Biochemistry End-of-Year 2014 — Past Paper OmpathStudy

Bchem EOY 2014 — Past Paper Questions & Answers This scan bundles pages from two different University of Nairobi Biochemistry exam sittings photographed together: one stray page from the August 5, 2014 First Year paper (HBC 100/UPC 100/VBC 100), and the complete August 19, 2013 Special/Supplementary paper for the same course. Each is presented separately below. This paper carries no printed answer key for the essay/fill-in sections; the MCQ section's original scan is marked with the grader's answers (cross-checked against standard biochemistry references), which are followed here. --- Paper 1: August 5, 2014 (partial — Section A, Questions 1–2 only) Question 1 The Henderson-Hasselbalch equation is important in the calculation of a pH of a buffer solution. (a) With a specific, physiological example, explain the term "buffer solution". [2 Marks] (b) Derive the Henderson-Hasselbalch equation using a hypothetical weak acid HA. [3 Marks] (c) How many grams of acetic acid (CH₃CO₂H) and sodium acetate are needed to make up to one litre of a 50 mM acetate buffer with a pH of 5.0? Ka = 1.75×10⁻⁵. [4 Marks] Model answer: (a) A buffer solution resists changes in pH upon addition of small amounts of acid or base. Physiological example: the bicarbonate buffer system (H₂CO₃/HCO₃⁻) in blood plasma, which keeps blood pH stable around 7.4 despite continuous metabolic acid production. (b) For HA ⇌ H⁺ + A⁻: Ka = [H⁺][A⁻]/[HA] → [H⁺] = Ka·[HA]/[A⁻] → taking -log of both sides: pH = pKa + log([A⁻]/[HA]) . (c) pKa = -log(1.75×10⁻⁵) ≈ 4.76. Using pH = pKa + log([A⁻]/[HA]): 5.0 = 4.76 + log([A⁻]/[HA]) → [A⁻]/[HA] ≈ 1.74. With total [A⁻]+[HA] = 50 mM: [HA] ≈ 18.2 mM, [A⁻] ≈ 31.8 mM. Mass of acetic acid (MW 60): 0.0182 mol × 60 ≈ 1.09 g . Mass of sodium acetate (MW 82): 0.0318 mol × 82 ≈ 2.61 g . Question 2 (a) State 4 functions of carbohydrates. (b) Explain the basis of reduction tests as a means of identifying sugars in unknown solutions. [2 Marks] Model answer: (a) Functions of carbohydrates: energy source and storage (glucose, glycogen, starch); structural role (cellulose, chitin, peptidoglycan); cell-cell recognition and signalling (glycoproteins, glycolipids, e.g. ABO blood group antigens); components of nucleic acids (ribose, deoxyribose) and of several coenzymes. (b) Reduction tests (e.g. Benedict's/Fehling's test) rely on the free/potential aldehyde or ketone group of a reducing sugar . In an alkaline medium containing Cu²⁺, a reducing sugar's open-chain carbonyl is oxidized (to a carboxylic acid) while Cu²⁺ is reduced to Cu⁺, precipitating as brick-red copper(I) oxide (Cu₂O) . Sugars that lack a free anomeric carbon (non-reducing sugars, e.g. sucrose) cannot reduce Cu²⁺ and give a negative test. --- Paper 2: August 19, 2013 — Special/Supplementary Examination (complete) Section A: Essay Questions (Attempt any THREE) — 30 Marks Question 1 Hyperammonemia is lethal and can result in serious mental retardation. Describe the main pathway for removal of ammonia from the body. Model answer: Ammonia is removed via the urea cycle , which converts toxic free NH₃/NH₄⁺ into non-toxic, water-soluble urea for excretion. Free ammonia (from amino acid deamination, primarily via glutamate dehydrogenase) plus CO₂, ATP, and (in the mitochondrion) is first converted to carbamoyl phosphate by carbamoyl phosphate synthetase I (activated by N-acetylglutamate). Carbamoyl phosphate + ornithine → citrulline (ornithine transcarbamylase), which exits to the cytosol. Citrulline + aspartate (contributing the second nitrogen) + ATP → argininosuccinate (argininosuccinate synthetase). Argininosuccinate → arginine + fumarate (argininosuccinase/argininosuccinate lyase); the fumarate re-enters the TCA cycle. Arginine + H₂O → urea + ornithine (arginase); ornithine returns to the mitochondrion to restart the cycle, and urea is excreted by the kidneys. --- Question 2 With regards to glycolysis: (a) Describe in detail the reactions catalysed by kinases. (b) With relevant reaction(s), distinguish between aerobic and anaerobic glycolysis. (c) Which of the two types of glycolysis proceeds with a higher overall standard free energy (ΔG°′)? Why? Model answer: (a) Glycolysis has four kinase-catalyzed steps: Hexokinase/glucokinase (Glucose + ATP → Glucose-6-P + ADP); Phosphofructokinase-1 (Fructose-6-P + ATP → Fructose-1,6-bisP + ADP, the committed, rate-limiting step); Phosphoglycerate kinase (1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP, substrate-level phosphorylation); Pyruvate kinase (Phosphoenolpyruvate + ADP → Pyruvate + ATP, substrate-level phosphorylation, the final irreversible step). (b) Aerobic glycolysis: pyruvate is fully oxidized via the TCA cycle and oxidative phosphorylation (requires O₂ as final electron acceptor); NADH generated is reoxidized by the electron transport chain. Anaerobic glycolysis: in the absence of O₂ (or in cells lacking mitochondria, e.g. RBCs), pyruvate is instead reduced to lactate by lactate dehydrogenase (Pyruvate +