Biomolecules
Origin: Primarily produced by plants.
Common Examples: Cane sugar, glucose, starch.
General Formula: Cₓ(H₂O)ᵧ (considered as hydrates of carbon).
● Example: Glucose (C₆H₁₂O₆) fits into this formula.
● Exceptions: Acetic acid (C₂H₄O₂) fits the formula but is not a carbohydrate;
Rhamnose (C₆H₁₂O₅) is a carbohydrate but doesn't fit the formula.
Definition: Chemically defined as optically active polyhydroxy aldehydes or
ketones or compounds that produce such units upon hydrolysis.
Sugars:
● Carbohydrates that are sweet in taste.
● Common Sugars:
○ Sucrose: Common household sugar.
○ Lactose: Sugar present in milk.
● Also called saccharides (from Greek: sakcharon meaning sugar).
Classification of
Carbohydrates
Carbohydrates are classified based on their hydrolysis behavior into three groups:
1. Monosaccharides:
○ Cannot be hydrolyzed further into simpler units.
○ Examples: Glucose, fructose, ribose.
○ About 20 naturally occurring monosaccharides are known.
2. Oligosaccharides:
● Yield 2 to 10 monosaccharide units upon hydrolysis.
● Further classified into disaccharides, trisaccharides, etc.
● Disaccharides: The most common, yield two monosaccharides.
○ Example: Sucrose hydrolyzes into glucose and fructose; Maltose
hydrolyzes into two glucose molecules.
3. Polysaccharides:
● Yield a large number of monosaccharide units upon hydrolysis.
● Examples: Starch, cellulose, glycogen, gums.
● Polysaccharides are not sweet, hence called non-sugars.
Reducing and Non-Reducing Sugars:
● Reducing Sugars: Carbohydrates that reduce Fehling’s solution and Tollen’s
reagent.
○ All monosaccharides (both aldoses and ketoses) are reducing sugars.
● Non-Reducing Sugars: Do not reduce Fehling’s solution or Tollen’s reagent.
Classification of Monosaccharides:
Monosaccharides are classified based on:
1. Number of Carbon Atoms.
2. Functional Group (Aldehyde or Keto).
● Aldose: Monosaccharides containing an aldehyde group.
● Ketose: Monosaccharides containing a keto group.
Preparation of Glucose
Structure of Glucose
1. Its molecular formula was found to be C6H12O6.
2. On prolonged heating with HI, it forms n-hexane, suggesting that all the six
carbon atoms are linked in a straight chain.
3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of
hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a
carbonyl group (>C = O) in glucose.
4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction
with a mild oxidising agent like bromine water. This indicates that the carbonyl
group is present as an aldehydic group.
5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which
confirms the presence of five –OH groups. Since it exists as a stable compound,
five –OH groups should be attached to different carbon atoms.
6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a
dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic
(–OH) group in glucose.
Glucose is a six carbon straight chain aldose which has one aldehydic group
(—CHO), one primary hydroxyl group (—CH2OH) and four secondary hydroxyl
groups (—CHOH). If the —OH group attached to C-5 is on the right side, the
glucose is assigned D–configuration; if the —OH group attached to C-5 is on the
left side, it is assigned L–configuration. The (+) and (–) signs represent the optical
rotation as dextro and laevo, respectively and have no relationship with D and L
configuration.
Cyclic Structure of
Glucose
1. Despite having the aldehyde group, glucose does not give Schiff’s test and it
does not form the hydrogensulphite addition product with NaHSO3 .
2. The pentaacetate of glucose does not react with hydroxylamine indicating the
absence of free —CHO group.
3. Glucose is found to exist in two different crystalline forms which are named as
a and b. The 𝜶-form of glucose (m.p. 419 K) is obtained by crystallisation from
concentrated solution of glucose at 303 K while the 𝛽-form (m.p. 423 K) is
obtained by crystallisation from hot and saturated aqueous solution at 371 K.
4. Such isomers, i.e., 𝜶-form and 𝛽-form, are called anomers. The six membered
cyclic structure of glucose is called pyranose structure (𝜶– or 𝛽–), in analogy with
pyran.
Haworth Structures
Fructose
Fructose is a ketohexose and has the molecular formula C6H12O6. It belongs to
D-series and is a laevorotatory compound.
The cyclic structures of two anomers of fructose are represented by Haworth
structures as given below.
Disaccharides
The sugar which on hydrolysis gives two units of monosaccharides is called
disaccharide.
The two monosaccharides are joined together by an oxide linkage formed by the
loss of a water molecule. Such a linkage between two monosaccharide units
through oxygen atom is called glycosidic linkage.
Sucrose
Non reducing sugar.
Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and
laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than
dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis
of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo
(–) and the product is named as invert sugar.
Maltose
Reducing sugar.
lactose
Reducing sugar.
Polysaccharides
Polysaccharides are the carbohydrates which yield a large number of
monosaccharide molecules upon hydrolysis. Starch, cellulose and glycogen are
examples of polysaccharides.
Starch
It is a polymer of 𝜶-glucose and consists of two components— Amylose and
Amylopectin. Amylose is water soluble component which constitutes about
15-20% of starch.
Amylopectin is insoluble in water and constitutes about 80- 85% of starch.
Cellulose
Cellulose is a linear polymer of 𝛽-D-glucose
Proteins
Amino acids
● Those compounds, whose molecule contains both the carboxylic acid group
and the amino group are called amino acids.
● There are twenty amino acids which form protein.
● The amino acids which are synthesised in body are known as non-essential
amino acids, e.g., glycine, alanine.
● Those amino acids which cannot be synthesised in body and must be
obtained through diet are known as essential amino acids, e.g., valine, lysine.
● Amino acids have also been classified as neutral, acidic and basic amino
acids.
● Amino acids like glycine, valine, etc. which contain one —NH2 and one
—COOH group are called neutral amino acids.
● Those amino acids such as aspartic acid, glutamic acid, etc. which contain
one —NH2 group and two —COOH groups are called acidic amino acids and
amino acids such as lysine, histidine, etc., which contain two —NH2 groups
and one —COOH group are called basic amino acids.
