Biomolecules — NCERT Solutions

The following reactions/observations of D-glucose cannot be explained by its open-chain (aldehyde) structure:

1. Existence of two anomers (α and β forms):
D-Glucose exists in two crystalline forms — α-D-glucose (m.p. 419 K) and β-D-glucose (m.p. 423 K). An open-chain aldehyde can give only one form. The existence of two forms is explained only by the cyclic hemiacetal structure with an anomeric carbon (C-1).

2. Mutarotation:
When either α-D-glucose or β-D-glucose is dissolved in water, the optical rotation changes and reaches an equilibrium value of +52.7°. This interconversion (mutarotation) cannot be explained by a fixed open-chain structure; it is explained by the ring-opening and ring-closing equilibrium between α and β cyclic forms.

3. Reaction with acetic anhydride to give pentaacetate:
D-Glucose reacts with acetic anhydride to give a pentaacetate (5 acetyl groups). The open-chain structure has only 4 –OH groups and 1 –CHO group. Acetic anhydride does not acetylate –CHO. The pentaacetate is explained by the cyclic form which has 5 –OH groups (including the anomeric –OH at C-1) and no free –CHO.

4. Failure to give Schiff's test and no reaction with NaHSO₃:
D-Glucose does not give Schiff's test (with fuchsin-aldehyde reagent) and does not form a crystalline addition product with NaHSO₃, as expected of a free aldehyde. This is because in the cyclic form, the –CHO is converted to a hemiacetal and is not freely available.

5. Existence of two methyl glucosides:
When D-glucose reacts with methanol in the presence of dry HCl, it forms two methyl glucosides (α-methyl glucoside and β-methyl glucoside). A simple aldehyde would give only one acetal. The two glucosides correspond to the two anomeric forms of cyclic glucose.

Essential Amino Acids:
Amino acids that cannot be synthesised by the human body and must be supplied through diet are called essential amino acids. There are 10 essential amino acids.

Examples:

1. Valine
2. Leucine

(Others: Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine, Arginine)

Non-Essential Amino Acids:
Amino acids that can be synthesised by the human body itself and do not need to be supplied through diet are called non-essential amino acids.

Examples:

1. Glycine
2. Alanine

(Others: Serine, Aspartic acid, Glutamic acid, etc.)

Summary:

(i) Peptide Linkage:
A peptide linkage (peptide bond) is the amide bond formed between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of another amino acid with the elimination of a water molecule.
$$\text{–NH}_2 + \text{HOOC–} \rightarrow \text{–NH–CO–} + \text{H}_2\text{O}$$
The –CO–NH– group is called the peptide linkage. It is the bond that links amino acid residues in a protein chain.

(ii) Primary Structure:
The primary structure of a protein refers to the specific sequence of amino acids in the polypeptide chain. It describes the number, type, and order (sequence) of amino acid residues linked by peptide bonds from the N-terminal (free –NH₂ end) to the C-terminal (free –COOH end). The primary structure is unique for each protein and determines its higher-order structure and function.

(iii) Denaturation:
Denaturation is the process by which a protein loses its native three-dimensional structure (secondary, tertiary, and quaternary structures) without breaking the peptide bonds (primary structure remains intact). It is caused by changes in pH, temperature, addition of organic solvents, heavy metal salts, etc.

During denaturation:

• Hydrogen bonds, disulfide bonds, and other non-covalent interactions maintaining the 3D structure are disrupted.
• The protein unfolds and loses its biological activity.
• Example: Coagulation of egg white on boiling; curdling of milk.

The secondary structure of a protein refers to the regular, repeating local folding patterns of the polypeptide backbone, stabilised mainly by hydrogen bonds between the –C=O and –N–H groups of the peptide backbone.

The two most common types of secondary structures are:

1. α-Helix:

• The polypeptide chain coils into a right-handed helical structure.
• It is stabilised by intramolecular hydrogen bonds formed between the –C=O group of one amino acid residue and the –N–H group of the amino acid four residues ahead in the chain.
• There are 3.6 amino acid residues per turn of the helix.
• Example: α-keratin (hair, wool, nails).

2. β-Pleated Sheet (β-sheet):

• The polypeptide chains are extended and lie side by side (parallel or antiparallel).
• It is stabilised by intermolecular hydrogen bonds between –C=O and –N–H groups of adjacent polypeptide chains (or different segments of the same chain).
• The structure resembles a pleated (folded) sheet.
• Example: Silk fibroin (β-keratin).

The α-helix structure of proteins is stabilised by intramolecular hydrogen bonds.

