Biochemistry

Chemistry, HSSC-KPK Second Year. This chapter is split into 31 topics and has 756 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Enzymes: introduction, biocatalysis, and digestion 27 questions
2. Nucleic acids: structural components and nucleotides 27 questions
3. Physical and chemical properties, and importance of lipids 28 questions
4. Functions and uses of carbohydrates in health, disease, and nutrition 29 questions
5. Proteins: introduction, functions, and classification 27 questions
6. DNA/RNA comparison, genetic information, and minerals of biological significance 27 questions
7. Factors affecting enzyme activity, enzyme inhibition, and industrial applications 30 questions
8. Structure and properties of proteins 27 questions
9. Lipids: classification and structure 27 questions
10. Carbohydrates: introduction, classification, and glycosidic bond 27 questions
11. Carbohydrates - sources, examples and the hydrate formula 23 questions
12. Monosaccharides - functional groups, carbon count and structures 23 questions
13. Oligosaccharides, polysaccharides and the glycoside bond 25 questions
14. Carbohydrate uses and nutritional importance - energy and ATP arithmetic 22 questions
15. Lactose, glycogen and blood glucose - carbohydrates in health 20 questions
16. Proteins - composition, nitrogen content and diversity arithmetic 21 questions
17. Conjugated and derived proteins - prosthetic groups and examples 22 questions
18. Protein structure levels - bonding, helices, sheets and the figure 22 questions
19. Enzymes in digestion and enzyme inhibition - additional angles 28 questions
20. Temperature and pH curves - figures 21.1 and 21.2 23 questions
21. Industrial enzymes - detergents, juice, syrup and paper making 22 questions
22. Lipid classes - simple, compound and derived groups and ester arithmetic 23 questions
23. Lipid structures from the table - fatty acids, wax, phospholipid, cholesterol 24 questions
24. Hydrolysis, saponification, hydrogenation and oxidation of fats - stoichiometry 22 questions
25. Lipid roles and the Technology-Science reading - energy arithmetic and hormones 20 questions
26. Nucleic acid history, bases and sugars - components in detail 20 questions
27. DNA base pairing and the flow of genetic information - arithmetic 30 questions
28. Minerals - natural sources, classification and iron nutrition 24 questions
29. Calcium and phosphorus - amounts, sources, functions and disorders 26 questions
30. Zinc and mineral disorders - sources, enzymes and deficiency or excess 20 questions
31. Biomolecules and the unit's key points and exercise items 20 questions

Sample questions: Biochemistry

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which correctly compares the bonding responsible for secondary structure versus that giving rise to tertiary structure?

• A. Secondary structure arises only from association of multiple separate polypeptide chains
• B. Both secondary and tertiary structure are held together by identical, single types of bonds with no distinction
• C. Tertiary structure is simply the linear amino acid sequence with no folding
• D. Secondary structure is held together primarily by hydrogen bonding along the backbone; tertiary structure arises from folding driven by attractions between alpha helices and pleated sheets (side-chain interactions)(correct answer)

Answer: D.Secondary structure results from hydrogen bonding between backbone atoms creating the alpha-helix or pleated-sheet patterns, while tertiary structure occurs when certain attractions are present between the alpha helices and pleated sheets, folding the chain into a compact three-dimensional shape.

Q2. Which of these correctly matches a protein structural level with its defining feature?

• A. Primary structure defines the amino acid sequence; quaternary structure defines the assembly of multiple subunits(correct answer)
• B. Primary structure defines the assembly of multiple subunits; quaternary structure defines the amino acid sequence
• C. Both primary and quaternary structure describe exactly the same feature
• D. Secondary structure describes the assembly of multiple polypeptide subunits

Answer: A.Primary structure is the linear sequence of amino acids along the peptide chain, while quaternary structure is the highest level, describing how multiple individually-folded protein subunits assemble into a functional complex.

Q3. Which best explains why coagulation of proteins is generally considered an irreversible change to the protein's function?

• A. Coagulation makes proteins more soluble and more biologically active
• B. Coagulation only affects the primary structure, which has no bearing on function
• C. Strong heat or certain reagents disrupt the secondary/tertiary structural organization that the protein's biological function depends on, and this organization is not readily restored(correct answer)
• D. Coagulation converts proteins into carbohydrates

Answer: C.Since a protein's biological function depends on its specific secondary, tertiary, and quaternary folding, disrupting this organization through coagulation (via heat or reagents) destroys the function, and the original folded structure is not readily restored.

Q4. Which correctly summarizes why the digestion of carbohydrates, fats, and proteins each requires different, specific enzymes at different digestive sites?

