I85PREP
⌘ K
Tamil Nadu HSE First Year Biochemistry study materials
Tamil Nadu HSE First Year Biochemistry study resources

Tamil Nadu HSE First Year Biochemistry practice questions

One question at a time. Build your Tamil Nadu HSE First Year Biochemistry understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Introduction to Biochemistry and Biomolecules

Which element is present in all organic biomolecules but absent from water and from most inorganic salts of the cell?

  1. Phosphorus
  2. Carbon Correct answer.
  3. Sulphur
  4. Iron

Explanation

Carbon forms the tetravalent backbone of every organic biomolecule, allowing long chains and rings. Sulphur, phosphorus and iron occur only in certain biomolecules or as trace elements, so they are not universal.

Question 2Introduction to Biochemistry and Biomolecules

In a biochemical reaction, which statement best describes the role of a catalyst such as an enzyme?

  1. It supplies the energy needed to drive an endergonic reaction
  2. It permanently changes the equilibrium constant of the reaction
  3. It is used up stoichiometrically during the formation of product
  4. It increases the rate by lowering the activation energy without being consumed Correct answer.

Explanation

A catalyst provides an alternative pathway of lower activation energy, so more molecules react per unit time. It is regenerated at the end and does not alter the position of equilibrium, which depends only on thermodynamics.

Question 3Introduction to Biochemistry and Biomolecules

Which bond links two monosaccharide units in a disaccharide such as sucrose or maltose?

  1. Phosphodiester bond
  2. Peptide bond
  3. Glycosidic bond Correct answer.
  4. Ester bond

Explanation

A glycosidic bond forms when the anomeric hydroxyl of one sugar condenses with a hydroxyl of another, releasing water. Peptide bonds join amino acids, phosphodiester bonds join nucleotides and ester bonds join fatty acids to glycerol.

Question 4Introduction to Biochemistry and Biomolecules

A solution of a protein is heated to 90 degrees Celsius and a heavy precipitate appears. Which explanation best accounts for this observation?

  1. The peptide bonds are hydrolysed into free amino acids
  2. The protein is converted into a carbohydrate by loss of nitrogen
  3. The secondary and tertiary structures unfold and hydrophobic groups aggregate Correct answer.
  4. The protein is oxidised and its amino acids are converted to fatty acids

Explanation

Heat disrupts the weak non-covalent forces such as hydrogen bonds and hydrophobic interactions that maintain the folded structure. The unfolded chains expose hydrophobic patches that clump together and precipitate.

Question 5Introduction to Biochemistry and Biomolecules

Which property of water allows it to absorb a large amount of heat with only a small rise in temperature, helping to keep body temperature steady?

  1. High specific heat capacity Correct answer.
  2. Neutral pH
  3. High surface tension
  4. Low viscosity

Explanation

Water's extensive hydrogen bonding means much energy is needed to raise its temperature, so it buffers thermal changes. This high specific heat is central to thermoregulation in living organisms.

Question 6Introduction to Biochemistry and Biomolecules

Which of the following is a pentose sugar found in both RNA and DNA nucleotides?

  1. Glucose
  2. Ribose Correct answer.
  3. Galactose
  4. Sucrose

Explanation

Ribose is the five-carbon sugar of ribonucleotides; deoxyribose, also a pentose, occurs in DNA. Glucose, sucrose and galactose are six-carbon sugars or disaccharides and are not part of nucleotides.

Question 7Introduction to Biochemistry and Biomolecules

A fatty acid with the formula CH3(CH2)14COOH contains no double bonds in its hydrocarbon chain. How is it classified?

  1. Saturated fatty acid Correct answer.
  2. Polyunsaturated fatty acid
  3. Essential fatty acid
  4. Monounsaturated fatty acid

Explanation

A fatty acid whose hydrocarbon chain has only single bonds between carbon atoms is saturated, because each carbon carries the maximum possible hydrogen. Unsaturated acids contain one or more carbon-carbon double bonds.

