Class 9 Biology Notes Chapter 6

Chapter 6: Biomolecules

Exercise Solution

A. Select the Correct Answers for the Following Questions

1. What is the primary function of carbohydrates?

a) Provide energy
b) Act as enzymes
c) Regulate processes
d) Make membranes

Correct Answer: a) Provide energy

2. How will you differentiate between monosaccharides and polysaccharides?

a) Monosaccharides are single sugars.
b) Polysaccharides are sweet in taste.
c) Monosaccharides are present in plant cell walls.
d) Polysaccharides dissolve easily.

Correct Answer: a) Monosaccharides are single sugars.

3. What is true about cellulose?

a) It is sweet in taste.
b) It is digestible by the human digestive system.
c) It provides structural support in plants.
d) It is soluble in water.

Correct Answer: c) It provides structural support in plants.

4. Which of the following proteins is involved in oxygen transport?

a) Insulin
b) Haemoglobin
c) Collagen
d) Keratin

Correct Answer: b) Haemoglobin

5. Which component of an amino acid determines its unique properties?

a) Amino group
b) Carboxyl group
c) R group (side group)
d) Hydrogen group

Correct Answer: c) R group (side group)

6. Which proteins are involved in defence against pathogens?

a) Antibodies
b) Myosin
c) Fibrinogen
d) Haemoglobin

Correct Answer: a) Antibodies

7. Which of the following are the units of most lipids?

a) Amino acids
b) Fatty acids and glycerol
c) Nucleotides
d) Simple sugars

Correct Answer: b) Fatty acids and glycerol

8. How do unsaturated fatty acids differ from saturated fatty acids?

a) They have more hydrogen atoms.
b) They contain double bonds in their hydrocarbon chains.
c) They are solid at room temperature.
d) They are found only in animal fats.

Correct Answer: b) They contain double bonds in their hydrocarbon chains.

9. Which of the following is NOT a function of proteins?

a) Transport oxygen in the blood.
b) Carry genetic information.
c) Help in digesting food.
d) Fight against pathogens.

Correct Answer: b) Carry genetic information.

10. Which components make up a nucleotide?

a) Sugar, phosphate, nitrogenous base
b) Amino acid, sugar, nitrogenous base
c) Fatty acid, phosphate, nitrogenous base
d) Protein, sugar, nitrogenous base

Correct Answer: a) Sugar, phosphate, nitrogenous base

11. Which nitrogenous base is found in RNA but not in DNA?

a) Adenine
b) Thymine
c) Uracil
d) Guanine

Correct Answer: c) Uracil

B. Write Short Answers

1. What are the main functions of carbohydrates in the body?

Answer: Carbohydrates serve as the primary and immediate source of cellular energy. They also act as structural components in cell walls (cellulose) and exoskeletons, and function as recognition molecules on cell surfaces.

2. How do the three groups of carbohydrates differ in taste?

Answer:

• Monosaccharides: Sweet in taste and highly soluble in water.

• Disaccharides: Also sweet in taste and soluble in water.

• Polysaccharides: Generally bland or non-sweet in taste and usually insoluble or poorly soluble in water.

3. Name two common monosaccharides and two disaccharides.

Answer:

• Monosaccharides: Glucose and fructose.

• Disaccharides: Sucrose and lactose.

4. Which monosaccharides make a sucrose molecule?

Answer: Sucrose is composed of one molecule of glucose and one molecule of fructose linked together.

5. Give an example of a storage polysaccharide in plants.

Answer: Starch.

6. Define amino acid and draw its structure.

Answer: An amino acid is an organic molecule containing an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group) attached to a central carbon atom.

General Structure of an Amino Acid:

    H
    |
H₂N—C—COOH
    |
    R

7. What are the basic components of lipids? Draw their structures.

Answer: The basic components of most common lipids, such as triglycerides, are glycerol and three fatty acid chains. Glycerol is a three-carbon alcohol with three hydroxyl groups, while fatty acids consist of long hydrocarbon chains ending in a carboxyl group.

General Structure of Glycerol:

CH₂OH
  |
CHOH
  |
CH₂OH

General Structure of a Fatty Acid:

CH₃—(CH₂)n—COOH

8. What are the types of RNA? Write their functions.

Answer:

1. mRNA (Messenger RNA): Carries copies of the genetic code from DNA in the nucleus to ribosomes for protein synthesis.

2. tRNA (Transfer RNA): Transports specific amino acids to the ribosomes during protein translation.

3. rRNA (Ribosomal RNA): Forms the structural core of ribosomes and helps catalyse peptide bond formation.

9. Briefly describe the function of DNA.

Answer: DNA stores, preserves, and transmits hereditary genetic information from generation to generation. It also directs cellular activities by controlling protein synthesis.

C. Write Answers in Detail

1. Write a comprehensive note on the structures and roles of the three classes of carbohydrates.

Answer:

1. Monosaccharides (Simple Sugars): These consist of single sugar units containing three to seven carbon atoms. Examples include glucose and ribose. They provide immediate cellular energy and serve as building blocks for larger molecules.

2. Disaccharides: These are formed when two monosaccharides join through a glycosidic bond in a condensation reaction. Examples include sucrose, maltose, and lactose. They serve as transportable sources of energy.

3. Polysaccharides: These are complex polymers composed of many monosaccharide units linked in long chains. Examples include starch, glycogen, and cellulose. They function as energy-storage molecules in plants (starch) and animals (glycogen), and as structural components in plant cell walls (cellulose).

