CBSE Class 12 Chemistry Chapter 14 Biomolecules NCERT Solutions

Solution: Monosaccharides represent the simplest form of carbohydrates. They are defined as carbohydrates that cannot undergo further hydrolysis to yield simpler units of polyhydroxy aldehydes or ketones. These fundamental carbohydrate units are classified based on the number of carbon atoms they contain and the type of functional group present. If a monosaccharide contains an aldehyde functional group, it is termed an aldose, whereas if it contains a keto functional group, it is called a ketose. Further classification includes trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and heptoses (7 carbons). For instance, a monosaccharide with a keto group and three carbon atoms is known as a ketotriose, and one with an aldehyde group and three carbon atoms is an aldotriose.

Solution: Reducing sugars are a class of carbohydrates characterized by their ability to reduce certain chemical reagents, specifically Fehling's solution and Tollen's reagent. This reducing property arises from the presence of a free aldehyde or ketone functional group within their molecular structure, which can be oxidized. All monosaccharides, whether they are aldoses or ketoses, exhibit this property. Additionally, most disaccharides, such as maltose and lactose, are also reducing sugars. However, sucrose is a notable exception; it is a non-reducing sugar because its glycosidic linkage involves the anomeric carbons of both monosaccharide units, leaving no free aldehyde or ketone group.

Solution: Carbohydrates perform several vital functions in plants. Two primary roles are:

1. Energy Storage: Polysaccharides like starch serve as the main storage form of energy in plants. Starch granules are typically found in various plant parts, such as roots, seeds, and stems, and are mobilized when the plant requires energy for growth or metabolic processes.
2. Structural Support: The polysaccharide cellulose is a crucial component of plant cell walls. It provides rigidity, structural integrity, and protection to the plant cells, enabling plants to maintain their shape and withstand environmental stresses.

Solution: The given carbohydrates can be classified as follows:

Monosaccharides: These are simple sugars that cannot be hydrolyzed into simpler carbohydrates. From the list provided, the monosaccharides are:

Ribose

2-deoxyribose

Galactose

Fructose

Disaccharides: These sugars are formed by the combination of two monosaccharide units through a glycosidic linkage. From the list provided, the disaccharides are:

Maltose

Lactose

Solution: A glycosidic linkage, also known as a glycosidic bond, is a type of covalent bond that connects a carbohydrate molecule to another group, which may or may not be another carbohydrate molecule. Specifically, in the context of carbohydrates, it refers to the linkage formed between two monosaccharide units through an oxygen atom. This bond is created via a dehydration reaction, where a molecule of water is eliminated. For example, in the disaccharide sucrose, the monosaccharide units of $\backslash alpha$-glucose and $\backslash beta$-fructose are joined together by a glycosidic linkage. The structure shows the connection formed between the anomeric carbon of one monosaccharide and a hydroxyl group of the other, releasing H$\_2$O.

$$CH\_2OH$$

$$HOH\_2C$$

$$UL\; Glycosidic\; OH,\; CH\_2OH\; linkage\; H\; OH\; OH\; H$$

$$-\; D\; -\; glucose\; -\; D\; -\; fructose$$

$$Surcose$$

Solution: Glycogen is a complex carbohydrate, specifically a polysaccharide, that serves as the primary form of glucose storage in animals, fungi, and certain bacteria. It is predominantly found and stored in the liver and muscle cells of animals.

Starch, on the other hand, is the main storage polysaccharide in plants. It is a mixture of two components: amylose (typically 15-20%) and amylopectin (typically 80-85%).

The key differences between glycogen and starch are:

• Location: Glycogen is found in animals, while starch is found in plants.
• Structure: Both glycogen and amylopectin (a component of starch) are branched polysaccharides. However, glycogen is significantly more branched than amylopectin. This higher degree of branching in glycogen allows for quicker mobilization of glucose units when energy is needed.
• Components: Starch has two distinct components (amylose and amylopectin), whereas glycogen is considered a single type of polysaccharide structurally similar to amylopectin but with more branching.

Solution: The hydrolysis of disaccharides yields their constituent monosaccharides. The products are:

1. Sucrose: Sucrose is a non-reducing disaccharide composed of glucose and fructose units linked by a glycosidic bond. Upon hydrolysis, sucrose breaks down into one molecule of $\backslash alpha$-D-glucose and one molecule of $\backslash beta$-D-fructose.

   $$Sucrose\; +\; H\_2O\; \backslash rightarrow\; \backslash alpha-D-Glucose\; +\; \backslash beta-D-Fructose$$

2. Lactose: Lactose, also known as milk sugar, is a disaccharide composed of galactose and glucose units. Hydrolysis of lactose yields one molecule of D-galactose and one molecule of D-glucose.

   $$Lactose\; +\; H\_2O\; \backslash rightarrow\; D-Galactose\; +\; D-Glucose$$