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આ MCQ મોડ્યુલ આના પર આધારિત છે: Vitamins Nucleic Acids Hormones
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Vitamins Nucleic Acids Hormones
🎓 Class 12
Chemistry
CBSE
Theory
Ch 10 – Biomolecules
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Vitamins Nucleic Acids Hormones
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Vitamins, Nucleic Acids and Hormones
This part completes the survey of biomolecules. Vitamins are needed in tiny amounts yet their absence causes specific diseases. Nucleic acids carry heredity. Hormones act as chemical messengers. All three show the same lesson as carbohydrates and proteins — structure determines function.
10.4 Vitamins
Certain organic compounds are required in small amounts in our diet, but their deficiency causes specific diseases. These compounds are called vitamins. Most vitamins cannot be synthesised in our body, but plants can synthesise almost all of them, so they are considered essential food factors. However, the bacteria of the gut can produce some of the vitamins required by us.
Definition. Vitamins are organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism. Different vitamins belong to various chemical classes and it is difficult to define them on the basis of structure.
Vitamins are designated by alphabets A, B, C, D and so on, and some are further named as sub-groups such as B₁, B₂, B₆ and B₁₂.
Excess of vitamins is also harmful, and vitamin pills should not be taken without the advice of a doctor.
Where the name came from. The term "vitamine" was coined from vital + amine, since the earlier identified compounds had amino groups. Later work showed that most of them did not contain amino groups, so the letter 'e' was dropped and the term vitamin is used these days.
10.4.1 Classification of vitamins
Vitamins are classified into two groups depending upon their solubility in water or fat.
| Group | Members | Storage in body | Dietary requirement |
|---|---|---|---|
| Fat soluble — soluble in fat and oils but insoluble in water | A, D, E and K | stored in liver and adipose (fat storing) tissues | need not be supplied daily |
| Water soluble | B group vitamins and vitamin C | cannot be stored (except vitamin B₁₂); readily excreted in urine | must be supplied regularly in diet |
Table 10.3 — Some important vitamins, their sources and deficiency diseases
| Sl. No. | Vitamin | Sources | Deficiency disease |
|---|---|---|---|
| 1 | Vitamin A | fish liver oil, carrots, butter and milk | Xerophthalmia (hardening of cornea of eye); night blindness |
| 2 | Vitamin B₁ (Thiamine) | yeast, milk, green vegetables and cereals | Beri beri (loss of appetite, retarded growth) |
| 3 | Vitamin B₂ (Riboflavin) | milk, egg white, liver, kidney | Cheilosis (fissuring at corners of mouth and lips), digestive disorders and burning sensation of the skin |
| 4 | Vitamin B₆ (Pyridoxine) | yeast, milk, egg yolk, cereals and grams | Convulsions |
| 5 | Vitamin B₁₂ | meat, fish, egg and curd | Pernicious anaemia (RBC deficient in haemoglobin) |
| 6 | Vitamin C (Ascorbic acid) | citrus fruits, amla and green leafy vegetables | Scurvy (bleeding gums) |
| 7 | Vitamin D | exposure to sunlight, fish and egg yolk | Rickets (bone deformities in children) and osteomalacia (soft bones and joint pain in adults) |
| 8 | Vitamin E | vegetable oils like wheat germ oil, sunflower oil | Increased fragility of RBCs and muscular weakness |
| 9 | Vitamin K | green leafy vegetables | Increased blood clotting time |
10.5 Nucleic Acids
Every generation of each species resembles its ancestors in many ways. It has been observed that the nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity. The particles in the nucleus responsible for heredity are called chromosomes, which are made up of proteins and another type of biomolecule called nucleic acids.
These are mainly of two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long chain polymers of nucleotides, they are also called polynucleotides.
10.5.1 Chemical composition of nucleic acids
Complete hydrolysis of DNA (or RNA) yields three things: a pentose sugar, phosphoric acid, and nitrogen-containing heterocyclic compounds called bases.
| Component | DNA | RNA |
|---|---|---|
| Sugar | β-D-2-deoxyribose | β-D-ribose |
| Bases | adenine (A), guanine (G), cytosine (C), thymine (T) | adenine (A), guanine (G), cytosine (C), uracil (U) |
| Acid | phosphoric acid in both | |
The one-word difference. DNA and RNA share three bases — A, G and C. The fourth base is thymine in DNA and uracil in RNA. And the sugar differs by a single oxygen atom: ribose in RNA, 2-deoxyribose in DNA.
10.5.2 Structure of nucleic acids
Nucleoside versus nucleotide — the distinction most often asked.
