Biological Molecules

 Biology XI Notes - Biological Molecules - Short Questions Answers

Chapter # 02 - Biology - XI

Q.1: What is biochemistry? What is the importance of biochemistry?

Ans:
Introduction of Biochemistry:
Biochemistry is a branch of biology that deals with the study of chemical processes in the bodies of living organisms.

Importance of Biochemistry:

• It helps to understand all chemical processes that occur in the bodies of living organisms, such as the formation of body structures, protein synthesis, and various metabolic processes.
• It explains abnormal reactions responsible for diseases.
• It provides information about cell differentiation. After fertilization, the zygote undergoes changes that lead to the formation of different organs in the body, such as the brain, muscles, lungs, kidneys, alimentary canal, and other organs.
• It explains cell growth.
• It provides knowledge about organs, tissues, cells, their molecules, and chemically bonded atoms.

Q.2: What are the biologically important properties of water?

Ans:
The biologically important properties of water are as follows:

• Best Solvent:
  Water is the best solvent for many substances. Most important processes occur in the presence of water.

• High Heat Capacity:
  Water absorbs and releases heat slowly. Its specific heat capacity is high, meaning it requires more heat to increase its temperature.

• High Heat of Vaporization:
  Water requires a high amount of heat energy to convert into vapor, so water molecules remain in a stable form.

• An Amphoteric Molecule:
  Water can act as both an acid and a base, making it amphoteric. In acidic conditions, it donates electrons to form H⁺ ions, and in basic conditions, it gains electrons to form OH⁻ ions.

• Due to this property, water acts as the best medium for chemical processes in living cells.

  Water acts as a buffer; it is neutral, neither acidic nor basic, with a pH of 7. This buffering helps prevent pH changes in a solution when an acid or alkali is added, helping to maintain pH stability in cellular metabolism.

  Cohesive Force in Water Molecules:
  There is an attraction among water molecules known as cohesive force, which keeps water molecules together due to hydrogen bonding. This cohesion assists in transporting substances both outside and inside cells.

Q.3: Describe the formation of organic molecules (condensation)?

Ans:
Formation of Large Molecules:
The molecules in living organisms are typically larger in size, called macromolecules (or polymers). These are formed by combining smaller molecules called micromolecules (or monomers).

Macromolecules are formed by a process known as condensation. When two micromolecules (monomers) join, a hydroxyl group (OH) from one monomer combines with a hydrogen (H) from another monomer, forming water. By releasing water, the two monomers bond to form a macromolecule. This condensation process (also known as dehydration) removes water, requiring a specific enzyme and energy.

Q.4: Describe the breaking of molecules (Hydrolysis)?

Ans:
Hydrolysis is the process in which macromolecules or polymers break down into smaller monomers or sub-units by adding water.

During hydrolysis, a water molecule splits into H⁺ and OH⁻ ions with the help of an enzyme. The H⁺ group attaches to one monomer, while the OH⁻ group attaches to another monomer, breaking the bond between the two. As a result, the two monomers separate, releasing energy.

Q.5: What is a chemical bond? Describe its types?

Ans:
A chemical bond is the attractive force that combines atoms together. Chemical bonds are of two main types.

• Ionic Bond:
  This bond is formed due to the loss or gain of electrons.

• Covalent Bond:
  This bond is formed by the mutual sharing of electrons between two atoms. In organic compounds, covalent bonds are formed between elements, and these bonds store energy.

Types of Covalent Bonds: Carbon can form three types of covalent bonds:

• Single Covalent Bond:
  This bond is formed by the mutual sharing of one electron pair between atoms.
  Example: $CH_3 - CH_3$ (Ethane)

• Double Covalent Bond:
  This bond is formed by the mutual sharing of two electron pairs between two atoms.
  Example:

  mathematica
  
      H   H
       \ /
        C = C
       / \
      H   H
  

  (Ethene)

• Triple Covalent Bond:
  This bond is formed by the mutual sharing of three electron pairs between atoms.
  Example: $H - C \equiv C - H$ (Ethyne)

Q.6: Write a note on Proteins?

Ans:
Proteins:
Proteins are complex organic compounds consisting of carbon, hydrogen, oxygen, nitrogen, sulfur, and sometimes phosphorus. They are formed by the combination of amino acids.

The amino acids are of twenty types, allowing various proteins to form through the linkage of amino acids. When amino acids are joined together, they create a polypeptide chain.

R is a side chain attached to a protein. This side chain provides additional characteristics to the compound. When two amino acids are joined by a chemical process, a dipeptide compound is formed.

When three amino acids combine, a tri-peptide compound is created, and linking many amino acids together results in a synthesized protein.

Structure of Proteins:
According to the structure of proteins, there are four types:

• Primary Structure:
  In this structure, amino acids are arranged in a linear manner in a polypeptide chain, known as the primary structure. This protein also contains disulfide (S-S) bonds.

• Secondary Structure:
  In a polypeptide chain, when amino acids are arranged in a spiral manner, it forms the secondary structure of protein, creating a helical (screw-like) structure.

• Tertiary Structure:
  When amino acids are folded or super-folded, displaying a three-dimensional structure, it is referred to as the tertiary structure of protein.

• Quaternary Structure:
  When two or more polypeptide chains are combined to form a large-sized molecule, it is known as the quaternary structure of protein (e.g., hemoglobin in blood). There are two types of quaternary proteins.

• When the peptide chains are similar, it is called homogeneous quaternary structure.
• When peptide chains are different, it is known as heterogeneous quaternary structure.  

Q.7: What are the Functions of Proteins?

Ans:
Functions of Proteins:

• Proteins form the cell structure and body of an organism.
• They form seeds and other parts in plants.
• They produce plasma membrane and other cell organs.
• Proteins create substances in animals that contribute to the formation of skin, muscles, nails, and hair.
• Hemoglobin in blood contains protein.
• Proteins assist in food digestion, muscle expansion and contraction, and blood clotting.
• They act as enzymes, which are organic catalysts that increase the rate of chemical reactions.
• Proteins function as hormones, growth factors, and gene activators.
• They also serve as antibodies, antigens, and fibrin.

Q.8: What are carbohydrates? What are their types?

Ans:
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, making up about 1% of protoplasm. They are present in all living organisms.

Based on molecular size, carbohydrates are classified into three groups:

• Mono-saccharides
• Oligo-saccharides
• Poly-saccharides

Mono-saccharides:
These are the carbohydrates that cannot be further broken down into simpler sugars, e.g., Glucose and Fructose.

Types of Mono-saccharides (Based on Carbon Atoms):

• Trioses: Sugars with 3 carbon atoms.
• Tetroses: Sugars with 4 carbon atoms.
• Pentoses: Sugars with 5 carbon atoms.
• Hexoses:
  These sugars have 6 carbon atoms.

All mono-saccharides are found in white solid crystals, have a sweet taste, and are easily soluble in water.

Mono-saccharides are commonly found in various fruits and vegetables. Glucose and fructose are present in grapes. Fructose, the sweetest sugar, is also found in honey. Lactose sugar is present in milk, with galactose as part of it. Glucose, fructose, and galactose are hexose sugars.

Oligo-Saccharides:
These carbohydrates are composed of two to ten mono-saccharide compounds. Upon hydrolysis, oligo-saccharides yield two to ten mono-saccharide molecules.

