Mixed & Higher-Order Questions

CHEExam Practice & Reasoning

These questions combine clues across carbohydrates, proteins, nucleic acids and vitamins, often as identify-the-biomolecule, give-reason, or distinguish formats. Success depends on recalling a few sharp facts per class and applying tests (Fehling's, Tollens', iodine) and structural logic correctly.

Hexose molecular formula

C_6H_{12}O_6 \quad (\text{glucose, fructose, galactose are isomers})

Aldohexoses (glucose, galactose) and the ketohexose fructose all share

C_6H_{12}O_6

Glycine (achiral amino acid)

H_2N\text{-}CH_2\text{-}COOH

The

\alpha

-carbon carries two H atoms, so there is no chiral centre and glycine is optically inactive.

Reducing-sugar oxidation (Tollens')

R\text{-}CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow R\text{-}COO^- + 2Ag\downarrow + 4NH_3 + 2H_2O

Free

\text{-}CHO

(or

\alpha

-hydroxy ketone via enediol) reduces

Ag^+

to a silver mirror.

Reducing-sugar test: a free aldehyde or an $\alpha$ -hydroxy ketone group reduces Tollens' and Fehling's; in sucrose both anomeric carbons are locked in the glycosidic bond, so it is non-reducing.
Fructose is a ketose yet reduces Fehling's/Tollens' because in alkaline medium it tautomerises through an enediol to aldoses (glucose/mannose).
Glycine, $H_2N\text{-}CH_2\text{-}COOH$ , is the only amino acid that is optically inactive because its $\alpha$ -carbon bears two H atoms (no chiral centre).
Glucose and fructose are structural isomers (both $C_6H_{12}O_6$ ): glucose is an aldohexose, fructose a ketohexose.
Proteins/amino acids are amphoteric (zwitterionic) — used in electrophoresis: at a pH away from its isoelectric point a protein carries a net charge and migrates toward the oppositely charged electrode.
Denaturation breaks H-bonds and disulphide/ionic interactions (secondary and tertiary structure) but does NOT break peptide bonds, so the primary sequence is unchanged.
DNA-specific facts to memorise: sugar is 2-deoxy-D-ribose; thymine is unique to DNA, uracil unique to RNA; nucleotides are joined by phosphodiester linkages between the C3 and C5 positions.
Identify-the-sugar: an optically active $C_6H_{12}O_6$ that reduces Fehling's but is a ketose is fructose; the aldose that also reduces Fehling's and is oxidised by mild bromine water to an acid is glucose.
Iodine test clue: a polysaccharide giving a blue (blue-black) colour with iodine is starch (amylose helix); cellulose gives no colour, while amylopectin/glycogen give only a faint red-violet.
Disaccharide-in-milk clue: lactose (galactose + glucose, $\beta\text{-}1,4$ ) is the reducing disaccharide of milk; the nucleoside of adenine in RNA is adenosine (adenine + ribose).
Give-reason staples: amino acids have high melting points because of their ionic zwitterionic lattice; glucose gives a negative Schiff's test because it exists mainly in the cyclic hemiacetal form with no free $\text{-}CHO$ .
Distinguish staples to keep ready: $\alpha$ -helix (intra-chain H-bonds, rod-like) vs $\beta$ -pleated sheet (inter-chain H-bonds, sheet-like); amylose (linear $\alpha\text{-}1,4$ ) vs glycogen (highly branched $\alpha\text{-}1,4$ + $\alpha\text{-}1,6$ ).

Where the marks go

Calling fructose non-reducing because it is a ketone — under the alkaline test conditions it isomerises to an aldose via an enediol and does reduce Fehling's/Tollens'.
Claiming denaturation changes the amino-acid sequence — only secondary/tertiary/quaternary structure is lost; peptide bonds (primary structure) survive.
Mixing up which base is DNA-only and which is RNA-only — thymine is in DNA, uracil is in RNA (both pair with adenine).
Saying glucose gives a positive Schiff's/ $NaHSO_3$ test like a normal aldehyde — the masked cyclic hemiacetal makes these tests negative.
Writing maltose or lactose as non-reducing — only sucrose among the common disaccharides is non-reducing; maltose and lactose retain a free anomeric $\text{-}OH$ .

How the board asks it

Identify / classifyreducing-sugar logic and the iodine test
An optically active compound $X$ of formula $C_6H_{12}O_6$ reduces Tollens' reagent yet is a ketohexose, and a polysaccharide $Y$ gives a blue-black colour with iodine. Identify $X$ and $Y$ and give one reason for each.
Give reasonsnon-reducing sucrose versus reducing maltose
Give a reason why sucrose is a non-reducing sugar whereas maltose is a reducing sugar, referring to the anomeric carbons and the glycosidic linkage in each.
Distinguishprotein secondary structure and polysaccharide branching
Distinguish between an $\alpha$ -helix and a $\beta$ -pleated sheet structure of proteins, and between amylose and glycogen, in terms of their bonding and shape.
Give reasonsfructose enediol tautomerism and glucose cyclic hemiacetal
Account for the fact that fructose, although a ketose, reduces Fehling's solution, whereas glucose gives a negative Schiff's test because it exists mainly as a cyclic hemiacetal with no free $-CHO$ group.
Define / statedenaturation and DNA-specific facts
Define denaturation of a protein and state which levels of structure are destroyed and which survives. State the nitrogenous base unique to DNA and the sugar present in DNA.

PreviousReasoning & Distinguish Questions

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.