Carbohydrates
Carbohydrates — Study Notes
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10.1 Carbohydrates
Explanation10.1 Carbohydrates
Carbohydrates are a large group of naturally occurring organic compounds primarily produced by plants. They are essential biomolecules that serve as a major source of energy and structural components in living organisms. Common examples include cane sugar (sucrose), glucose, and starch. Chemically, carbohydrates were initially thought to be hydrates of carbon, having a general formula Cx(H2O)y. For example, glucose has the molecular formula C6H12O6, which fits the formula C6(H2O)6. However, not all compounds fitting this formula are carbohydrates; for instance, acetic acid (CH3COOH) fits C2(H2O)2 but is not a carbohydrate. Conversely, rhamnose (C5H12O5) is a carbohydrate but does not fit the simple hydrate definition. Carbohydrates are optically active polyhydroxy aldehydes or ketones or compounds that yield such units upon hydrolysis. Some carbohydrates are sweet and are called sugars, such as sucrose (common table sugar) and lactose (milk sugar). The term saccharides is also used for carbohydrates, derived from the Greek word 'sakcharon' meaning sugar. Carbohydrates play vital roles in biological systems, including energy storage, structural functions, and as components of nucleic acids and cell membranes.
- Carbohydrates are primarily plant-produced organic compounds.
- General formula often approximated as Cx(H2O)y but with exceptions.
- Defined chemically as optically active polyhydroxy aldehydes or ketones or their hydrolysis products.
- Some carbohydrates are sweet and called sugars; sucrose and lactose are common examples.
- Also called saccharides from Greek origin.
- They are essential biomolecules involved in energy and structural roles.
- 📌 Carbohydrates: Optically active polyhydroxy aldehydes or ketones or compounds yielding such units on hydrolysis.
- 📌 Saccharides: Another term for carbohydrates, meaning sugars.
- 📌 Sugars: Sweet-tasting carbohydrates like sucrose and lactose.
10.1.1 Classification of Carbohydrates
Explanation10.1.1 Classification of Carbohydrates
Carbohydrates are classified based on their behavior upon hydrolysis into three main groups: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed further to simpler polyhydroxy aldehydes or ketones. About 20 monosaccharides are known in nature, including glucose, fructose, and ribose. Oligosaccharides yield two to ten monosaccharide units upon hydrolysis and include disaccharides, trisaccharides, and tetrasaccharides. Disaccharides are the most common oligosaccharides. For example, sucrose hydrolyzes to glucose and fructose, while maltose yields two glucose molecules. Polysaccharides yield many monosaccharide units on hydrolysis and include starch, cellulose, glycogen, and gums. Polysaccharides are generally non-sweet and called non-sugars. Carbohydrates are also classified as reducing or non-reducing sugars based on their ability to reduce Fehling's solution or Tollens' reagent. All monosaccharides, whether aldose or ketose, are reducing sugars.
- Monosaccharides: simplest carbohydrates, not hydrolyzable further.
- Oligosaccharides: yield 2-10 monosaccharides on hydrolysis; includes disaccharides.
- Polysaccharides: yield many monosaccharides; usually non-sweet.
- Reducing sugars reduce Fehling's and Tollens' reagents; all monosaccharides are reducing sugars.
- Non-reducing sugars do not reduce these reagents, e.g., sucrose.
- Classification is based on hydrolysis behavior.
- 📌 Monosaccharides: Carbohydrates that cannot be hydrolyzed further.
- 📌 Oligosaccharides: Carbohydrates yielding 2-10 monosaccharides on hydrolysis.
- 📌 Polysaccharides: Carbohydrates yielding many monosaccharides on hydrolysis.
10.1.2 Monosaccharides
Explanation10.1.2 Monosaccharides
Monosaccharides are classified based on the number of carbon atoms and the functional group present. If the monosaccharide contains an aldehyde group, it is called an aldose; if it contains a keto group, it is called a ketose. The number of carbon at
Practice Questions — Carbohydrates
Includes NCERT exercise questions with answers
Q1.Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.
Answer:
Glucose and sucrose are carbohydrates that contain multiple hydroxyl (-OH) groups, which can form hydrogen bonds with water molecules, making them soluble in water. Cyclohexane and benzene are non-polar hydrocarbons without polar functional groups and cannot form hydrogen bonds with water. Since water is a polar solvent, it dissolves polar substances like glucose and sucrose but not non-polar substances like cyclohexane or benzene.
