Biomolecules — MHT-CET Chemistry Formulas and Traps

Carbohydrates: Classification, Structure and Reactions

Classification, and the Structures of Glucose and Fructose

Glucose and fructose rings

\text{Glucose: C-1 (CHO)} + \text{C-5 OH} \to \text{pyranose (hemiacetal)};\quad \text{Fructose: C-2 (C=O)} + \text{C-5 OH} \to \text{furanose (hemiketal)}

Reactions of Glucose: What Each One Proves

Mild and strong oxidation

\text{Glucose} \xrightarrow{\text{Br}_2/\text{H}_2\text{O}} \text{gluconic acid (1 COOH)};\quad \text{Glucose} \xrightarrow{\text{conc. HNO}_3} \text{saccharic acid (2 COOH)}

Common traps

Hemiacetal for fructose, three chiral carbons for glucose

Glucose (aldehyde) gives a hemiACETAL and has FOUR chiral carbons; fructose (ketone) gives a hemiKETAL. Both wrong versions are offered as options in the same question.

Counting the carbonyl carbon as an OH

An aldotriose has 3 carbons but only 2 OH — the CHO carbon carries none. 84 u is 2 × 42, so the answer is the aldotriose, not the tetrose.

Glycosidic Linkages in Di- and Polysaccharides

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Starch, Glycogen and Cellulose

Three glucose polymers

\text{amylose: } \alpha\text{-1,4};\quad \text{amylopectin / glycogen: } \alpha\text{-1,4} + \alpha\text{-1,6};\quad \text{cellulose: } \beta\text{-1,4}

Sucrose, Maltose, Lactose, Raffinose, Stachyose: the Linkage and the Hydrolysis Count

Sugar	Units	Linkage	Reducing?	Glucose per mole
Maltose	Glucose + glucose	alpha-1,4	Yes	2
Lactose	Galactose + glucose	beta-1,4	Yes	1
Sucrose	Glucose + fructose	C-1 (alpha-Glc) to C-2 (beta-Fru)	No	1 Both anomeric carbons are in the bond — hence non-reducing and invert sugar on hydrolysis.
Raffinose	Gal + Glc + Fru	Trisaccharide	No	1
Stachyose	2 Gal + Glc + Fru	Tetrasaccharide	No	1

Maltose is the only common disaccharide that gives two glucose per mole.

Common traps

Lactose as the double-glucose sugar

Lactose gives ONE glucose and one galactose. Only maltose gives two glucose per mole; the 'double quantity compared with sucrose' answer is maltose.

Amylose with a 1,6 branch

Amylose is the UNBRANCHED half of starch — alpha-1,4 only. The option 'alpha-1,4 and alpha-1,6' describes amylopectin and glycogen.

Amino Acids, Peptides and Proteins

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Peptide Bonds, Protein Shape and Structure Levels

Peptide-bond count

n\ \text{amino acids} \to (n-1)\ \text{peptide bonds};\qquad \alpha\text{-helix: } 3.6\ \text{residues per turn}

The Amino Acids by Side Chain: Class, Code and the Essential Ones

Amino acid	3-letter / 1-letter	Side chain R	Class
Glycine	Gly / G	–H	Neutral, achiral
Alanine	Ala / A	–CH₃	Neutral
Serine	Ser / S	–CH₂OH	Neutral
Threonine	Thr / T	–CH(OH)CH₃	Neutral, essential
Methionine	Met / M	–CH₂CH₂SCH₃	Neutral, essential, has S
Leucine / Valine	Leu / L · Val / V	isobutyl · isopropyl	Neutral, essential
Tryptophan	Trp / W	indolylmethyl	Neutral, essential, heterocyclic
Aspartic acid	Asp / D	–CH₂COOH	Acidic D is aspartic acid; E is glutamic acid.
Glutamic acid	Glu / E	–CH₂CH₂COOH	Acidic
Lysine	Lys / K	–(CH₂)₄NH₂	Basic, essential
Arginine	Arg / R	guanidino	Basic
Histidine	His / H	imidazolylmethyl	Basic, essential, heterocyclic The one that is basic, essential and heterocyclic at once — the favourite answer.

Class follows the side chain: a second COOH is acidic, a second basic N is basic, everything else neutral.

Common traps

Gln and Asn read as acidic

Glutamine and asparagine are the AMIDES of the acidic pair — neutral. Only Glu and Asp, the free side-chain acids, are acidic; the three-letter codes differ by one letter.

Insulin as fibrous

Insulin, albumins and legumelin are globular. In every 'NOT globular' or 'identify fibrous' question the answer has been myosin.

Nucleic Acids: Nucleotides, Bases and the Double Helix

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Nucleotide Anatomy: Sugar, Base and Phosphate by Position

Ring nitrogens and the glycosidic N

\text{purine: N-1, 3, 7, 9 (sugar at N-9)};\quad \text{pyrimidine: N-1, 3 (sugar at N-1)};\quad \text{phosphate at C-5}'

The Phosphodiester Backbone and the Watson–Crick Double Helix

Base pairing

\text{A}=\text{T}\ (2\ \text{H-bonds}),\quad \text{G}\equiv\text{C}\ (3\ \text{H-bonds});\quad 5'\text{-phosphate} \cdots 3'\text{-OH}

Common traps

Purine N at 1, 3, 5

There is no N-5 in a purine — the six-membered ring's nitrogens are 1 and 3, the five-membered ring's 7 and 9. Position 5 is the shared carbon.

–C–O–C– as the backbone

An ether link is what a glycosidic bond is; the nucleic-acid backbone runs through PHOSPHORUS — sugar 3′-O–P–O-5′ sugar. The statement 'backbone formed of –C–O–C– linkage' is the false one.

Lipids, Enzymes and Other Biomolecules

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Triglycerides, Fatty Acids, Soap and Enzymes

Triglyceride hydrolysis

\text{triglyceride} + 3\,\text{H}_2\text{O} \to \text{glycerol} + 3\,\text{RCOOH};\qquad \text{linolenic acid: } \text{C}_{18},\ 3\ \text{C=C}

Common traps

Linoleic for linolenic

One letter apart: linoleic acid has TWO C=C, linolenic THREE. The '-trien-' in 9,12,15-octadecatrienoic acid gives the count.

Biomolecules formulas

Carbohydrates: Classification, Structure and Reactions

Classification, and the Structures of Glucose and Fructose

Reactions of Glucose: What Each One Proves

Common traps

Glycosidic Linkages in Di- and Polysaccharides

Starch, Glycogen and Cellulose

Sucrose, Maltose, Lactose, Raffinose, Stachyose: the Linkage and the Hydrolysis Count

Common traps

Amino Acids, Peptides and Proteins

Peptide Bonds, Protein Shape and Structure Levels

The Amino Acids by Side Chain: Class, Code and the Essential Ones

Common traps

Nucleic Acids: Nucleotides, Bases and the Double Helix

Nucleotide Anatomy: Sugar, Base and Phosphate by Position

The Phosphodiester Backbone and the Watson–Crick Double Helix

Common traps

Lipids, Enzymes and Other Biomolecules

Triglycerides, Fatty Acids, Soap and Enzymes

Common traps

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