These carbohydrates form by the union of two similar or different sugars through a condensation process, releasing a water molecule. Examples of di-saccharides are as follows:

• Glucose + Glucose → Maltose + H₂O
  $(C_6H_{12}O_6) + (C_6H_{12}O_6) \rightarrow (C_{12}H_{22}O_{11})$
• Glucose + Fructose → Sucrose + H₂O
• Glucose + Galactose → Lactose + H₂O

Sucrose, a common sugar, is widely used and obtained from sugar cane or beet. It is formed by combining glucose and fructose. Sugar cane and beet are cultivated for sucrose production.

Poly-Saccharides:
Poly-saccharides are carbohydrates formed by combining many mono-saccharides, e.g., Starch, Glycogen, Cellulose, etc.

Q.9: What is starch?

Ans:
Starch:
Starch consists of many glucose molecules linked together in a straight or branched manner.

Starch is found in leaves, roots, and seeds in granular form, such as in wheat, potatoes, rice, etc. It is insoluble in water but becomes soluble when boiled. Through hydrolysis, it breaks down into simple sugars and participates in energy production through oxidation.

Q.10: What is Glycogen?

Ans:
Glycogen:
It is the animal starch. When many glucose molecules link together in a chain-like manner, glycogen forms. It is present in the muscles and liver of animals, as well as in bacteria and fungi.

Glycogen is produced by the liver, which converts excess glucose into glycogen. This glycogen is stored in the liver and muscles. When there is a shortage of glucose in the body, glycogen is converted back into glucose to provide energy. This process helps maintain the glucose balance in the blood. Glycogen is insoluble in water.

Q.11: What is Cellulose?

Ans:
Cellulose:
Cellulose is composed of many glucose molecules, about 2000 to 3000. It is only produced in plants and is a key component of the cell wall.

In animals, cellulose cannot be digested by enzymes. However, specific bacteria in the caecum of the alimentary canal assist in digesting cellulose through the action of certain enzymes.

Q.12: What are the functions of Carbohydrates?

Ans:
Functions of Carbohydrates:

• Carbohydrates produce energy through oxidation, which is used for various functions.
• Carbohydrates can be converted into other substances. In plants, excess glucose is stored as starch, which can be converted back to glucose when needed. In animals, glucose is converted into glycogen.
• Complex carbohydrate molecules form the structure of living organisms and strengthen organs. In plants, cellulose is formed from carbohydrates.

Q.13: What are Lipids? Name the different types of Lipids?

Ans:
Lipids are organic compounds such as oil, butter, waxes, fats, natural rubber, and vitamins A, E, and K, as well as steroids like cholesterol. They are composed of carbon, hydrogen, and a minimal amount of oxygen.

Bloor proposed the term "lipid" in 1943. Lipids are defined as organic compounds that are insoluble in water but soluble in Bloor’s reagent mixture of diethyl ether and ethyl alcohol (2:1 ratio) or other fat solvents like benzene, acetone, chloroform, and ether. Lipids form fatty acids upon hydrolysis.

Lipids are found in both plants and animals. In plants, they are usually present in seeds, fruits, and nuts.

Classification of Lipids:
Lipids are divided into the following groups:

• Acylglycerol (Fats and Oils)
• Waxes

Q.14: Write a note on Acylglycerol (Fats and Oils)?

Ans:
Acylglycerol (Fats and Oils):
These lipids may be neutral fats or oils.

Neutral Fats and Oils:
These are esters of fatty acids and glycerol, also known as triglycerides. When three molecules of fatty acids combine with one molecule of glycerol, a lipid molecule, triglyceride, is formed.

Oils are liquid at room temperature because they contain unsaturated fatty acids, whereas fats are solid because they contain saturated fatty acids.

Oils are rich in unsaturated fatty acids, such as groundnut oil, mustard oil, cotton oil, and olive oil. Animal fats are a mixture of saturated and unsaturated fatty acids. Oils are typically found in plants. Cotton seeds contain linolein, which has linoleic acid.

Q.15: What are Waxes?

Ans:
Waxes are esters of fatty acids with monohydroxy alcohol of long chains or cholesterol instead of alcohol. They contain an odd number of carbon atoms (C₂₅ to C₃₅). They are water-insoluble and flexible. Waxes are found in the coatings of stems, stalks, leaves, and fruits, and can be used in cosmetics, creams, ointments, and polishes.

Waxes are also used as machine lubricants. In the early days, wax was obtained from whales, but it is now sourced from a desert plant called Simmondsia chinensis or jojoba. Jojoba is rich in wax esters as storage lipids in its seeds.

Q.16: What are Phospholipids?

Ans:
Phospholipids:
Phospholipids are esters of fatty acids with alcohol, containing phosphorus. They are found in all living cells and help regulate cell permeability and transport processes.

Phospholipid molecules have two ends:

• Hydrophilic end (head): Upper part, considered water-loving.
• Hydrophobic end (tail): Lower part, considered water-fearing.

• Phospholipids are actually compound lipids because one fatty acid in the lipid is replaced by a phosphate group. Other compound lipids include Glycolipids and Sphingolipids.

  Glycolipids:
  These lipids are linked to a sugar molecule. Upon hydrolysis, they release sugar, fatty acid, and a nitrogenous compound. They are found in chloroplasts.

  Sphingolipids:
  These are found in animals but not in plants.

Q.17: Write a note on Terpenoids?

Ans:
Terpenoids:
These are large lipid compounds composed of isoprenoid units (C₅H₈ units). The main classes of terpenoids are:

• Terpenes
• Steroids
• Carotenoids

Terpenes:
Terpenes are an important class of lipids obtained through the steam distillation of plants. Some terpenoids are essential oils responsible for the fragrance of various plants, such as roses, peppermint, camphor, cloves, sandalwood, eucalyptus, citrus, and Ocimum basilicum (Niazbo).

Importance of Terpenes:

• Some terpenes are used in perfumes, e.g., Myrcene from bay oil, Geraniol from rose oil, Limonene from lemon oil, and Menthol from peppermint oil.
• Some terpenes' derivatives are present in vitamins A₁ and A₂.
• They are a significant component of chlorophyll.
• They act as intermediate compounds in cholesterol formation.
• Terpenes are also used in the synthesis of rubber and latex.

Steroids:
These fats do not form soap with NaOH or KOH. Steroids consist of three six-membered rings (A, B, and C) and one five-membered ring (D), forming a structure known as perhydro-cyclopentano-phenanthrene.

These rings are connected and contain a total of 17 carbon atoms, called the steroid nucleus. Common examples of steroidal lipids include cholesterol, vitamin D, and cortisone.

Cholesterol is found in animal tissues, nerve tissues, and the spinal cord. It is also present in egg yolks, various oils, and fats. Cholesterol is synthesized in the liver and acts as a precursor in the biosynthesis of sex hormones, vitamins, and steroidal hormones, such as cortisone.

Carotenoids:
Carotenoids are also included in this group. They are pigments that produce red, orange, yellow, and brown colors in plants. Vitamin A is a derivative of carotene, found in carrots, tomatoes, and leaves.

The red pigment carotene and yellow pigment xanthophyll are present in many plants. Tetrapyrrole pigment is present in chlorophyll and cytochromes.

Q.18: What are the functions of Lipids?

Ans:
Functions of Lipids:

• They are the main source of energy, providing double the energy of carbohydrates and proteins.
• Lipids serve as stored energy in the body, available for use when needed.
• They contribute to the structural framework of living tissues, forming plasma membranes, vacuole membranes, mitochondria, nuclei, chloroplasts, etc.
• Lipids help dissolve vitamins A and D, preventing their decomposition.
• They are a component of the electron transport system in mitochondria.
• Lipids are found in the outer cuticle of insects, making them waterproof.
• They form a protective covering on the surface of stems, leaves, and fruits.
• Waxes are used in cosmetics, ointments, creams, and polishes.

