Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

Which are incorrect statements? (a) amylose is a component of cellulose (b) starch is a polymer of \(\alpha\) - glucose (c) proteins are compounds of only one type of amino acids (d) in cyclic structure of fructose, there are four carbon atoms and one oxygen atom. (a) 1,2 and 4 (b) 1,3 and 4 (c) 1,2 and 3 (d) 2,3 and 4

Step-by-Step Solution

Answer

Option (b) is correct; statements 1, 3, and 4 are incorrect.

Step 1: Analyze Statement (a)

Amylose is actually a component of starch, not cellulose. Cellulose and starch are both polysaccharides but differ in their structures and components. Thus, statement (a) is incorrect.

Step 2: Analyze Statement (b)

Starch is indeed a polymer of \( \alpha \)-glucose. Starch consists of two types of glucose units, amylose, and amylopectin, both of which are derived from \( \alpha \)-glucose. Thus, statement (b) is correct.

Step 3: Analyze Statement (c)

Proteins are made of various types of amino acids, not just one type. Proteins are polymers composed of up to 20 different kinds of amino acids, which make up the protein structure by forming peptide bonds. Thus, statement (c) is incorrect.

Step 4: Analyze Statement (d)

The cyclic structure of fructose, known as the furanose form, typically includes five carbon atoms and one oxygen atom, forming a five-membered ring. Therefore, statement (d) is incorrect.

Step 5: Determine Incorrect Statements

From the analysis above, the incorrect statements are (a), (c), and (d). Therefore, the answer is option (b), which lists statements 1, 3, and 4 as incorrect.

Key Concepts

Carbohydrates

Carbohydrates are essential biomolecules consisting of carbon, hydrogen, and oxygen, often in the formula \\( C_n(H_2O)_n \) where \( n \) is a variable.
They play a pivotal role in providing energy and structural support in plants and animals. Carbohydrates are classified based on the number of sugar units they contain:

• Monosaccharides: Single sugar units such as glucose and fructose.
• Disaccharides: Composed of two sugar units, like sucrose and lactose.
• Polysaccharides: Long chains of sugar units, like starch, cellulose, and glycogen.

Carbohydrates are crucial not only for immediate energy release through glycolysis but also as energy stores (e.g., glycogen in animals and starch in plants).
Additionally, some carb structures, like chitin and cellulose, provide support and protection to plants and various organisms.

Proteins

Proteins are complex molecules vital for countless biological processes. They are composed of long chains of amino acids linked together by peptide bonds.
The sequence and structure of amino acids determine the protein's function and properties.
Proteins play an array of roles, including:

• Enzymatic activity: Acting as catalysts to speed up biochemical reactions.
• Transport: Hemoglobin, for instance, transports oxygen in the blood.
• Structural support: Collagen and keratin provide tensile strength to connective tissues.
• Defense: Antibodies help fight infections.

Understanding proteins involves analyzing their different levels of structure:

• Primary structure: The sequence of amino acids.
• Secondary structure: Includes alpha helices and beta sheets formed through hydrogen bonding.
• Tertiary structure: The 3D shape of a single polypeptide chain.
• Quaternary structure: The structure formed by multiple polypeptide chains.

Polysaccharides

Polysaccharides are carbohydrate molecules composed of long chains of monosaccharide units, often more than ten.
They have diverse functions and are crucial for energy storage and structural integrity.

• Starch: A major energy store in plants, composed of amylose and amylopectin, both derived from \( \alpha \)-glucose. Amylose is a linear polymer, while amylopectin is branched.
• Cellulose: Provides structural strength to the plant cell wall. Unlike starch, cellulose is made of \( \beta \)-glucose units linked in a straight chain, providing high tensile strength.
• Glycogen: The primary energy storage polysaccharide in animals, similar to amylopectin but more extensively branched.

Polysaccharides do not dissolve readily in water, making them ideal for energy storage or structural roles like forming cell walls.

Amino Acids

Amino acids are the building blocks of proteins, each containing an amino group \( (-NH_2) \), a carboxyl group \( (-COOH) \), and a unique side chain or R group that determines the properties of the amino acid.
They are linked by peptide bonds to form proteins, influencing protein characteristics based on their order and properties.
Of the 20 standard amino acids, nine are essential, meaning they must be obtained through the diet as the body cannot synthesize them.
Amino acids have various functions in the body:

• Building blocks for proteins: Assemble into polypeptides that fold into functional proteins.
• Neurotransmitters: Some serve roles in signaling in the nervous system, such as glutamate and glycine.
• Intermediates in metabolism: Participates in pathways like the Krebs cycle for energy production.

The specific way amino acids connect and interact affects a protein's overall function and effectiveness in its role.