Explanation:
The solubility of a compound in water depends on its ability to interact with water molecules. Polar compounds with -OH groups can hydrogen bond with water, increasing solubility. Non-polar compounds lack such groups and are insoluble in water.
Q2.What are the expected products of hydrolysis of lactose?
Answer:
Lactose is a disaccharide composed of one molecule of glucose and one molecule of galactose linked by a β-1,4-glycosidic bond. Upon hydrolysis (acidic or enzymatic), lactose breaks down into its constituent monosaccharides: glucose and galactose.
Explanation:
Hydrolysis of disaccharides involves breaking the glycosidic bond by addition of water, catalyzed by acid or enzyme (lactase in the case of lactose). This yields the monosaccharide units.
Q3.How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?
Answer:
In D-glucose, the aldehyde group is in equilibrium with the cyclic hemiacetal form. When glucose is converted to its pentaacetate derivative, all the hydroxyl groups are acetylated, and the aldehyde group is converted into an acetal (specifically a cyclic acetal) due to intramolecular reaction. This acetal formation masks the aldehyde functionality, so the pentaacetate of D-glucose does not show the typical aldehyde group reactions.
Explanation:
The aldehyde group in glucose is reactive and forms a cyclic hemiacetal in solution. Upon acetylation, the hemiacetal oxygen and the aldehyde carbon form an acetal, which is stable and does not behave like a free aldehyde. Hence, the aldehyde group is not detected in the pentaacetate derivative.
Q4.The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.
Answer:
Amino acids have both an amino group (-NH2) and a carboxyl group (-COOH) which can form strong intermolecular hydrogen bonds with water molecules. This leads to higher solubility in water. Additionally, amino acids exist as zwitterions in solid state, which have strong ionic interactions, resulting in higher melting points. On the other hand, halo acids contain a halogen atom instead of the amino group, which does not form hydrogen bonds as effectively and lacks zwitterionic nature, leading to lower melting points and solubility.
Explanation:
The presence of both acidic (-COOH) and basic (-NH2) groups in amino acids allows them to form internal salts (zwitterions) and extensive hydrogen bonding with water, increasing melting points and solubility. Halo acids lack the amino group and thus have weaker intermolecular forces and less solubility.
Q5.Where does the water present in the egg go after boiling the egg?
Answer:
When an egg is boiled, the heat causes the proteins in the egg white and yolk to denature and coagulate, trapping water inside the solidified structure. Some water may evaporate as steam if the egg is boiled for a long time or if the shell cracks, but most of the water remains inside the egg in a bound form within the coagulated proteins.
Explanation:
Boiling causes protein denaturation and coagulation, which traps water molecules inside the solid matrix. The water does not disappear but is held within the solidified egg. If the shell cracks, some water may escape as steam.
Q6.Why cannot vitamin C be stored in our body?
Answer:
Vitamin C cannot be stored in our body because it is a water-soluble vitamin. Water-soluble vitamins dissolve in water and are not stored in large amounts in the body. Excess amounts are excreted through urine. Therefore, regular intake of vitamin C through diet is necessary to maintain adequate levels.
Explanation:
Vitamin C is water soluble and not stored in fat tissues or other storage sites. Unlike fat-soluble vitamins (A, D, E, K), it is excreted quickly, so the body requires a continuous supply from dietary sources.
Q7.What products would be formed when a nucleotide from DNA containing thymine is hydrolysed?
Answer:
When a nucleotide from DNA containing thymine is hydrolysed, it breaks down into three components: a nitrogenous base (thymine), a pentose sugar (2-deoxyribose), and a phosphate group. Thus, the products formed are thymine, 2-deoxyribose sugar, and phosphate ion.
Explanation:
DNA nucleotides consist of a nitrogenous base, a sugar, and a phosphate group. Hydrolysis cleaves the phosphodiester bonds and glycosidic bonds, releasing the base, sugar, and phosphate separately.
Q8.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?
Answer:
The fact that there is no relationship among the quantities of different bases obtained on hydrolysis of RNA suggests that RNA is a single-stranded molecule with a random sequence of bases. Unlike DNA, which has complementary base pairing and a double-stranded structure, RNA does not have a fixed ratio of bases, indicating its single-stranded nature.
Explanation:
In DNA, the amount of adenine equals thymine and guanine equals cytosine due to base pairing in double strands. The absence of such proportionality in RNA indicates it is single-stranded and does not have complementary base pairing.
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Chemistry · Class 12