In fibrous proteins, polypeptide chains are held together by ____. (Note: one or more of the following options

NCERT Exemplar · Q60

Q. In fibrous proteins, polypeptide chains are held together by _______. (Note: one or more of the following options may be correct.)

(i) van der Waals forces

(ii) disulphide linkage

(iii) electrostatic forces of attraction

(iv) hydrogen bonds

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Concept understanding — Biochemical Bonds

Biochemical Bonds: The Glue That Holds Life Together

Imagine you're building with LEGO bricks. Some bricks click together tightly and never come apart unless you really yank them. Others snap together lightly and can be pulled apart with a gentle tug. Some bricks don't even click — they just stick because of static cling or magnetism.

Biochemical bonds are exactly like that. They are the forces that hold atoms together inside the molecules of your body — your DNA, proteins, fats, and carbohydrates. Without these bonds, you'd literally fall apart into a pile of individual atoms.

The Core Idea

Atoms bond because being bonded is more stable (lower energy) than being alone. Think of it like this: a single atom is like a person standing alone in a cold room. Bonding is like huddling together for warmth — you lose some freedom of movement, but you gain stability.

In biochemistry, we care about four main types of bonds. They differ in strength, how they form, and what they do in living systems.

1. Covalent Bonds — The Strong, Permanent LEGO Clicks

This is the strongest bond in biology. Two atoms share electrons — like two people holding the same umbrella. Each atom contributes one or more electrons, and they both "own" the pair.

Key properties:

Very strong (100–400 kJ/mol)
Forms the backbone of all biomolecules
Takes a lot of energy (or enzymes) to break

Where you find it:

The carbon-carbon bonds in your DNA's sugar-phosphate backbone
The peptide bonds linking amino acids into proteins
The bonds within a glucose molecule

Note

A single covalent bond shares 2 electrons. A double bond shares 4. Triple bonds are rare in biology but exist (e.g., in cyanide).

2. Ionic Bonds — The Static Cling of Opposites

Some atoms steal electrons from others. When that happens, one atom becomes positively charged (lost an electron) and the other becomes negatively charged (gained one). Opposite charges attract — that's an ionic bond.

Key properties:

Moderate strength (5–100 kJ/mol in dry conditions)
Very weak in water (because water molecules get in between)
Easily broken by changes in pH or salt concentration

Where you find it:

In salt bridges that help proteins fold into their correct shape
Between the phosphate groups of DNA and positively charged proteins (histones)

Watch out

Ionic bonds are often called "bonds" but in water they behave more like attractions. Don't confuse them with covalent bonds — they're much weaker in biological fluids.

3. Hydrogen Bonds — The Gentle, Reversible Magnets

This is the most important weak bond in biology. A hydrogen atom that's already covalently bonded to an electronegative atom (like oxygen or nitrogen) gets a slight positive charge. It then gets attracted to another electronegative atom nearby.

Think of it like a weak magnet — it holds things together but can be easily undone.

Key properties:

Weak individually (5–30 kJ/mol)
But many together can be very strong
Easily broken by heat or changes in pH
Directional — they only work when atoms are properly aligned

Where you find it:

Between the two strands of DNA (this is what holds the double helix together)
In protein folding (between amino acids in the backbone)
Between water molecules (giving water its unique properties)

Important

Hydrogen bonds are the reason DNA can unzip for replication. If DNA used covalent bonds between strands, it would be impossible to separate without destroying the molecule.

4. Van der Waals Interactions — The Fleeting, Accidental Touches

Even neutral atoms have temporary, uneven distributions of electrons. These create tiny, momentary charges that attract nearby atoms. It's like two people accidentally brushing shoulders in a crowd — brief, weak, but real.

Key properties:

Extremely weak (0.5–5 kJ/mol per interaction)
Only work when atoms are very close (within 0.3–0.4 nm)
Add up significantly when many atoms are packed together

Where you find it:

In the hydrophobic core of proteins (where oily amino acids pack tightly)
Between lipid tails in cell membranes
In enzyme-substrate binding (helps "grip" the substrate)

Putting It All Together: A Biological Example

Consider a protein in your body. It's a long chain of amino acids held together by covalent peptide bonds. That chain then folds into a specific shape. The folding is guided by:

Hydrogen bonds between backbone atoms (forming alpha helices and beta sheets)
Ionic bonds between charged side chains …

Why this formula?

