bchem EOY 2020 — Past Paper Questions & Answers

Question 1

0 The synthesis of Thymidine Monosphosphate (TMP) requires all BUT one;

Accepted Answer

Folic acid — 5-Fluorouracil is a chemotherapy drug that INHIBITS thymidylate synthase; it is not a requirement for TMP synthesis. Folate, NADPH, thymidylate synthase and dihydrofolate reductase are all genuinely needed to convert dUMP to dTMP. ---

Question 2

Which of the following statements is TRUE about Molisch’s test for carbohydrates?

Accepted Answer

It is negative for non-reducing sugars — Molisch's test (α-naphthol + concentrated H2SO4, forming a purple ring) is a general test positive for essentially all carbohydrates, since it depends on acid-catalysed dehydration to furfural/hydroxymethylfurfural. ---

Question 3

Question text In reference to Lewis structures of PCl3 and SCl4;

Accepted Answer

Dative bond exists in the latter but not in the former — Both PCl3 and SCl4 have exactly one lone pair on the central atom (P and S respectively), which is the one universally true structural fact among the options. ---

Question 4

Question text The following are components of Coenzyme A, EXCEPT

Accepted Answer

Ribose — Coenzyme A is built from pantothenic acid + β-mercaptoethylamine (pantetheine) linked to 3′-phosphoadenosine diphosphate (ADP with a 3′-phosphate) and its ribose. Glutamate is not part of its structure. ---

Question 5

Question text Macromolecules’ three-dimensional structure is formed and maintained by the following noncovalent interactions, except.

Accepted Answer

Hydrogen bonds — Carbon-carbon bonds are covalent, not noncovalent — 3D macromolecular structure is instead held together by hydrogen bonds, hydrophobic effects, ionic interactions and van der Waals forces. ---

Question 6

Question text Saliva has a [H3O+] of 10-8 while compound B has pOH of 6. Which one is more acidic?

Accepted Answer

Saliva, pH 8 — [H3O+]=10⁻⁸ M gives pH 8. pOH 6 gives pH = 14 − 6 = 8. Identical pH means identical acidity. ---

Question 7

Question text A prosthetic group of a protein is a non-protein structure that is:

Accepted Answer

Permanently associated with the protein. — A prosthetic group is a non-protein component permanently/tightly bound to a protein (e.g. heme in hemoglobin), as opposed to a substrate or transiently-bound ligand. ---

Question 8

Question text A solution with pH = 5 is than a solution with pH = 7.

Accepted Answer

200 times more basic — Each pH unit is a 10-fold difference in [H+]; pH 5 vs pH 7 is a 2-unit difference, i.e. 10² = 100 times more acidic. ---

Question 9

Inadequate tryptophan in the diet can lead to low levels of;

Accepted Answer

Pyridoxine — Tryptophan is a precursor for endogenous niacin (vitamin B3) synthesis, so dietary tryptophan deficiency can precipitate niacin deficiency (pellagra). ---

Question 10

Glucokinase is to be found in the liver and in the pancreatic β-cells. Which of the following is NOT true about glucokinase?

Accepted Answer

It is induced by insulin — This is the false statement, which is what the question asks for: glucokinase actually has LOWER affinity for glucose (higher Km) than hexokinase, not higher. It is correctly not inhibited by G6P, is induced by insulin, and has a higher Vmax. ---

Question 11

Which of the following is associated with type I GSD?

Accepted Answer

Hyperuricemia — Hypoglycemia is the cardinal, unifying feature of Type I glycogen storage disease (von Gierke's, glucose-6-phosphatase deficiency) — the liver cannot release free glucose from either glycogenolysis or gluconeogenesis. ---

Question 12

Aspirin:

Accepted Answer

Inhibits cyclooxygenase — Aspirin's core mechanism is irreversible acetylation/inhibition of cyclooxygenase (COX); the downstream effects on thromboxane and prostaglandin levels follow from this. ---

Question 13

Which of the following is INCORRECT about glycogenin?

