Lesson 13 of 13 · 18 min
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Must-know facts
25 facts
- 01Carbohydrates: 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.
- 02Mono (no hydrolysis; about 20 in nature), oligo (2–10 units), poly (many units; non-sugars).
- 03All monosaccharides, aldoses and ketoses, are reducing sugars.
- 04Glucose from starch: dilute H₂SO₄, 393 K, 2–3 atm. From sucrose: dilute HCl or H₂SO₄ in alcoholic solution, equal glucose and fructose.
- 05Glucose evidence: HI → n-hexane (straight chain); oxime + cyanohydrin (C=O); Br₂ water → gluconic acid (–CHO); pentaacetate (5 –OH); HNO₃ → saccharic acid (1° alcohol).
- 06D/L compares the lowest asymmetric carbon with glyceraldehyde; –OH on the right = D. It says nothing about (+) or (–): D-(–)-fructose.
- 07Glucose has no free –CHO: no Schiff's test, no NaHSO₃ adduct, pentaacetate ignores NH₂OH. C5–OH closes a six-membered pyranose ring.
- 08α-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.
- 09Fructose: 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
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.