Biomolecules

The chapter on Biomolecules in the 12th standard NCERT Chemistry textbook explores the structure, functions, and importance of the major classes of organic compounds found in living organisms. Here’s a summary:

Key Concepts:

• Carbohydrates: These are polyhydroxy aldehydes or ketones, or substances that yield them on hydrolysis. They serve as a primary source of energy and also have structural roles. The chapter covers:
  
  • Classification: Monosaccharides (glucose, fructose), disaccharides (sucrose, lactose, maltose), and polysaccharides (starch, cellulose, glycogen).
  • Structure: Open-chain and cyclic forms of monosaccharides (Haworth projections).
  • Reactions: Oxidation, reduction, and glycoside formation.
  • Importance: Energy storage, structural components of cell walls.
• Proteins: These are polymers of amino acids. They perform a vast array of functions in living systems. The chapter covers:
  
  • Amino Acids: The building blocks of proteins, containing both amino (-NH2) and carboxyl (-COOH) groups. Classification based on their side chains.
  • Structure: Primary (sequence of amino acids), secondary (α-helix, β-pleated sheet), tertiary (overall 3D shape), and quaternary (arrangement of multiple polypeptide chains).
  • Denaturation: Loss of protein’s 3D structure due to heat, pH changes, etc.
  • Enzymes: Biological catalysts, proteins that speed up biochemical reactions.
  • Hormones: Chemical messengers, some of which are proteins.
• Nucleic Acids: These are polymers of nucleotides, which consist of a pentose sugar, a phosphate group, and a nitrogenous base. They carry genetic information. The chapter covers:
  
  • Types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
  • Structure: Double helix structure of DNA, different types of RNA (mRNA, tRNA, rRNA).
  • Functions: DNA stores genetic information, RNA plays a role in protein synthesis.
• Lipids: These are nonpolar molecules, including fats, oils, phospholipids, and steroids. They serve as energy reserves, structural components of cell membranes, and hormones. The chapter covers:
  
  • Fats and Oils: Triglycerides, esters of glycerol and fatty acids.
  • Phospholipids: Lipids containing a phosphate group, important components of cell membranes.
  • Steroids: Lipids with a characteristic four-ring structure, including cholesterol and some hormones.
• Enzymes: Biological catalysts that speed up biochemical reactions. The chapter covers:
  
  • Mechanism of enzyme action: Lock-and-key model, induced-fit model.
  • Factors affecting enzyme activity: Temperature, pH, inhibitors.
• Hormones: Chemical messengers produced by endocrine glands that regulate various physiological processes.
  
• Importance of Biomolecules: Biomolecules are essential for life. They provide energy, build and repair tissues, catalyze reactions, store and transmit genetic information, and regulate various bodily functions.

Exercise

1. What are monosaccharides ?

Ans : Monosaccharides are the simplest form of carbohydrates, often referred to as “simple sugars.” They are the basic building blocks (monomers) of more complex carbohydrates like disaccharides and polysaccharides.

Key features:

• Sweet taste: Many monosaccharides have a sweet taste, although not all of them do.
• Water-soluble: They are generally water-soluble due to the presence of multiple hydroxyl (-OH) groups in their structure.  
• Crystalline solids: In their pure form, they are typically colorless, crystalline solids.  
• Aldehydes or ketones: They contain either an aldehyde (-CHO) or a ketone (C=O) functional group, along with multiple hydroxyl groups.  
• Chirality: Most monosaccharides are chiral, meaning they have at least one asymmetric carbon atom. This gives rise to different stereoisomers (molecules with the same chemical formula but different spatial arrangements of atoms).  