Q.19: What are Nucleic acids?

Ans:
Nucleic Acids:
Nucleic acids are organic substances containing carbon, hydrogen, oxygen, nitrogen, and phosphorus. There are two types of nucleic acids.

Deoxyribo-Nucleic Acid (DNA):
DNA is present in the nucleus. It transfers hereditary characteristics to new cells. It contains a sugar molecule called deoxyribose.

Ribo-Nucleic Acid (RNA):
RNA is formed in the nucleus and migrates to the cytoplasm. It contains ribose sugar and participates in protein synthesis.

Structure of DNA and RNA:
DNA and RNA are composed of small units called nucleotides, which consist of three parts:

• A pentose sugar (containing 5 carbon atoms)
• A phosphate group
• Nitrogen bases

• Pentose sugar is of two types:

  • Ribose Sugar: Present in RNA.
  • De-Oxyribose Sugar: Found in DNA.

  The nitrogen bases are of the following types:

  • Adenine
  • Guanine
    (These are purine-type compounds)
  • Cytosine
  • Thymine
    (These are pyrimidine-type compounds)

  Uracil:
  It is the nitrogen base present in RNA, replacing thymine.

  Function:

• DNA is located in the nucleus. It serves as the genetic material, transferring hereditary characteristics to new cells.
• RNA is formed in the nucleus by DNA and then transferred into the cytoplasm, where it participates in protein synthesis.

Q.20: What is the difference between DNA and RNA?

Ans:
Difference Between DNA & RNA:

• DNA is composed of two filaments that form a double-helix structure, linked together like a spring. RNA consists of separate filaments and does not form a double helix.
• DNA contains de-oxyribose sugar, while RNA contains ribose sugar.
• In DNA, the nitrogen base thymine is present, while in RNA, thymine is replaced by uracil.
• DNA acts as genetic material, helping to transfer hereditary traits, while RNA participates in protein synthesis.

Q.21: Write a note on Nucleotides?

Ans:
Nucleotides:
These compounds are the subunits of nucleic acids. In cells, there are three types of nucleotides:

• Mononucleotides
• Dinucleotides
• Polynucleotides

• Mononucleotides:
  These nucleotides are found singly in the cell or as part of other molecules. They are not attached to DNA or RNA. Mononucleotides contain additional phosphate groups. For example, ATP (Adenosine triphosphate) contains three phosphates and is produced from ADP (Adenosine diphosphate), which contains two phosphates. ATP is an energy-rich compound, used in various bodily functions.

  Dinucleotide:
  When two nucleotides are attached by a covalent bond, a dinucleotide molecule forms, e.g., NAD (Nicotinamide adenine dinucleotide). The two nucleotides in NAD are connected by phosphate groups.

  NAD acts as a co-enzyme, a non-protein organic part of an enzyme. It helps remove hydrogen atoms from substrates and, by gaining hydrogen, converts into NADH.

  Polynucleotide:
  When many nucleotides are linked together, they form polynucleotides. DNA and RNA are examples of polynucleotides. DNA nucleotides transfer genetic information to new cells, while RNA participates in protein synthesis.

  Genetic information is encoded in a "genetic code." Nitrogen bases in nucleic acids form different codes that interact with amino acids to control the arrangement of amino acids and the amount of protein produced.

Q.22: Explain that DNA acts as a genetic material?

Ans:
DNA as Hereditary Material:
DNA, a component of genes, contains hereditary characteristics passed to new cells or generations, establishing it as hereditary material.

Griffith demonstrated DNA as genetic material through transformation in bacteria. Living bacteria acquire DNA from dead bacteria, transforming from harmless to virulent, proving DNA’s role as genetic material.

Another proof by Hershey and Chase showed that DNA from one bacterial cell could transfer to another via bacteriophage virus. When a bacteriophage infects a bacterial cell, only its DNA enters the cell, while the outer protein coat remains outside. The viral DNA takes control of the host cell's biochemical functions.

Q.23: How DNA behaves as genetic material?

Ans:
DNA is a polynucleotide composed of deoxyribose sugar, a phosphate group, and four types of nitrogen bases. The sugar and phosphate do not play any role in forming the genetic code. The four nitrogen bases are arranged in a specific sequence, encoding information. These bases can form multiple combinations of genetic codes from 20 amino acids. DNA strands consist of billions of nucleotides, allowing for the encoding of vast amounts of information in genetic codes.

Q.24: How RNA acts as a carrier of information?

Ans:
Ribonucleic acid (RNA) serves as an information carrier from DNA to ribosomes for protein synthesis.

RNA is crucial for protein synthesis and exists in three types:

• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)

These RNA types are formed in the nucleus by DNA and transferred to the cytoplasm to participate in protein synthesis within ribosomes.

The genetic information is transferred from DNA to mRNA and then to the cytoplasm for protein synthesis in two steps:

• Transcription
• Translation

Transcription:
Transcription is the first step in protein synthesis, occurring in the nucleus. During this process, DNA produces a single-stranded RNA molecule from one of its strands. This RNA molecule is called messenger RNA (mRNA) as it carries the genetic message from DNA.

The mRNA then transports the message from the nucleus to the ribosome in the cytoplasm, indicating the formation of a specific type of protein.

Translation:
During translation, tRNA and rRNA play key roles. rRNA attaches to the ribosome, while tRNA in the cytoplasm transfers specific amino acids to the ribosome. This process translates the mRNA’s information into an amino acid sequence, producing a particular type of protein.

Q.25: What are conjugated molecules? Describe briefly the different types of conjugated molecules?

Ans:
Conjugated Molecules:
When biomolecules from two different groups combine, conjugated molecules are formed. Different types of conjugated molecules include:

• Glycolipids
• Glycoproteins (Mucoids)
• Nucleoproteins
• Lipoproteins

Glycolipids:
These are formed when fatty acids and carbohydrates combine. They also contain nitrogen and are derivatives of carbohydrate glycerides. Examples include galactolipids and sulpholipids found in chloroplasts, and cerebrosides found in the brain.

Glycoproteins (Mucoids):
When carbohydrates and proteins link, glycoproteins are formed. They are commonly found in animal and plant cells as oligo- and polysaccharides. Examples include egg albumin and gonadotropic hormone. Some glycoproteins are present in cell membranes.

Nucleoproteins:
These are combinations of proteins with nucleic acids in the nucleus. They are less acidic and soluble in water.

Lipoproteins:
When lipids and proteins link, lipoproteins form, such as cholesterol and lecithin lipids linked to simple proteins. These compounds help transport lipids in blood plasma.

Lipoproteins are found in the membranes of the endoplasmic reticulum, nuclei, mitochondria, and in the nerve sheath, chloroplasts, and bacterial membranes.

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• Phospholipids
• Terpenoids

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Biology XI Notes - The Biology - Short Questions Answers

Chapter # 01 - Biology - XI

Short Questions Answers Section I - Introduction

Q.1: What is Biology?

Ans:
Biology is one of the natural sciences, which deals with the study of living organisms. It is known as the study of life. The word "biology" is derived from Greek words "Bios" meaning life and "Logos" meaning study or knowledge.

Formerly, living organisms were classified into two kingdoms:

• Plant Kingdom:
  In this kingdom, plants were included. The study of plants is called Botany.

• Animal Kingdom:
  In this kingdom, animals were included. The study of animals is known as Zoology.

Q.2: Write a note on Five Kingdoms Classification?

Ans:
According to modern research, the old system of classification has been discarded. Now, all living organisms are classified into five kingdoms. This system was proposed by Robert Whittaker in 1969 and was later modified by two American biologists, L. Margulis and K. Schwartz.

• Kingdom - Monera:
  This kingdom includes all the prokaryotes. These are simple living organisms which do not contain a complete nucleus in their cells, e.g., Blue-green algae, Bacteria.

• Kingdom - Protoctista (Protista):
  This kingdom includes three kinds of living organisms:

  • Animal-like:
    Unicellular protozoan organisms like Amoeba, Euglena.

  • Plant-like:
    Algae - simple water-living organisms that contain chlorophyll.

  • Fungi-like:
    Slime mold, water mold.

• Kingdom - Fungi:
  This kingdom includes multi-cellular, non-green thallophytes that have a very simple body called mycelium. They have a cell wall, and their body may be coenocytic (multinucleate). Due to the absence of chlorophyll, they cannot manufacture their own food, so they are either parasites or saprophytes, e.g., Agaricus (Mushroom), Yeast.

• Kingdom - Plantae:
  This kingdom includes multi-cellular, eukaryotic organisms that have a complete nucleus in their cells. They usually contain chlorophyll and can manufacture their own food, e.g., Mustard, Sunflower, Apple.

  Kingdom - Animalia:
  This kingdom includes multi-cellular eukaryotic organisms with a complete nucleus in their cells. They do not contain chlorophyll and have no cell wall, e.g., Hydra, Earthworm, insects, Frog, Birds, Fishes, Man.

Q.3: What are the major branches or fields of Specialization of biology?

Ans:
Some major branches or fields of specialization in biology are as follows:

• Molecular Biology:
  It is a modern branch of biology focusing on the structure and function of molecules that support biological processes in living organisms, such as nucleic acids, gene structure, proteins, and protein synthesis. It is the foundation of genetic engineering.

• Microbiology:
  It deals with the study of micro-organisms, such as viruses, bacteria, and protozoans.

• Environment Biology:
  This branch studies the environment and its impact on organisms.

• Marine Biology:
  It focuses on organisms found in seawater or ocean water and examines their physical and chemical environmental characteristics.

• Fresh Water Biology:
  This branch studies life found in freshwater environments like rivers, lakes, ponds, and streams, including the physical and chemical characteristics affecting life.

• Parasitology:
  This branch examines parasitic organisms, their lifecycle, disease transmission, and interactions with hosts.

• Human Biology:
  It covers all aspects of human life, including anatomy, physiology, health, inheritance, and evolution.

• Social Biology:
  It studies the social activities of animals within populations, especially humans, and considers behavior that may be inherited from parents or developed due to environmental factors.

• Biotechnology:
  It is a very modern and recent branch of biology. It deals with the study of (i) the use of data and techniques of engineering; and (ii) technology for studying and solving problems related to living organisms, especially in human beings.

Q.4: What are the different levels of biological organization?

Ans:
All living organisms have well-organized and highly complex bodies. This biological organization is not simple and is composed of different levels, starting from the basic level of sub-atomic and atomic particles up to the high level of individual whole organisms.

The levels of biological organization are as follows:

• Atomic and sub-atomic level
• Molecular level
• Cell and organelles level
• Tissue level
• Organ and organ system level
• Individual whole organism level
• Broader levels of organization:
  • Species population
  • Community
  • Ecosystem
  • Biosphere

Q.5: Define the following terms:

• Symbiosis
• Commensalism
• Mutualism
• Parasitism

Ans:
Defining the following:

• Symbiosis:
  When two living organisms live together in a way that is beneficial to both, it is called symbiosis.

• Commensalism:
  The association between two organisms in which one benefits, while the other remains unaffected or gains no harm or benefit, is called commensalism. For example, saprophytic bacteria in the animal gut.

• Mutualism:
  This is an association between two or more organisms in which both benefit from the relationship. When separated, both species can survive independently.

• Parasitism:
  When two living organisms live together in such a way that one organism benefits and the other is harmed, it is called parasitism. For example, Plasmodium causes malaria in humans.

Q.6: What is the biological method? Explain hypothesis.

Biological Method:
The method used to solve problems in biology through observation, data collection, and experimentation is called the biological method.

Steps of the biological method are as follows:

• Observation:
  It is the identification of a biological problem. Through deep observation, the problem can be understood properly.

• Data Collection:
  Deep observation helps to gather all facts and information about the work, which has been reported by others. This process is called data collection.

• Hypothesis:
  Based on these facts and information, a tentative statement is prepared by the scientist, known as a hypothesis.

• Reasoning:
  The hypothesis can guide further observations and experiments. Reasoning can be of two types:

  • Inductive Reasoning:
    Isolated facts are used to form a general idea to explain a phenomenon.
    Example: In 1665, Robert Hooke observed a piece of cork under a microscope and identified small chambers, which he called cells. This work contributed to further cell research, leading to the cell theory by M.J. Schleiden and T. Schwann.

  • Deductive Reasoning:
    This involves general assumptions and experiments to reach a conclusion.

• Experimentation:
  In the biological method, further experiments are necessary to achieve accurate results.

• Conclusion:
  On the basis of experiments, observations, and new data collection, conclusions are drawn.

  Theory:
  A theory is presented to the world based on conclusions. If the hypothesis is true, the theory is accepted; otherwise, it is rejected.

  Law:
  When a theory is accepted and proven true, it is considered a general principle, i.e., a law.

Q.7: Write a note on the importance of biology?
OR
Application of biology for the welfare of mankind?

Ans:
Application of Biology for the Welfare of Mankind:
Biology is a very important field of science with great significance for the welfare of mankind. Its applications include:

• Helping to improve the standard of life.
• Promoting better health.
• Assisting in the protection and conservation of the environment.
• Applying modern technology in agriculture to improve the quality and quantity of crops, solving food and other essential issues.
• Utilizing modern technology in medical sciences.

Some applications of biology in medical sciences are as follows:

• Immunization:
  Immunization, or resistance against diseases, is achieved through vaccination worldwide. This technique has helped control diseases like polio, smallpox, and hepatitis, significantly reducing infection and infant mortality rates. Edward Jenner first introduced vaccination in 1795 to protect against dangerous diseases like polio, hepatitis, and smallpox.

• Antibiotics:
  Antibiotics are substances used to inhibit the growth of microorganisms. The first antibiotic, Penicillin, was isolated from the fungus Penicillium notatum. This work, done by Fleming, Flory, and Chain, earned them the Nobel Prize. Antibiotics are widely used to control diseases such as TB, cholera, leprosy, and anthrax.

• Chemotherapy:
  Biology constantly aims to develop new medicines for disease treatment. Recently, harmful diseases like AIDS and cancer are treated with certain chemicals in a process called chemotherapy.

• Radiotherapy:
  The use of radioactive rays (X-rays) is widely used in treating diseases. This technique is called radiotherapy, which is also useful for diagnosing diseases. Radiotherapy is primarily used to treat cancer, but it is expensive and painful.

  Food Shortage Due to Population Increase:
  Due to the increase in population, there is always a shortage of food and other necessities. Modern technology in agriculture and related fields can help increase food production.

Q.8: What is Hydroponics?

Ans:
Hydroponics:
It is a soil-less or water culture technique in which terrestrial plants are grown in an aerated solution. This technique allows vegetables and other plants to grow, helping meet the food needs of a particular area. Tomato crops and other vegetables are cultivated in greenhouses using this technique, yielding satisfactory production.

Advantages of Hydroponics:
Using this technique, crops can be protected from soil diseases and weeds. In dry regions, some crops can be grown successfully. For example, tomatoes and other crops are cultivated in greenhouses for production.

Q.9: What is Cloning?

Ans:
Cloning:
Cloning is a modern technique in biological science that produces duplicate copies of genetic material. It is a method of asexual reproduction, with all cloned members being genetically identical. Examples include regeneration, asexual reproduction in animals and plants, human twins, and cancer tumors.

In 1996, scientists in Scotland successfully cloned a sheep named "Dolly." This technique is successfully applied in lower mammals.

Procedure of Cloning:
In cloning, the nucleus of an egg is removed, and a nucleus from a fully developed individual is introduced into the egg.

The modified egg is then implanted into a female's womb for complete development. The resulting individual is very similar to the one whose nucleus was used.

Importance of Cloning:

• By cloning, different kinds of human cells can be prepared, such as liver cells, skin cells, and blood cells. This may enable the development of human body organs, allowing defective organs to be replaced by cloned ones.
• This technique can improve quality in agriculture and medical sciences.
• Growth hormones, insulin, and other substances can be produced through cloning.
• Cloning can reduce the area needed for cultivation.
• It can also help in determining essential minerals and understanding plant structures.

Q.10: Write a note on Protection and Conservation of Environment?

Ans:
Human beings and other organisms live in a particular environment that provides nourishment and basic needs. This environment faces damage in various ways.

• Pollution:
  Pollution harms our environment and exists in multiple forms, such as air, water, and land pollution. Acid rain, greenhouse effects, and waste matter with toxic substances contribute to pollution, directly affecting the lives of organisms.

  To protect the environment, practical methods to reduce or minimize pollution are essential. A healthy environment is necessary for all living organisms.

• Deforestation & Industrialization:
  Activities like deforestation, industrialization, and other human interventions disturb the natural biological systems in the environment.

  Conserving forests helps reduce soil erosion and flooding. By protecting various plant and animal species, a stable and balanced ecosystem can be maintained.

THE ATOMIC NUCLEUS

 

Chapter – 19

Q.1: How many neutrons and protons do the following nuclei contain?

Nuclide	Protons	Neutrons
$^{27}_{13}Al$	13	$27 - 13 = 14$
$^{40}_{18}Ar$	18	$40 - 18 = 22$
$^{138}_{56}Ba$	56	$138 - 56 = 82$
$^{207}_{82}Pb$	82	$207 - 82 = 125$
$^{28}_{14}Si$	14	$28 - 14 = 14$
$^{238}_{92}U$	92	$238 - 92 = 146$

Q.2: Do $\alpha$, $\beta$, and gamma rays come from the same element? Why do we find all three in many radioactive samples?
Ans: A radioactive element either emits $\alpha$-particles or $\beta$-particles, but never both. Gamma radiations generally accompany $\beta$-emission and, in some cases, with $\alpha$-emission.

A radioactive element (or sample) is a mixture of various nuclides of different relative abundances and with different modes of disintegration. Hence, we can find all the three types of radiations in a radioactive sample at the same time. For example, R-226 is an $\alpha$-emitter, but Ra-25 is a $\beta$-emitter.

Q.3: It is more difficult to start fusion reaction than fission. Why?
Ans: Fission is caused by captured neutrons by heavy nuclei. Neutrons, being electrically neutral, are highly penetrating particles for nuclei. But in fusion of two light nuclei, the positively charged nuclei are repelled by the repulsive forces. So work has to be done against the repulsive forces of the two nuclei.

Q.4: Is it possible that fusion of two small nuclei may occur without collision at extremely high energy?
Ans: No. Two nuclei must collide with sufficient kinetic energy to penetrate their mutual “Coulomb Barrier” and come within the range of the nuclear forces.

Q.5: Explain how a nuclear reactor produces heat as a result of fission?
Ans: In fission, the difference of binding energies of reactants and products is converted into energy. The difference of mass (0.22u) appears as energy (200 MeV). If fission takes place in a bulk solid, most of the disintegration energy appears as an increase in the internal energy of the solid, which shows a corresponding rise in temperature. This thermal energy is carried away to the heat exchanger by circulating the coolant through the reactor.

Q.6: What are the benefits and risks of nuclear reactors? Which reactor is relatively better from the point of safety?
Ans: Nuclear reactors are used to produce (i) electricity, (ii) nuclear fuels, and (iii) radioisotopes. These are peaceful uses of nuclear energy; the reactor fuel is clean burning and relatively easy to transport.

The risks of reactors include the possibility of safety hazards for the workers, environmental damage near the plant, the problem of storing highly radioactive wastes, and a limited supply of raw materials. Nuclear reactors have built-in safety devices. The accidental problems, such as leakage of radioactive substances, could occur if safety features malfunctioned. Pressurized water reactors, using water as a moderator and coolant, are safer with shut-off control rods and liquid "poison."

Q.7: Both fission and fusion apparently produce energy. How can you reconcile this with the law of conservation of energy?
Ans: In fission of U-235 with thermal neutrons, the loss of mass (0.2153 u) is converted into energy, producing about 200 MeV per fission.

In fusion, four protons may be combined to produce one helium nucleus and two positrons. Here, the losses of mass (0.027 u) are converted into energy, producing about 26 MeV. Thus, in both cases, the total "mass-energy" remains conserved.

Q.8: When a photon disappears in producing an electron and a positron, is the energy of a photon equivalent to that of the particles produced? Explain.
Ans: No, the energy of the photon is always greater than the rest mass energy of elements (electron and positron pair, 1.02 MeV). The surplus energy is taken by the two particles as their kinetic energy.

Q.9: When a neutron decays into a proton and an electron, there would be a loss of mass. What would be the energies of the products and their relative directions of motion?
Ans: Neutron is not a stable particle outside nuclei. It decays into a proton, an electron, and an antineutrino. The half-life of the free neutron is 10.8 min.

$$^0n^1 \rightarrow ^1p^1 + e^0 + \nu$$

• Mass of neutron = 1.008649 u = 939.58 MeV
• Mass of proton = 1.0072766 u = 938.23 MeV
• Mass of electron = 0.000549 u = 0.511 MeV
• Mass of proton + electron = 1.0078256 u
• Loss of mass = 1.0086469 - 1.0078256 = 0.0008393 u = 0.78 x 18 MeV

These would be the energies of the products. Due to their kinetic energies, the two particles will move apart (and not be attracted).

Q.10: Why do most moderators, used in nuclear reactors, are light atoms like $H^1, H^2,$ $C^{12}$ slow down the neutrons, and hence they are slowed?
Ans: Fast moving neutrons can be stopped when they make elastic collisions with stationary particles of the same mass. Since the mass of protons, deuterons, or graphite nuclei is comparable to the mass of neutrons, hence they are slowed.

Q.11: Can a conventional fission reactor ever explode like a bomb does? Why?
Ans: In a nuclear reactor, a fission explosion is not possible because the amount of fuel (e.g., U-235 or Pu-239) is of sub-critical mass and it can shut off control rods in emergencies. Also, liquid "poison" can be inserted directly into the moderator if other safety devices fail.

Q.12: In LMFBR, would you expect the radioactivity of the sodium coolant to include the life time of the reactor?
Ans: Yes, because sodium can capture neutrons.

$$^{11}\text{Na}^{12} + ^0\text{n}^1 \rightarrow ^{11}\text{Na}^{24} + \gamma$$

Here, Na-24 is radioactive (beta and gamma emitter) with a half-life of 15.0 h.

