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Definition
• Carbohydrates, or carbs, are sugar molecules. Along with proteins and fats,
carbohydrates are one of three main nutrients found in foods and drinks.
• Carbohydrates are found in a wide array of both healthy and unhealthy foods—
bread, beans, milk, popcorn, potatoes, cookies, spaghetti, soft drinks, corn, and
cherry pie. They also come in a variety of forms. The most common and abundant
forms are sugars, fibers, and starches.
• A carbohydrate is a naturally occurring compound, or a derivative of such a
compound, with the general chemical formula Cx(H2O)y, made up
of molecules of carbon (C), hydrogen (H), and oxygen (O). Carbohydrates are the
most widespread organic substances and play a vital role in all life.
• The chemical formula of a carbohydrate is Cx(H2O)y, which denotes
some carbons (C) with some water molecules (H2O) attached—hence the
word carbohydrate, which means “hydrated carbon.”
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Classification
Although a number of classification schemes have been devised for
carbohydrates, the division into four major groups are as follows-
monosaccharides
disaccharides
oligosaccharides
Polysaccharides
A saccharide is the unit structure of carbohydrates. In biochemistry,
saccharides are the carbohydrates or sugars that serve as the main source
of energy that fuels diverse biochemical processes
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Monosaccharide
• Most monosaccharides, or simple sugars, are found in grapes, other
fruits, and honey.
• Although they can contain from three to nine carbon atoms, the most
common representatives consist of five or six joined together to form a
chainlike molecule.
• Three of the most important simple sugars—glucose (also known as
dextrose, grape sugar, and corn sugar), fructose (fruit sugar),
and galactose—have the same molecular formula, (C6H12O6), but,
because their atoms have different structural arrangements, the sugars
have different characteristics; i.e., they are isomers.
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Disaccharide
• Two molecules of a simple sugar that are linked to each other form
a disaccharide, or double sugar.
• The disaccharide sucrose, or table sugar, consists of one molecule of
glucose and one molecule of fructose; the most familiar sources of
sucrose are sugar beets and cane sugar.
• Milk sugar, or lactose, and maltose are also disaccharides.
• Before the energy in disaccharides can be utilized by living things, the
molecules must be broken down into their respective monosaccharides
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Oligo saccharide
Oligosaccharides, which consist of three to six monosaccharide units,
are rather infrequently found in natural sources, although a few plant
derivatives have been identified.
They are normally present as glycans
Oligosaccharide chains are linked to lipids or to compatible amino
acid side chains in proteins, by N- or O-glycosidic bonds.
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POLYSACCHARIDE
• Polysaccharides (the term means many sugars) represent most of the
structural and energy-reserve carbohydrates found in nature.
• Large molecules that may consist of as many as 10,000
monosaccharide units linked together, polysaccharides vary
considerably in size, in structural complexity, and in sugar content;
several hundred distinct types have thus far been identified.
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Functions
• Carbohydrates are widely distributed molecules in plant and animal tissues. In
plants and arthropods, carbohydrates from the skeletal structures, they also
serve as food reserves in plants and animals. They are important energy sources
required for various metabolic activities, the energy is derived by oxidation.
• Living organisms use carbohydrates as accessible energy to fuel cellular
reactions. They are the most abundant dietary source of energy (4kcal/gram) for
all living beings.
• Carbohydrates along with being the chief energy source, in many animals, are
instant sources of energy. Glucose is broken down by glycolysis/ Kreb’s
cycle to yield ATP.
• Serve as energy stores, fuels, and metabolic intermediates. It is stored as
glycogen in animals and starch in plants.
• Stored carbohydrates act as an energy source instead of proteins.
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• They form structural and protective components, like in the cell wall of
plants and microorganisms. Structural elements in the cell walls of bacteria
(peptidoglycan or murein), plants (cellulose), and animals (chitin).
• Carbohydrates are intermediates in the biosynthesis of fats and proteins.
• Carbohydrates aid in the regulation of nerve tissue and is the energy source
for the brain.
• Carbohydrates get associated with lipids and proteins to form surface
antigens, receptor molecules, vitamins, and antibiotics.
• Formation of the structural framework of RNA and DNA (ribonucleic acid
and deoxyribonucleic acid).
• They are linked to many proteins and lipids. Such linked carbohydrates are
important in cell-cell communication and in interactions between cells and
other elements in the cellular environment.
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Physical Properties of Carbohydrates
1.Solubility: Carbohydrates are generally soluble in water due to their polar nature.
This property makes them readily available for metabolic processes within the cell.
2.Taste and Sweetness: Many carbohydrates, particularly monosaccharides (simple
sugars) and some disaccharides (double sugars), have a sweet taste. For example,
glucose, fructose, and sucrose are sweet-tasting sugars.
