Biomolecules flash cards

1. 1.What are biomolecules?

   Complex organic molecules present in living cells that build up and maintain living organisms — e.g. carbohydrates, proteins, lipids, nucleic acids, enzymes, vitamins and hormones.

   Hint: Molecules of life.

2. 2.Define carbohydrates in chemical terms.

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

   Hint: Hydrates of carbon.

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

   Some carbohydrates (e.g. rhamnose $C_6H_{12}O_5$) do not fit $C_x(H_2O)_y$, and some compounds fitting the formula (e.g. acetic acid, formaldehyde) are not carbohydrates.

   Hint: Formula fails both ways.

4. 4.How are carbohydrates classified based on hydrolysis behaviour?

   Monosaccharides (not hydrolysable), oligosaccharides (2–10 units on hydrolysis) and polysaccharides (many units on hydrolysis).

   Hint: Three broad groups.

5. 5.What are monosaccharides? Give examples.

   Simplest carbohydrates that cannot be hydrolysed further to smaller sugars. Examples: glucose, fructose, ribose, galactose.

   Hint: Building-block sugars.

6. 6.Define oligosaccharides and give the sub-type for two units.

   Carbohydrates giving 2–10 monosaccharide units on hydrolysis. Two units = disaccharide (e.g. sucrose, maltose, lactose).

   Hint: Oligo = few.

7. 7.What is the difference between reducing and non-reducing sugars?

   Reducing sugars reduce Tollens' and Fehling's reagents (have a free aldehydic or ketonic group), e.g. all monosaccharides and maltose, lactose. Non-reducing sugars do not, e.g. sucrose.

   Hint: Free anomeric —OH matters.

8. 8.Classify sugars as aldose or ketose with examples.

   Aldose = monosaccharide with an aldehyde group (e.g. glucose); Ketose = with a keto group (e.g. fructose).

   Hint: —CHO vs C=O.

9. 9.What is the molecular formula of glucose and its other common names?

   $C_6H_{12}O_6$. Also called dextrose, grape sugar or blood sugar.

   Hint: Aldohexose.

10. 10.How is glucose prepared industrially?

    By hydrolysis of starch: boiling starch with dilute $H_2SO_4$ at 393 K under pressure. Also from sucrose by hydrolysis with dilute acid.

    Hint: Starch + dilute acid.

11. 11.State the open-chain structure of glucose.

    An aldohexose: $CHO-(CHOH)_4-CH_2OH$; a straight-chain molecule with one —CHO, four —CHOH and one primary —CH_2OH group.

    Hint: 6 carbons, one aldehyde.

12. 12.Give two reactions of glucose that confirm its straight-chain structure.

    Forms glucose pentaacetate with acetic anhydride (5 —OH groups), and on oxidation with HNO_3 gives saccharic (glucaric) acid, and with HI/red P gives n-hexane (six C in a chain).

    Hint: Pentaacetate + n-hexane.

13. 13.Which reactions show glucose contains a carbonyl (—CHO) group?

    It forms an oxime with $NH_2OH$ and adds HCN to give a cyanohydrin — reactions characteristic of a carbonyl group; further, it reduces Tollens'/Fehling's confirming aldehyde.

    Hint: Oxime and cyanohydrin.

14. 14.What is the cyclic (pyranose) structure of glucose?

    Glucose exists mainly as a six-membered ring formed when the C5 —OH adds to the C1 —CHO, giving a cyclic hemiacetal called glucopyranose.

    Hint: Six-membered oxygen ring.

15. 15.What are anomers? Name the anomers of glucose.

    Isomers differing only in configuration at the anomeric carbon (C1). Glucose has $\alpha$-D-glucose and $\beta$-D-glucose.

    Hint: Differ at C1 only.

16. 16.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 through the open-chain form.

    Hint: $\alpha \rightleftharpoons \beta$ in water.

17. 17.Why does glucose not give certain characteristic aldehyde tests (e.g. does not react with $NaHSO_3$, Schiff's) despite having —CHO?

    Because in solution glucose exists mostly in the cyclic hemiacetal form, so only a small fraction has a free —CHO group at any time.

    Hint: Cyclic form dominates.

18. 18.What is the molecular formula and functional group of fructose?

    $C_6H_{12}O_6$ — a ketohexose containing a keto group (at C2) and a primary —CH_2OH; the rest are —CHOH groups.

    Hint: Ketose, same formula as glucose.

19. 19.What ring form does fructose adopt in the cyclic structure?

    A five-membered ring (furanose) — fructofuranose — formed by addition of C5 —OH to the C2 keto group.

    Hint: Five-membered furanose.

20. 20.Why does fructose, a ketose, still reduce Tollens' and Fehling's reagents?

    In basic medium fructose isomerises to glucose (and mannose) via an enediol, generating an aldehyde group that reduces the reagents.

    Hint: Base-catalysed to aldose.

21. 21.Both glucose and fructose are D-sugars. What does the 'D' denote?

    'D' refers to configuration (—OH on right in Fischer projection) of the reference carbon (highest-numbered chiral carbon) relative to D-glyceraldehyde, not the direction of optical rotation.

    Hint: Configuration, not rotation.

22. 22.What is a glycosidic linkage?

    The ether-type C—O—C bond formed by loss of a water molecule between the anomeric carbon of one monosaccharide and an —OH of another, joining the two sugar units.

    Hint: Ether bridge between sugars.

23. 23.Give the composition and glycosidic linkage of sucrose.

    Sucrose = $\alpha$-D-glucose + $\beta$-D-fructose joined by a C1–C2 glycosidic linkage between their anomeric carbons.

    Hint: Glucose + fructose, both anomeric C used.

24. 24.Why is sucrose a non-reducing sugar?

    Its glycosidic bond links the anomeric carbon of glucose to the anomeric carbon of fructose, so no free aldehyde/keto group remains to reduce Tollens'/Fehling's.

    Hint: Both reducing groups are locked.

25. 25.What is invert sugar?

    The equimolar mixture of glucose and fructose formed on hydrolysis of sucrose; the sign of rotation changes from $(+)$ to $(-)$, hence 'inversion'.

    Hint: Hydrolysed sucrose.

26. 26.Explain the term 'inversion' in sucrose hydrolysis.

    Sucrose is dextrorotatory $(+66.5^\circ)$; hydrolysis gives glucose $(+52.5^\circ)$ and fructose $(-92^\circ)$, whose mixture is laevorotatory, so the net rotation inverts from $+$ to $-$.

    Hint: Fructose's strong laevo dominates.

27. 27.Give the composition and linkage of maltose.

    Maltose = two $\alpha$-D-glucose units joined by a C1–C4 glycosidic linkage. It is a reducing sugar (one free anomeric —OH remains).

    Hint: Glucose + glucose, malt sugar.

28. 28.Give the composition and linkage of lactose.

    Lactose (milk sugar) = $\beta$-D-galactose + $\beta$-D-glucose joined by a C1–C4 glycosidic linkage. It is a reducing sugar.

    Hint: Galactose + glucose.

29. 29.Name the two components of starch and their structural differences.

    Amylose — a linear polymer of $\alpha$-D-glucose with C1–C4 links (water-soluble, ~15–20%); Amylopectin — a branched polymer with C1–C4 chains and C1–C6 branch links (insoluble, ~80–85%).

    Hint: Linear + branched.

30. 30.Which polysaccharide is the reserve carbohydrate in animals, and how is it structured?

    Glycogen ('animal starch'), stored in liver and muscles; it is a highly branched polymer of $\alpha$-D-glucose, more branched than amylopectin.

    Hint: Animal starch.

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