Q.13: Consider a sample of 1000 radioactive nuclei with a half-life T. Approximately, how many will be left after a time 3T?
Ans: The number of nuclei decayed in one half-life (T = T) are 500. Also, the number of nuclei that decay in three periods of half-life are $1000/2^3$. Hence, the number of nuclei left undecided is 125.

Q.14: What is the condition for “critical mass?”
Ans: If the mass of fissile material is such that the multiplication factor k > 1, then fission is said to occur in a critical mass. The multiplication factor is the ratio of the number of neutrons in any particular generation to the number of neutrons in the preceding generation. In a reactor, it is slightly above 1; but in a fission bomb, it is about 2.5.

Q.15: Why is heavy water more efficient as a moderator than ordinary water?
Ans: Heavy water (D₂O) has a much lower probability of capturing neutrons, but it can slow down neutrons. In fact, heavy water is 1600 times more efficient as a moderator than ordinary water (H₂O).

Q.16: In LMFBR, why is water not used as a coolant instead of liquid metal?
Ans: If water is used as a coolant in LMFBR, it slows down the neutrons through collisions and hinders the process of breeding (which requires less neutrons to convert U-238 into Pu-239). Also, the probability of capturing neutrons for water is high. Moreover, high pressure is needed to stop vaporization of water, and the core is heated up.

Sodium is a solid at room temperature but becomes liquid at 98°C. Hence, there is no need to pressurize the reactor to keep the sodium from vaporizing. Sodium is highly valued for thermal conductivity and heat transfer coefficient.

Q.17: Why are breeder reactors a necessity?
Ans: The world’s deposit of fossil fuels may not last more than 500 years, and nuclear fuels may not last for more than 5000 years. So, reactors that generate more nuclear fuels than they consume—breeder reactors—are a necessity.

Nuclear Radiations - Question Answers - Physics XII

 Q.1: Explain how you would test whether the radiation from a radioactive source is α, β or Gamma radiation?

Ans: When radiations are allowed to pass through a magnetic field, the α and β particles are deflected while γ-rays pass through undeflected. This technique helps to identify the radiation.

Q.2: A particle which produces more ionization is less penetrating. Why?
Ans: When a particle ionizes an atom, it loses a part of its energy. Since the greater the ionizing power, the greater is the loss of energy; and hence, the smaller is its penetrating power.

Q.3: It is said that α or β particles carry an atom without colliding with its electrons. How can each do so?
Ans: An α-particle is positively charged and a β particle is negatively charged. So an α particle ionizes an atom by attraction while a β particle ionizes an atom by repulsion.

Q.4: In how many ways can Gamma rays produce ionization of the atom?
Ans: Gamma rays only ionize an atom by collision. Being a high-energy photon, it can produce ionization in three ways:
i. it may lose all its energy in a single collision with the electron of an atom (photoelectric effect);
ii. it may lose only a part of its energy in a collision (Compton effect);
iii. it may be stopped by a heavy nucleus giving rise to electron-position pair (materialization of energy).

Q.5: In what way does a neutron produce ionization of an atom?
Ans: A neutron collides with a substance containing a large number of hydrogen atoms and knocks out a proton. In this way, it causes ionization.

Q.6: Name different electromagnetic radiations that are capable of producing ionization of atoms. By what process do they ionize?
Ans:
i. Ultraviolet rays
ii. X-rays
iii. Gamma rays

The rays interact with matter inelastically. They remove electrons from the atoms of the target material.

Q.7: Why is lead a better shield against α, β, and gamma radiations than an equal thickness of a water column?
Ans: α and β particles do not travel far enough in water due to intense ionization they produce. Reduction of gamma rays' beam intensity is a measure of its range, which is considerably more. However, materials having large numbers of electrons per unit volume are more effective absorbers of gamma radiations. When gamma rays are incident on lead, then, because of the photoelectric effect, they lose their energy in a single encounter and travel only a small distance. But as water has fewer electrons than lead, so gamma rays lose less energy and penetrate through a larger distance in water. Hence, lead is a better shield against gamma rays than water.

Q.8: Lead is heavier and denser than water. Yet water is more effective as a shield against neutrons?
Ans: To be stopped or slowed down, a neutron must undergo a direct collision (elastic) with a nucleus or some other particle that has a mass comparable to that of the neutron. Water contains hydrogen. Thus nuclear protons of hydrogen atoms, after collision, move; while the neutron is slowed down. But when neutrons collide with the nucleus of lead, it bounces neutrons back almost with the same speed. Hence, water is a better shield against neutrons than lead.

Q.9: In an X-ray photograph, bones show up very clearly, but the fleshy part shows very faintly. Why?
Ans: X-rays can be stopped by bones, but they can penetrate flesh.

Q.10: In a cloud chamber photograph, the path of an α particle is a thick and continuous line, whereas that of a β particle is a thin and broken line. Why?
Ans: An α-particle is highly ionizing than a β-particle.

Q.11: Why do gamma rays not give line tracks in the cloud chamber photograph?
Ans: Gamma rays do not produce ionization directly. They interact with atoms to eject electrons. These electrons, like β particles, produce irregular cloud tracks of their own, which branch out from the direction of gamma rays.

Q.12: A neutron can produce little ionization. Is there any sure chance of getting a cloud chamber track for it to count in the Geiger counter?
Ans: Neutrons are unable to ionize a gas. However, ionization is only produced when a neutron strikes directly a nucleus or a hydrogenous material, e.g., body tissues. The knocked-out proton produces ionization in the Geiger counter.

Q.13: A cloud chamber track of an α particle sometimes shows an abrupt bend accompanied by a small branched track. What could possibly be the cause of this forked track?
Ans: When an α-particle strikes a nucleus, the recoiling nucleus leaves a track. This is the cause of a forked track.

Q.14: Why is the recommended maximum dose for radiation a bit higher for women beyond the childbearing age than for young women?
Ans: It has been found that ovary and grown follicular cells are most sensitive cells for radiation. But primordial follicles and oocytes are more radiation repellent, and they grow even after irradiation. Also, the fertility of ovary is much affected when the whole body is irradiated by a specific dose of radiation (e.g., 200 RAD) than when ovary alone is irradiated by the same dose.

Q.15: It is possible for a man to burn his hand with x- or γ-rays so seriously that he must have it amputated and yet may suffer no other consequence. However, a whole-body x- or γ-ray overexposure so slight as to cause no detectable damage might cause birth deformity in one of its subsequent children. Explain. Why?
Ans: The damage to body cells, caused by very high doses of radiation, can be as serious as to stop them from working and multiplying. Widespread damage of cells may kill people. Delayed effects, such as cancer, leukemia, deformity, and mental retardation in children and grandchildren, may take place due to genetic syndromes.

Q.16: Which of α, β, and γ rays would you advise for the treatment of (i) skin cancer (ii) the cancer of flesh just under the skin (iii) a cancerous tumor deep inside the body?
Ans:
i. For the treatment of skin cancer, we use α-particles, as their penetration is small.
ii. For the treatment of cancer of flesh just under the skin, β-particles should be used because of their medium penetration power.
iii. For the treatment of deep infection in the body, γ rays should be used, as they are highly penetrating.

Q.17: Two radioisotopes of an element are available: one of long half-life and the other of short half-life. Which isotope is advisable for the treatment of a patient and why?
Ans: For the treatment, radioisotopes of short half-life should be used so that any material remaining in the body quickly decays away.

Q.18: Why are many artificially prepared radioisotopes of elements rare in nature?
Ans: Many artificially prepared radioisotopes of elements are rare in nature because of their extremely small half-life.

Q.19: Can radiocarbon dating be used to measure the age of stone walls of ancient civilizations?
Ans: No, radiocarbon dating cannot be used to measure the age of stone walls. "Carbon-14 clock" can be used for organic archaeological samples (i.e., matter that was once living). However, a "uranium clock" can be used for this purpose.

Q.20: How can a radioisotope be used to determine the effectiveness of a fertilizer?
Ans: When P-32 is given to a plant mixed with water, the amount of the chemical absorbed by various parts of the plant is checked by a G.M. counter. This technique helps to find the exact amount of the fertilizer required.

THE ATOMIC NUCLEUS

Chapter – 19

Q.1: How many neutrons and protons do the following nuclei contain?

Nuclide	Protons	Neutrons
$^{27}_{13}Al$	13	$27 - 13 = 14$
$^{40}_{18}Ar$	18	$40 - 18 = 22$
$^{138}_{56}Ba$	56	$138 - 56 = 82$
$^{207}_{82}Pb$	82	$207 - 82 = 125$
$^{28}_{14}Si$	14	$28 - 14 = 14$
$^{238}_{92}U$	92	$238 - 92 = 146$

Q.2: Do $\alpha$, $\beta$, and gamma rays come from the same element? Why do we find all three in many radioactive samples?
Ans: A radioactive element either emits $\alpha$-particles or $\beta$-particles, but never both. Gamma radiations generally accompany $\beta$-emission and, in some cases, with $\alpha$-emission.

A radioactive element (or sample) is a mixture of various nuclides of different relative abundances and with different modes of disintegration. Hence, we can find all the three types of radiations in a radioactive sample at the same time. For example, R-226 is an $\alpha$-emitter, but Ra-25 is a $\beta$-emitter.

Q.3: It is more difficult to start fusion reaction than fission. Why?
Ans: Fission is caused by captured neutrons by heavy nuclei. Neutrons, being electrically neutral, are highly penetrating particles for nuclei. But in fusion of two light nuclei, the positively charged nuclei are repelled by the repulsive forces. So work has to be done against the repulsive forces of the two nuclei.

Q.4: Is it possible that fusion of two small nuclei may occur without collision at extremely high energy?
Ans: No. Two nuclei must collide with sufficient kinetic energy to penetrate their mutual “Coulomb Barrier” and come within the range of the nuclear forces.

Q.5: Explain how a nuclear reactor produces heat as a result of fission?
Ans: In fission, the difference of binding energies of reactants and products is converted into energy. The difference of mass (0.22u) appears as energy (200 MeV). If fission takes place in a bulk solid, most of the disintegration energy appears as an increase in the internal energy of the solid, which shows a corresponding rise in temperature. This thermal energy is carried away to the heat exchanger by circulating the coolant through the reactor.

Q.6: What are the benefits and risks of nuclear reactors? Which reactor is relatively better from the point of safety?
Ans: Nuclear reactors are used to produce (i) electricity, (ii) nuclear fuels, and (iii) radioisotopes. These are peaceful uses of nuclear energy; the reactor fuel is clean burning and relatively easy to transport.

The risks of reactors include the possibility of safety hazards for the workers, environmental damage near the plant, the problem of storing highly radioactive wastes, and a limited supply of raw materials. Nuclear reactors have built-in safety devices. The accidental problems, such as leakage of radioactive substances, could occur if safety features malfunctioned. Pressurized water reactors, using water as a moderator and coolant, are safer with shut-off control rods and liquid "poison."

Q.7: Both fission and fusion apparently produce energy. How can you reconcile this with the law of conservation of energy?
Ans: In fission of U-235 with thermal neutrons, the loss of mass (0.2153 u) is converted into energy, producing about 200 MeV per fission.

In fusion, four protons may be combined to produce one helium nucleus and two positrons. Here, the losses of mass (0.027 u) are converted into energy, producing about 26 MeV. Thus, in both cases, the total "mass-energy" remains conserved.

Q.8: When a photon disappears in producing an electron and a positron, is the energy of a photon equivalent to that of the particles produced? Explain.
Ans: No, the energy of the photon is always greater than the rest mass energy of elements (electron and positron pair, 1.02 MeV). The surplus energy is taken by the two particles as their kinetic energy.

Q.9: When a neutron decays into a proton and an electron, there would be a loss of mass. What would be the energies of the products and their relative directions of motion?
Ans: Neutron is not a stable particle outside nuclei. It decays into a proton, an electron, and an antineutrino. The half-life of the free neutron is 10.8 min.

$$^0n^1 \rightarrow ^1p^1 + e^0 + \nu$$

• Mass of neutron = 1.008649 u = 939.58 MeV
• Mass of proton = 1.0072766 u = 938.23 MeV
• Mass of electron = 0.000549 u = 0.511 MeV
• Mass of proton + electron = 1.0078256 u
• Loss of mass = 1.0086469 - 1.0078256 = 0.0008393 u = 0.78 x 18 MeV

These would be the energies of the products. Due to their kinetic energies, the two particles will move apart (and not be attracted).

Q.10: Why do most moderators, used in nuclear reactors, are light atoms like $H^1, H^2,$ $C^{12}$ slow down the neutrons, and hence they are slowed?
Ans: Fast moving neutrons can be stopped when they make elastic collisions with stationary particles of the same mass. Since the mass of protons, deuterons, or graphite nuclei is comparable to the mass of neutrons, hence they are slowed.

Q.11: Can a conventional fission reactor ever explode like a bomb does? Why?
Ans: In a nuclear reactor, a fission explosion is not possible because the amount of fuel (e.g., U-235 or Pu-239) is of sub-critical mass and it can shut off control rods in emergencies. Also, liquid "poison" can be inserted directly into the moderator if other safety devices fail.

Q.12: In LMFBR, would you expect the radioactivity of the sodium coolant to include the life time of the reactor?
Ans: Yes, because sodium can capture neutrons.

$$^{11}\text{Na}^{12} + ^0\text{n}^1 \rightarrow ^{11}\text{Na}^{24} + \gamma$$

Here, Na-24 is radioactive (beta and gamma emitter) with a half-life of 15.0 h.

Q.13: Consider a sample of 1000 radioactive nuclei with a half-life T. Approximately, how many will be left after a time 3T?
Ans: The number of nuclei decayed in one half-life (T = T) are 500. Also, the number of nuclei that decay in three periods of half-life are $1000/2^3$. Hence, the number of nuclei left undecided is 125.

Q.14: What is the condition for “critical mass?”
Ans: If the mass of fissile material is such that the multiplication factor k > 1, then fission is said to occur in a critical mass. The multiplication factor is the ratio of the number of neutrons in any particular generation to the number of neutrons in the preceding generation. In a reactor, it is slightly above 1; but in a fission bomb, it is about 2.5.

Q.15: Why is heavy water more efficient as a moderator than ordinary water?
Ans: Heavy water (D₂O) has a much lower probability of capturing neutrons, but it can slow down neutrons. In fact, heavy water is 1600 times more efficient as a moderator than ordinary water (H₂O).

Q.16: In LMFBR, why is water not used as a coolant instead of liquid metal?
Ans: If water is used as a coolant in LMFBR, it slows down the neutrons through collisions and hinders the process of breeding (which requires less neutrons to convert U-238 into Pu-239). Also, the probability of capturing neutrons for water is high. Moreover, high pressure is needed to stop vaporization of water, and the core is heated up.

Sodium is a solid at room temperature but becomes liquid at 98°C. Hence, there is no need to pressurize the reactor to keep the sodium from vaporizing. Sodium is highly valued for thermal conductivity and heat transfer coefficient.

