Biomolecules flash cards

1. 1.What are biomolecules?

   Complex organic molecules that build up living systems and are essential for life processes, e.g. carbohydrates, proteins, nucleic acids, lipids, vitamins and hormones.

   Hint: Molecules of life

2. 2.Define carbohydrates chemically.

   Optically active polyhydroxy aldehydes or ketones, or compounds that produce these on hydrolysis. General formula often $C_x(H_2O)_y$.

   Hint: Polyhydroxy aldehyde/ketone

3. 3.Why is the name 'carbohydrate' (hydrate of carbon) considered a misnomer?

   Some like rhamnose $C_6H_{12}O_5$ do not fit $C_x(H_2O)_y$, and some non-carbohydrates like acetic acid $C_2H_4O_2$ do fit, so the H:O = 2:1 criterion is not exclusive.

   Hint: Rhamnose vs acetic acid

4. 4.Classify carbohydrates by hydrolysis into three classes.

   Monosaccharides (cannot be hydrolysed), oligosaccharides (2–10 units on hydrolysis) and polysaccharides (many units on hydrolysis).

   Hint: Based on hydrolysis products

5. 5.What is a monosaccharide? Give examples.

   A carbohydrate that cannot be hydrolysed further to simpler units. Examples: glucose, fructose, ribose, galactose.

   Hint: Simplest sugar unit

6. 6.What is an oligosaccharide?

   A carbohydrate giving 2 to 10 monosaccharide units on hydrolysis, e.g. sucrose (di-), maltose (di-), raffinose (tri-).

   Hint: 2–10 units

7. 7.Define disaccharide with examples.

   An oligosaccharide giving two monosaccharide units on hydrolysis. Examples: sucrose, maltose, lactose.

   Hint: Two units

8. 8.How are monosaccharides classified by carbonyl group and carbon number?

   By functional group: aldose (–CHO) or ketose (>C=O); by carbon number: triose (3C), tetrose (4C), pentose (5C), hexose (6C). Combined, e.g. aldohexose, ketohexose.

   Hint: Aldo/keto + triose…hexose

9. 9.Glucose is an aldohexose; fructose is a ketohexose. Explain the terms.

   Aldohexose = 6-carbon sugar with an aldehyde group (glucose). Ketohexose = 6-carbon sugar with a keto group (fructose).

   Hint: CHO vs C=O, both 6C

10. 10.What are reducing sugars?

    Sugars that reduce Tollens' reagent and Fehling's solution; all monosaccharides and those disaccharides with a free aldehyde/ketone (hemiacetal) group, e.g. maltose, lactose.

    Hint: Free anomeric –OH reduces Tollens/Fehling

11. 11.Which common disaccharide is non-reducing and why?

    Sucrose — its two anomeric carbons (C1 of glucose, C2 of fructose) are joined in the glycosidic linkage, so no free hemiacetal/reducing group remains.

    Hint: Both reducing groups locked

12. 12.What is the molecular formula and origin of the name of glucose?

    $C_6H_{12}O_6$; also called dextrose (dextrorotatory) or grape sugar. It is an aldohexose.

    Hint: Dextrose / grape sugar

13. 13.Name two common laboratory preparations of glucose.

    (1) From sucrose: boiling with dilute HCl/$H_2SO_4$ in alcohol gives glucose + fructose. (2) From starch: hydrolysis with dilute $H_2SO_4$ at 393 K under pressure.

    Hint: Sucrose or starch hydrolysis

14. 14.How many chiral carbons does open-chain glucose have and how many optical isomers?

    4 chiral (asymmetric) carbons (C2, C3, C4, C5), giving $2^4 = 16$ optical isomers.

    Hint: $2^n$, n = 4

15. 15.Give evidence that glucose contains a straight chain of 6 carbons.

    On prolonged heating with HI it gives n-hexane, showing all six carbons are in a straight, unbranched chain.

    Hint: HI → n-hexane

16. 16.Which reactions show glucose has an aldehyde group?