Question 8Introduction to Biochemistry and Biomolecules

Which statement correctly distinguishes a cofactor from a coenzyme?

  1. A cofactor is a protein, and a coenzyme is a carbohydrate attached to an enzyme
  2. A cofactor is a non-protein helper, and a coenzyme is an organic cofactor often derived from a vitamin Correct answer.
  3. A cofactor is always a metal ion, and a coenzyme is always a metal ion bound to a protein
  4. A cofactor is permanently destroyed, and a coenzyme is permanently retained by the enzyme

Explanation

Cofactors are the broad class of non-protein substances that enzymes require for activity, and coenzymes are the organic subset such as NAD+ or coenzyme A. Metal ions are inorganic cofactors, so the two categories are not identical.

Question 9Introduction to Biochemistry and Biomolecules

Which structural feature is common to all amino acids that occur in proteins?

  1. A ring containing nitrogen and a metal ion
  2. An alpha carbon bearing an amino group, a carboxyl group and a hydrogen Correct answer.
  3. A disulphide bridge joining two cysteine residues
  4. Two carboxyl groups separated by one carbon atom

Explanation

Every standard amino acid has a central alpha carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable side chain. The side chain is what distinguishes one amino acid from another.

Question 10Introduction to Biochemistry and Biomolecules

A polypeptide has a free amino group at one end and a free carboxyl group at the other. What is this linear arrangement called?

  1. Quaternary structure
  2. Secondary structure
  3. Tertiary structure
  4. Primary structure Correct answer.

Explanation

Primary structure is the sequence of amino acid residues in the chain, held together by peptide bonds, with defined amino and carboxyl termini. Higher levels of structure describe folding and subunit assembly.

Question 11Introduction to Biochemistry and Biomolecules

When a globular protein folds in aqueous solution, which interaction contributes most to stabilising the interior of the molecule?

  1. Hydrogen bonds between polar side chains and water
  2. Ionic bonds between charged side chains
  3. Peptide bonds between successive residues
  4. Hydrophobic interactions between nonpolar side chains Correct answer.

Explanation

Water excludes nonpolar groups, so hydrophobic side chains cluster in the interior, a major driving force for folding. Ionic and hydrogen bonds contribute, but burial of hydrophobic surfaces dominates in aqueous media.

Question 12Introduction to Biochemistry and Biomolecules

Which statement about the reducing property of glucose is correct?

  1. Glucose reduces Fehling's solution only after hydrolysis with dilute acid
  2. Glucose reduces Fehling's solution because it contains a ketone group at carbon one
  3. Glucose cannot reduce Fehling's solution because its anomeric carbon is locked in the ring
  4. Glucose reduces Fehling's solution because the ring can open to expose a free aldehyde group Correct answer.

Explanation

In solution the cyclic hemiacetal of glucose opens to give the open-chain aldehyde form, which can be oxidised and thereby reduce cupric ions to cuprous oxide. This is why glucose is called a reducing sugar.

Question 13Introduction to Biochemistry and Biomolecules

Sucrose gives a negative result in Benedict's test, whereas maltose gives a positive result. What is the best explanation?

  1. Sucrose is a ketose and ketoses never reduce Benedict's reagent
  2. Sucrose is a polysaccharide and cannot dissolve in the reagent
  3. In sucrose both anomeric carbons are involved in the glycosidic bond, so no free aldehyde is available Correct answer.
  4. Maltose contains a ketone group that reacts with the reagent

Explanation

Sucrose is a non-reducing disaccharide because its glycosidic bond links the anomeric carbon of glucose to the anomeric carbon of fructose. Maltose retains a free anomeric carbon on its glucose unit, so it can reduce the reagent.

Question 14Introduction to Biochemistry and Biomolecules

Which polysaccharide serves as the principal storage carbohydrate in animal liver and muscle?

  1. Glycogen Correct answer.
  2. Chitin
  3. Starch
  4. Cellulose

Explanation

Glycogen is a highly branched polymer of glucose stored in liver and muscle and mobilised when blood glucose falls. Cellulose and chitin are structural polymers, and starch is the plant storage form.