Accepted Answer

It has an inherent catalytic power — This is the incorrect statement (as asked): glycogenin sits at the single reducing end of the glycogen molecule, not the non-reducing end — glycogen chains elongate outward from glycogenin at their non-reducing ends. ---

Question 14

What type of inhibitors does the pesticides belong to?

Accepted Answer

Arsenic acid — Organophosphate compounds are the classic pesticide class that act as (irreversible) enzyme inhibitors, most notably of acetylcholinesterase. ---

Question 15

Which of the following enzymes does not catalyze an anaplerotic reaction?

Accepted Answer

Malic enzyme (ME) — Malate dehydrogenase simply interconverts malate and oxaloacetate as an internal step of the TCA cycle itself; it does not add net carbon to replenish the cycle from an outside source the way pyruvate carboxylase, PEPCK (reverse), malic enzyme, or GOT can. ---

Question 16

Which of the following metabolites would not yield glucose?

Accepted Answer

Glucogenic amino acids — Even-chain fatty acids are broken down entirely to acetyl-CoA, and animals cannot achieve net conversion of acetyl-CoA to glucose (the two carbons lost as CO2 in the TCA cycle offset the two that entered), so they cannot yield net new glucose. ---

Question 17

A is associated with favism

Accepted Answer

Kinase — Favism is caused by glucose-6-phosphate dehydrogenase (G6PD) deficiency — G6PD is a dehydrogenase. ---

Question 18

Galactosemia:

Accepted Answer

is manifested by improved vision — Classic (Type I) galactosemia is caused by deficiency of galactose-1-phosphate uridyltransferase (GALT). ---

Question 19

The following are all true representations of the Lineweaver-Burk equation from the transformed Hyperbolic Michaelis-Menten equation into a straight line equation, y = mx + b, EXCEPT:

Accepted Answer

x = 1/[S] — In the Lineweaver-Burk plot (1/v vs 1/[S]), the slope is Km/Vmax, not Vmax/Km as stated — this is the incorrect statement the question asks for. ---

Question 20

Dopamine

Accepted Answer

does not require pyridoxal phosphate in its synthesis — Dopamine is a catecholamine neurotransmitter; it requires PLP (vitamin B6) for its synthesis via DOPA decarboxylase, does not cross the blood-brain barrier well (hence L-DOPA is used therapeutically instead), and its underproduction is linked to Parkinson's disease, not schizophrenia. ---

Question 21

Cysteine synthesis requires the following EXCEPT:

Accepted Answer

homocysteine — α-Ketobutyrate is a byproduct released when cystathionine is cleaved to cysteine, not a required input for the synthesis pathway (which needs homocysteine, serine, cystathionine synthase and vitamin B6). ---

Question 22

Bonding between atoms of elements is influenced by the following forces EXCEPT;

Accepted Answer

Shielding effect of electrons — Like-charged nuclei repel rather than attract one another; nuclear-nuclear attraction is not a real force in chemical bonding. ---

Question 23

Which of the following properties of water does not contribute to the fitness of the aqueous environment for living organisms?

Accepted Answer

High heat of vaporization. — Water's very low molecular weight is simply a fact about the molecule, not itself a 'fitness for life' property in the classic sense — unlike its high heat of vaporization, high specific heat, hydrogen-bond-driven cohesion, and the density anomaly that makes ice float, which are the properties classically cited as making water suited to life. ---

Question 24

Amino acids are said to be conditionally essential if:

Accepted Answer

the body is not able to synthesize them — Conditionally essential amino acids are only required from the diet in specific circumstances, such as during rapid growth in infancy or metabolic stress/illness, when endogenous synthesis cannot keep pace with demand. ---

Question 25

A patient with methanol poisoning was prescribed oral or intravenous ethanol administration at high doses. What is biochemical mechanism of such treatment?