Examples:

• Glucose: A very common and important monosaccharide, also known as dextrose or blood sugar. It’s a hexose (contains 6 carbon atoms) and an aldose (contains an aldehyde group).  
• Fructose: Also a hexose, but it’s a ketose (contains a ketone group). It’s found in fruits and honey and is sweeter than glucose.  
• Galactose: Another hexose, an epimer of glucose (differs in the position of one hydroxyl group). It’s a component of lactose (milk sugar).  
• Ribose: A pentose (contains 5 carbon atoms) and an aldose. It’s a crucial component of RNA (ribonucleic acid).  
• Deoxyribose: A pentose, derived from ribose by the loss of one oxygen atom.
  It’s a crucial component of DNA (deoxyribonucleic acid).

2. What are reducing sugars?

Ans : Reducing sugars are carbohydrates that can act as reducing agents, meaning they can donate electrons to other substances. This ability is due to the presence of a free aldehyde (-CHO) or ketone (C=O) group in their structure.

Examples of Reducing Sugars:

• All monosaccharides: Glucose, fructose, galactose, ribose, etc., are all reducing sugars because they have a free aldehyde or ketone group.  
• Some disaccharides: Maltose and lactose are reducing sugars because they have one free anomeric carbon that can open up to form an aldehyde group.

3. Write two main functions of carbohydrates in plants.

Ans : 

Energy Source: Just like we need food to fuel our bodies, plants need energy to grow, reproduce, and carry out all their essential functions. Carbohydrates, particularly in the form of sugars like glucose and sucrose, are the primary source of energy for plants. They are produced through photosynthesis, the remarkable process where plants convert sunlight, water, and carbon dioxide into energy-rich carbohydrates. These carbohydrates are then used to power various cellular activities, ensuring the plant has the energy it needs to flourish.  

Structural Support: Imagine a plant without its sturdy frame – it would be a floppy mess! Carbohydrates, especially in the form of cellulose, play a vital role in providing structural support and rigidity to plant cell walls.
Cellulose is a complex carbohydrate that forms long, strong fibers, acting like the plant’s backbone. It gives plant cells their shape, prevents them from bursting due to internal pressure, and allows plants to grow tall and strong, reaching for the sunlight they need.

4. Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.

Ans : 

Monosaccharides:

• Ribose
• 2-deoxyribose
• Fructose
• Galactose

Disaccharides:

• Maltose
• Lactose

 5. What do you understand by the term glycosidic linkage?

Ans : A glycosidic linkage is the covalent bond that joins a carbohydrate (sugar) molecule to another group, which can be another carbohydrate or a non-carbohydrate molecule. It’s the key connection that holds together disaccharides, oligosaccharides, and polysaccharides.

6. What is glycogen? How is it different from starch?

Ans : Glycogen is the main storage form of glucose in animals and fungi.

It’s a polysaccharide, meaning it’s made up of many glucose units linked together. Think of it as the animal equivalent of starch in plants.

Feature	Glycogen	Starch
Structure	Highly branched	Less branched (amylopectin) and unbranched (amylose) components
Branching	More frequent (every 8-12 glucose units)	Less frequent (every 24-30 glucose units in amylopectin)
Solubility	More soluble in water	Less soluble in water
Source	Animals and fungi	Plants
Function	Energy storage in animals	Energy storage in plants

7. What are the hydrolysis products of (i) sucrose, and (ii) lactose?

Ans : 

Sucrose hydrolysis produces glucose and fructose.  

Lactose hydrolysis produces glucose and galactose.

8. What is the basic structural difference between starch and cellulose?

Ans : The key structural difference between starch and cellulose lies in the type of glycosidic linkage between their glucose units.

This difference leads to variations in their shape, packing, and digestibility, ultimately determining their distinct roles in plants and our diets.

9. What happens when D-glucose is treated with . the following reagents.

(i) HI

(ii) Bromine water

(iii) HNO3

Ans : 

10. Enumerate the reactions of D-glucose which cannot be explained with open chain structure.

Ans : The open-chain structure of D-glucose is useful for understanding some of its basic reactions, but it fails to explain phenomena like mutarotation, its weak reactivity with Schiff’s reagent, and the formation of a stable pentaacetate. These observations strongly support the existence of a cyclic hemiacetal structure for D-glucose, where the aldehyde group is involved in the ring formation.