Question 15Introduction to Biochemistry and Biomolecules

A triglyceride is formed when one glycerol molecule reacts with three fatty acid molecules. Which type of reaction occurs?

  1. Amidation with elimination of three water molecules
  2. Phosphorylation with addition of three phosphate groups
  3. Hydrogenation with addition of three hydrogen molecules
  4. Esterification with elimination of three water molecules Correct answer.

Explanation

Each fatty acid carboxyl group condenses with a glycerol hydroxyl group to form an ester linkage, releasing one water molecule per bond. Three ester bonds and three water molecules result.

Question 16Introduction to Biochemistry and Biomolecules

Which feature distinguishes a phospholipid from a simple triacylglycerol?

  1. Three fatty acids are attached to a single glycerol molecule
  2. A phosphate-containing polar head group replaces one fatty acid Correct answer.
  3. The molecule contains glycerol and no nitrogen or phosphorus
  4. The fatty acid chains are always saturated

Explanation

In a phospholipid one glycerol hydroxyl is esterified to a phosphate group bearing a polar alcohol, giving a hydrophilic head and two hydrophobic tails. Triacylglycerols have three fatty acids and no phosphate group.

Question 17Introduction to Biochemistry and Biomolecules

Which nitrogenous base is found in RNA but not in DNA?

  1. Thymine
  2. Guanine
  3. Uracil Correct answer.
  4. Adenine

Explanation

RNA contains uracil in place of the thymine found in DNA, while adenine, guanine and cytosine occur in both nucleic acids. The difference arises because uracil lacks the methyl group present on thymine.

Question 18Introduction to Biochemistry and Biomolecules

In a DNA double helix, adenine pairs with thymine and guanine pairs with cytosine. Which force holds these base pairs together?

  1. Covalent phosphodiester bonds between bases
  2. Peptide bonds between adjacent nucleotides
  3. Hydrogen bonds between complementary bases Correct answer.
  4. Ionic bonds between the sugar and phosphate groups

Explanation

Complementary bases are held by hydrogen bonds: two between adenine and thymine, three between guanine and cytosine. Phosphodiester bonds instead link nucleotides covalently along each strand.

Question 19Introduction to Biochemistry and Biomolecules

A DNA sample contains 30 per cent adenine. According to Chargaff's rules, what percentage of its bases is guanine?

  1. 40 per cent
  2. 30 per cent
  3. 20 per cent Correct answer.
  4. 70 per cent

Explanation

Adenine equals thymine, so thymine is also 30 per cent, totalling 60 per cent for A plus T. The remaining 40 per cent is shared equally by guanine and cytosine, giving 20 per cent guanine.

Question 20Introduction to Biochemistry and Biomolecules

Which statement best explains why DNA is a more stable store of genetic information than RNA?

  1. DNA is single-stranded, so its bases are less exposed to damage
  2. DNA contains ribose, which protects the backbone from chemical attack
  3. DNA contains uracil, which is repaired more efficiently than thymine
  4. DNA lacks the 2'-hydroxyl group, making it less prone to hydrolysis, and thymine replaces uracil Correct answer.

Explanation

The absence of a 2'-hydroxyl group in deoxyribose makes the DNA backbone resistant to alkaline hydrolysis. Thymine in DNA also allows cytosine deamination to be recognised as damage, since uracil is not a normal DNA base.

Question 21Introduction to Biochemistry and Biomolecules

Which of the following is a water-soluble vitamin that functions as a coenzyme in many oxidation-reduction reactions?

  1. Vitamin D
  2. Vitamin A
  3. Riboflavin Correct answer.
  4. Vitamin K

Explanation

Riboflavin (vitamin B2) is the precursor of FMN and FAD, coenzymes that accept and donate electrons in redox reactions. Vitamins A, D and K are fat-soluble and act in vision, calcium regulation and clotting respectively.

Question 22Introduction to Biochemistry and Biomolecules

Which class of biomolecules includes enzymes, antibodies and haemoglobin, and is built from amino acids joined by peptide bonds?

