3D shape of the entire chain.

Let’s examine each option:

Option (A) describes “a long chain of amino acids linked in a specific sequence.” That is the primary structure — the linear order of amino acids held together by peptide bonds. This is not secondary structure.

Option (B) says “a folding of the polypeptide chains which exists as fibrous and globular.” This refers to the tertiary structure (the overall 3D shape of a single polypeptide chain) or even the classification of proteins based on shape. Fibrous and globular are categories of tertiary/quaternary structure, not secondary. …
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COMEDK 2025Set 2025-E1 markMCQ
$Q. Two statements, one Assertion and the other Reason are given. Choose the right option. Assertion : Insulin is called a protein whereas Glycyl alanine is not called a protein Reason : A polypeptide with amino acid residue less than 100 can also be called as a protein if it has a well-defined conformation of a protein. (A) Assertion is correct but Reason is incorrect (B) Both Assertion and Reason are incorrect (C) Both Assertion and Reason are correct (D) Assertion is incorrect but Reason is correct$
›Reveal solutionSolution

Both statements are correct: insulin (51 residues, well-defined conformation) qualifies as a protein even though it has fewer than 100 residues, whereas glycyl-alanine (a dipeptide) does not.

Assertion: Insulin is a protein; glycyl-alanine (a dipeptide) is not. This is true — insulin, though only 51 amino-acid residues, has a definite three-dimensional structure and biological function, so it is classed as a protein, while a simple dipeptide is not. …
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COMEDK 2025Set 2025-M1 markMCQ
$A$
›Reveal solutionSolution
The question asks for the incorrect statement. Glucocorticoids regulate metabolism and inflammation, not water/salt excretion (that’s mineralocorticoids like aldosterone). So option (A) is false; the others are true.

Concept & Intuition

This is a biology fact-checking problem. You need to recall specific functions of hormones, properties of vitamins, protein denaturation, and RNA hydrolysis. The trick is to spot the mismatch between a hormone’s actual role and the statement. Glucocorticoids (e.g., cortisol) are often confused with mineralocorticoids (e.g., aldosterone) because both come from the adrenal cortex. The key distinction: glucocorticoids handle stress, metabolism, and immune suppression; mineralocorticoids handle electrolyte and water balance.

Step-by-step reasoning

Evaluate statement (A):

Glucocorticoids (like cortisol) primarily regulate glucose metabolism, suppress inflammation, and help the body respond to stress. They do not control water and salt excretion by the kidney. That job belongs to mineralocorticoids (e.g., aldosterone), which promote sodium retention and potassium excretion. Therefore, statement (A) is incorrect.

Evaluate statement (B):

Vitamins A, D, E, and K are fat-soluble. Vitamin A is stored in the liver (as retinol esters), and vitamin K is also stored in the liver (though in smaller amounts). This statement is correct.

Evaluate statement (C): …
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COMEDK 2024Set 2024-A1 markMCQ
$Q. Among the following peptides, identify the pair where the name and its structure are correctly matched. (A) (B) (C) (D)$
›Reveal solutionSolution
The key is to match the side‑chain sequence in the condensed structure with the standard three‑letter amino‑acid abbreviations (N‑terminal to C‑terminal). Only option (C) correctly names the tripeptide Ala‑Gly‑Phe.

Concept & Intuition

Peptide names are written from the N‑terminal (free amino end) to the C‑terminal (free carboxyl end). Each three‑letter code corresponds to a specific side chain:

Gly (glycine): side chain = H (i.e., the α‑carbon is –CH₂–).

Ala (alanine): side chain = –CH₃.

Phe (phenylalanine): side chain = –CH₂C₆H₅ (benzyl).

To check correctness, read the structure from left (N‑terminus) to right (C‑terminus) and list the side chains in order. Then compare that sequence to the name.

Step‑by‑step reasoning

Option (A):

Structure: H₂N–CH₂–C(=O)–NH–CH(–CH₂SH)–COOH

First residue: α‑carbon has no side chain (just –CH₂–) → Gly.

Second residue: α‑carbon has –CH₂SH (cysteine side chain) → Cys, not Ala. Name given: “Gly‑Ala” → mismatch (should be Gly‑Cys). ✗ Incorrect.

Option (B):

Structure: H₂N–CH₂–C(=O)–NH–CH(–CH₃)–COOH

First residue: –CH₂– → Gly.

Second residue: –CH₃ → Ala. Name given: “Ala‑Gly” → order reversed (N‑terminal is Gly, not Ala). ✗ Incorrect.