11. What are essential and non-essential amino acids? Give two examples of each type.

Ans : 

Essential Amino Acids:

These are amino acids that the human body cannot synthesize in sufficient quantities (or at all) and therefore must be obtained from the diet. There are nine essential amino acids.  

• Examples:
  • Lysine: Involved in muscle protein synthesis, calcium absorption, and enzyme and hormone production.  
  • Tryptophan: A precursor for serotonin (a neurotransmitter) and melatonin (a hormone).  

Non-Essential Amino Acids:

These are amino acids that the human body can synthesize in sufficient quantities and therefore do not need to be obtained directly from the diet. “Non-essential” doesn’t mean they are less important; they are just as crucial for protein synthesis and other bodily functions.  

• Examples:
  • Alanine: Involved in glucose metabolism and plays a role in the citric acid cycle.  
  • Glutamic acid: A neurotransmitter and involved in various metabolic pathways.

12. Define the following as related to proteins:

(i) Peptide linkage

(ii) Primary structure

(iii) Denaturation

Ans : 

(i) Peptide Linkage:

A peptide linkage (or peptide bond) is the covalent chemical bond that connects amino acids together in a protein chain. It’s specifically an amide bond (-CO-NH-) formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another amino acid. This happens through a dehydration reaction (removal of a water molecule). The peptide linkage is the backbone of the protein chain, linking the amino acid residues together in a specific sequence.

(ii) Primary Structure:

The primary structure of a protein refers to the linear sequence of amino acids that make up the polypeptide chain. It’s the most fundamental level of protein structure. Think of it as the specific order of amino acids, like the letters in a very long word. The primary structure is determined by the genetic code (DNA). It dictates all the higher levels of protein structure.

(iii) Denaturation:

Denaturation is the loss of the three-dimensional (3D) structure of a protein, resulting in the loss of its biological function. Proteins have a specific folded shape that is essential for their proper functioning. Denaturation disrupts the non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds, etc.) that maintain this folded structure.

13. What are the common types of secondary structure of proteins?

Ans : 

α-helix:

• A right-handed spiral structure where the polypeptide backbone coils around an imaginary axis.  
• Stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amino hydrogen of another amino acid four residues down the chain.  

β-pleated sheet:

• Consists of extended polypeptide chains arranged side by side, forming a sheet-like structure.  
• Stabilized by hydrogen bonds between the backbones of adjacent strands.

14. What types of bonding helps in stabilising the α-helix structure of proteins?

Ans : The α-helix, a common secondary structure in proteins, is primarily stabilized by hydrogen bonds.  

Here’s a breakdown:

• Hydrogen Bonds: The α-helix is held together by hydrogen bonds between the carbonyl oxygen (C=O) of one amino acid residue and the amino hydrogen (N-H) of another amino acid residue four residues down the chain. This creates a sort of spiral staircase effect, with the hydrogen bonds forming the “steps” that hold the helix together.
  
• Other Interactions: While hydrogen bonds are the primary stabilizing force, other interactions also contribute to the α-helix’s stability:
  
  • Van der Waals Forces: These weak, short-range attractions between atoms contribute to the overall stability of the helix.
  • Dipole-Dipole Interactions: The arrangement of the peptide bonds in the α-helix creates a net dipole moment, which can contribute to the helix’s stability.

15. Differentiate between globular and fibrous proteins.

Ans : 

Feature	Globular Proteins	Fibrous Proteins
Shape	Spherical, compact	Elongated, fibrous
Structure	Mix of secondary structures, complex tertiary structure	Dominated by one type of secondary structure, often repeated
Solubility	Usually soluble	Usually insoluble
Function	Primarily functional (enzymes, hormones, transport, etc.)	Primarily structural (support, strength, protection)
Examples	Hemoglobin, insulin, enzymes	Collagen, keratin, elastin

16. How do you explain the amphoteric behaviour of amino acids?

Ans : Amino acids are considered amphoteric because they can act as both acids and bases. This is due to the presence of both acidic and basic functional groups within their structure:  

• Carboxyl group (-COOH): This group can donate a proton (H⁺), acting as an acid.  
• Amino group (-NH₂): This group can accept a proton (H⁺), acting as a base.

