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Chapter 10 Biomolecules
Carbohydrates
Classification Of Carbohydrates
Carbohydrates are classified based on their hydrolysis behavior:
- Monosaccharides: Cannot be hydrolyzed further into simpler units (e.g., glucose, fructose).
- Oligosaccharides: Yield 2-10 monosaccharide units upon hydrolysis (e.g., disaccharides like sucrose, maltose).
- Polysaccharides: Yield a large number of monosaccharide units upon hydrolysis (e.g., starch, cellulose, glycogen).
They are also classified as reducing or non-reducing sugars based on their ability to reduce Fehling's solution and Tollens' reagent. All monosaccharides and some disaccharides (like maltose, lactose) are reducing.
Monosaccharides
Monosaccharides are classified based on the number of carbon atoms (triose, tetrose, pentose, hexose) and the presence of an aldehyde (aldose) or keto (ketose) group. Glucose ($C_6H_{12}O_6$) is an aldohexose, while fructose is a ketohexose. Glucose exists in open-chain and cyclic (pyranose) forms, with α and β anomers differing at the anomeric carbon (C1). Fructose exists in cyclic (furanose) forms.
Disaccharides
Composed of two monosaccharide units linked by a glycosidic bond. Examples include sucrose (glucose + fructose, non-reducing), maltose (glucose + glucose, reducing), and lactose (galactose + glucose, reducing).
Polysaccharides
Long chains of monosaccharide units. They serve as storage (starch in plants, glycogen in animals) or structural (cellulose in plants) molecules. Starch consists of amylose (unbranched, $C_1-C_4$ linkage) and amylopectin (branched, $C_1-C_4$ and $C_1-C_6$ linkages). Cellulose is a straight chain polymer of β-D-glucose units linked by $C_1-C_4$ glycosidic bonds.
Importance Of Carbohydrates
Carbohydrates are essential for life, serving as a primary food source (energy), storage molecules (starch, glycogen), and structural components (cellulose in cell walls, chitin in arthropod exoskeletons). They are also raw materials for industries like textiles and paper.
Proteins
Amino Acids
Proteins are polymers of α-amino acids, linked by peptide bonds. Amino acids have an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon (α-carbon), along with a side chain (R group). They are zwitterionic in solution, amphoteric, and mostly optically active (L-configuration). Amino acids are classified as essential (cannot be synthesized by the body) or non-essential (can be synthesized).
Classification Of Amino Acids
Amino acids are classified as neutral, acidic, or basic based on the relative number of amino and carboxyl groups in their side chain.
Structure Of Proteins
Proteins exhibit four levels of structure:
- Primary Structure: The specific sequence of amino acids in a polypeptide chain.
- Secondary Structure: Localized folding of the polypeptide backbone into α-helices or β-pleated sheets, stabilized by hydrogen bonds.
- Tertiary Structure: Overall three-dimensional folding of the polypeptide chain, forming fibrous or globular shapes, stabilized by hydrogen bonds, disulfide linkages, van der Waals forces, and electrostatic interactions.
- Quaternary Structure: Arrangement of multiple polypeptide subunits in a protein.
Denaturation Of Proteins
Denaturation involves the loss of secondary, tertiary, and quaternary structures due to heat or changes in pH, while the primary structure remains intact. This unfolding process leads to the loss of biological activity (e.g., coagulation of egg white).
Enzymes
Enzymes are biocatalysts, primarily globular proteins, that speed up biochemical reactions by lowering activation energy. They are highly specific for their substrates and reactions, often named after the substrate or reaction type (ending in '-ase').
Mechanism Of Enzyme Action
Enzymes bind to substrates at their active sites, forming enzyme-substrate complexes. This interaction lowers the activation energy, facilitating the conversion of substrate to product.
Vitamins
Vitamins are essential organic compounds required in small amounts for normal growth and health, but not synthesized by the body (except for some B vitamins synthesized by gut bacteria). They are classified based on solubility:
Classification Of Vitamins
- Fat-Soluble Vitamins: A, D, E, K. Stored in adipose tissue and liver.
