Karen A. Beitler
Polymers are long molecules made up of repeating chains of monomers. Add glucose to fructose, the sugar in fruit, and you have table sugar (sucrose). Pretty simple so far! Add many more sugar molecules, also known as saccharides, and you come up with a polymer called starch. Carbohydrates make ideal molecules for storing energy because they are large, making them insoluble in water. They are also known to fold into many shapes and are easily converted into other sugars when needed.
The monomer of carbohydrates is a monosaccharide with the basic formula of a carbon, two hydrogen and an oxygen molecule (CH
2
0)
12
. Glucose, a product of photosynthesis, is simple sugar and a monomer with the chemical formula C
6
H
12
O
6.
Glucose is commonly known as corn sugar or blood sugar. Glucose is a monosaccharide or simple sugar made by plants and some prokaryotes. This molecule is used in cellular respiration by the mitochondria in the cell and produces energy needed to carry out cell processes. Glucose (also known as dextrose) is a basic monomer of carbohydrates
13
. Carbohydrates are classified based on the number of carbons they have. Glucose, galactose and fructose all have a six-carbon ring and the same formula; they are hexoses and isomers of one another. An isomer has the same formula but a different structure. Ribose, arabinose, and xylose have five carbons in their formula and are known as pentoses. There are many more, they generally differ in the number of carbons or in the orientation of the hydroxyl (-OH) group. These seemingly small differences affect a sugar's biochemical properties such as taste, or physical properties, such as the temperature at which they melt
14
. This can make a recipe succeed or fail, if you are cooking, and using a different type of sugar than was called for.
Disaccharides are double sugars (C
12
H
22
O
11
). One molecule of water is missing from the formula. A few examples are sucrose (glucose+ 2 fructose), lactose (4 glucose + galactose) and maltose (glucose +4 glucose)
15
. Sucrose is table sugar, lactose is the main sugar in milk
11
. Maltose is not naturally occurring and is the product of hydrolysis. Maltose is the by-product of the principal plant polysaccharide called cellulose. Cellulose is an important carbohydrate in nature.
Polysaccharides are large polymers of sugars and macromolecules (C
6
H
10
O
5
)
n.
Starch is a king-size polymer of many glucose molecules. A complex form of starch is amylopectin, a branched network of glucose molecules. A simpler form is amylose, which has between 200 and 20,000 glucose units
16
. Starch is the main component of potatoes, bread, rice and corn that gives these foods their sweet taste. Starch is the storage molecule of plants, a main constituent of seeds. Since starch is insoluble (can not be dissolved) in water or alcohol this molecule is easily hydrolyzed by enzymes called amylases. Amylases will break down this gigantic polymer into monomers for energy use quickly and efficiently. Animals store a polysaccharide called glycogen. Glycogen is a polymer of glucose like the amylopectin that makes up starch, but glycogen has many more branches. Hydrolysis of glycogen releases glucose for quick use in animal cells. Cells utilize glucose for energy. When glucose cannot be stored as glycogen it is stored as fat
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. Stored glycogen does not last long in humans or an animal, that is the reason for the need to eat.
A cousin to starch is cellulose, the connective tissue in plants. Starch and cellulose are both polymers of the monomer glucose and have the same repeated glucose-based monomers. Cellulose is a structural polysaccharide of plants. Cellulose is linear and indigestible in human digestive system, which helps keep it working.
There are two major biological structural polysaccharides; chitin and cellulose. Cellulose may contain more than half of the world's total organic carbon. Wood is mostly cellulose, and cotton is almost pure cellulose
18
. Cellulose is digested by the microorganisms in termites and the stomach of ruminants. Cellulose aids in the smooth working of the digestive track of humans by providing indigestible fiber
19
. Cellulose is an unbranched natural polysaccharide like chitin.
Chitin is a natural polysaccharide and makes up the shells of lobsters, shrimp and insects. The difference between cellulose and chitin is mobility in the molecule. Chitin is an unbranched polysaccharide that is hard and brittle. Chitin's brittleness may be attributed to a side chain containing nitrogen that cellulose lacks. Chitin's extra side chain also increases its size and mobility. Cellulose is soft, but when layered, as seen in the trunks of trees, cellulose can be very strong. Cellulose has a sister component in animals called collagen.