Biochemical Bonds: Why the Key Formulas Hold

Biochemical bonds are the forces that hold atoms together in biomolecules. The key formulas come from electrostatics and quantum mechanics — not from biology itself. Let's break down the why behind the most important ones.

1. Ionic Bond Energy: Coulomb's Law

Formula:

E = \frac{k \cdot q_1 \cdot q_2}{r}

Why it holds:

Opposite charges attract — this is a fundamental law of physics (Coulomb's law).
In a biochemical context, consider a sodium ion ( $\text{Na}^+$ ) and a chloride ion ( $\text{Cl}^-$ ). The energy released when they come together is directly proportional to the product of their charges ( $q_1 q_2$ ) and inversely proportional to the distance ( $r$ ) between them.
The constant $k$ accounts for the medium (water vs. vacuum). In water, the effective force is weaker because water molecules partially shield the charges — this is why ionic bonds in biology are often weaker in aqueous environments.

Key insight: The formula is not arbitrary — it's derived from the inverse-square law of electrostatics, integrated over the distance the charges move toward each other.

2. Covalent Bond Energy: The Morse Potential (Approximation)

Formula (simplified):

E = D_e \left(1 - e^{-a(r - r_0)}\right)^2

Why it holds:

Covalent bonds arise from shared electrons between atoms. The energy is not a simple inverse-square law because electrons are delocalized.
The Morse potential is an empirical formula that captures two key observations:
1. At equilibrium distance ( $r_0$ ): Energy is minimum ( $E = 0$ in this form).
2. If atoms are pulled apart ( $r \to \infty$ ): Energy approaches $D_e$ (the bond dissociation energy).
3. If atoms are pushed too close ( $r \to 0$ ): Energy skyrockets due to Pauli repulsion (electrons can't occupy the same space).
The exponential term $e^{-a(r - r_0)}$ models the rapid drop in attractive force as distance increases — this comes from quantum mechanical overlap of electron clouds.

Key insight: The formula is a curve fit to quantum mechanical calculations, not a first-principles derivation. But it works because it respects the physics: attraction at long range, repulsion at short range, and a stable minimum.

3. Hydrogen Bond Energy: Dipole-Dipole Interaction

Formula (approximate):

E \approx -\frac{2 \mu_1 \mu_2}{4 \pi \epsilon_0 r^3} \cdot \cos\theta

Why it holds:

A hydrogen bond (e.g., between water molecules) is not a true bond — it's a strong dipole-dipole interaction.
The dipole moment ( $\mu$ ) arises because oxygen is more electronegative than hydrogen, creating partial charges ( $\delta^+$ and $\delta^-$ ).
The energy depends on:
- Strength of dipoles ( $\mu_1 \mu_2$ )
- Distance ( $r$ ) — falls off as $1/r^3$ , much faster than ionic bonds ( $1/r$ )
- Orientation ( $\cos\theta$ ) — strongest when dipoles are aligned head-to-tail

Key insight: The $1/r^3$ dependence comes from the derivative of the dipole field. Unlike point charges, dipoles have a field that decays faster — this is why hydrogen bonds are directional and weaker than covalent bonds.

4. Van der Waals Interaction: Lennard-Jones Potential

Formula:

E = 4\epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right]

Why it holds:

Van der Waals forces arise from temporary fluctuations in electron distribution — even nonpolar molecules have instantaneous dipoles. …

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Fibrous proteins are held together mainly by hydrogen bonds between backbone amide groups and disulphide linkages between cysteine residues — so the correct options are (ii) and (iv).

The question asks what holds polypeptide chains together in fibrous proteins. Fibrous proteins — like collagen, keratin, and myosin — are structural proteins with long, rod-like shapes. Their strength and stability come from two dominant interactions.