  1. Carbohydrates
  2. Proteins Correct answer.
  3. Nucleic acids
  4. Lipids

Explanation

Proteins are polymers of amino acids linked by peptide bonds and perform catalytic, defensive and transport roles. Carbohydrates, lipids and nucleic acids are built from sugars, fatty acids and nucleotides respectively.

Question 23Introduction to Biochemistry and Biomolecules

An enzyme is added to a reaction mixture at a concentration far below that of the substrate. What happens to the enzyme after the reaction reaches completion?

  1. It is permanently inactivated and must be replaced by new enzyme
  2. It becomes part of the product molecule
  3. It remains unchanged in amount and can catalyse further substrate molecules Correct answer.
  4. It is converted into product along with the substrate

Explanation

Enzymes are true catalysts: they participate in the reaction cycle but are regenerated in their original form. Therefore a small amount of enzyme can convert many substrate molecules over time.

Question 24Introduction to Biochemistry and Biomolecules

Which observation would best support the conclusion that a substance is a protein rather than a carbohydrate?

  1. It gives a brick-red precipitate with Benedict's reagent
  2. It dissolves readily in water at room temperature
  3. It gives a violet colour with biuret reagent Correct answer.
  4. It gives a blue-black colour with iodine solution

Explanation

The biuret test detects peptide bonds and gives a violet colour with proteins. Benedict's reagent detects reducing sugars, and iodine detects starch, so those results indicate carbohydrates.

Question 25Carbohydrates

Glucose and fructose both have the molecular formula C6H12O6 but differ in the position of the carbonyl group. Glucose is classified as which type of sugar?

  1. A disaccharide
  2. A ketohexose
  3. An aldopentose
  4. An aldohexose Correct answer.

Explanation

In glucose the carbonyl group is an aldehyde at carbon 1, so it is an aldohexose. Fructose carries a ketone group at carbon 2 and is therefore a ketohexose. Both have six carbons, which rules out a pentose, and a single sugar unit rules out a disaccharide.

Question 26Carbohydrates

Which monosaccharide is the standard reference sugar for defining D and L configuration?

  1. Erythrose
  2. Dihydroxyacetone
  3. Glyceraldehyde Correct answer.
  4. D-glucose

Explanation

Glyceraldehyde, the simplest aldotriose, was chosen as the reference compound for stereochemical families. The arrangement of groups around its single chiral carbon defines the D and L series. All larger sugars are related to it by the configuration of their highest-numbered chiral centre.

Question 27Carbohydrates

Sucrose gives a negative result with Fehling's solution and does not reduce Tollens' reagent. What is the best explanation?

  1. It is hydrolysed too slowly to react
  2. It is a ketose and ketoses never reduce
  3. It has no free anomeric hydroxyl group Correct answer.
  4. It contains no glucose unit

Explanation

In sucrose the anomeric carbons of glucose and fructose are both locked in the glycosidic bond, so no free reducing group is available. Reducing behaviour requires a free anomeric hydroxyl that can open to the aldehyde form. Hydrolysis with acid or invertase releases glucose and fructose, which then reduce the reagents.

Question 28Carbohydrates

Starch consists of amylose and amylopectin. Which statement describes the glycosidic linkages in amylopectin?

  1. α-1,4 linkages with α-1,6 branch points Correct answer.
  2. Only α-1,4 linkages in a single unbranched chain
  3. Only β-1,4 linkages in a linear chain
  4. Alternating α-1,4 and β-1,6 linkages

Explanation

Amylopectin is a branched glucan whose main chains are joined by α-1,4 bonds, with α-1,6 bonds creating the branch points. Amylose is the essentially unbranched α-1,4 component. Cellulose, not starch, contains β-1,4 linkages.

Question 29Carbohydrates

A sugar solution is treated with bromine water and gives a carboxylic acid, while the same sugar with strong nitric acid gives a dicarboxylic acid. The sugar is most likely which of the following?

