Chapter 10 : Biomolecules

Internal Questions of Chapter 10 Biomolecules Questions and Answers :

14.1 Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.

Answer : The solubility differences between glucose or sucrose and cyclohexane or benzene are due to their molecular structures and the types of intermolecular interactions they can form with water.

Glucose and Sucrose:  Glucose and sucrose are soluble in water because they can form hydrogen bonds with water molecules. Glucose is a hydrophilic molecule due to the presence of hydroxyl (-OH) groups that can engage in hydrogen bonding with water. Similarly, sucrose is composed of glucose and fructose units, both containing hydroxyl groups. These hydrogen bonds between the hydroxyl groups of glucose, fructose, and water allow glucose and sucrose to dissolve in water.

Cyclohexane and Benzene:  Cyclohexane and benzene, being simple six-membered ring hydrocarbons, lack hydroxyl groups or other polar functional groups. As a result, they do not form strong hydrogen bonds with water. Instead, the interactions between nonpolar hydrocarbons and water are generally weaker van der Waals forces. These weak interactions do not favor the dissolution of cyclohexane or benzene in water. Instead, these hydrocarbons tend to cluster together, minimizing their interactions with water molecules.

14.2 What are the expected products of hydrolysis of lactose?

Answer :  The hydrolysis of lactose, a disaccharide composed of glucose and galactose, results in the formation of its monosaccharide components. The expected products of hydrolysis of lactose are:

Glucose: One molecule of glucose is obtained.

Galactose: One molecule of galactose is obtained.

Hydrolysis breaks the glycosidic bond between glucose and galactose, releasing these monosaccharides as separate molecules.

14.3 How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?

Answer : The absence of the aldehyde group in the pentaacetate of D-glucose is due to the reaction that occurs during the acetylation process.

When D-glucose undergoes acetylation, typically using acetic anhydride or acetyl chloride, the hydroxyl groups () on the glucose molecule react with acetyl groups () from the acetylating agent. This reaction involves the replacement of hydroxyl groups with acetyl groups. One of the hydroxyl groups that is commonly acetylated is the one on the anomeric carbon (), which is the aldehyde group in glucose. This reaction effectively converts the aldehyde group into an acetyl group, resulting in the formation of the pentaacetate derivative of glucose.

14.4 The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.

Answer : Amino acids have higher melting points and solubility in water compared to the corresponding halo acids due to differences in their molecular structures and intermolecular forces.

Higher Melting Points:  Amino acids have complex structures that include polar functional groups like amino () and carboxyl () groups. These groups can form hydrogen bonds with each other, leading to a network of intermolecular forces. In contrast, halo acids (halogenated carboxylic acids) typically have weaker intermolecular forces due to the presence of halogen atoms, which are less electronegative than oxygen or nitrogen. The stronger hydrogen bonding in amino acids results in higher melting points compared to halo acids.

Higher Solubility in Water:  Amino acids are hydrophilic (water-loving) due to the presence of polar functional groups. Their amino and carboxyl groups can interact with water molecules through hydrogen bonding. This interaction allows amino acids to dissolve readily in water. In contrast, halo acids, while also having polar functional groups, may have reduced solubility in water due to the larger nonpolar halogen atoms interfering with hydrogen bonding interactions.

14.5 Where does the water present in the egg go after boiling the egg?

Answer :  When you boil an egg, the water present in the egg undergoes a phase change from liquid to gas due to the heat. This phase change is known as evaporation. The water molecules absorb energy from the heat source, gain enough kinetic energy, and transition from the liquid state to the gaseous state (water vapor).

As a result, the water vapor is released into the surrounding air. This is why you might observe steam rising from a boiling egg. The water vapor disperses into the atmosphere, and what remains is the cooked egg with its solid proteins and other biomolecules.

14.6 Why cannot vitamin C be stored in our body?

Answer :  Vitamin C, also known as ascorbic acid, cannot be stored in our body to a significant extent because humans lack the enzyme L-gulonolactone oxidase, which is required for the final step in the synthesis of vitamin C. This enzyme is responsible for converting glucose to ascorbic acid in other animals that can synthesize their own vitamin C.