    It reduces Tollens' reagent (silver mirror) and Fehling's solution, is oxidised by bromine water to gluconic acid, and forms an oxime/cyanohydrin with $NH_2OH$/HCN.

    Hint: Tollens, Fehling, Br2 water, oxime

17. 17.What does glucose form with acetic anhydride, and what does this indicate?

    It forms glucose pentaacetate, indicating the presence of five –OH groups.

    Hint: Penta-acetate ⇒ 5 OH

18. 18.Why must the five –OH groups of glucose be on different carbons?

    Because multiple –OH groups on the same carbon (gem-diol) would be unstable, so each of the five hydroxyls sits on a separate carbon.

    Hint: Stable ⇒ one OH per C

19. 19.Write the open-chain (Fischer) structure summary of D-glucose.

    CHO–(CHOH)$_4$–$CH_2OH$ with –OH on C2, C4, C5 to the right and C3 to the left in Fischer projection (D-configuration: bottom chiral C5 –OH on right).

    Hint: CHO at top, CH2OH at bottom

20. 20.What is meant by the D/L configuration of a sugar?

    It refers to the configuration of the lowest chiral carbon (farthest from carbonyl): –OH on the right in Fischer = D, on the left = L. It is a relative reference, not the sign of rotation.

    Hint: Reference glyceraldehyde, lowest chiral C

21. 21.Which glucose reaction gave evidence against a simple open-chain aldehyde structure?

    Glucose does not give the Schiff's test, does not react with $NaHSO_3$ or $NH_3$, and pentaacetate does not react with $NH_2OH$ — pointing to a cyclic (hemiacetal) structure.

    Hint: Fails some carbonyl tests

22. 22.What is the cyclic hemiacetal structure of glucose called?

    A six-membered pyranose ring formed by intramolecular reaction of the C5 –OH with the C1 aldehyde, creating a new –OH at C1 (anomeric carbon).

    Hint: Pyranose, C5–OH attacks C1

23. 23.What are anomers? Name the glucose anomers.

    Cyclic sugars differing only in configuration at the anomeric (C1) carbon. Glucose: $\alpha$-D-glucose (C1 –OH down/trans to $CH_2OH$) and $\beta$-D-glucose (C1 –OH up).

    Hint: Differ at C1 only

24. 24.What is mutarotation?

    The gradual change in specific rotation of a freshly prepared sugar solution to an equilibrium value, due to interconversion of $\alpha$ and $\beta$ anomers via the open-chain form.

    Hint: $\alpha \rightleftharpoons$ open $\rightleftharpoons \beta$

25. 25.Specific rotations: $\alpha$-D-glucose $= +111^\circ$, $\beta$-D-glucose $= +19.2^\circ$. What is the equilibrium value?

    About $+52.7^\circ$, the value reached by any glucose solution after mutarotation.

    Hint: Weighted equilibrium mix

26. 26.What is the Haworth structure?

    A representation of cyclic sugars as flat rings (pyranose = six-membered, furanose = five-membered) drawn perpendicular to the plane, showing groups above and below the ring.

    Hint: Flat-ring cyclic depiction

27. 27.Molecular formula and nature of fructose.

    $C_6H_{12}O_6$, a ketohexose; also called laevulose/fruit sugar because it is strongly laevorotatory (levorotatory).

    Hint: Laevulose, ketohexose

28. 28.Which carbons bear the carbonyl in fructose, and what ring does it form?

    Keto group at C2; C5 –OH attacks C2 to form a five-membered furanose ring (fructofuranose).

    Hint: C2 keto, furanose ring

29. 29.Fructose has a keto group yet reduces Tollens'/Fehling's. Why?

    In alkaline medium fructose undergoes tautomerisation (Lobry de Bruyn rearrangement) to an enediol and then to glucose/mannose, giving a free aldehyde that reduces the reagents.

    Hint: Base → enediol → aldose

30. 30.How many chiral carbons does open-chain fructose have?

    Three chiral carbons (C3, C4, C5), so $2^3 = 8$ optical isomers.

    Hint: Keto C2 not chiral

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