• A. Because digestion does not actually require any enzymes
• B. Because a single universal enzyme handles all digestion regardless of substrate
• C. Because carbohydrates, fats, and proteins are chemically identical and require the same enzyme
• D. Because enzymes are highly specific and can act on only one substrate/reaction type, different macronutrients require different enzymes (amylases for carbohydrates, lipases for fats, proteases for proteins), each released where conditions (like pH) favor their activity(correct answer)

Answer: D.Since enzymes are highly specific (each works for only one type of reaction), the body uses distinct enzymes — amylases for carbohydrates, lipases for fats, and proteases like pepsin/trypsin for proteins — each secreted and activated at the digestive site (mouth, stomach, intestine) where the local conditions (e.g., pH) favor that enzyme's activity.

Q5. In fruit juice production, enzyme preparations containing cellulases and hemicellulases are added to:

• A. Break down the plant cell wall and release more juice(correct answer)
• B. Color the juice artificially
• C. Kill harmful bacteria in the juice
• D. Add artificial sweetness to the juice

Answer: A.In fruit juice production, enzyme preparations containing cellulases and hemicellulases are added to break down the tough plant cell wall (built of cellulose fibres held together by pectins and hemicelluloses), releasing most of the liquid.

Q6. Which correctly summarizes the structural relationship between cholesterol and beta-carotene, both shown as example lipid structures in the chapter?

• A. Both are derived (unsaponifiable) lipids with complex ring or chain structures distinct from the simple ester-based structure of triglycerides(correct answer)
• B. Both are phospholipids found only in cell membranes
• C. Cholesterol and beta-carotene are chemically identical compounds
• D. Both are simple triglycerides with three fatty acid chains

Answer: A.Cholesterol (a steroid, with its characteristic four-ring structure) and beta-carotene (a long conjugated chain terpenoid) are both unsaponifiable, derived lipids, structurally very different from the simple ester-linked structure of triglycerides.

Q7. Insulin, a peptide hormone secreted by the beta cells of pancreatic islets of Langerhans, helps maintain blood glucose level by facilitating:

• A. The digestion of cellulose in the intestine
• B. The synthesis of new DNA strands
• C. Cellular glucose uptake and regulating carbohydrate, lipid, and protein metabolism(correct answer)
• D. The absorption of fat-soluble vitamins exclusively

Answer: C.Insulin is a peptide hormone secreted by the beta cells of pancreatic islets of Langerhans that maintains blood glucose level by facilitating cellular glucose uptake, regulating carbohydrate, lipid, and protein metabolism.

Q8. Which correctly distinguishes the chemical composition of a hormone derived from cholesterol (a steroid hormone) versus one derived from an amino acid (like tyrosine)?

• A. Both types of hormones have identical multi-ring steroid structures
• B. Steroid hormones are built from amino acids, while amino-acid-derivative hormones are built from cholesterol
• C. A steroid hormone (e.g., progesterone) has a multi-ring cholesterol-based structure, while an amino-acid-derivative hormone (e.g., thyroid hormone, catecholamines) is built from a modified single amino acid(correct answer)
• D. Neither hormone type has any defined chemical structure

Answer: C.Steroid hormones like progesterone, glucocorticoids, mineralocorticoids, androgen, and estrogen are built on cholesterol's ring structure, while amino-acid-derivative hormones (thyroid hormones and catecholamines) are chemically modified forms of single amino acids like tyrosine or tryptophan — two structurally distinct classes of signaling molecules.

Q9. Which correctly summarizes the full compositional distinction between DNA and RNA at the level of the nucleotide?

• A. DNA uses deoxyribose sugar and thymine (not uracil); RNA uses ribose sugar and uracil (not thymine); both share adenine, guanine, and cytosine(correct answer)
• B. DNA contains uracil exclusively, while RNA contains thymine exclusively
• C. DNA and RNA use identical sugars and identical sets of all four bases
• D. DNA uses ribose sugar and RNA uses deoxyribose sugar

Answer: A.DNA's nucleotides use deoxyribose sugar and the pyrimidine thymine (not uracil), while RNA's nucleotides use ribose sugar and uracil (not thymine); both nucleic acids share the same purines (adenine, guanine) and the pyrimidine cytosine.

Q10. A DNA molecule is formed by two antiparallel long polydeoxyribonucleotide chains, which are held together throughout their whole length by:

• A. Covalent bonds between the sugar backbones directly
• B. Ionic bonds between phosphate groups
• C. Hydrogen bondings present between their nitrogenous bases(correct answer)
• D. No bonds at all; they are held only by gravity

Answer: C.A DNA molecule is formed by two antiparallel long polydeoxyribonucleotide chains which wind on each other; these two chains are held together throughout the whole length by hydrogen bondings present between their nitrogenous bases.