Problem 156 - Objective Biology for NEET

Question

Assertion: Starch forms complex with \(\mathrm{I}_{2}\) Reason: Starch forms secondary helical structure.

Step-by-Step Solution

Answer

The assertion and reason are correct, and the reason accurately explains the assertion. Starch, due to its secondary helical structure, forms a complex with iodine.

Step 1: Understanding the structure of starch

Starch is a polysaccharide that is made up of a large number of glucose units connected by glycosidic bonds. The structure of starch includes two types of molecules: linear and helical amylose, and branched amylopectin. The helical structure is important for its interaction with iodine.

Step 2: Understanding the interaction between starch and iodine

When iodine is mixed with starch, it gets trapped inside the helical structure of the amylose molecules, forming a starch-iodine complex. This reaction is specific to starch and iodine.

Step 3: Correlating assertion and reason

The assertion states that starch forms a complex with iodine, which is true because the iodine atoms become trapped within the helical structure of the amylose molecules. The reason states that the complex forms because starch has a secondary helical structure, this is indeed the reason for the formation of the complex. The reason correctly explains the assertion.

Key Concepts

Polysaccharides

Polysaccharides are large, complex carbohydrates composed of long chains of monosaccharide units linked together by glycosidic bonds. In simple terms, they are made up of many simple sugars bonded together. Starch is one of the most common polysaccharides found in plants and serves as a storage form of energy. Human diets include starch from foods like rice, potatoes, and wheat.

There are several key features of polysaccharides that make them unique:

• Size and Complexity: Because they consist of many sugar units, polysaccharides can be very large and complex.
• Insolubility: Due to their size, many polysaccharides do not dissolve well in water, which is why they are ideal for energy storage.
• Variety of Functions: Besides energy storage, polysaccharides also provide structural support and protection in organisms.

Understanding polysaccharides requires looking at how their structure determines their function in biological systems.

Helical Structure

The concept of helical structure in biochemistry refers to a specific shape that certain molecular chains form, resembling a spring or a corkscrew. In the case of starch, the helical structure is essential because it allows certain interactions with other molecules, like iodine.

Helical structures can be attributed to the way the individual sugar units are arranged and bonded, causing the chain to coil:

• Stabilization: Hydrogen bonds often stabilize the helical structure, maintaining its shape.
• Function: The helix provides a compact and efficient way to store energy and is crucial for the interaction with iodine in the formation of the starch-iodine complex.

The secondary helical structure of starch is crucial for its interaction with iodine, allowing iodine molecules to fit snugly within the coil, leading to the characteristic blue-black color seen in the starch-iodine test.

Amylose and Amylopectin

Starch consists of two types of molecules: amylose and amylopectin. Understanding their distinct structures helps explain why starch behaves as it does, especially in forming complexes with iodine.

Amylose is a long, unbranched polysaccharide that coils into a helical shape. This spiral arrangement is what enables the trapping of iodine molecules, forming the starch-iodine complex. The linear nature of amylose allows for a more uniform helix.

• Simplicity: Because of its linear, helical structure, amylose is particularly efficient at interacting with iodine.

Amylopectin, on the other hand, is highly branched, making up the majority of the starch structure. Its branches prevent it from forming helices that can trap iodine effectively:

• Complexity: Due to its branched nature, amylopectin does not interact with iodine in the same way as amylose does.

Both amylose and amylopectin are crucial for the properties and behavior of starch, especially in how they contribute to the formation of the starch-iodine complex.