Accepted Answer

Uncompetitive inhibition of alcohol dehydrogenase — Ethanol is a competitive substrate for alcohol dehydrogenase, outcompeting methanol for the enzyme and slowing methanol's conversion to its toxic metabolites (formaldehyde, formic acid). ---

Question 26

blocks oxidative phosphorylation by inhibiting ATP-synthetase

Accepted Answer

Oligomycin — Oligomycin binds and blocks ATP synthase directly, halting oxidative phosphorylation while still permitting some proton leak. Cyanide, CO, antimycin A and amytal instead block electron transport chain complexes. ---

Question 27

The difference between Group IV and Group VI elements is that;

Accepted Answer

None of the choices is correct — Group VI elements (O, S...) are generally more reactive than Group IV elements (C, Si), which tend to be relatively inert/stable due to their tetravalent, fully-shared-bond covalent chemistry. ---

Question 28

Which of the following enzymes require a primer to function?

Accepted Answer

Glycogen phosphorylase — Glycogen synthase cannot initiate a new glucose chain de novo — it requires an existing primer (built by glycogenin) of at least ~4 glucose residues to extend. ---

Question 29

One of the following compounds is NOT a glycosaminoglycan.

Accepted Answer

Dermatan sulfate — Keratin is a structural intermediate-filament protein, not a glycosaminoglycan. Dermatan sulfate, chondroitin sulfate, hyaluronic acid and heparan sulfate are all genuine GAGs. ---

Question 30

The following amino acids are synthesized from α-ketoglutarate EXCEPT:

Accepted Answer

Proline — Alanine is synthesized from pyruvate by transamination, not from α-ketoglutarate. Glutamate, glutamine, proline and arginine are all part of the glutamate family, synthesized from/via α-ketoglutarate. ---

Question 31

One of the following carbohydrates is a heteropolysaccharide

Accepted Answer

Cellulose — Hyaluronate (hyaluronic acid) is built from alternating glucuronic acid and N-acetylglucosamine — two different sugars, making it a heteropolysaccharide. Cellulose, glycogen, chitin and starch are each built from a single repeating sugar unit (homopolysaccharides). ---

Question 32

The following are correct about vitamin B12 EXCEPT

Accepted Answer

it is also called cobalamin — This is the false statement (as asked): vitamin B12 (cobalamin) participates in methylmalonyl-CoA mutase and methionine synthase reactions (isomerization and methyl transfer), not hydroxylation reactions. ---

Question 33

Between (CH3)3NH+ and (CH3)2NH2+, the N in

Accepted Answer

in the latter is a stronger Nucleophile — Additional methyl groups on nitrogen donate electron density inductively (+I effect), making the more heavily methylated amine's nitrogen a stronger Lewis base/nucleophile in simplified inductive-effect teaching (ignoring solvation effects). ---

Question 34

Lack of vitamin C leads to

Accepted Answer

well deposited collagen — Vitamin C deficiency causes scurvy — impaired collagen hydroxylation leads to fragile connective tissue, bleeding gums, and poor wound healing. ---

Question 35

From ribose-5-phosphate, humans may synthesize the following compounds EXCEPT:

Accepted Answer

CoA-SH — Biotin is not synthesized from ribose-5-phosphate (or by humans at all — it must be obtained from diet/gut flora); it has an entirely separate biosynthetic origin unrelated to the pentose phosphate pathway. ATP, NAD, CoA-SH and nucleic acids all incorporate a ribose-5-phosphate-derived nucleotide backbone. ---

Question 36

The following are functions of nucleotides EXCEPT:

Accepted Answer

they are structural components of cell membranes — Nucleotides are not structural components of cell membranes — that role belongs to phospholipids. Nucleotides do serve as energy carriers, coenzyme components, nucleic acid building blocks, and cell-signalling regulators. ---

Question 37

The end product of glycolytic pathway during strenuous activity in skeletal muscle is;

Accepted Answer

Acetyl CoA — Under strenuous anaerobic conditions, skeletal muscle regenerates NAD+ via lactate dehydrogenase, converting pyruvate to lactate. ---

Question 38

Which of the following statements is NOT true about anabolic processes?