  1. Fructose
  2. Glucose Correct answer.
  3. Starch
  4. Sucrose

Explanation

Bromine water is a mild oxidant that converts only the aldehyde group of an aldose into a carboxyl group, giving an aldonic acid. Nitric acid is stronger and oxidises both the aldehyde at C-1 and the primary alcohol at C-6, giving glucaric acid. Ketoses and non-reducing sugars are not oxidised by bromine water in this way.

Question 30Carbohydrates

Which pair of monosaccharides makes up the disaccharide lactose?

  1. Glucose and mannose
  2. Glucose and fructose
  3. Glucose and galactose Correct answer.
  4. Glucose and glucose

Explanation

Lactose is formed from one galactose unit joined by a β-1,4 linkage to one glucose unit. It is the characteristic sugar of milk. Maltose is glucose plus glucose, and sucrose is glucose plus fructose.

Question 31Carbohydrates

Two samples of glucose are dissolved in water and allowed to stand. The optical rotation of each gradually changes until both reach the same value. This phenomenon is called:

  1. Mutarotation Correct answer.
  2. Inversion
  3. Racemisation
  4. Epimerisation

Explanation

Freshly dissolved glucose exists as a mixture of α and β anomers that slowly interconvert through the open-chain form until equilibrium is reached. The accompanying change in optical rotation is mutarotation. Inversion refers specifically to the change in sign of rotation when sucrose is hydrolysed.

Question 32Carbohydrates

Which reagent is used to distinguish an aldose from a ketose, since only the aldose is oxidised by it?

  1. Iodine solution
  2. Tollens' reagent
  3. Fehling's solution
  4. Bromine water Correct answer.

Explanation

Bromine water is a mild oxidant that converts the aldehyde group of an aldose into a carboxyl group but leaves ketoses unchanged. Fehling's and Tollens' reagents are alkaline and isomerise ketoses, so both sugar types react. Iodine solution tests for starch rather than for the aldose or ketose class.

Question 33Carbohydrates

Cellulose is not digested by humans although starch is. Which structural difference accounts for this?

  1. Cellulose is a branched polymer and starch is linear
  2. Cellulose contains fructose units instead of glucose
  3. Cellulose has β-1,4 linkages that human amylases cannot hydrolyse Correct answer.
  4. Cellulose has α-1,4 linkages that humans cannot cleave

Explanation

Human digestive enzymes such as amylase are specific for α-glycosidic bonds. Cellulose is a linear polymer of glucose joined by β-1,4 linkages, which these enzymes cannot cleave. Starch, with α-1,4 and α-1,6 bonds, is therefore digested while cellulose passes through as fibre.

Question 34Carbohydrates

The ring form of glucose is a six-membered ring containing one oxygen atom. This ring is named as which type of structure?

  1. Lactone
  2. Furanose
  3. Anhydride
  4. Pyranose Correct answer.

Explanation

A six-membered sugar ring containing one oxygen atom is called a pyranose ring, formed when the C-5 hydroxyl attacks the C-1 aldehyde. A five-membered ring of this kind is a furanose. The names are retained from the parent heterocycles pyran and furan.

Question 35Carbohydrates

On treatment with phenylhydrazine, glucose forms an osazone. Which carbons of the sugar are involved in this reaction?

  1. C-5 and C-6
  2. C-1 and C-6
  3. C-2 and C-3
  4. C-1 and C-2 Correct answer.

Explanation

Osazone formation involves the carbonyl carbon and the adjacent carbon bearing a hydroxyl group, that is C-1 and C-2 of glucose. The reaction oxidises C-2 to a carbonyl and attaches phenylhydrazone groups at both positions. Because fructose and mannose share the same C-1 and C-2 arrangement, they give the same osazone as glucose.

Question 36Carbohydrates

Which of the following is a pentose sugar of importance in nucleic acids?

  1. Maltose
  2. Glucose
  3. Galactose
  4. Ribose Correct answer.

Explanation

Ribose is a five-carbon aldose that forms part of RNA, and its deoxy derivative forms part of DNA. Glucose and galactose are six-carbon sugars, and maltose is a disaccharide. Pentoses are also involved in coenzymes such as ATP and NAD.

