Although glucose, galactose, and fructose all have the same chemical formula C 6 H 12 O 6 , they differ structurally and stereochemically. This makes them different molecules despite sharing the same atoms in the same proportions, and they are all isomers of one another, or isomeric monosaccharides.
Glucose and galactose are aldoses, and fructose is a ketose. During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule in this case, between two monosaccharides is known as a glycosidic bond. Glycosidic bonds also called glycosidic linkages can be of the alpha or the beta type.
Disaccharides : Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group.
In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose. Common disaccharides include lactose, maltose, and sucrose.
Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules.
The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose. The chain may be branched or unbranched, and it may contain different types of monosaccharides. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Plants are able to synthesize glucose, and the excess glucose is stored as starch in different plant parts, including roots and seeds.
The starch in the seeds provides food for the embryo as it germinates while the starch that is consumed by humans is broken down by enzymes into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. Glycogen is the storage form of glucose in humans and other vertebrates.
It is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis. Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose and provides structural support to the cell. Every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains.
This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure. Carbohydrates serve various functions in different animals. The enzymes that build up and break down glycogen and starch act on the free ends of the polysaccharides. Having a great deal of branching ensures that plants and animals can quickly add to their energy supply when energy is plentiful, or break it down the storage molecules when energy is in short supply.
Polysaccharides Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Cellulose is the most abundant organic molecule on earth, since it is the main component of plant cell walls. Polysaccharides are very large, high molecular weight biological molecules that are almost pure carbohydrate.
They are constructed by animals and plants from simpler, monosaccharide molecules, by joining together large numbers of the simpler molecules using glycosidic bonds -O-. In some of the largest polysaccarhide structures there can be 10, individual units joined together. There is a large diversity of polysaccharide form; they can differ in the type of sugar, the connections between the sugars and the complexity of the overall molecule. Sometimes known as glycans , there are three common and principal types of polysaccharide, cellulose, starch and glycogen , all made by joining together molecules of glucose in different ways.
This molecule is synthesized, stored, modified and used as a building material by plants. It is certainly the most abundant of all the polysaccharides. In the cellulose molecule the individual glucose monosaccharides are all linked to one another in the form of a long, long, linear chain.
The carbon atom number 1 C1 in one sugar is linked to the fourth carbon atom C4 of the next sugar in an extended array.
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