Specifically:

• Hydrogen bonds are formed between the carbonyl oxygen (–C=O) of one amino acid residue and the imino hydrogen (–N–H) of the amino acid residue that is four residues ahead in the same polypeptide chain.

$$\text{–C=O} \cdots \text{H–N–}$$

• These hydrogen bonds run roughly parallel to the helix axis.
• The regular repetition of these hydrogen bonds along the entire length of the helix stabilises the coiled structure.

Note: The side chains (R groups) of amino acids project outward from the helix and do not participate in the hydrogen bonding that stabilises the helix backbone.

Amino acids contain two functional groups:

• An acidic group: –COOH (carboxyl group)
• A basic group: –NH₂ (amino group)

Because of the presence of both an acidic and a basic group in the same molecule, amino acids can act as both acids and bases — i.e., they are amphoteric.

As a base (accepting a proton): The –NH₂ group accepts H⁺ from an acid:
$$\text{H}_2\text{N–CHR–COOH} + \text{H}^+ \rightarrow \text{H}_3\text{N}^+\text{–CHR–COOH}$$

As an acid (donating a proton): The –COOH group donates H⁺ to a base:
$$\text{H}_2\text{N–CHR–COOH} + \text{OH}^- \rightarrow \text{H}_2\text{N–CHR–COO}^- + \text{H}_2\text{O}$$

Zwitter ion formation: In aqueous solution, the –COOH group donates a proton to the –NH₂ group within the same molecule, forming a dipolar ion (zwitter ion):
$$\text{H}_2\text{N–CHR–COOH} \rightleftharpoons \text{H}_3\text{N}^+\text{–CHR–COO}^-$$

This internal salt (zwitter ion) can donate a proton (act as acid) or accept a proton (act as base), confirming the amphoteric nature of amino acids.

Definition: Enzymes are biological catalysts (biocatalysts) that speed up the rate of biochemical reactions in living organisms without themselves being consumed in the reaction.

Key features:

• Almost all enzymes are globular proteins in nature (a few RNA molecules called ribozymes also have catalytic activity).
• They are highly specific — each enzyme catalyses only one particular reaction or acts on one particular type of substrate (lock and key specificity).
• They are required only in small quantities.
• They function by lowering the activation energy of the reaction.
• They are sensitive to temperature and pH — they work best at an optimum temperature and pH.
• They are generally named after the substrate or the reaction they catalyse, with the suffix -ase (e.g., maltase, sucrase, urease).

Example: Maltase catalyses the hydrolysis of maltose to glucose:
$$\text{Maltose} \xrightarrow{\text{Maltase}} 2\,\text{Glucose}$$

Effect of denaturation on protein structure:

When a protein is denatured (by heat, change in pH, organic solvents, heavy metal salts, etc.):

1. Secondary structure is destroyed: The hydrogen bonds stabilising the α-helix or β-pleated sheet are broken. The regular folding pattern is lost.

2. Tertiary structure is destroyed: The non-covalent interactions (hydrogen bonds, hydrophobic interactions, van der Waals forces) and disulfide bonds (in some cases) that maintain the three-dimensional folded shape are disrupted. The protein unfolds.

3. Quaternary structure is destroyed (if present): The association between multiple polypeptide subunits is disrupted.

4. Primary structure remains intact: The peptide bonds (covalent bonds) linking amino acids in the polypeptide chain are NOT broken during denaturation. The sequence of amino acids is unchanged.

5. Loss of biological activity: Since the specific 3D shape (active site) is lost, the protein can no longer perform its biological function.

Example: Boiling of egg white causes coagulation — the albumin protein denatures and precipitates. Curdling of milk is another example.

Classification of Vitamins:

Vitamins are classified into two groups based on their solubility:

1. Fat-soluble vitamins:

• Soluble in fats and oils; insoluble in water.
• Can be stored in the body (liver and adipose tissue).
• Vitamins: A, D, E, and K

2. Water-soluble vitamins:

• Soluble in water; cannot be stored in the body (excess is excreted in urine).
• Must be supplied regularly through diet.
• Vitamins: B group (B₁, B₂, B₆, B₁₂, etc.) and C

Vitamin responsible for coagulation (clotting) of blood:
Vitamin K is responsible for the coagulation (clotting) of blood. It is essential for the synthesis of prothrombin and other clotting factors in the liver. Deficiency of Vitamin K leads to increased bleeding time.