- Water-Soluble Vitamins: B complex group and Vitamin C. Readily excreted, require regular intake.
Deficiency of vitamins leads to specific diseases (e.g., Vitamin A deficiency causes night blindness, Vitamin C deficiency causes scurvy).
Nucleic Acids
Nucleic acids (DNA and RNA) are polymers of nucleotides, responsible for heredity and protein synthesis.
Chemical Composition Of Nucleic Acids
Nucleotides consist of a nitrogenous base (Purines: Adenine, Guanine; Pyrimidines: Cytosine, Thymine in DNA; Uracil in RNA), a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Nucleosides are bases linked to sugar.
Structure Of Nucleic Acids
Nucleotides are linked by phosphodiester bonds between the 5' and 3' carbons of sugars, forming polynucleotide chains. DNA has a double-stranded helical structure where two polynucleotide chains are held together by hydrogen bonds between complementary bases (A with T, G with C). RNA is typically single-stranded but can form complex secondary structures by folding.
Biological Functions Of Nucleic Acids
DNA serves as the reserve of genetic information and is responsible for heredity, undergoing self-replication. RNA molecules (mRNA, tRNA, rRNA) are involved in protein synthesis, carrying genetic information from DNA, linking amino acids, and forming ribosomal structure/catalysis.
Hormones
Hormones are intercellular messengers produced by endocrine glands, transported by the bloodstream to target cells. They regulate various biological activities. Chemically, they can be steroids (estrogen, testosterone), polypeptides (insulin, endorphins), or amino acid derivatives (epinephrine). Hormones like insulin and glucagon regulate blood glucose, while others control growth, development, stress responses, and reproductive functions.
Intext Questions
Question 10.1. Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.
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Question 10.2. What are the expected products of hydrolysis of lactose?
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Question 10.3. How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?
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Question 10.4. The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.
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Question 10.5. Where does the water present in the egg go after boiling the egg?
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Question 10.6. Why cannot vitamin C be stored in our body?
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Question 10.7. What products would be formed when a nucleotide from DNA containing thymine is hydrolysed?
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Question 10.8. When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?
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Exercises
Question 10.1. What are monosaccharides?
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Question 10.2. What are reducing sugars?
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Question 10.3. Write two main functions of carbohydrates in plants.
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Question 10.4. Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.
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Question 10.5. What do you understand by the term glycosidic linkage?
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Question 10.6. What is glycogen? How is it different from starch?
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Question 10.7. What are the hydrolysis products of
(i) sucrose and
(ii) lactose?
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Question 10.8. What is the basic structural difference between starch and cellulose?
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Question 10.9. What happens when D-glucose is treated with the following reagents?
(i) HI
(ii) Bromine water
(iii) $HNO_3$
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Question 10.10. Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.
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Question 10.11. What are essential and non-essential amino acids? Give two examples of each type.
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Question 10.12. Define the following as related to proteins
(i) Peptide linkage
(ii) Primary structure
(iii) Denaturation.
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Question 10.13. What are the common types of secondary structure of proteins?
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Question 10.14. What type of bonding helps in stabilising the $\alpha$-helix structure of proteins?
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Question 10.15. Differentiate between globular and fibrous proteins.
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Question 10.16. How do you explain the amphoteric behaviour of amino acids?
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Question 10.17. What are enzymes?
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Question 10.18. What is the effect of denaturation on the structure of proteins?
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Question 10.19. How are vitamins classified? Name the vitamin responsible for the coagulation of blood.
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Question 10.20. Why are vitamin A and vitamin C essential to us? Give their important sources.
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Question 10.21. What are nucleic acids? Mention their two important functions.
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Question 10.22. What is the difference between a nucleoside and a nucleotide?
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Question 10.23. The two strands in DNA are not identical but are complementary. Explain.
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Question 10.24. Write the important structural and functional differences between DNA and RNA.
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Question 10.25. What are the different types of RNA found in the cell?
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