Question

(a) Mention any two types of RNA molecules.

(b) How is glycogen different from starch ?

(c) Explain the \( \beta \)-pleated sheet structure of proteins.

Hint:

Name two of m-RNA, t-RNA and r-RNA. Glycogen is the more branched animal polymer, while starch is the plant polymer. In the beta sheet, extended chains are held by hydrogen bonds between chains.

Answer 1

Step 1: Understanding the Question:
This question has three short parts from the chapter on Biomolecules. Part (a) is about nucleic acids, part (b) is about polysaccharides, and part (c) is about the secondary structure of proteins.

Step 2: Part (a) - Types of RNA:
RNA is a nucleic acid that carries out protein synthesis in the cell. It occurs in three main forms.
1. Messenger RNA (m-RNA) carries the genetic message from DNA to the ribosome.
2. Transfer RNA (t-RNA) brings the correct amino acid to the ribosome during protein synthesis.
3. Ribosomal RNA (r-RNA) is a structural part of the ribosome.
Any two of these are enough for the answer.

Step 3: Part (b) - Glycogen versus starch:
Both are polymers of \( \alpha \)-D-glucose, but they differ in source and in structure.
Starch is the storage carbohydrate of plants. It is a mixture of amylose (unbranched, about 15-20%) and amylopectin (branched, about 80-85%).
Glycogen is the storage carbohydrate of animals, also called animal starch. It is found in the liver, muscles and brain.
Glycogen has a structure like amylopectin, but it is more highly branched. It has shorter chains and more branch points than amylopectin.

Step 4: Part (c) - Beta-pleated sheet structure:
This is a secondary structure of proteins. The polypeptide chains lie side by side, almost fully stretched.
Neighbouring chains are held together by intermolecular hydrogen bonds between the C=O group of one chain and the N-H group of the other.
The chains can run in the same direction or in opposite directions. The sheet looks like a pleated (folded) surface.
The side chains (R groups) point above and below the sheet. Silk fibroin is a good example of this structure.

Final Answer:
(a) m-RNA and t-RNA (or r-RNA). (b) Starch is a plant polymer of amylose and amylopectin; glycogen is an animal polymer that is more branched than amylopectin. (c) In the \( \beta \)-pleated sheet, almost fully extended chains lie side by side and are joined by intermolecular hydrogen bonds.