Ncert Books Class 11 Biology Chapter 9 Free Download
FAQs on Ncert Books Class 11 Biology Chapter 9 Free Download
1. Which topics in Biomolecules Class 11 are most important for the 2025-26 board exams?
For the CBSE Class 11 Biology exam, certain topics from Biomolecules carry higher weightage and are frequently asked. Students should focus on:
- Protein Structure: The four levels of protein organisation (Primary, Secondary, Tertiary, and Quaternary) are a very important 5-mark question.
- Enzymes: The mechanism of enzyme action, including the concept of the active site, and factors affecting enzyme activity (temperature, pH) are critical.
- Carbohydrates: Differentiating between monosaccharides, disaccharides, and polysaccharides with examples like glucose, sucrose, starch, and cellulose.
- Nucleic Acids: The structural differences between DNA and RNA often appear as a 3-mark question.
2. What kind of questions can be expected from the topic of 'Enzymes' in an exam?
Questions on enzymes are a staple in the Biomolecules chapter and can be framed for different marks:
- 1-Mark Questions: Definitions of terms like 'enzyme', 'active site', 'substrate', or 'denaturation'.
- 3-Mark Questions: Explaining the lock-and-key hypothesis or induced-fit model of enzyme action. A question asking to differentiate between competitive and non-competitive inhibition is also common.
- 5-Mark Questions: A detailed question on the factors affecting enzyme activity, such as temperature, pH, and substrate concentration, often requiring graphical representations.
3. How should I answer a 5-mark question on the different levels of protein structure?
To secure full marks on a question about protein structure, your answer should be well-structured and include diagrams. Follow this format:
- Primary Structure: Define it as the linear sequence of amino acids linked by peptide bonds. Draw a simple chain.
- Secondary Structure: Explain the formation of Alpha-helix and Beta-pleated sheets due to hydrogen bonding. Illustrate both.
- Tertiary Structure: Describe it as the 3D folding of the polypeptide chain, stabilized by various bonds (hydrogen, ionic, disulfide bridges). This structure is crucial for biological function.
- Quaternary Structure: Explain that it involves the assembly of multiple polypeptide chains (subunits). Use haemoglobin as a key example.
Always link the structure to its biological stability and function for higher marks.
4. What is the difference between DNA and RNA? Frame an answer for a 3-mark question.
The key differences between DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are crucial for exams. For a 3-mark question, presenting the answer in a table or points is effective:
- Sugar: DNA contains Deoxyribose sugar, whereas RNA contains Ribose sugar.
- Nitrogenous Bases: DNA has Adenine, Guanine, Cytosine, and Thymine (T). In RNA, Thymine is replaced by Uracil (U).
- Structure: DNA is typically a double-stranded helix, while RNA is usually single-stranded.
- Function: DNA serves as the primary carrier of genetic information. RNA has diverse roles, including acting as a messenger (mRNA), transferring amino acids (tRNA), and forming ribosomes (rRNA).
5. Why is the tertiary structure of a protein considered essential for its biological activity?
The tertiary structure is essential because it represents the final, specific three-dimensional shape of a single polypeptide chain. This 3D conformation is what creates the functional sites of a protein. For instance, in enzymes, the folding creates a unique pocket or cleft called the active site. Only when this active site has the correct shape can it bind to its specific substrate and catalyse a reaction. If the tertiary structure is disrupted (a process called denaturation), the active site is lost, and the protein loses its biological function, even if the primary sequence of amino acids remains intact.
6. How are polysaccharides like starch and cellulose structurally different, and why does this matter for digestion in humans?
This is a classic higher-order thinking question. The primary difference lies in the glycosidic bonds linking their glucose monomers:
- Starch: Composed of α-glucose monomers linked by α-1,4 glycosidic bonds. This creates a helical structure that is easily accessible to human digestive enzymes like amylase.
- Cellulose: Composed of β-glucose monomers linked by β-1,4 glycosidic bonds. This results in straight, fibrous chains that are packed tightly together.
This structural difference is critical because humans possess the enzyme (amylase) to break the α-1,4 bonds in starch but lack the enzyme cellulase, which is required to break the β-1,4 bonds in cellulose. Consequently, we can digest starch for energy, but cellulose passes through our digestive tract as indigestible dietary fibre.
7. What are some common mistakes students should avoid when answering questions from the Biomolecules chapter?
To maximize scores in this chapter, students must be precise. Common pitfalls to avoid include:
- Confusing Structures: Mixing up the four levels of protein structure or confusing the structures of starch, glycogen, and cellulose.
- Vague Definitions: Not clearly defining terms like 'peptide bond', 'glycosidic bond', or 'active site'.
- Forgetting Examples: Failing to provide mandatory examples, such as haemoglobin for quaternary structure or insulin as a protein hormone.
- Omitting Diagrams: Questions on protein structure, enzyme action, or DNA structure often require simple, labelled diagrams. Omitting them can lead to a loss of marks.
- Not Explaining 'Why': For HOTS questions, it's not enough to state a fact; you must explain the underlying reason, like why a denatured enzyme is non-functional.
