Biomolecules 1 Flashcards
what are carbohydrates
Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
Some of the carbohydrates,
which are sweet in taste, are also called sugars.
The most common sugar, used in our homes is named as sucrose whereas the sugar present in milk is known as lactose.
Carbohydrates are also called saccharides
general formula of carbohydrates
Most of them have a general formula, Cx(H2O)y, and were considered as hydrates of carbon from where the name carbohydrate was derived.
For example acetic acid (CH3COOH) fits into
this general formula, C2
(H2O)2 but is not a carbohydrate.
Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition.
monosaccharides
A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide.
About 20 monosaccharides are known to occur in nature.
Some common examples are glucose, fructose, ribose, etc.
oligosaccharides
Carbohydrates that yield two to ten
monosaccharide units, on hydrolysis, are called oligosaccharides.
They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide
on hydrolysis.
Amongst these the most common are disaccharides.
The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different.
For example, one molecule of sucrose on
hydrolysis gives one molecule of glucose and one molecule of fructose
whereas maltose gives two molecules of only glucose.
polysaccharides
Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides.
Some common examples are starch, cellulose, glycogen, gums,
etc.
Polysaccharides are not sweet in taste, hence they are also
called non-sugars.
reducing sugars and non reducing sugars
. All those carbohydrates which reduce Fehling’s
solution and Tollens’ reagent are referred to as reducing sugars.
All monosaccharides whether aldose or ketose are reducing sugars.
Sucrose is non reducing disaccharide. All polysaccharides are on reducing.
what is glucose
Glucose is an aldohexose and is also known as dextrose. It is the
monomer of many of the larger carbohydrates, namely starch, cellulose.
It is probably the most abundant organic compound on earth.
Its structure was elucidated using evidences:
i) Its molecular formula is C6H12O6
reactions
full name of glucose and its meaning
Glucose is correctly named as D(+)-glucose. ‘D’ before the name
of glucose represents the configuration whereas ‘(+)’ represents
dextrorotatory nature of the molecule.
D and L configuration
The letters ‘D’ or ‘L’ before the name of any compound indicate the relative configuration of a particular stereoisomer of a compound with respect to configuration of some other compound, configuration of
which is known.
In the case of carbohydrates, this refers to their
relation with a particular isomer of glyceraldehyde.
In D form ( + glyceraldehyde), the OH group lies on the right hand side. In L form ( - glyceraldehyde), the -Oh group lies on the left hand side.
what properties of glucose could not be explained by open chain structure
- Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with
NaHSO3. - The pentaacetate of glucose does not react with hydroxylamine
indicating the absence of free —CHO group. - Glucose is found to exist in two different crystalline forms which are named as a and b. The a-form of glucose (m.p. 419 K) is obtained by crystallisation from concentrated solution of glucose at 303 K while
the b-form (m.p. 423 K) is obtained by crystallisation from hot and
saturated aqueous solution at 371 K.
what is anomeric carbon in glucose
The two cyclic hemiacetal forms of glucose differ only in the
configuration of the hydroxyl group at C1, called anomeric carbon.
(the aldehyde carbon before cyclisation). Such isomers, i.e., a-form
and b-form, are called anomers.
fructose
Fructose is an important ketohexose. It is obtained along with glucose
by the hydrolysis of disaccharide, sucrose. It is a natural
monosaccharide found in fruits, honey and vegetables. In its pure
form it is used as a sweetener. It is also an important ketohexose.
what is glycosidic linkage
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.
reducing and non reducing disaccharides
In disaccharides, if the reducing groups of monosaccharides i.e.,
aldehydic or ketonic groups are bonded, these are non-reducing sugars,
e.g., sucrose. On the other hand, sugars in which these functional groups
are free, are called reducing sugars, for example, maltose and lactose.
what is starch
Starch is the main storage polysaccharide of plants. It is
the most important dietary source for human beings. High content
of starch is found in cereals, roots, tubers and some vegetables. It
is a polymer of a-glucose and consists of two components—
Amylose and Amylopectin.
