Previous year questions: Biomolecules

Previous year questions: biomolecules

All ten items are from the official NCERT Exemplar Problems in Biology for Class 11. The exemplar does not print marks against individual questions, so the marks shown here follow the section the question is printed in: one mark for a multiple choice item, two marks for a very short answer, three marks for a short answer and five marks for a long answer. Each heading names the exemplar chapter and the question number inside its section. Work each answer before reading the model answer, because several items test the same vocabulary from different angles.

Q1 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 9, Multiple Choice Questions, Q1. Living organisms and inanimate objects contain the same major elements. What is the difference between the two groups?

Answer: Living organisms have more carbon, oxygen and hydrogen per unit mass than inanimate objects. The same elements appear in both, so the difference lies in relative abundance, not in the list of elements. Silicon is a case in point: it dominates the crust but is negligible in the human body. The comparison is about proportion, not about which elements are present, and that is the exact distinction the question tests. A one-mark answer simply names carbon, hydrogen and oxygen and says per unit mass.

Q2 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 9, Multiple Choice Questions, Q3. Which of these is an amino acid?

Answer: Glycine is an amino acid, because it has an amino group and a carboxyl group on the same alpha carbon. Formic acid, glycerol and glycolic acid carry other functional groups and are not amino acids. The presence of both an amino group and a carboxyl group on one carbon is the test, and glycine passes it.

Q3 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 9, Multiple Choice Questions, Q4. An amino acid carrying both a positive and a negative charge at the same time is in which form?

Answer: That form is the zwitterionic form. The ionisable amino and carboxyl groups of the amino acid can both be charged at the same pH, and this dipolar state is called the zwitterion. The form matters because the charge changes with pH, which is why an amino acid moves in an electric field at different pH values. In the zwitterionic state the molecule carries both charges and behaves as a neutral particle overall, and that is the basis of electrophoretic separation of amino acids.

Q4 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 9, Multiple Choice Questions, Q5. Which of these is a hexose?

Answer: Fructose is a hexose, with six carbons, twelve hydrogens and six oxygens. Erythrose is a tetrose, and ribose and ribulose are pentoses. Counting carbons is the quick test, and the formula C6H12O6 marks a hexose such as glucose or fructose. Both have six carbons, so both belong to the hexose group even though they differ in the position of the functional group. Ribose and ribulose have five carbons, so they cannot be the answer even though they are sugars.

Q5 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 9, Multiple Choice Questions, Q8. What is the most abundant component of living organisms?

Answer: Water is the most abundant component of living organisms. Proteins come next, then nucleic acids, carbohydrates and lipids, and the chapter's composition table ranks them in that order. That order is worth remembering because a one-mark question can ask for any one of the positions.

Q6 (NCERT Exemplar, 2 marks)

NCERT Exemplar Class 11 Biology, Chapter 9, Very Short Answer Type Questions, Q2. Select the appropriate bond for a polysaccharide, a protein, a fat and water among ester, glycosidic, peptide and hydrogen bonds.

Answer: Polysaccharides are joined by glycosidic bonds, proteins by peptide bonds, fats by ester bonds and water molecules by hydrogen bonds. The bond names follow the chemistry of the linking groups, so the matching is one-to-one. A glycosidic bond joins two sugars, a peptide bond joins two amino acids, an ester bond joins a fatty acid to glycerol, and hydrogen bonds hold water molecules to one another.

Q7 (NCERT Exemplar, 2 marks)

NCERT Exemplar Class 11 Biology, Chapter 9, Very Short Answer Type Questions, Q6. Glycine and alanine differ in one substituent on the alpha carbon. What are the other common substituent groups?

Answer: Glycine has hydrogen and alanine has a methyl group as the variable R group, but both also carry a hydrogen atom, a carboxyl group and an amino group on the alpha carbon. Those three groups are common to the two amino acids, and with the R group they make the four substituents of the alpha carbon. The question is a reminder that the R group alone differs, and that difference is enough to make two distinct amino acids out of one skeleton.

Q8 (NCERT Exemplar, 3 marks)

NCERT Exemplar Class 11 Biology, Chapter 9, Short Answer Type Questions, Q2. Is rubber a primary or a secondary metabolite? Write four sentences about rubber.

Answer: Rubber is a secondary metabolite, because it is not part of the core physiology that every cell needs. It is a polymeric substance produced by plants, and it appears among the secondary metabolites listed in the chapter. It is useful to people as a raw material for industry. Its ecological role in the plant is not fully understood, which is why the chapter groups it with compounds whose functions are still being studied. Rubber is a polymeric substance, and it is classed as a secondary metabolite rather than a primary one because the plant can live without making it, even though people value it highly. Four short sentences are enough for the three marks: the class, the chemistry, the human use and the uncertain ecological role. Write the class first, because a three-mark item on rubber usually begins by asking whether it is primary or secondary. The chemistry sentence can then name rubber as a polymeric substance that the plant makes from its own carbon compounds.

Q9 (NCERT Exemplar, 5 marks)

NCERT Exemplar Class 11 Biology, Chapter 9, Long Answer Type Questions, Q1. Formation of the enzyme-substrate complex is the first step in a catalysed reaction. Describe the steps up to product formation.

Answer: The substrate diffuses to the active site and binds there, forming the enzyme-substrate complex. Binding reshapes the enzyme and grips the substrate firmly. The active site then brings the reacting groups together, breaking and forming bonds, so the substrate takes the transition state and turns into the product, giving an enzyme-product complex. The product is released, and the unchanged enzyme is free to bind another substrate molecule. Throughout this cycle the enzyme lowers the activation energy, so the reaction reaches the product far faster than it would without catalysis. The enzyme itself is unchanged at the end, which is why a small amount of enzyme can convert a large amount of substrate, and why the same enzyme can run the cycle again and again.

Q10 (NCERT Exemplar, 5 marks)

NCERT Exemplar Class 11 Biology, Chapter 9, Long Answer Type Questions, Q5. Describe the various forms of lipids with examples.

Answer: A fatty acid is a carboxyl group with a hydrocarbon chain, and it may be saturated, like palmitic acid, or unsaturated. Glycerol is a simple lipid with three hydroxyl groups. When fatty acids are esterified to glycerol the product is a mono-, di- or triglyceride; triglycerides are the fats and oils, and the melting point separates the solid fats from the liquid oils. Phospholipids carry phosphorus and a phosphorylated organic compound and are built into membranes; lecithin is one. Cholesterol is a steroid, and neural tissue holds lipids with more complex structures. Lipids share one property, insolubility in water, and they are counted with the macromolecular fraction because membrane fragments are insoluble. A five-mark answer should therefore name each form, give an example and add that shared property at the end. Triglycerides are the store, phospholipids are the membrane, cholesterol is the steroid, and fatty acids are the building blocks that all of them use. The answer also earns a mark for saying that all these forms are insoluble in water, which is the common property that defines the class.