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Biomolecules: one-page cheat sheet

NEET Chemistry · Unit 19 · Organic Chemistry

ScoreChemby CSJ Sir
Weightage: 2–3 questions · 8–12 marks

1 · Carbohydrates

Mono-no further hydrolysis: glucose, fructose, galactose, ribose

Oligo-2 to 10 units: sucrose, maltose, lactose

Poly-starch, cellulose, glycogen

Reducingall monosaccharides, maltose, lactose; sucrose is non-reducing

Fructoseketohexose, laevorotatory (

- 92. 4^{\circ}

), furanose; in honey

2 · Glucose structure proofs

Reagent	Shows
HI / red P	n-hexane: straight C $_{6}$ chain
NH $_{2}$ OH, HCN	oxime, cyanohydrin: C=O
Br $_{2}$ water	gluconic acid: –CHO
Ac $_{2}$ O	pentaacetate: five –OH
conc. HNO $_{3}$	saccharic acid: –CH $_{2}$ OH at C-6

No Schiff's test, no NaHSO $_{3}$ adduct, pentaacetate does not react with hydroxylamine: the ring is a cyclic hemiacetal.

3 · Rings & mutarotation

Pyranose–OH at C-5 attacks C-1: six-membered ring

Anomers

α

: C-1 OH down;

β

: up (Haworth)

Rotation

α

+ 11 2^{\circ}

β

+ 1 9^{\circ}

both go to

+ 52. 5^{\circ}

Mixabout 36%

α

, 64%

β

, under 0.02% open chain

4 · Di- & polysaccharides

Sugar	Units and link	Reducing
Sucrose	$α$ -Glc C1 – $β$ -Fru C2	no
Maltose	two $α$ -Glc, C1–C4	yes
Lactose	$β$ -Gal + $β$ -Glc, C1–C4	yes

Invert sugarsucrose

+ 66. 5^{\circ}

gives glucose

+ 52. 5^{\circ}

+ fructose

- 92. 4^{\circ}

: net

- 2 0^{\circ}

Amylose15–20%, linear

α

-1,4, soluble

Amylopectin80–85%, branched (

α

-1,6)

Cellulose

β

-glucose,

β

-1,4, unbranched

Glycogenlike amylopectin, branches every 8–12 units

5 · Amino acids

NaturalL-configuration; glycine is achiral

AcidicAsp, Glu

BasicLys, Arg, His

RCH(NH_{2}) COOH ⇌ RCH(NH_{3}^{+}) COO^{-} (zwitterion)

pInet charge zero; least soluble; no migration

6 · Proteins

Peptide bond–CO–NH–, planar and rigid (partial double bond)

1°amino acid sequence

2°

α

-helix (N–H of

n

to C=O of

n + 4

);

β

-sheet

3°folding; H-bonds, S–S, ionic, van der Waals

4°subunits: haemoglobin

2 α + 2 β

Fibrouskeratin, myosin; Globular: insulin, albumin

Denaturation2°, 3°, 4° lost; primary stays intact

7 · Enzymes & vitamins

Enzymes are globular proteins that lower activation energy, not polysaccharides.

Vitamin	Deficiency
A (retinol)	night blindness, xerophthalmia
B $_{1}$ (thiamine)	beri-beri
B $_{2}$ (riboflavin)	cheilosis
B $_{6}$	convulsions
B $_{12}$	pernicious anaemia
C	scurvy
D	rickets, osteomalacia
E	increased RBC fragility
K	slow blood clotting

Fat-soluble: A, D, E, K. Water-soluble: B and C (B $_{12}$ is stored in the liver).

8 · Nucleic acids

SugarDNA 2-deoxyribose; RNA ribose

Basespurines A, G; pyrimidines C, T (DNA), C, U (RNA)

Nucleosidebase at C-1′ of sugar

Nucleotidenucleoside + H

_{3}

_{4}

at C-5′

Linkage3′–5′ phosphodiester

A = T (2 H-bonds), G \equiv C (3 H-bonds)

RNAsingle strand: mRNA, rRNA, tRNA

9 · Biomolecule tests

Molischcarbohydrates: violet ring

Benedict / Fehlingreducing sugars: red Cu

_{2}

Iodinestarch: blue-black

Ninhydrinamino acids, proteins: purple

Biuretpeptide bonds: violet

Seen in NEET 2024, 2020

Glucose does not give Schiff's test or the NaHSO $_{3}$ adduct.

Sucrose hydrolysis gives $α$ -D-glucose + $β$ -D-fructose.

Denaturation leaves the primary structure intact. Lysine is a basic amino acid.

Seen in NEET 2021, 2022, 2025

Starch gives the Tyndall effect (colloid); glucose, urea and NaCl do not.

Enzymes are not polysaccharides.

Keto sugar in honey, laevorotatory: D-fructose.

Six traps to remember

Sucrose is non-reducing; maltose and lactose are reducing.
Glucose gives no Schiff's test and no NaHSO $_{3}$ adduct, so it exists as a cyclic hemiacetal.
Invert sugar rotates to the left because fructose ( $- 92. 4^{\circ}$ ) outweighs glucose ( $+ 52. 5^{\circ}$ ).
Glycine is the only achiral natural amino acid.
Denaturation destroys 2°, 3° and 4° structure but not the primary.
A nucleotide is a nucleoside plus phosphoric acid, not phosphorous acid.

Biomolecules: free one-page cheat sheet