Question 37Carbohydrates

A glycosidic bond is formed when two monosaccharides react. What is released in this condensation?

  1. Hydrogen gas
  2. Oxygen
  3. Water Correct answer.
  4. Carbon dioxide

Explanation

Joining two monosaccharides through a glycosidic linkage is a condensation in which the anomeric hydroxyl of one sugar reacts with a hydroxyl of the other, releasing one molecule of water. Hydrolysis reverses the process by adding water back. No gas is evolved in this reaction.

Question 38Carbohydrates

In the open-chain form of D-glucose, the hydroxyl group on the highest-numbered chiral carbon lies on the right. What does the D designation describe?

  1. The direction in which the solution rotates plane-polarised light
  2. The configuration at the highest-numbered chiral centre Correct answer.
  3. The size of the sugar ring formed in solution
  4. The position of the carbonyl group in the chain

Explanation

D and L describe the absolute configuration at the chiral carbon farthest from the carbonyl group, compared with glyceraldehyde. D means the hydroxyl at that centre is on the right in the Fischer projection. The prefix does not predict the sign of optical rotation or the ring size.

Question 39Carbohydrates

Which enzyme hydrolyses maltose into two glucose units?

  1. Invertase
  2. Zymase
  3. Lactase
  4. Maltase Correct answer.

Explanation

Maltase, present in intestinal juice, cleaves the α-1,4 bond of maltose to release two molecules of glucose. Invertase acts on sucrose and lactase on lactose. Zymase is a yeast enzyme complex that ferments glucose to ethanol and carbon dioxide.

Question 40Carbohydrates

A patient lacks the enzyme needed to break down lactose and develops symptoms after drinking milk. Which bond must the missing enzyme cleave?

  1. An α-1,4 bond between two glucose units
  2. An α-1,6 branch point in a glucan
  3. An α-1,2 bond between glucose and fructose
  4. A β-1,4 bond between galactose and glucose Correct answer.

Explanation

Lactose consists of galactose joined to glucose by a β-1,4 glycosidic bond, and lactase cleaves this linkage. Failure to hydrolyse it leaves lactose to be fermented by gut bacteria, causing the symptoms of intolerance. The other linkages describe maltose, sucrose and amylopectin respectively.

Question 41Carbohydrates

Which property of glucose is demonstrated by its reaction with hydroxylamine to give an oxime?

  1. Presence of an amino group
  2. Presence of a double bond in the ring
  3. Presence of a carbonyl group Correct answer.
  4. Presence of a carboxyl group

Explanation

Hydroxylamine reacts with carbonyl compounds to form oximes, so a positive reaction shows that glucose can exist in an open-chain form with a free aldehyde group. This supports the presence of a carbonyl group. Glucose has no carboxyl or amino group in its structure.

Question 42Carbohydrates

Which statement about the sweetness and source of fructose is correct?

  1. Fructose is a pentose found mainly in milk
  2. Fructose is a disaccharide found in cane sugar
  3. Fructose is a hexose found in honey and fruit and is sweeter than glucose Correct answer.
  4. Fructose is a hexose that is less sweet than glucose

Explanation

Fructose is a ketohexose present in honey, fruits and invert sugar, and it tastes distinctly sweeter than glucose. It is a monosaccharide, not a disaccharide, although it is a component of sucrose. Its six-carbon skeleton makes it a hexose rather than a pentose.

Question 43Carbohydrates

A sample of honey is found to contain a sugar that gives the same osazone as glucose. Which sugar in honey most directly explains this observation?

  1. Sucrose
  2. Cellulose
  3. Fructose Correct answer.
  4. Starch

Explanation

Glucose, fructose and mannose differ only in the arrangement around C-1 and C-2 or beyond, so all three yield the same osazone with phenylhydrazine. Fructose is a normal constituent of honey and accounts for the shared osazone. Sucrose, starch and cellulose do not give the glucose osazone directly.