In zwitter ionjc form, a-amino acid show amphoteric behaviour as they react with both acids and bases.

17.  What are enzymes?

Ans : Enzymes are biological catalysts, meaning they are substances that speed up chemical reactions in living organisms without being consumed in the process.

18. What is the effect of denaturation on the structure of proteins?

Ans : Protein denaturation, achieved by altering temperature (heating) or pH, disrupts the hydrogen bonds that maintain a protein’s specific folded shape. This unfolding leads to a loss of biological activity and a change in the protein’s properties. Denaturation affects both the tertiary and secondary structures, while the primary structure (the amino acid sequence) remains unchanged.

19.  How are vitamins classified? Name the vitamin responsible for the coagulation of blood.

Ans : 

Fat-soluble vitamins:

• These vitamins (A, D, E, and K) dissolve in fat and are stored in the body’s fatty tissues and liver.  
• They are absorbed best when consumed with foods containing fat.  
• Because they can be stored, excessive intake can lead to toxicity. 

Water-soluble vitamins:

• These vitamins (C and all the B vitamins) dissolve in water and are not stored in the body to a significant extent.  
• Excess amounts are excreted through urine.  
• They need to be consumed regularly as the body cannot store them (with the exception of vitamin B12, which can be stored in the liver for many years).

20. Why are vitamin A and vitamin C essential to us? Give their important sources.

Ans : 

Vitamin A

Sources:

• Animal Sources: Liver, oily fish (like salmon), eggs, cheese, milk, and yogurt.  
• Plant Sources: The body can also convert beta-carotene into Vitamin A. Good sources of beta-carotene include:
  • Yellow, orange, and red vegetables (carrots, sweet potatoes, peppers)  
  • Green leafy vegetables (spinach, kale)  
  • Yellow fruits (mangoes, apricots)

Vitamin C

Sources:

• Fruits: Citrus fruits (oranges, lemons, grapefruits), berries (strawberries, blueberries), kiwi, and many others.  
• Vegetables: Broccoli, Brussels sprouts, peppers (especially red and yellow), tomatoes, and potatoes.

21. What are nucleic acids ? Mention their two important functions.

Ans : Nucleic acids are large biomolecules essential for all known forms of life. They are polymers of nucleotides, which are themselves composed of three parts:   

1. A phosphate group: One or more phosphate groups.  
2. A nitrogenous base: A molecule containing nitrogen and having basic properties. There are five main nitrogenous bases: adenine (A), guanine (G), cytosine (C), thymine (T) (found in DNA), and uracil (U) (found in RNA).

Storage and Transmission of Genetic Information: DNA (deoxyribonucleic acid) is the primary carrier of genetic information in most organisms. The sequence of nitrogenous bases in DNA encodes the instructions for building and maintaining an organism. This information is passed from one generation to the next through DNA replication. RNA (ribonucleic acid) plays a crucial role in the decoding and expression of this genetic information.  

Protein Synthesis: RNA is directly involved in protein synthesis. There are different types of RNA, each with a specific function:  

• Messenger RNA (mRNA): Carries the genetic code from DNA to the ribosomes (the protein synthesis machinery).  
• Transfer RNA (tRNA): Brings specific amino acids to the ribosome during protein synthesis, based on the mRNA code.  
• Ribosomal RNA (rRNA): A structural component of ribosomes, which are essential for protein synthesis.