A nucleoside is formed by the attachment of a base to the 1′ position of the sugar.
A nucleotide is formed when a nucleoside is linked to phosphoric acid at the 5′ position of the sugar moiety.
In nucleosides the sugar carbons are numbered 1′, 2′, 3′ and so on, in order to distinguish them from the bases.
A nucleoside is formed by the attachment of a base to the 1′ position of the sugar.
A nucleotide is formed when a nucleoside is linked to phosphoric acid at the 5′ position of the sugar moiety.
In nucleosides the sugar carbons are numbered 1′, 2′, 3′ and so on, in order to distinguish them from the bases.
Nucleotides are joined together by a phosphodiester linkage between the 5′ and 3′ carbon atoms of the pentose sugar. The formation of a typical dinucleotide follows this pattern, and the chain extends as an alternating backbone of sugar and phosphate with the bases projecting from it.
Primary structure of a nucleic acid. Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary structure.
Secondary structure — the Watson and Crick double helix
Nucleic acids have a secondary structure also. James Watson and Francis Crick gave a double strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases.
Complementary base pairing. The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases: adenine forms hydrogen bonds with thymine, whereas cytosine forms hydrogen bonds with guanine.
In the secondary structure of RNA a single stranded helix is present, which sometimes folds back on itself. RNA molecules are of three types and perform different functions: messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).
10.5.3 Biological functions of nucleic acids
DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self duplication during cell division, and identical DNA strands are transferred to daughter cells.
Another important function of nucleic acids is protein synthesis in the cell. Actually, the proteins are synthesised by various RNA molecules in the cell, but the message for the synthesis of a particular protein is present in DNA.
DNA fingerprinting. Every individual has unique fingerprints at the tips of the fingers, but these can be altered by surgery. A sequence of bases on DNA is also unique for a person, and information regarding this is called DNA fingerprinting. It is the same for every cell and cannot be altered by any known treatment. It is used (i) in forensic laboratories for identification of criminals, (ii) to determine paternity of an individual, (iii) to identify dead bodies in accidents by comparing the DNA of parents or children, and (iv) to identify racial groups to rewrite biological evolution.
10.6 Hormones
Hormones are molecules that act as intercellular messengers. They are produced by endocrine glands in the body and are poured directly into the blood stream, which transports them to the site of action.
Chemical nature
| Chemical class | Examples |
|---|---|
| Steroids | estrogens and androgens |
| Polypeptides | insulin and endorphins |
| Amino acid derivatives | epinephrine and norepinephrine |
Functions
Hormones help to maintain the balance of biological activities in the body. The role of insulin in keeping the blood glucose level within a narrow limit is an example: insulin is released in response to a rapid rise in blood glucose level, while the hormone glucagon tends to increase the glucose level. The two hormones together regulate glucose in the blood. Epinephrine and norepinephrine mediate responses to external stimuli, and growth hormones and sex hormones play a role in growth and development.
Thyroxine and iodine
Thyroxine, produced in the thyroid gland, is an iodinated derivative of the amino acid tyrosine. An abnormally low level of thyroxine leads to hypothyroidism, characterised by lethargy and obesity, while an increased level causes hyperthyroidism. A low level of iodine in the diet may lead to hypothyroidism and enlargement of the thyroid gland — a condition largely controlled by adding sodium iodide to commercial table salt, that is, iodised salt.
Steroid hormones
Steroid hormones are produced by the adrenal cortex and gonads (testes in males and ovaries in females). Glucocorticoids control carbohydrate metabolism, modulate inflammatory reactions and are involved in reactions to stress. Mineralocorticoids control the level of excretion of water and salt by the kidney. If the adrenal cortex does not function properly, one result may be Addison's disease, characterised by hypoglycemia, weakness and increased susceptibility to stress; the disease is fatal unless treated by glucocorticoids and mineralocorticoids.
Hormones released by the gonads are responsible for the development of secondary sex characters. Testosterone is the major sex hormone produced in males and is responsible for development of secondary male characteristics. Estradiol is the main female sex hormone, responsible for development of secondary female characteristics and participating in the control of the menstrual cycle. Progesterone is responsible for preparing the uterus for implantation of the fertilised egg.
🧪 Activity 10.5 — Reading a DNA strand and writing its partnerL3 Apply
Complementary base pairing is not just a fact to memorise — it is the mechanism by which genetic information is copied. This activity makes that concrete.
Predict: One strand of a DNA fragment reads A–T–G–C–C–A–T–G. Before reading on, write the sequence of the partner strand, and then predict what the corresponding RNA sequence would be.
