Carbohydrates
Carbohydrates are a class of molecules that are composed of carbon, hydrogen and oxygen. They have the empirical formula
CH2O. Carbohydrates are produced in green plants in the presence of chlorophyll and sunlight in a process known as
photosynthesis. They serve as food sources for living organisms and provide the structural support for plants. Many of the
carbohydrates are large polymers composed of repeating units of simple sugars.
Monosaccharides
Monosaccharides or simple sugars are the smallest molecular units of carbohydrates. The general la of monosaccharides is
(CH2O)n where n = 3 to 9. Monosaccharides are crystalline substances that are generally very sweet. They are fairly polar and
dissolve readily in water
Many monosaccharides including glucose, fructose, and galactose have 6 carbon atoms and the molecular formula C6H12O6. They
are known as hexose sugars. Monosaccharides with 5 carbon atoms such as ribose are known as pentoses. Pentoses and hexoses
are the most common monosaccharides found in nature.
Monosaccharides contain a carbonyl group ( C=O ) and several hydroxyl groups (-OH) in their molecules. If the carbonyl is located
on terminal carbon atom , the monosaccharide is an alkanal or aldehyde. They are often referred to as aldose or reducing sugars. If
the carbonyl is located on an internal carbon atom, the sugar is an alkanone or ketone. Such sugars are sometimes called ketose
sugars
The straight chain structures for D-glucose and D-fructose are shown below:
Glucose is an aldehyde while fructose is a ketone. The carbon atoms in glucose are numbered, starting with 1 at the top and ending
with 6 at the bottom. In each of the above sugars there are several asymmetric or chiral carbon atoms. Therefore there are many
possible optical isomers. In addition both glucose and fructose form ring structures shown below.
The six-member ring for glucose contains five carbon atoms and 1 oxygen atom. The carbons are numbered starting at the right of
the oxygen in the ring and continuing clockwise. A six member-ring is known as a pyranose sugar. The five-member ring in fructose
is known as a furanose
Glucose is usually found in a ring or cyclic (hemiacetal) structure in aqueous solution due to intramolecular reaction between the
alkanal group at the first carbon and the hydroxyl group on the fifth carbon to form a 6-member ring. . Thus, there are two common
ring structure isomers of glucose called α-D-glucose where the –OH on carbon 1 is below the ring and β-D-glucose where the –OH
group on carbon 1 is above the ring.
The only difference between the α and β forms is the location of the H and OH groups on the first carbon. This difference is
significant and gives the two isomers different physical and chemical properties
Dissacharides
Two monosaccharides can combine to form a more complex molecule known as a disaccharide. When two monosaccharides
combine in a condensation reaction, a molecule of water is eliminated. Sucrose or common table sugar, has the molecular formula
C12H22O11. It results from the combination of α-D-glucose and β-D-fructose.
The C-O-C bond that forms between the rings is an ether structure. This link is called the glucoside linkage. It is common in all
disaccharides and polysaccharides. In the case of sucrose the glucoside link forms between Carbon 1 of the glucose and carbon 2
of the fructose. In many structures, however, the glucoside link forms between carbon atoms 1 and 4 of neighboring units. Therefore
it is often referred to as a 1,4 bond. When one consumes sucrose the glucoside linkage undergoes hydrolysis takes place. This
reaction is catalyzed by an enzyme in saliva and produces the two monosaccharides. These monosaccharides are eventually
broken down and oxidized to carbon dioxide and water vapor with the release of energy. Like monosaccharides some disaccharides
are aldose or reducing sugars, while others are ketose or non-reducing sugars.
Some other common disaccharides include
α-D-glucose + β-D-galactose = lactose
Polysaccharides
Polysaccharides are complex molecules that are made up of multiple monosaccharide units joined together in a polymer
chain. These molecules have large molar masses. Unlike monosaccharides or disaccharides there substances are not sweet, are
generally insoluble or only slightly soluble in water. Because it is unlikely that they will have an alkanal unit in the polymer chain they
are non-reducing and do not react with Benedicts reagent.
Most plants store glucose as a polysaccharide , which is commonly called starch. There are two forms of starch. Both forms are
polymers of α-D-glucose. Amylose has a straight chain structure consisting of thousands of long chains with 1,4 glycosidic linkages.
Typically the molar mass of amylase ranges from 10,000 to as high as 50,000.
The amylose molecule contains only 1,4 linkages between repeating glucose units
Amylopectin has two types of glycosidic linkages. It has 1,4 links as in amylase, but also has branches of 1,6 linkages as well.
Amylopectin has about twice as may glucose units as amylose. Its molecular mass may range from 50,000 – 100 million
A portion of an amylopectin molecule containing 1,4,& 1,6 linkages
Cellulose is very similar to amylose but the linkage is a β linkage instead of an α linkage
The cellulose showing 1,4 β linkages
Human beings cannot digest cellulose because they lack the enzyme necessary to break up the
beta linkage.
Functions of Carbohydrates
Carbohydrates serve as energy sources, energy reserves (e.g., glycogen) and as precursors for other biologically important
molecules, such as DNA. In plants cellulose serves as a structural material in the cell walls
1. Energy Sources: During metabolism animals break down carbohydrates to carbon dioxide and water vapor.
Monosaccharides and dissaccharides break down quickly and provide quick energy sources. Starches take longer to
metabolize but the end products are the same. Human beings cannot break down cellulose, since we lack the appropriate
enzyme to breakdown the β 1-4 linkage
2. Storage The main storage polysaccharides are starches and glycogen. While plants use starch as their storage
polysaccharides, animals use glycogen. Glycogen is stored in liver. When the body has a high glucose concentration, the
pancreas releases insulin, which converts glucose into glycogen and stores it in the liver. When the glucose concentration is
low, the hormone glucagon converts glycogen back into glucose.
Glycogen is the primary energy reserve in human beings . Metabolism of glucose provides the energy necessary for our bodies
to function and carry out daily activities. When it is broken down into glucose and oxidized, ultimately to CO2 and H2O, through
cellular respiration. Large amounts of energy are released. One molecule of glucose yields 36 units of energy.
3. Structure Cellulose is a major component of plant cell walls. It is a polymer of β-D-glucose and forms a very strong fiber,
which is excellent building material in plants. Cows and other ruminants have enzymes that break down cellulose. In humans it
is primarily bulk or roughage.
Some insects have exoskeletons that are composed of a polysaccharide known as chitin. Chitin is a leather like structural
substance that eventually hardens when it is shed. Chitin is often used in medicine for sutures because it is both strong and
flexible, but it also decomposes over time.
4. Precursor to other biomolecules Carbohydrates, namely ribose and deoxyribose are part of the skeleton of RNA
and DNA respectively.
5. Dietary Fiber. Dietary fiber is primarily cellulose. It is not digested for energy but plays an important role in assisting food
through the alimentary canal. Lack of fiber in the diet can lead todiverticulosis, and other diseases of the large intestine.