Because humans lack this enzyme, they are unable to produce vitamin C internally and must obtain it from their diet. Any excess vitamin C that is consumed is not effectively stored and is excreted from the body through urine. As a result, a regular intake of vitamin C through dietary sources is necessary to maintain adequate levels of this important nutrient in the body.

14.7 What products would be formed when a nucleotide from DNA containing thymine is hydrolysed?

Answer : When a nucleotide from DNA containing thymine is hydrolyzed, the products formed would include:

Thymine (base):  The nitrogenous base thymine would be released as a result of the hydrolysis.

Deoxyribose (sugar):  The deoxyribose sugar molecule that was part of the nucleotide would also be released.

Phosphate Group:  The phosphate group, which is part of the nucleotide structure, would be detached as well.

14.8 When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?

Answer :  The fact that there is no consistent relationship among the quantities of different bases obtained when RNA is hydrolyzed suggests that RNA is often single-stranded and doesn't have a strict complementary base pairing pattern like DNA. In DNA, the base pairs (adenine with thymine, and cytosine with guanine) create a specific and consistent ratio of adenine to thymine and cytosine to guanine. However, the absence of a consistent pattern in RNA hydrolysis products indicates that RNA molecules typically lack a complementary partner to form stable base pairs throughout their entire length. This supports the understanding that RNA can fold back on itself, form hairpin loops, and exhibit structural diversity, which is essential for its various biological functions, including gene expression and catalysis.

Class 12 Chemistry Chapter 14 Biomolecules Exercise Answers :

Question14.1 What are monosaccharides?

Answer : A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide.  Monosaccharides are essential for providing energy to living organisms and are important in various cellular processes.  About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.

14.2 What are reducing sugars?

Answer : Reducing sugars are carbohydrates that have the ability to reduce certain chemical solutions, such as Fehling's solution and Tollens' reagent. These solutions contain metal ions that can be reduced by the aldehyde or ketone functional groups present in reducing sugars.

14.3 Write two main functions of carbohydrates in plants.

Answer :  Two main functions of carbohydrates in plants  are :

Energy Source:  Carbohydrates in plants serve as a vital source of energy. Through photosynthesis, plants convert sunlight into glucose and other sugars, which are stored as energy reserves. When needed, these carbohydrates can be broken down to release energy, powering various plant activities.

Structural Support:  Carbohydrates, such as cellulose, play a key role in providing structural support to plants. Cellulose forms the cell walls, giving plants their shape and strength. This structural framework enables plants to stand upright and maintain their form as they grow.

14.4 Classify the following into monosaccharides and disaccharides.

Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.

Answer : The classification of the given compounds into monosaccharides and disaccharides:

Monosaccharides:   Ribose , 2-deoxyribose , Galactose  and Fructose

Disaccharides: Maltose and  Lactose

Monosaccharides are single sugar molecules, while disaccharides are composed of two monosaccharide units linked together.

14.5 What do you understand by the term glycosidic linkage?

Answer : A glycosidic linkage refers to a type of chemical bond that connects two sugar molecules (monosaccharides) together in carbohydrates. It forms when the hydroxyl group  of one sugar molecule reacts with the hydroxyl group of another sugar molecule, resulting in the elimination of a water molecule (dehydration reaction).

Glycosidic linkages play a crucial role in forming various carbohydrate structures, such as disaccharides (two sugars linked together), oligosaccharides (a small number of sugar units), and polysaccharides (long chains of sugar units). The specific type and arrangement of glycosidic linkages determine the properties and functions of these carbohydrate molecules.

14.6 What is glycogen? How is it different from starch?

Answer :   Glycogen is a complex polysaccharide that serves as a storage form of glucose in animals, including humans. It is primarily stored in the liver and muscles and functions as a readily accessible energy reserve. Glycogen is composed of a large number of glucose molecules linked together through α-1,4-glycosidic bonds, as well as α-1,6-glycosidic branches that occur at regular intervals. This branching structure allows for rapid and efficient breakdown of glycogen to release glucose when the body needs energy.

Starch, on the other hand, is a similar polysaccharide but is found in plants. Like glycogen, starch serves as a storage form of glucose. Starch is also composed of glucose molecules linked together through α-1,4-glycosidic bonds, forming linear chains. However, the key difference between starch and glycogen lies in their branching patterns. Starch has fewer and less frequent branches compared to glycogen.

14.7 What are the hydrolysis products of : (i) sucrose and (ii) lactose?

Answer : (i) The hydrolysis products of sucrose are glucose and fructose.

(ii) The hydrolysis products of lactose are glucose and galactose.

14.8 What is the basic structural difference between starch and cellulose?

Answer :  The basic structural difference between starch and cellulose lies in the orientation of the glucose molecules and the type of glycosidic bonds they form.

In starch:

(i) Starch is made up of glucose molecules linked together by α-1,4-glycosidic bonds.

(ii) It forms helical or spiral chains.

(iii) It is an energy storage polysaccharide in plants.

In cellulose:                                         

(i) Cellulose is also made up of glucose molecules, but they are linked together by β-1,4-glycosidic bonds.

(ii) It forms straight and linear chains.

(iii) It serves as a structural component in plant cell walls.

The difference in the type of glycosidic bonds (α in starch, β in cellulose) leads to distinct structural properties and functions for these two polysaccharides.

14.9 What happens when D-glucose is treated with the following reagents?

(i) HI  (ii) Bromine water  (iii)

Answer : (i) HI (Hydroiodic Acid): When D-glucose reacts with HI, the glycosidic bonds between the glucose molecules break, and the glucose molecules get reduced. The products of this reaction are n-hexane and n-pentane. This reaction involves the cleavage of the glycosidic bonds and reduction of the sugar molecules.

(ii) Bromine Water: D-glucose does not readily react with bromine water under normal conditions. However, if the reaction is carried out in the presence of concentrated hydrobromic acid (Brönsted acid catalyst), it can lead to the oxidation of glucose to form D-gluconic acid. This reaction involves the substitution of hydroxyl groups by bromine atoms.

(iii) (Nitric Acid): D-glucose reacts with nitric acid to form various oxidation products, including saccharic acid. This reaction involves the oxidation of the glucose molecule's hydroxyl groups.

14.10 Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.

Answer : Glycosidic Bond Formation:  The open-chain structure doesn't account for the formation of glycosidic bonds between glucose molecules to create disaccharides and polysaccharides.

Mutarotation: The equilibrium between α and β anomeric forms in solution isn't explained by the open-chain structure.

Cyclic Acetal Formation: The formation of cyclic acetals, like the pyranose ring, isn't addressed by the linear structure.

Reduction to Sugar Alcohols: The open-chain structure doesn't predict the reduction of glucose to sugar alcohols.

Enzyme Recognition: Enzyme interactions with glucose often involve its cyclic form rather than the linear structure.

Oxidation Reactions: Reactions leading to products like gluconic acid or saccharic acid are not accounted for by the open-chain structure.

These reactions highlight the significance of the cyclic hemiacetal structure in explaining various properties and behaviors of D-glucose.

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

Answer : Essential Amino Acids: Essential amino acids are amino acids that the body cannot produce and must be obtained from the diet.

They are vital for proper health and functioning. Examples : Leucine and  Phenylalanine

Non-Essential Amino Acids: Non-essential amino acids are amino acids that the body can synthesize on its own from other compounds.

They are not required to be obtained directly from the diet. Examples : Alanine and Glutamine

14.12 Define the following as related to proteins

(i) Peptide linkage (ii) Primary structure (iii) Denaturation.

Answer : (i) Peptide Linkage:  The peptide linkage is the covalent bond formed between the amino group of one amino acid and the carboxyl group of another amino acid. It connects amino acids in a protein chain.

(ii) Primary Structure:  The primary structure of a protein refers to the specific sequence of amino acids in the protein chain. It's the linear arrangement that forms the backbone of the protein.

(iii) Denaturation:   Denaturation is the process in which a protein loses its natural structure and function due to changes in temperature, pH, or other external factors. It disrupts the protein's shape and can lead to loss of function.

14.13 What are the common types of secondary structure of proteins?

Answer :  The common types of secondary structure in proteins are:

(i) Alpha Helix

(ii) Beta Sheet

14.14 What type of bonding helps in stabilising the α-helix structure of proteins?

Answer :  Hydrogen bonding helps in stabilizing the α-helix structure of proteins.

14.15 Differentiate between globular and fibrous proteins.

Answer : difference between globular and fibrous proteins are :

Characteristic	Globular Proteins	Fibrous Proteins
Structure	Compact , folded structure	Elongated, thread-like structure
Solubility	Generally soluble in water	Ofter insoluble in water
Functions	Diverse functions, enzymes ,antibodies	Structural support ,connective tissues
Shape	Varied ,not limited to a specific shape	Long and repetitive shape
Location	Ofter found in cellular fluids	Commonly found in tissue
Example	Hemoglobin ,enzymes ,antibodies	Collagen, keratin ,fibrin

14.16 How do you explain the amphoteric behaviour of amino acids?

Answer :  Amino acids exhibit amphoteric behavior due to the presence of both acidic (carboxyl) and basic (amino) functional groups within their molecular structure. These functional groups can donate or accept protons (H+ ions), enabling amino acids to act as both acids and bases in different chemical environments. In an acidic environment, the amino group can accept a proton, becoming positively charged, while in a basic environment, the carboxyl group can donate a proton, becoming negatively charged. This ability to gain or lose protons allows amino acids to maintain a neutral charge at a certain pH level, known as their isoelectric point (pI). This amphoteric nature makes amino acids key components in protein chemistry and their interactions with various biological systems.

14.17 What are enzymes?

Answer : Enzymes are specialized biomolecules, usually proteins, that act as biological catalysts. They speed up chemical reactions in living organisms by lowering the activation energy required for these reactions to occur. Enzymes are highly specific, each catalyzing a particular reaction or group of reactions. They play a crucial role in various biological processes, including digestion, metabolism, and cellular regulation.

14.18 What is the effect of denaturation on the structure of proteins?

Answer : Denaturation disrupts the three-dimensional structure of proteins. It causes the protein to lose its original shape and functional conformation. This often occurs due to factors like high temperature, extreme pH, or exposure to chemicals. Denaturation leads to the unfolding of the protein's secondary, tertiary, and quaternary structures, but the primary structure (sequence of amino acids) remains unchanged. As a result, the protein may lose its biological activity and become non-functional.

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

Answer : Vitamins are classified into two main groups based on their solubility:

Water-Soluble Vitamins:  These vitamins dissolve in water and include vitamin C and the B-complex vitamins (such as  ,   ,  ,   , , … etc.).

Fat-Soluble Vitamins: These vitamins dissolve in fats and oils. They include vitamins A, D, E, and K.

The vitamin responsible for the coagulation of blood is Vitamin K. It plays a crucial role in the synthesis of certain proteins required for blood clotting or coagulation.

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

Answer : Vitamin A and vitamin C are essential to us because they play vital roles in maintaining our health and supporting various physiological processes.

Vitamin A:

Importance: Vitamin A is essential for good vision, healthy skin, proper immune function, and the growth and development of various tissues.

Sources: Important sources of vitamin A include foods like carrots, sweet potatoes, spinach, kale, eggs, and liver.

Vitamin C:                              

Importance: Vitamin C is important for collagen synthesis (which is crucial for skin, bones, and connective tissues), antioxidant protection, immune function, and the absorption of iron from plant-based foods.

Sources: Good sources of vitamin C include citrus fruits (oranges, lemons), strawberries, bell peppers, broccoli, and kiwi.

Both vitamins A and C are necessary for our overall health and well-being.

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

Answer : Nucleic acids are biomolecules that serve as the genetic information storage and transmission molecules in living organisms. They include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Two important functions of nucleic acids are:

Genetic Information Storage:  Nucleic acids, especially DNA, store genetic information in the sequence of their nucleotide units. This information carries the instructions for the development, growth, functioning, and reproduction of organisms.

Protein Synthesis:  Nucleic acids, primarily RNA, are involved in protein synthesis. They provide the template and instructions for assembling amino acids in the correct order to form proteins, which are essential for various cellular functions.

Nucleic acids are fundamental to the continuity of life and the transmission of genetic traits from one generation to the next.

14.22 What is the difference between a nucleoside and a nucleotide?

Answer : A nucleoside is composed of a nitrogenous base (purine or pyrimidine) and a sugar molecule (ribose or deoxyribose), but it lacks a phosphate group.

A nucleotide consists of a nitrogenous base, a sugar molecule (ribose or deoxyribose), and one or more phosphate groups. It's the basic building block of nucleic acids (DNA and RNA) and plays a crucial role in genetic information storage and transmission.

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

Answer : In DNA, the two strands are not identical but complementary due to the specific base pairing between adenine (A) and thymine (T), as well as between cytosine (C) and guanine (G). This is known as the base-pairing rule. Each base can only form hydrogen bonds with its complementary partner: A with T (or U in RNA) and C with G. These hydrogen bonds create a stable double-stranded structure. As a result, if you know the sequence of bases on one strand, you can determine the sequence of bases on the other strand.

This complementary base pairing is the foundation of DNA replication and transcription, allowing for accurate copying of genetic information and the synthesis of RNA molecules from DNA templates.

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

Answer :  Structural Differences:

Sugar: DNA contains deoxyribose sugar, while RNA contains ribose sugar.

Bases: DNA has adenine (A), cytosine (C), guanine (G), and thymine (T) as bases. RNA has adenine (A), cytosine (C), guanine (G), and uracil (U) as bases (uracil replaces thymine).

Double Strand vs. Single Strand: DNA is typically double-stranded, forming a double helix structure. RNA is usually single-stranded.

Stability: DNA is relatively stable due to the absence of a hydroxyl group on the 2' carbon of deoxyribose. RNA is more prone to degradation due to the presence of a hydroxyl group on the 2' carbon of ribose.

Functional Differences:       

Genetic Information: DNA stores genetic information and transmits it from one generation to the next. RNA participates in protein synthesis and other cellular processes.

Protein Synthesis: DNA provides the template for protein synthesis through the process of transcription. RNA is involved in translation, where it carries the genetic information from DNA to the ribosome and helps assemble amino acids into proteins.

Types and Functions: DNA exists in one form (double-stranded) and mainly functions as a long-term genetic storage molecule. RNA exists in several forms (mRNA, tRNA, rRNA) with roles in various cellular activities, including protein synthesis, gene regulation, and enzymatic functions.

Location: DNA is mainly located in the cell nucleus. RNA is found both in the nucleus and the cytoplasm of the cell.

14.25 What are the different types of RNA found in the cell?

Answer :  In a cell, the following types of RNA are found:

Messenger RNA (mRNA):  mRNA carries the genetic information from DNA to the ribosome, where it serves as a template for protein synthesis (translation).

Transfer RNA (tRNA): tRNA brings amino acids to the ribosome during protein synthesis. Each tRNA molecule is specific for a particular amino acid and has an anticodon region that base pairs with the codon on mRNA.

Ribosomal RNA (rRNA): rRNA is a structural component of ribosomes, which are responsible for protein synthesis. It helps catalyze the formation of peptide bonds between amino acids.

These three types of RNA work together to ensure the accurate and efficient synthesis of proteins in the cell .