Hydrogen bonds are the backbone of secondary structure. In α-keratin (a fibrous protein), the α-helices are stabilized by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen of another, four residues away. In collagen, hydrogen bonds form between the three polypeptide strands of the triple helix. Without hydrogen bonds, the chains would simply drift apart. So (iv) is correct.
Disulphide linkages are covalent bonds between the sulfur atoms of two cysteine residues. These are especially important in keratin — the protein in hair, nails, and skin. The more disulphide cross-links, the harder the material (think horn vs. soft fur). So (ii) is correct.
Van der Waals forces (i) are weak, non-specific attractions that occur between all atoms when they are close together. They exist in every protein, but they are far too weak and non-directional to be the defining force holding fibrous-protein chains together, so this is not the answer the question is looking for. …

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Here are the common mistakes students make on this question about Biochemical Bonds in Fibrous Proteins, along with how to avoid each.

Mistake 1: Selecting only one option (e.g., only hydrogen bonds)

Why it happens:

Students often memorize that fibrous proteins (like collagen, keratin, or silk fibroin) are stabilized by hydrogen bonds between polypeptide chains. They stop there, forgetting that fibrous proteins are multiply stabilized.

How to avoid:

Always recall the structural hierarchy of fibrous proteins. They are not just held by one type of bond. In a typical fibrous protein:

Hydrogen bonds (between N–H and C=O groups) give strength and elasticity.
Disulphide linkages (covalent –S–S– bonds between cysteine residues) provide permanent cross-linking (e.g., in keratin).
Electrostatic forces (ionic bonds between charged side chains) also contribute.
van der Waals forces (hydrophobic interactions) are present but are weak and not the primary stabilizers.

Key takeaway: The question says “one or more options may be correct.” Always check all options for relevance.

Mistake 2: Including van der Waals forces as a primary stabilizer

Why it happens:

Students think “all non-covalent interactions matter equally.” In fibrous proteins, van der Waals forces are present but are not the main forces holding polypeptide chains together — they are too weak and non-directional.

How to avoid:

Remember the relative strength of interactions in protein structure:

Strongest: Covalent bonds (disulphide linkages)
Moderate: Hydrogen bonds, electrostatic forces
Weakest: van der Waals forces

For fibrous proteins, the primary stabilizers are hydrogen bonds, disulphide bonds, and electrostatic attractions. van der Waals forces are secondary and not exam-worthy as a “holding” force here.

Mistake 3: Confusing fibrous proteins with globular proteins

Why it happens:

Students mix up the bonding patterns. In globular proteins, hydrophobic interactions (van der Waals) are crucial for folding. In fibrous proteins, the chains are extended and parallel — so inter-chain hydrogen bonds dominate.

How to avoid:

Use a simple mnemonic:

Fibrous → F for Filamentous, Forces = Hydrogen, Disulphide, Electrostatic.
Globular → G for Globular, Gets Van der Waals (hydrophobic core).

Mistake 4: Forgetting that disulphide linkages are covalent and permanent

Why it happens: …

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Q.In fibrous proteins, polypeptide chains are held together by _______. (Note: one or more of the following options may be correct.)

Concept understanding — Biochemical Bonds

Biochemical Bonds: The Glue That Holds Life Together

The Core Idea

1. Covalent Bonds — The Strong, Permanent LEGO Clicks

2. Ionic Bonds — The Static Cling of Opposites

3. Hydrogen Bonds — The Gentle, Reversible Magnets

4. Van der Waals Interactions — The Fleeting, Accidental Touches

Putting It All Together: A Biological Example

Why this formula?

Biochemical Bonds: Why the Key Formulas Hold

1. Ionic Bond Energy: Coulomb's Law

2. Covalent Bond Energy: The Morse Potential (Approximation)

3. Hydrogen Bond Energy: Dipole-Dipole Interaction

4. Van der Waals Interaction: Lennard-Jones Potential

Method: Conceptual Elimination by Bond Type & Protein Structure

Step 1 – Recall the defining feature of fibrous proteins

Step 2 – List the bonds that stabilize any protein structure

Step 3 – Identify which bonds are especially important in fibrous proteins

Mistake 1: Selecting only one option (e.g., only hydrogen bonds)

Mistake 2: Including van der Waals forces as a primary stabilizer

Mistake 3: Confusing fibrous proteins with globular proteins

Mistake 4: Forgetting that disulphide linkages are covalent and permanent

Q.In fibrous proteins, polypeptide chains are held together by _______. (Note: one or more of the following options may be correct.)

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