Option (C):

Structure: H₂N–CH(–CH₃)–C(=O)–NH–CH₂–C(=O)–NH–CH(–CH₂C₆H₅)–COOH

First residue: –CH₃ → Ala.

Second residue: –CH₂– (no side chain) → Gly.

Third residue: –CH₂C₆H₅ → Phe. Name given: “Ala‑Gly‑Phe” → matches exactly. ✓ Correct.

Option (D): …
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COMEDK 2024Set 2024-M1 markMCQ
$Q. Given below are 4 statements about Insulin. Which of these statement/(s) is/are correct? [A] Insulin is a globular protein consisting of 51 Amino acids. [B] Insulin is constituted of 3 polypeptide chains linked together. [C] Insulin is constituted of 2 polypeptide chains linked together by disulphide bonds. [D] The 3 polypeptide chains in Insulin are linked together by Hydrogen bonding. (A) [D] (B) [A] & [C] (C) [B] & [D] (D) [B]$
›Reveal solutionSolution
Insulin is a globular protein with 51 amino acids arranged in two polypeptide chains (A and B) linked by disulphide bonds, so only statements [A] and [C] are correct.

Concept and Intuition

Insulin is a classic example of a small, well-studied protein. Its structure is fundamental in biochemistry: it is synthesized as a single chain (proinsulin) that is later cleaved to remove a connecting peptide (C-peptide), leaving two chains — the A chain (21 amino acids) and the B chain (30 amino acids). These two chains are held together by disulphide bonds (covalent bonds between cysteine residues), not by hydrogen bonds between separate chains. Hydrogen bonds are important for the protein’s three-dimensional folding, but they do not link the two chains together. Knowing this, we can evaluate each statement.

Step-by-step reasoning

Statement [A]: “Insulin is a globular protein consisting of 51 Amino acids.”

Insulin is indeed a globular protein (soluble, roughly spherical shape). The total number of amino acids in the active form is 21 (A chain) + 30 (B chain) = 51.

This statement is correct.

Statement [B]: “Insulin is constituted of 3 polypeptide chains linked together.”

Mature insulin has only two chains (A and B). The third chain (C-peptide) is present only in the precursor proinsulin and is removed during processing.

This statement is incorrect.

Statement [C]: “Insulin is constituted of 2 polypeptide chains linked together by disulphide bonds.”

Exactly right: the A and B chains are connected by two interchain disulphide bonds (between A7–B7 and A20–B19). There is also one intrachain disulphide bond within the A chain (A6–A11).

This statement is correct.

Statement [D]: “The 3 polypeptide chains in Insulin are linked together by Hydrogen bonding.” …
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KCET 2023Set B-41 markMCQ
Q.Match List-I and List-II with respect to proteins and their functions and select the correct option. List-I | List-II
1. Collagen | p. Fights infectious agents
2. Trypsin | q. Hormone
3. Insulin | r. Enzyme
4. Antibody | s. Intercellular ground substance (A) 1-s, 2-p, 3-r, 4-p (B) 1-q, 2-r, 3-q, 4-s (C) 1-s, 2-q, 3-r, 4-p (D) 1-s, 2-r, 3-q, 4-p
›Reveal solutionSolution
Assign each protein its biological role: collagen → ground substance, trypsin → enzyme, insulin → hormone, antibody → defence.

Concept — Proteins and their functions (Biomolecules). NCERT gives a table of proteins with their functions; each protein here has exactly one role.

Collagen — the most abundant protein in the animal world; it is the fibrous protein of the intercellular ground substance of connective tissue. → s

Trypsin — a proteolytic enzyme secreted (as trypsinogen) by the pancreas; it hydrolyses proteins in the small intestine. → r

Insulin — a peptide hormone secreted by the β-cells of the islets of Langerhans; it lowers blood glucose. → q

Antibody — an immunoglobulin that fights infectious agents (antigens/pathogens). → p …
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KCET 2019Set A-11 markMCQ
$Q. Which of the following is generally water insoluble? (A) Fibrous protein (B) Amylose (C) Vitamin-C (D) Glycine$
›Reveal solutionSolution
The key is solubility in water: fibrous proteins are structural and hydrophobic, while amylose, vitamin-C, and glycine are all water-soluble. The insoluble one is fibrous protein.

Concept & Intuition

Solubility in water depends on how well a molecule can interact with water molecules through hydrogen bonding or ionic interactions. Water is polar, so "like dissolves like" — polar and charged substances tend to dissolve, while nonpolar, hydrophobic ones do not.

Let's look at each option:

Fibrous protein — These are structural proteins (like collagen, keratin, elastin) that form long, sheet-like or rope-like fibers. Their amino acid sequences are rich in hydrophobic residues, and they are tightly packed with extensive cross-linking. They are designed to be insoluble in water, providing mechanical strength to tissues. They do not form colloidal solutions in water; they remain as solid fibers.

Amylose — This is a linear polymer of glucose (a polysaccharide), a component of starch. Each glucose unit has many –OH groups, which form hydrogen bonds with water. Amylose is actually water-soluble (it forms a colloidal solution in hot water, and even in cold water it disperses to some extent). So it is not generally insoluble.

Vitamin-C (ascorbic acid) — This is a small, polar molecule with multiple –OH groups and a lactone ring. It is highly water-soluble. In fact, it is one of the most water-soluble vitamins. Definitely not insoluble.

Glycine — The simplest amino acid. It exists as a zwitterion in water ( $\ce{^+H3N-CH2-COO-}$ ), making it highly polar and very soluble in water. It dissolves readily.

Watch out

A common mistake is to think that all proteins are water-soluble. Many globular proteins (like enzymes, hemoglobin) are soluble, but fibrous proteins are specifically designed to be insoluble — that's their biological function. …
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KCET 2018Set A-11 markMCQ
$Q. The following graph shows concentration of substrate on enzyme activity: (see the graph described below) What does the Y-axis represent? (A) Temperature (B) Velocity of reaction (C) pH (D) Pressure$
›Reveal solutionSolution
The plateau is labelled $V_{max}$ , and $V$ stands for velocity — so the Y-axis is the velocity (rate) of the reaction.

Step 1 — Recognise the curve. A plot of substrate concentration against enzyme activity that saturates at a maximum is the Michaelis–Menten hyperbola:
$v = \frac{V_{max}\,[S]}{K_m + [S]}.$
Step 2 — Read the axes.

X-axis = substrate concentration $[S]$ (stated in the stem).

Y-axis = the velocity (rate) of the reaction $v$ ; it rises steeply at low $[S]$ and asymptotes to the horizontal line marked $V_{max}$ , the maximum velocity reached when every active site is occupied. …
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KCET 2018Set A-11 markMCQ
$\mathrm{CH_4, NH_3, H_2O, H_2}$
›Reveal solutionSolution
Miller's spark-discharge experiment used a reducing atmosphere: $\mathrm{CH_4}$ , $\mathrm{NH_3}$ , $\mathrm{H_2}$ and water vapour.

Step 1 — The hypothesis being tested.

Oparin and Haldane proposed that the first organic molecules formed abiotically from simple inorganic molecules in a reducing (oxygen-free, hydrogen-rich) primitive atmosphere, with energy supplied by lightning and heat. Miller set out to test exactly this.

Step 2 — The apparatus and the gases.

In 1953 S. L. Miller built a closed flask containing:

$\mathrm{CH_4}$ (methane) — the carbon source

$\mathrm{NH_3}$ (ammonia) — the nitrogen source

$\mathrm{H_2}$ (hydrogen) — makes the mixture strongly reducing

water vapour ( $\mathrm{H_2O}$ ) — from a flask of boiling water

Electric discharge (~800 °C, simulating lightning) was passed through the mixture.

Step 3 — Why the other options fail. …
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Q.Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be _______.

Concept understanding — Protein Structure Levels

Protein Structure Levels: From a String to a Working Machine

Level 1: Primary Structure — The Sequence

Level 2: Secondary Structure — Local Folding Patterns

Level 3: Tertiary Structure — The Global 3D Shape

Level 4: Quaternary Structure — Multiple Chains Working Together

Why this formula?

Protein Structure Levels: Understanding the "Why" Behind the Hierarchy

1. Primary Structure: The Sequence "Formula"

2. Secondary Structure: Local Folding Patterns

The $\alpha$ -helix

The $\beta$ -sheet

3. Tertiary Structure: The 3D Fold

4. Quaternary Structure: Multiple Subunits

Concept: Levels of Protein Structure

Method: Definition Matching

The Correct Answer

Common Mistake #1: Confusing "Sequence" with "Folding"

Common Mistake #2: Mixing Up "Polypeptide" vs. "Protein"

Common Mistake #3: Forgetting That "Sequence" is the Key Word

Q.Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be _______.

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