Vitamin A:

Why essential:

• Vitamin A (retinol) is essential for normal vision, especially in dim light (night vision). It is a component of the visual pigment rhodopsin.
• It is necessary for the growth and maintenance of epithelial tissues (skin, mucous membranes).
• It is important for normal bone and tooth development.
• Deficiency causes: Night blindness (nyctalopia), xerophthalmia (dryness of eyes), and increased susceptibility to infections.

Important sources: Fish liver oil, liver, butter, milk, eggs, carrots, green leafy vegetables (as β-carotene, a precursor).

Vitamin C:

Why essential:

• Vitamin C (ascorbic acid) is essential for the synthesis of collagen (a structural protein in connective tissue, bones, and blood vessels).
• It acts as an antioxidant, protecting cells from oxidative damage.
• It is necessary for the proper functioning of the immune system.
• It enhances the absorption of iron from food.
• Deficiency causes: Scurvy — characterised by bleeding gums, swollen joints, slow wound healing, and anaemia.

Important sources: Citrus fruits (lemon, orange, amla/Indian gooseberry), tomatoes, green leafy vegetables, guava, capsicum.

Nucleic Acids:
Nucleic acids are high molecular weight, naturally occurring biopolymers (polynucleotides) found in the nuclei of all living cells. They are polymers of nucleotides linked by 3',5'-phosphodiester bonds.

There are two types:

1. DNA (Deoxyribonucleic acid) — contains 2-deoxyribose sugar
2. RNA (Ribonucleic acid) — contains ribose sugar

Two Important Functions of Nucleic Acids:

1. Storage and transmission of genetic information (Heredity):

DNA is the chemical basis of heredity. It carries the genetic information (coded in the sequence of bases) that is passed from parents to offspring during reproduction. The sequence of bases in DNA encodes the information for the synthesis of all proteins in the cell.

2. Protein synthesis:

RNA molecules (mRNA, tRNA, rRNA) are directly involved in the synthesis of proteins (translation). mRNA carries the genetic message from DNA to the ribosomes; tRNA brings the appropriate amino acids; rRNA forms part of the ribosome structure. Thus, nucleic acids control the synthesis of proteins that determine the structure and function of the organism.

In short:
$$\text{Nucleoside} = \text{Base} + \text{Sugar}$$
$$\text{Nucleotide} = \text{Base} + \text{Sugar} + \text{Phosphate}$$
$$\text{Nucleotide} = \text{Nucleoside} + \text{Phosphate}$$

Given: DNA is a double-stranded molecule.

Explanation:

In the double helix structure of DNA (Watson-Crick model):

• The two strands run antiparallel to each other (one strand runs 5'→3' and the other runs 3'→5').
• The two strands are held together by hydrogen bonds between specific pairs of nitrogenous bases — this is called complementary base pairing:
• Adenine (A) always pairs with Thymine (T) — connected by 2 hydrogen bonds
• Guanine (G) always pairs with Cytosine (C) — connected by 3 hydrogen bonds

This means:

• If one strand has the sequence 5'-A-T-G-C-3', the other strand must have 3'-T-A-C-G-5'.
• The two strands are NOT identical (they have different base sequences).
• But they are COMPLEMENTARY — the base sequence of one strand determines the base sequence of the other strand.

This complementarity is the basis of:

1. DNA replication (each strand serves as a template for a new complementary strand)
2. Transcription (DNA strand serves as template for mRNA synthesis)

Chargaff's rule confirms this: In any DNA, [A] = [T] and [G] = [C].

Structural Differences:

Functional Differences:

There are three main types of RNA found in the cell, all involved in protein synthesis:

1. Messenger RNA (mRNA):

• It is synthesised in the nucleus using DNA as a template (transcription).
• It carries the genetic message (codons — triplet base sequences) from DNA to the ribosomes in the cytoplasm.
• It serves as the template for protein synthesis (translation).
• It is the largest RNA but present in the smallest amount.

2. Transfer RNA (tRNA) — also called Soluble RNA (sRNA):

• It is the smallest RNA molecule.
• It acts as an adaptor molecule — it reads the codons on mRNA and brings the specific amino acid to the ribosome during translation.
• Each tRNA is specific for one amino acid.
• It has a cloverleaf secondary structure with an anticodon loop (complementary to mRNA codon) and an amino acid attachment site at the 3'-end.

3. Ribosomal RNA (rRNA):

• It is the most abundant RNA in the cell.
• It is a structural and functional component of ribosomes (along with ribosomal proteins).
• Ribosomes are the sites of protein synthesis.
• rRNA plays a catalytic role in peptide bond formation (ribozyme activity).

Summary: