Which of the following best describes the amphoteric nature of amino acids?

They can act as both acids and bases.

They are neutral in all conditions.

Why does glucose not give Schiff's test?

Because it exists in a cyclic hemiacetal form

Because it lacks an aldehyde group

Because it exists in a cyclic hemiacetal form

Because it forms a cyanohydrin with HCN

Because it forms glucose pentaacetate

Which of the following is a characteristic of amylopectin?

Contains α(1→6) glycosidic bonds

Which of the following statements about lactose is correct?

Lactose is a reducing sugar composed of $3b2$-D-galactose and $3b2$-D-glucose linked through C1–C4.

Lactose is a non-reducing sugar composed of $3b1$-D-glucose and $3b2$-D-fructose.

Lactose is a reducing sugar composed of $3b2$-D-galactose and $3b2$-D-glucose linked through C1–C4.

Lactose is a reducing sugar composed of $3b1$-D-glucose and $3b2$-D-fructose linked through C1–C2.

Lactose is a non-reducing sugar composed of $3b2$-D-galactose and $3b2$-D-glucose linked through C1–C4.

During enzyme-catalyzed reactions, what happens to the enzyme after the reaction is complete?

The enzyme remains unchanged and can be reused.

The enzyme is permanently altered and cannot be reused.

The enzyme is consumed as part of the reaction.

The enzyme remains unchanged and can be reused.

The enzyme is converted into a product of the reaction.

Which of the following statements about peptide bonds is true?

A peptide bond is formed between the amino group of one amino acid and the carboxyl group of another.

Peptide bonds are ionic bonds that link amino acids together.

Peptide bonds are formed by the hydrolysis of amino acids.

A peptide bond is a type of hydrogen bond.

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Class 12 Science (PCB)

Chemistry Part 2

Biomolecules

Learning Objectives

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Explain the characteristics of biomolecules such as carbohydrates, proteins, nucleic acids, and hormones.
Classify carbohydrates, proteins, nucleic acids, and vitamins based on their structures.
Explain the difference between DNA and RNA.
Describe the role of biomolecules in biosystems.

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Revision Notes & Summary

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Notes on Biomolecules

10.1 Carbohydrates

Definition: Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules which provide such units on hydrolysis.
Classification: Broadly classified into three groups:
- Monosaccharides
- Disaccharides
- Polysaccharides
Examples: Cane sugar, glucose, starch.
General Formula: Cₓ(H₂O)ᵥ (e.g., glucose: C₆H₁₂O₆).
Functions:
- Energy source for mammals (e.g., glucose from starch digestion).
- Storage molecules (starch in plants, glycogen in animals).

10.2 Proteins

Definition: Proteins are polymers of about twenty different α-amino acids linked by peptide bonds.
Essential Amino Acids: Cannot be synthesized by the body; must be obtained from the diet (e.g., Isoleucine, Arginine).
Denaturation: Change in pH or temperature can disturb secondary and tertiary structures, affecting function.
Enzymes: Biocatalysts that speed up reactions; specific to substrates.

10.3 Nucleic Acids

Definition: Polymers of nucleotides, consisting of a base, a pentose sugar, and a phosphate moiety.
Types: DNA and RNA.
- DNA: Contains deoxyribose, double-stranded, includes bases adenine, guanine, cytosine, and thymine.
- RNA: Contains ribose, single-stranded, includes adenine, guanine, cytosine, and uracil.
Functions: Genetic information transfer, protein synthesis.

10.4 Vitamins

Definition: Organic compounds required in small amounts for specific biological functions.
Classification:
- Fat-soluble (A, D, E, K)
- Water-soluble (B group, C)
Importance: Deficiency can lead to diseases.

10.5 Hormones

Definition: Molecules acting as intercellular messengers produced by endocrine glands.
Types: Steroids (e.g., estrogens), polypeptides (e.g., insulin), amino acid derivatives (e.g., epinephrine).
Functions: Regulate biological activities, maintain homeostasis (e.g., insulin regulates blood glucose levels).

10.6 Key Concepts

Glycosidic Linkage: Bond between monosaccharides in carbohydrates.
Peptide Linkage: Bond between amino acids in proteins.
Denaturation: Loss of protein structure and function due to external stress.
Complementary Strands: DNA strands are not identical but complementary, crucial for replication.

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Practice Test – MCQs, True/False

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Multiple Choice Questions

Energy storage

Structural support

Water retention

Photosynthesis

Correct Answer: B

Solution:

Cellulose provides structural support to plant cell walls, making it the most abundant organic polymer in the plant kingdom.

Chapter Concept:

Polysaccharides: Starch, Cellulose and Glycogen

Vitamin B₆

Vitamin C

Vitamin D

Vitamin B₁₂

Correct Answer: C

Solution:

Vitamin D is a fat-soluble vitamin that is stored in the liver and adipose tissues.

Chapter Concept:

Vitamins: Classification, Sources and Deficiency Diseases

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Biomolecules

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True or False

Correct Answer: True

Solution:

Enzymes act as biocatalysts that lower the activation energy of reactions, facilitating the reaction to proceed under milder conditions.

Chapter Concept :

Enzyme Catalysis and Mechanism

Correct Answer: True

Solution:

Polypeptides with more than a hundred amino acid residues and a molecular mass above 10,000 u are typically referred to as proteins, although the distinction is not sharp.

Chapter Concept :

Peptide Linkage and Polypeptides

Correct Answer: True

Solution:

Starch is indeed a plant storage polysaccharide made of α-glucose. It consists of amylose, which is unbranched with C1–C4 links, and amylopectin, which is branched with C1–C4 and C1–C6 links.

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Biomolecules

Learning Objectives

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Revision Notes & Summary

Notes on Biomolecules

10.1 Carbohydrates

10.2 Proteins

10.3 Nucleic Acids

10.4 Vitamins

10.5 Hormones

10.6 Key Concepts

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Practice Test – MCQs, True/False

Multiple Choice Questions

Q1.What is the primary role of cellulose in plants?

Correct Answer: B

Q2.Which of the following vitamins is stored in the liver and adipose tissues?

Correct Answer: C

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Q1.Enzymes reduce the activation energy required for a reaction to occur.

Correct Answer: True

Q2.A polypeptide with more than about a hundred amino acid residues and a molecular mass above 10,000 u is generally called a protein.

Correct Answer: True

Q3.Starch is a plant storage polysaccharide composed of α-glucose units, and it includes both amylose and amylopectin.

Correct Answer: True

Q3.Which of the following best describes the amphoteric nature of amino acids?

Correct Answer: C

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Q4.Which sugar is present in the backbone of RNA?

Correct Answer: B

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Q5.Why does glucose not give Schiff's test?

Correct Answer: B

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Q6.Which of the following is a characteristic of amylopectin?

Correct Answer: C

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Q7.Which of the following statements about lactose is correct?

Correct Answer: B

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Q8.During enzyme-catalyzed reactions, what happens to the enzyme after the reaction is complete?

Correct Answer: C

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Q9.What type of linkage connects nucleotides in a DNA strand?

Correct Answer: C

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Q10.Which of the following statements about peptide bonds is true?

Correct Answer: A

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Q4.Glucose can form a cyanohydrin by reacting with hydrogen cyanide (HCNHCNHCN), confirming the presence of a carbonyl group in its structure.

Correct Answer: True

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Q5.Enzymes reduce the magnitude of activation energy for biochemical reactions, similar to chemical catalysts.

Correct Answer: True

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Q6.In DNA, the sugar moiety is β\betaβ-D-2-deoxyribose, whereas in RNA, it is β\betaβ-D-ribose.

Correct Answer: True

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Q7.Fructose forms five-membered cyclic furanose structures through the addition of the hydroxyl group at C-5 to the ketonic group.

Correct Answer: True

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Q8.Denaturation of proteins by heat or pH change destroys their secondary and tertiary structures while leaving the primary structure intact.

Correct Answer: True

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Q9.Thyroxine is an iodinated derivative of the amino acid tyrosine, and its deficiency can lead to hypothyroidism.

Correct Answer: True

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