22. What is the difference between a nucleoside and a nucleotide?

Ans : 

Feature	Nucleoside	Nucleotide
Components	Nitrogenous base + Pentose sugar	Nitrogenous base + Pentose sugar + Phosphate(s)
Phosphate Group	Absent	Present (one or more)
Bond	N-glycosidic bond (between base and sugar)	N-glycosidic bond + Phosphodiester bond(s)
Function/Role	Building block of nucleotides	Building block of nucleic acids (DNA, RNA), energy carrier (ATP), signaling molecule
Example	Adenosine (Adenine + Ribose)	Adenosine monophosphate (AMP) (Adenine + Ribose + Phosphate)

23. The two strands in DNA are not identical but are complementary. Explain.

Ans : The two strands of DNA are not identical but complementary, a crucial feature for its structure and function. Complementarity arises from the specific pairing of nitrogenous bases: adenine (A) always pairs with thymine (T) (or uracil (U) in RNA), and guanine (G) always pairs with cytosine (C). These pairings are dictated by the hydrogen bond donors and acceptors on the bases, with A and T (or U) forming two hydrogen bonds, and G and C forming three. This precise pairing ensures a snug fit between the strands and facilitates accurate replication. If the strands were identical, there would be no specific pairing mechanism, leading to random association and redundant information storage. Instead, complementarity allows for each strand to serve as a template for the synthesis of a new, complementary strand during replication, ensuring accurate copying of genetic information. For example, a strand with the sequence 5′-ATGC-3′ will pair with a complementary strand 3′-TACG-5′, demonstrating the antiparallel arrangement and the A-T and G-C pairings. This complementary structure also provides stability to the double helix and allows for a vast amount of genetic information to be stored within the specific sequence of base pairs. Therefore, the non-identical but complementary nature of DNA strands is essential for accurate replication, efficient information storage, and ultimately, genetic diversity.

24. Write the important structural and functional differences between DNA and RNA.

Ans : 

Feature	DNA	RNA
Sugar	Deoxyribose (lacks an oxygen atom at the 2′ carbon)	Ribose (has an -OH group at the 2′ carbon)
Strands	Double-stranded (double helix)	Single-stranded (can fold into complex 3D structures)
Bases	Adenine (A), Guanine (G), Cytosine (C), Thymine (T)	Adenine (A), Guanine (G), Cytosine (C), Uracil (U) (replaces Thymine)
Stability	More stable due to the absence of the 2′ -OH group	Less stable due to the presence of the 2′ -OH group, which is more reactive

25.  What are the different types of RNA found in the cell?

Ans : 

Messenger RNA (mRNA):

• Function: Carries the genetic code from DNA to the ribosomes, the protein synthesis machinery. It acts as a template for protein synthesis.  
• Structure: Single-stranded, varies in length depending on the protein it codes for. Contains codons (three-base sequences) that specify amino acids.  

Transfer RNA (tRNA):

• Function: Brings specific amino acids to the ribosome during protein synthesis. It “reads” the mRNA codons and delivers the corresponding amino acid.  
• Structure: Small, cloverleaf-shaped molecule. Has an anticodon (three-base sequence) that recognizes the mRNA codon.  

Ribosomal RNA (rRNA):

• Function: A major structural and catalytic component of ribosomes. It provides the site for protein synthesis and catalyzes the formation of peptide bonds.  
• Structure: Forms complexes with proteins to make up ribosomes.  

Small Nuclear RNA (snRNA):

• Function: Involved in RNA processing, such as splicing (removing introns from pre-mRNA). Forms complexes with proteins called snRNPs (small nuclear ribonucleoproteins).  
• Structure: Small RNA molecules found in the nucleus.  

MicroRNA (miRNA):

• Function: Small, non-coding RNAs that regulate gene expression at the post-transcriptional level (after mRNA is made). They can inhibit translation or degrade mRNA.  
• Structure: Short, single-stranded RNA molecules.  

Small Interfering RNA (siRNA):

• Function: Involved in RNA interference (RNAi), a process that silences genes by degrading mRNA or inhibiting translation. Often used in research to study gene function.  
• Structure: Short, double-stranded RNA molecules.