3.Physical State: Carbohydrates can exist in various physical states, including solid,
liquid, or amorphous forms. Common solid forms include granules (e.g., starch in
plants) and crystals (e.g., table sugar). Some carbohydrates can also be found in a
gel-like or syrupy state.
4.Melting and Boiling Points: Carbohydrates typically have high melting and boiling
points when compared to smaller organic molecules. However, the specific melting
and boiling points can vary depending on the type and size of the carbohydrate. For
example, glucose, a monosaccharide, has a melting point of around 146°C (295°F).
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5.Optical Activity: Many carbohydrates are optically active, meaning they can rotate the plane
of polarized light. This property is due to their chiral nature, as they often have asymmetrical
carbon atoms (chiral centers). Glucose, for instance, exhibits optical activity.
6.Color: Carbohydrates can vary in color depending on their structure and state. For example,
glucose and fructose are colorless, while some larger carbohydrates like caramel can be brown.
7.Hygroscopicity: Carbohydrates often have a high affinity for water and can readily absorb
moisture from the environment, leading to changes in their physical properties, such as texture
and consistency.
8.Crystalline Structure: Some carbohydrates, like sucrose and lactose, can form well-defined
crystalline structures. The arrangement of molecules in these crystals can affect their texture
and properties, such as solubility.
9.Viscosity: Carbohydrates in solution can exhibit varying degrees of viscosity (thickness or
resistance to flow). For example, solutions of polysaccharides like cellulose or pectin can be
quite viscous.
10.Molecular Weight: Carbohydrates can have a wide range of molecular weights, from small
monosaccharides with low molecular weights to large polysaccharides with high molecular
weights.
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Chemical properties of carbohydrates
1.Composition: Carbohydrates are composed of carbon, hydrogen, and oxygen atoms.
The ratio of hydrogen to oxygen is typically 2:1, which is the same as in water (H2O).
2.Functional Groups: Carbohydrates contain functional groups such as hydroxyl
groups (-OH) and carbonyl groups (C=O). The carbonyl group is either an aldehyde (-
CHO) or a ketone (C=O) group.
3.Isomerism: Carbohydrates can exist as isomers, which means they have the same
molecular formula but different structural arrangements. Common isomers include
glucose and fructose.
4.Hydrolysis: Carbohydrates can be hydrolyzed, which means they can be broken down
into smaller units (monosaccharides) by adding water and an enzyme. This process is
essential for the digestion of carbohydrates in the body.
5.Ring Structures: In aqueous solutions, many monosaccharides exist in a ring form
due to intramolecular reactions between the carbonyl and hydroxyl groups. This ring
structure is often in a six-membered (pyranose) or five-membered (furanose) ring.
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6.Redox Reactions: Carbohydrates can undergo redox
(oxidation-reduction) reactions. For example, in cellular
respiration, glucose is oxidized to produce energy.
7.Cyclization: Monosaccharides can undergo cyclization
reactions to form hemiacetals or hemiketals, resulting in the
formation of ring structures.
8.Tautomerism: Some monosaccharides can exist in different
tautomeric forms, where the positions of hydrogen atoms
and double bonds change within the molecule.
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SOME TERMS DISCUSSED
• Monosaccharide exists in both as straight chain structure and cyclic structure .
Sugars with five membered rings and with six membered rings are most stable.
Cyclic structures are the result of hemiacetal formation by intermolecular
reaction between carbonyl group and a hydroxyl group.
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CHIRAL CENTRE
• An atom with four different groups bonded to it in such a way that it has a non-
superimposable mirror image is called a chiral center.
• Chiral center means the carbon atom in the system that has all the different
substituents. So, in the ring structure, we have to find which carbon atom is attached to
four different compounds or elements.
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D and L isomers
• A D-isomer is defined as a stereoisomer which rotates light that is polarized
in a clockwise direction. This differs from an L-isomer which rotates light
in an anticlockwise direction. The pair are enantiomers of each other which
act as mirror images of each other and can also be known as optical
isomers.
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Optically active compound
• The compounds which are capable of optical rotation are said to be
optically active compounds. All the chiral compounds are optically
active. The chiral compound contains an asymmetric center where the
carbon is attached to four different atoms or groups. It forms two non-
superimposable mirror images.
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Enantiomer
• Enantiomers are a pair of molecules that exist in two forms that are
mirror images of one another but cannot be superimposed one upon
the other. Enantiomers are in every other respect chemically identical.
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Anomer
• Anomers are cyclic monosaccharides or glycosides that are epimers,
differing from each other in the configuration of C-1 if they are
aldoses or in the configuration at C-2 if they are ketoses.
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Epimer
• Epimers are carbohydrates that differ in the location of the -OH group
in one location. Both D-glucose and D-galactose are the best
examples. D-glucose and D-galactose epimers create a single
difference at C-4 carbon. They are not enantiomers, they are just
epimers, or diastereomers, or isomers.