- Write out the given strand with space beneath each base.
- Under each base write its complementary partner, using the rule that adenine pairs with thymine and cytosine pairs with guanine.
- Count how many A–T pairs and how many C–G pairs you have made.
- Now rewrite the partner strand as it would appear in RNA, remembering which base RNA uses in place of thymine.
- Explain in one sentence why this pairing rule allows DNA to be copied exactly during cell division.
Given strand: A–T–G–C–C–A–T–G
Complementary DNA strand: T–A–C–G–G–T–A–C
Count: four A–T pairs and four C–G pairs. Note that the partner strand is not identical to the original but is complementary to it — a distinction worth stating carefully in an examination answer.
As RNA: U–A–C–G–G–U–A–C. RNA contains adenine, guanine and cytosine just as DNA does, but its fourth base is uracil instead of thymine, so every T becomes U.
Why this permits exact copying. Because the pairing is specific — A only with T, C only with G — each strand carries complete information about the other. During cell division the two strands separate and each acts as a template on which its partner is rebuilt, so a DNA molecule is capable of self duplication and identical DNA strands are transferred to daughter cells. This is precisely how DNA functions as the chemical basis of heredity and the reserve of genetic information.
Intext questions
Intext 10.6 — Why cannot vitamin C be stored in our body?
Vitamin C (ascorbic acid) is a water soluble vitamin. Water soluble vitamins are readily excreted in urine and cannot be stored in the body — the only exception among them being vitamin B₁₂. They must therefore be supplied regularly in the diet. Fat soluble vitamins A, D, E and K behave differently because they are stored in the liver and adipose tissues.
Intext 10.7 — Products of hydrolysing a DNA nucleotide containing thymine
A nucleotide consists of a base attached to the 1′ position of the sugar and phosphoric acid attached at the 5′ position. Complete hydrolysis of a thymine-containing DNA nucleotide therefore gives three products: thymine, the sugar β-D-2-deoxyribose, and phosphoric acid.
Intext 10.8 — What the base ratios of RNA suggest about its structure
In DNA the strands are held together by hydrogen bonds between specific pairs of bases — adenine with thymine and cytosine with guanine — so the amount of adenine always equals that of thymine, and cytosine equals guanine. If hydrolysis of RNA shows no such relationship among the quantities of the different bases, it follows that RNA does not have two complementary strands paired in this way. This supports the conclusion that RNA has a single stranded helical structure, which sometimes folds back on itself.
Competency-Based Questions
A public health team is planning a nutrition programme in a coastal district. Surveys report three problems: children with bone deformities, adults with bleeding gums, and a high incidence of enlarged thyroid glands. The team must also advise a laboratory that is analysing a nucleic acid sample of unknown type.
1. Identify the vitamin deficiency behind each of the first two problems and name a dietary source for each. L2 Understand
Bone deformities in children is rickets, caused by deficiency of vitamin D; sources are exposure to sunlight, fish and egg yolk. In adults the corresponding condition is osteomalacia. Bleeding gums is scurvy, caused by deficiency of vitamin C (ascorbic acid); sources are citrus fruits, amla and green leafy vegetables.
2. Explain the chemical basis of the thyroid problem and the standard public health remedy. L3 Apply
Thyroxine, produced in the thyroid gland, is an iodinated derivative of the amino acid tyrosine. A low level of iodine in the diet means insufficient thyroxine can be made, leading to hypothyroidism and enlargement of the thyroid gland. The remedy, which has largely controlled the condition, is the addition of sodium iodide to commercial table salt — iodised salt.
3. The team must decide which vitamins need daily supply and which can be given periodically. Justify the split. L4 Analyse
Daily supply needed: the B group vitamins and vitamin C, because they are water soluble, are readily excreted in urine and cannot be stored in the body — with the single exception of vitamin B₁₂. Periodic supply acceptable: vitamins A, D, E and K, because they are fat soluble and are stored in the liver and adipose tissues. A caution should accompany the programme, since excess of vitamins is also harmful and vitamin pills should not be taken without medical advice.
4. The laboratory finds the sample contains ribose and uracil. Identify the nucleic acid and state the three types of this molecule. L2 Understand
The presence of β-D-ribose as the sugar and uracil as the fourth base identifies it as RNA. DNA would instead contain β-D-2-deoxyribose and thymine. The three types of RNA are messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA), and they perform different functions in protein synthesis.
5. A student states that the two strands of DNA are identical because DNA can copy itself exactly. Evaluate this statement. L5 Evaluate
The conclusion is right but the reasoning is wrong. The two strands of DNA are not identical — they are complementary. They are held together by hydrogen bonds formed between specific pairs of bases: adenine pairs only with thymine and cytosine only with guanine. So wherever one strand has A the other has T, and wherever one has C the other has G; reading the two strands gives different sequences. It is precisely this complementarity, not identity, that makes exact copying possible. Because each base determines its partner uniquely, each strand carries full information about the other, so when the strands separate during cell division each can serve as a template to rebuild its partner. The DNA molecule is therefore capable of self duplication and identical DNA strands are transferred to daughter cells. A worked check: if one strand reads A–T–G–C, its partner reads T–A–C–G, which is a different sequence — yet either one is sufficient to regenerate the pair.
Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Vitamin C must be supplied regularly in the diet.
Reason (R): Vitamin C is water soluble, is readily excreted in urine and cannot be stored in the body.
Answer: A. Water soluble vitamins — the B group and vitamin C — cannot be stored, except vitamin B₁₂, so they must be supplied regularly. Fat soluble vitamins A, D, E and K are stored in liver and adipose tissue.
Assertion (A): A nucleoside and a nucleotide are the same thing.
Reason (R): Both contain a nitrogenous base attached to a pentose sugar.
Answer: D. The assertion is false — a nucleoside is a base attached to the 1′ position of the sugar, while a nucleotide is a nucleoside additionally linked to phosphoric acid at the 5′ position. The reason is true of both, but the presence or absence of the phosphate is exactly what distinguishes them.
Assertion (A): Hydrolysis of RNA shows no fixed relationship among the quantities of the different bases.
Reason (R): RNA has a single stranded helical structure and so lacks the complementary base pairing found in DNA.
Answer: A. In double stranded DNA the specific A–T and C–G pairing forces the amounts of the paired bases to be equal. Since RNA is single stranded, no such constraint applies and the base quantities are unrelated.
Coming next. Part 6 gives the chapter summary and complete worked solutions to all twenty-five NCERT exercises of Chapter 10.
Frequently Asked Questions
How are vitamins classified and which ones must be taken daily?
Vitamins are classified by solubility. Fat soluble vitamins A, D, E and K dissolve in fat and oils but not water and are stored in the liver and adipose tissues. Water soluble vitamins — the B group and vitamin C — are readily excreted in urine and cannot be stored, except vitamin B₁₂, so they must be supplied regularly in the diet.
Which vitamin deficiencies cause scurvy, rickets, beri beri and night blindness?
Scurvy, with bleeding gums, is caused by deficiency of vitamin C, found in citrus fruits, amla and green leafy vegetables. Rickets in children and osteomalacia in adults are caused by deficiency of vitamin D, obtained from sunlight, fish and egg yolk. Beri beri, with loss of appetite and retarded growth, is caused by deficiency of vitamin B₁ (thiamine). Night blindness and xerophthalmia are caused by deficiency of vitamin A, found in fish liver oil, carrots, butter and milk.
What is the difference between a nucleoside and a nucleotide?
A nucleoside is formed by attaching a nitrogenous base to the 1′ position of the pentose sugar. A nucleotide is formed when that nucleoside is further linked to phosphoric acid at the 5′ position of the sugar. In short, nucleotide equals nucleoside plus phosphate, and it is the nucleotide that is the monomer unit of nucleic acids.
What are the differences between DNA and RNA?
DNA contains the sugar β-D-2-deoxyribose while RNA contains β-D-ribose. Both contain adenine, guanine and cytosine, but the fourth base is thymine in DNA and uracil in RNA. DNA has a double strand helical structure with complementary base pairing, whereas RNA is single stranded and sometimes folds back on itself. DNA is the chemical basis of heredity, while RNA molecules — m-RNA, r-RNA and t-RNA — carry out protein synthesis.
Why are the two strands of DNA described as complementary rather than identical?
The strands are held together by hydrogen bonds between specific pairs of bases: adenine pairs only with thymine and cytosine only with guanine. So where one strand has A the other has T, and where one has C the other has G — the sequences are different but each determines the other exactly. This complementarity is what allows each strand to act as a template so that DNA can self duplicate during cell division.
What are hormones and what are their chemical types?
Hormones are molecules that act as intercellular messengers. They are produced by endocrine glands and poured directly into the blood stream, which carries them to the site of action. Chemically some are steroids such as estrogens and androgens, some are polypeptides such as insulin and endorphins, and some are amino acid derivatives such as epinephrine and norepinephrine.
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