2. Discuss the functions of proteins.

Answer: Proteins perform diverse and essential roles in living organisms.

1. Enzymatic Catalysis: Enzymes accelerate metabolic and biochemical reactions.

2. Structural Support: Proteins provide mechanical strength and structural support. For example, collagen is found in skin and bones, while keratin is found in hair and nails.

3. Transport: Proteins transport vital molecules across membranes or through the blood. For example, haemoglobin transports oxygen.

4. Defence and Immunity: Antibodies help neutralize foreign pathogens and toxins.

5. Hormonal Regulation: Some proteins regulate physiological processes. For example, insulin regulates blood glucose levels.

6. Movement: Contractile proteins, such as actin and myosin, enable muscle contraction.

3. What are fatty acids? Explain their types.

Answer: Fatty acids are carboxylic acids containing long hydrocarbon chains. They are divided into two main types.

1. Saturated Fatty Acids: These contain only single covalent bonds between carbon atoms in their hydrocarbon chains. They generally pack closely together and are commonly found in animal fats and butter, which are often solid at room temperature.

2. Unsaturated Fatty Acids: These contain one or more double bonds in their carbon chains. The bends caused by many double bonds prevent tight packing, so foods rich in these fatty acids, such as many plant oils and olive oil, are generally liquid at room temperature.

4. Describe the basic structure of fats and oils. Explain how lipids play a role in energy storage.

Answer:

Structure: Fats and oils, commonly known as triglycerides, are formed from one glycerol molecule chemically bonded to three fatty acid molecules.

Role in Energy Storage: Lipids contain a high proportion of carbon–hydrogen bonds and store more than twice as much energy per gram as carbohydrates. Because they are hydrophobic (insoluble in water), they can be stored compactly in adipose tissues without adding substantial water weight. Thus, they serve as long-term energy reserves.

5. Explain the double-helix structure of DNA and discuss the base pairing in this structure.

Answer:

Double-Helix Structure: The double-helix model of DNA is associated with James Watson and Francis Crick. DNA consists of two polynucleotide strands wound around a central axis in a spiral shape. The backbone of each strand consists of alternating deoxyribose sugar and phosphate groups, while the nitrogenous bases project inward like the rungs of a ladder.

Base Pairing: Nitrogenous bases pair specifically through hydrogen bonds:

• Adenine (A) pairs with Thymine (T) through two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) through three hydrogen bonds.

These complementary base pairs are essential for accurate DNA replication and the reliable transmission of genetic information.

6. Explain how the information in DNA is converted to RNA and then to proteins.

Answer: This process is known as the central dogma of molecular biology.

1. Transcription: The enzyme RNA polymerase reads a DNA gene sequence and synthesizes a complementary messenger RNA (mRNA) strand.

2. RNA Processing: In eukaryotic cells, the initial RNA transcript is processed to produce mature mRNA. The mRNA then leaves the nucleus through nuclear pores and enters the cytoplasm, where it can reach a ribosome.

3. Translation: At the ribosome, the genetic code on mRNA is read in groups of three bases called codons. Transfer RNA (tRNA) molecules bring specific amino acids corresponding to these codons. The ribosome links the amino acids through peptide bonds to form a polypeptide chain, which folds into a functional protein.

D. Inquisitive Questions

1. Evaluate the importance of water in the functioning of biomolecules.

Answer: Water is a universal biological solvent in which many biomolecules dissolve and biochemical reactions occur. Its polarity allows it to dissolve ionic and polar substances, facilitating biochemical reactions. Water also participates in hydrolysis reactions and is released during many condensation reactions. Furthermore, water helps stabilize the three-dimensional structures of proteins and nucleic acids through hydrogen bonding and contributes to the organization of cell membranes through hydrophobic interactions.

2. Find the amount or percentage of carbohydrates, proteins, fats, and water in the following food products and compare them with each other.

Answer: The amounts of carbohydrates, proteins, fats, and water vary among food products. The following is a general comparison of common foods.

1. Bread: Bread is rich in carbohydrates, mainly starch. It contains moderate amounts of protein, small amounts of fat, and some water.

2. Burger: A burger contains carbohydrates from the bun, protein and fat from the meat or other filling, and water in varying amounts. Its nutritional composition depends on the ingredients and portion size.

3. Cucumber: Cucumber contains approximately 95–96% water. It has small amounts of carbohydrates, including sugars and fibre, and very low amounts of protein and fat.

4. Egg: A whole egg contains approximately 75% water, around 12–13% protein, and about 10–11% fat. It contains very little carbohydrate.

5. Rice (500 g): The nutritional composition depends on whether the rice is weighed dry or cooked. Cooked rice contains substantial water and is rich in carbohydrates, with smaller amounts of protein and very little fat. Dry rice contains a much higher percentage of carbohydrates because it has much less water.

6. Potato: Raw potato contains approximately 75–80% water and is rich in starch. It also contains small amounts of protein and very little fat.

Comparison:

• Carbohydrates: Rice, bread, and potatoes are important sources of carbohydrates and energy.

• Proteins: Eggs are a good source of protein. Burgers may also provide substantial protein, depending on their ingredients.

• Fats: Eggs and many burgers contain more fat than cucumber, potatoes, or plain cooked rice.

• Water: Cucumber has the highest water content among these examples. Eggs and potatoes also contain considerable water, while dry rice and bread contain much less.

Note: These percentages are approximate. Exact nutritional values depend on the food variety, recipe, serving size, and whether the food is raw, cooked, or processed.

Notes by Sir Dilawar

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