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Must-know facts
25 facts
- 1Carbohydrates: optically active polyhydroxy aldehydes or ketones, or compounds that hydrolyse to them. Cx(H₂O)y is not a test: acetic acid fits, rhamnose (C₆H₁₂O₅) does not.
- 2Mono (no hydrolysis; about 20 in nature), oligo (2–10 units), poly (many units; non-sugars).
- 3All monosaccharides, aldoses and ketoses, are reducing sugars.
- 4Glucose from starch: dilute H₂SO₄, 393 K, 2–3 atm. From sucrose: dilute HCl or H₂SO₄ in alcoholic solution, equal glucose and fructose.
- 5Glucose evidence: HI → n-hexane (straight chain); oxime + cyanohydrin (C=O); Br₂ water → gluconic acid (–CHO); pentaacetate (5 –OH); HNO₃ → saccharic acid (1° alcohol).
- 6D/L compares the lowest asymmetric carbon with glyceraldehyde; –OH on the right = D. It says nothing about (+) or (–): D-(–)-fructose.
- 7Glucose has no free –CHO: no Schiff's test, no NaHSO₃ adduct, pentaacetate ignores NH₂OH. C5–OH closes a six-membered pyranose ring.
- 8α-glucose m.p. 419 K (crystallised at 303 K); β-glucose m.p. 423 K (crystallised at 371 K). They are anomers, differing at C1.
- 9Fructose: ketohexose, keto at C2; C5–OH closes a five-membered furanose ring; D-(–).
- 10Sucrose: α-glucose C1 – β-fructose C2, non-reducing. Hydrolysis gives invert sugar because fructose (–92.4°) outweighs glucose (+52.5°).
- 11Maltose: α-glucose C1–C4 α-glucose, reducing. Lactose: β-galactose C1–C4 β-glucose, reducing.
- 12Starch = amylose (15–20%, soluble, unbranched, 200–1000 units, C1–C4) + amylopectin (80–85%, insoluble, branched C1–C6). Cellulose: β-glucose C1–C4, unbranched. Glycogen: like amylopectin, more branched.
- 13Proteins hydrolyse to α-amino acids only. Zwitter ion H₃N⁺–CHR–COO⁻ explains high m.p., water solubility and amphoteric behaviour.
- 14Glycine is the one optically inactive natural α-amino acid. Natural ones are mostly L.
- 15Ten essential amino acids: Val, Leu, Ile, Arg, Lys, Thr, Met, Phe, Trp, His.
- 16Peptide bond –CO–NH–; >10 amino acids = polypeptide; >100 residues and >10,000 u = protein; insulin has 51.
- 17Fibrous (keratin, myosin; insoluble) versus globular (insulin, albumins; soluble).
- 181° sequence; 2° α-helix (right-handed, intrachain H-bonds C=O···H–N) and β-pleated sheet (interchain H-bonds); 3° overall folding; 4° sub-unit arrangement.
- 19Denaturation (heat, pH) destroys 2° and 3° structure; 1° stays. Boiled egg, curdled milk.
- 20Enzymes: mostly globular proteins, specific, named with -ase; sucrose hydrolysis Ea 6.22 kJ mol⁻¹ with acid, 2.15 with sucrase.
- 21Fat-soluble A, D, E, K (stored); water-soluble B group and C (not stored, except B₁₂).
- 22A: xerophthalmia, night blindness. B₁: beri beri. B₂: cheilosis. B₆: convulsions. B₁₂: pernicious anaemia. C: scurvy. D: rickets, osteomalacia. E: fragile RBCs. K: longer clotting time.
- 23Nucleoside = base + sugar (at C1′); nucleotide = nucleoside + phosphate (at C5′); chain links 5′–3′ phosphodiester.
- 24DNA: 2-deoxyribose, A G C T, double helix, A–T and C–G. RNA: ribose, A G C U, single strand; m-, r-, t-RNA.
- 25Hormones: steroids (estrogens, androgens), polypeptides (insulin, endorphins), amino acid derivatives (epinephrine, norepinephrine; thyroxine from tyrosine).
Common traps
Where marks are lost
Calling any compound that fits Cx(H₂O)y a carbohydrate.
Reading D as dextrorotatory.
Deciding D or L from C2 of glucose.
Saying glucose is non-reducing because its ring has no free –CHO.
Calling α- and β-glucose enantiomers or open-chain isomers.
Thinking sucrose is reducing because it contains glucose.
Explaining invert sugar as sucrose becoming laevorotatory.
Separating starch and cellulose by their linkage positions.
Swapping the starch fractions.
Saying denaturation breaks peptide bonds.
Putting vitamin B₁₂ with vitamins that cannot be stored, or vitamin K with water-soluble ones.
Mixing up nucleoside and nucleotide, or pairing A with U in DNA.
Formulas
5 to know
Sucrose hydrolysis
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
Dilute HCl or H₂SO₄ in alcohol, or the enzyme sucrase; gives invert sugar.
Starch or cellulose to glucose
(C₆H₁₀O₅)n + nH₂O → nC₆H₁₂O₆
Dilute H₂SO₄, 393 K, 2–3 atm.
Maltase
C₁₂H₂₂O₁₁ (maltose) + H₂O → 2C₆H₁₂O₆ (glucose)
Enzyme named after its substrate.
Invert sugar rotation
equal masses: ½(+52.5°) + ½(–92.4°) = –19.95°
Glucose and fructose have the same molar mass, so equimolar = equal mass; the average is negative.
Peptide bonds in a chain
n amino acids → (n − 1) peptide bonds, (n − 1) H₂O lost
Tripeptide: 3 amino acids, 2 peptide bonds.
Key terms
15 terms
- Carbohydrate
- Optically active polyhydroxy aldehyde or ketone, or a compound that hydrolyses to one.
- Reducing sugar
- A sugar that reduces Fehling's solution and Tollens' reagent.
- Aldose / ketose
- Monosaccharide with an aldehyde / keto group.
- Anomers
- Cyclic forms of a sugar that differ only at the anomeric carbon (C1 in glucose).
- Pyranose / furanose
- Six-membered / five-membered sugar ring containing one oxygen.
- Glycosidic linkage
- Oxygen bridge between two sugar units, formed with loss of water.
- Invert sugar
- The laevorotatory glucose–fructose mixture from hydrolysing dextrorotatory sucrose.
- Zwitter ion
- Dipolar form of an amino acid, H₃N⁺–CHR–COO⁻, neutral overall.
- Essential amino acid
- One the body cannot make, so it must come from the diet.
- Peptide bond
- The amide link –CO–NH– between amino acids.
- Denaturation
- Loss of 2° and 3° structure and of activity on heating or pH change; 1° structure kept.
- Enzyme
- A biocatalyst, almost always a globular protein, specific to its reaction and substrate.
- Nucleotide
- Base + pentose sugar + phosphate (at C5′).
- Complementary strands
- DNA strands whose bases pair A with T and C with G.
- Hormone
- An intercellular messenger released by an endocrine gland into the blood.