Accepted Answer

It is a degradative phase of metabolism — This is the false statement (as asked): anabolism is the synthetic/building phase of metabolism (using energy to build larger molecules from smaller ones), not the degradative phase — that describes catabolism. ---

Question 39

With regards to H2O;

Accepted Answer

The angle H-O-H is = 180o — Water is bent with two lone pairs on oxygen; lone-pair repulsion compresses the H-O-H bond angle to about 104.5°, which is less than the ideal tetrahedral angle of 109.5°. ---

Question 40

The following are noble gases except;

Accepted Answer

Argon — Hydrogen is a diatomic, group 1 element, not a noble gas. Argon, neon, radon and helium are all noble gases (group 18). ---

Question 41

Moderately severe thiamine deficiency causes;

Accepted Answer

neurological disorders and oedema — Moderately severe thiamine deficiency classically produces the mixed neurological-and-oedema picture of 'wet' beriberi, distinct from the more severe, typically alcohol-associated Wernicke-Korsakoff encephalopathy. ---

Question 42

Both Serotonin and γ- amino-butyric acid:

Accepted Answer

are synthesized from aromatic amino acids — Both serotonin (from tryptophan, via aromatic L-amino-acid decarboxylase) and GABA (from glutamate, via glutamate decarboxylase) require a PLP-dependent decarboxylase step in their synthesis. ---

Question 43

Which of the following reactions is not endergonic?

Accepted Answer

C6H12O6 + C6H12O6 → C12H22O11 + H2O — Complete oxidation of glucose to CO2 and water (aerobic respiration) is strongly exergonic, releasing free energy — unlike the other listed condensation/biosynthetic reactions, which require energy input (endergonic). ---

Question 44

What functional groups are present in this molecule? HOCH2CH2CHO

Accepted Answer

Hydroxyl and ketone — HOCH2CH2CHO has a terminal hydroxyl group (HO-) and a terminal aldehyde group (-CHO). ---

Question 45

The glyoxylate cycle is also known as the

Accepted Answer

Kornberg cycle — The glyoxylate cycle is also called the Kornberg cycle, after Hans Kornberg who first described it. ---

Question 46

An inability to generate tetrahydrobiopterin might be expected to:

Accepted Answer

reduce the body’s ability to transfer one carbon fragments — Phenylalanine hydroxylase strictly requires tetrahydrobiopterin (BH4) as a cofactor to convert phenylalanine to tyrosine; without BH4, this normal degradative route for phenylalanine is blocked (producing a BH4-deficient form of hyperphenylalaninemia). ---

Question 47

The last irreversible reaction of gluconeogenesis is catalyzed by;

Accepted Answer

Phosphoglycerate mutase — Glucose-6-phosphatase catalyses the final, irreversible step of gluconeogenesis, releasing free glucose from glucose-6-phosphate. ---

Question 48

In the pyruvate dehydrogenase reaction, which of the following statements is INCORRECT?

Accepted Answer

E3 dehydrogenates reduced form of lipoate (reoxidation) — This is the incorrect statement (as asked): the pyruvate dehydrogenase complex reaction occurs in the mitochondrial matrix, not the cytosol. ---

Question 49

Enzymes are biological catalysts that enhance the rate of a reaction by:

Accepted Answer

Increasing the energy of the transition state. — Enzymes speed up reactions by lowering the activation energy needed to reach the transition state, not by increasing it or altering the reaction's overall free-energy change. ---

Question 50

The following enzymes participate in both Kreb’s cycle and glyoxylate cycle EXCEPT

Accepted Answer

Malate dehydrogenase — Isocitrate lyase is one of the two defining enzymes unique to the glyoxylate cycle (along with malate synthase) and does not participate in the standard TCA cycle. Malate dehydrogenase, aconitase, citrate synthase and succinate dehydrogenase are shared with the TCA cycle. ---

bchem EOY 2020 — Past Paper Questions & Answers

Questions, Answers & Explanations