Question 44Carbohydrates

Which structural feature makes amylose give a blue colour with iodine?

  1. Its free aldehyde end group
  2. Its branched α-1,6 structure
  3. Its coiled helical chain that traps iodine molecules Correct answer.
  4. Its β-1,4 linkages

Explanation

Amylose forms a helical coil in which iodine molecules become trapped, producing an intense blue complex. Amylopectin gives a reddish-violet colour because its branching limits helix formation. The colour does not depend on a free aldehyde group or on β-linkages.

Question 45Carbohydrates

Which of the following best describes the product when glucose is reduced with sodium borohydride?

  1. Sorbitol Correct answer.
  2. Gluconolactone
  3. Gluconic acid
  4. Glucaric acid

Explanation

Sodium borohydride reduces the aldehyde group of glucose to a primary alcohol, giving the sugar alcohol sorbitol. Oxidation, by contrast, gives gluconic acid or glucaric acid depending on the strength of the oxidant. A lactone is an internal ester formed by oxidation, not reduction.

Question 46Carbohydrates

Which sugar is described as a non-reducing disaccharide because both anomeric carbons are involved in the glycosidic bond?

  1. Sucrose Correct answer.
  2. Cellobiose
  3. Maltose
  4. Lactose

Explanation

In sucrose the anomeric carbon of glucose is joined to the anomeric carbon of fructose, leaving no free reducing group. Maltose, lactose and cellobiose each retain one free anomeric carbon and are therefore reducing sugars. This is why sucrose does not react with Fehling's solution.

Question 47Carbohydrates

A chemist needs to convert glucose into a derivative that still contains the six-carbon chain and one carboxyl group at C-1 only. Which reagent should be used?

  1. Nitric acid
  2. Bromine water Correct answer.
  3. Sodium borohydride
  4. Phenylhydrazine

Explanation

Bromine water oxidises only the aldehyde group at C-1 to a carboxyl group, leaving C-6 as an alcohol, so the product is gluconic acid. Nitric acid would oxidise both ends to give glucaric acid. Sodium borohydride reduces the aldehyde, and phenylhydrazine forms an osazone.

Question 48Carbohydrates

Which statement correctly compares the ring forms of glucose and fructose in solution?

  1. Glucose forms a pyranose ring and fructose forms a furanose ring Correct answer.
  2. Both sugars form only six-membered pyranose rings
  3. Glucose forms a furanose ring and fructose forms a pyranose ring
  4. Neither sugar can form a cyclic structure in water

Explanation

Glucose normally closes through its C-5 hydroxyl to give a six-membered pyranose ring, while fructose closes mainly through its C-5 hydroxyl to give a five-membered furanose ring. Both sugars do exist in cyclic forms in aqueous solution. The ring size depends on which hydroxyl group attacks the carbonyl carbon.

Question 49Carbohydrates

Glycogen differs from amylopectin mainly in which respect?

  1. It has more frequent α-1,6 branch points Correct answer.
  2. It contains no glycosidic bonds
  3. It contains fructose units
  4. It is joined by β-1,4 linkages

Explanation

Glycogen is a glucose polymer with α-1,4 chains and α-1,6 branches, but it is more highly branched than amylopectin. This compact, branched structure allows rapid release of glucose when needed. Both polymers are built from glucose and contain glycosidic bonds.

Question 50Carbohydrates

A carbohydrate with the molecular formula C6H12O6 does not react with bromine water and gives a positive Seliwanoff test. Which type of monosaccharide is it?

  1. An aldose such as glucose
  2. An amino sugar such as glucosamine
  3. A ketose such as fructose Correct answer.
  4. A disaccharide such as sucrose

Explanation

Bromine water oxidises aldoses to aldonic acids but leaves ketoses unchanged, so no reaction with bromine water points to a ketose. Seliwanoff reagent (resorcinol in acid) dehydrates ketoses rapidly to give a red colour, while aldoses react slowly, confirming a ketose such as fructose.