Q.17: Why are breeder reactors a necessity?
Ans: The world’s deposit of fossil fuels may not last more than 500 years, and nuclear fuels may not last for more than 5000 years. So, reactors that generate more nuclear fuels than they consume—breeder reactors—are a necessity.

THE ATOMIC SPECTRA

Q.1: The Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?
Ans: The assumption in Bohr’s theory that an electron moving around the nucleus in a certain orbit does not radiate energy is contrary to classical electrodynamics.

Q.2: Why does the hydrogen gas produced in the laboratory not glow and emit radiations?
Ans: A spectrum is given by the light emitted from an incandescent gas or vapour e.g., electric discharge through a gas or hydrogen-filled discharge tube.

Q.3: Why are the energy levels of the hydrogen atom less than zero?
Ans: The energy levels of the hydrogen atom are negative. This shows that the electron is bound (not free). Thus, one must do work (or expend energy) to remove it from the atom.

Q.4: If the hydrogen gas is bombarded by electrons of energy 13.6 eV, would you expect to observe all the lines of the hydrogen spectrum?
Ans: If a hydrogen atom is bombarded by electrons of energy 13.6 eV, it gets ionized; because 13.6 eV is the ground state energy, which is equivalent to the ionization energy. As such, no spectral lines of hydrogen will be observed.

Q.5: Hydrogen gas at room temperature absorbs light of wavelengths equal to the lines in the Lyman series but not those in the Balmer series. Explain?
Ans: Hydrogen gas at room temperature contains electrons in the ground state (p=1). If the energy supplied to the electron is such that the electron is lifted from its ground state to one of the higher allowed orbits, the atom will be excited, and it will absorb energy equal to the difference of the energies of the electron in the two states. Thus, light of wavelength equal to the lines in the Lyman series will be absorbed.

Q.6: How are x-rays different from visible radiations?
Ans: Both x-rays and visible radiations are electromagnetic waves, but x-rays differ from the visible radiations in the following features:
i. X-rays are highly penetrating. They can pass through many opaque solids such as wood or flesh but are stopped by bones and metals. Hence x-ray photographs are used in medicine.
ii. They cause ionization in gases.
iii. They can eject photoelectrons on striking some metals.
iv. They produce fluorescence in many substances like zinc sulphide, barium platinocyanide, etc.
v. They can damage living tissues if exposed to them for a longer duration.

Q.7: What property of x-rays makes them so useful in seeing otherwise invisible internal structures?
Ans: In solids, the atoms are grouped together in a regular manner. The interatomic distance in a crystal is of the order of the wavelength of x-rays. Hence a crystal is used as a 'transmission grating' to produce diffraction of x-rays. This x-ray crystallography has helped to locate the internal structure of crystal systems (called basic unit cells). Recently developed internal imaging devices (for the human body) include CT (computerized tomography) scanning, MRI (Magnetic Resonance Imaging), and PET (Positron Emission Tomography).

Q.8: Explain the difference between laser light and light from an incandescent lamp (or bulb)?
Ans:

Laser Light	Incandescent Light
i. Laser light is highly monochromatic.	i. Light from an incandescent bulb is a mixture of several wavelengths.
ii. It consists of parallel waves in a narrow beam and is highly directional (i.e., moves straight without spreading).	ii. It is emitted in all directions and spreads out.
iii. It is produced due to stimulated emission of radiation.	iii. It is produced due to spontaneous emission of radiation.

Q.9: Does the light emitted by a neon sign constitute a spectrum or only a few colors? Explain?
Ans: The luminous neon in a discharge tube has a reddish color. Its spectrum is composed of a few colors (line spectrum) of wavelength, very close to each other. So the spectral lines are closely spaced to form a band spectrum.

Q.10: Suppose you are given a glass tube having two electrodes sealed on both ends. The inside is either hydrogen or helium. How can you tell which one it is without breaking the tube?
Ans: The electrodes are connected across a voltage source. If the voltage is gradually increased, then the hydrogen-filled tube will become luminous first because its ionization potential is four times less than that of helium.

The gases can also be differentiated by taking the spectrum of each other.

Q.11: The hydrogen atom contains only a single electron and yet the hydrogen spectrum contains many lines. Why is this so?
Ans: The atoms of hydrogen can be excited to different energy levels. The excited electrons will not stay there. These will jump to the inner orbits. On de-excitation, an electron does not necessarily return to the ground state in a single jump. Rather, it may return by several jumps. Thus, several spectral lines of different frequencies are emitted, depending upon the differences of energies between the levels for the transitions. So, the spectrum of hydrogen contains many lines.

Q.12: The electron in a hydrogen atom requires energy of 10.2eV for the excitation to a higher energy level. A photon and an electron, each of energy 10.5eV, are incident on the atom. Which of these can excite the atom? Give an explanation in support of your answer.
Ans: To excite an electron, energy can be supplied to the electron by direct collision with accelerated particles as well as by the photons of energy hv. The energy of a photon must be exactly equal to the excitation energy (10.2 eV) for the bound electron; otherwise, it will not be absorbed (since it cannot transfer its energy in parts).

On the other hand, accelerated particles can give energy to the bound electron in full as well as in part. Hence an electron 10.5eV (a little higher than the excitation energy of 10.2 eV) can excite the hydrogen atom.

Q.13: Describe the atomic processes in the target of an x-ray tube whereby x-ray continuous spectra and characteristic spectra are produced?
Ans: The x-rays produced by an x-ray tube consist of two parts:
i. A series of un-interrupted wavelengths having a short cut-off wavelength (Xm) are produced when high-velocity electrons are decelerated by a heavy nucleus. This constitutes a continuous spectrum of photons, including x-rays. This process is called 'bremsstrahlung' (German for breaking radiation).
ii. A number of distinct and discreet wavelengths which constitute line (or discontinuous) spectrum of the x-rays are produced when electrons are dislodged from the inner most orbits, followed by electron jumps from the outer orbits. So characteristic spectra result from transitions to a 'hole' in an inner energy level.

Q.14: Explain clearly why x-ray emission lines in the range of 0.1nm are not observed from an x-ray tube when a low atomic number metal is used as the target in the tube?
Ans: For the production of most energetic x-rays, the electrons must be raised from deploying energy levels of the target atoms and certain electrons of innermost shell must be knocked out. The target metal with low atomic number will have x-rays of larger wavelengths. Hence, emission lines of x-rays in the range of 0.001 - 1 nm are not observed.

Q.15: Why do the frequencies of characteristic x-rays depend on the type of the material used for the target?
Ans: The transitions for the emission of characteristic x-rays depend upon the nature of the target material atoms, because frequency of x-rays (v) depends upon the atomic number (z) of the target material [v proportional z-according to Moseley’s law: v: (2.48 x 10^15 Hz) (Z - 1)^2]. Due to Moseley’s work, the characteristic x-ray spectrum became the universally accepted signature of an element.

Q.16: Does the maximum frequency in the ‘bremsstrahlung process’ depend on the nature of the target material?
Ans: No. the maximum frequency and minimum wavelength in the ‘bremsstrahlung process’ do not depend on the material.

Q.17: In laser operation, what process is required to be produced before ‘stimulated emission’?
Ans: Laser operation requires the creation of a non-equilibrium condition, called “population inversion” in which the number of atoms in a high energy state is greater than the number in a lower energy state.

Q.18: Why does laser usually emit only one particular color of light rather than several colors?
Ans: A laser beam is highly coherent and monochromatic, i.e., the emitted photons have the same frequency and wavelength. As each and every color has its own wavelength, so a laser, being monochromatic, emits only one particular color of light.