Although glucose, galactose, and fructose all have the same chemical formula C 6 H 12 O 6 , they differ structurally and chemically and are known as isomers because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon Figure 2.
Figure 2. Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula C6H12O6 but a different arrangement of atoms. Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms Figure 3.
Glucose in a ring form can have two different arrangements of the hydroxyl group -OH around the anomeric carbon carbon 1 that becomes asymmetric in the process of ring formation. Figure 3. Five and six carbon monosaccharides exist in equilibrium between linear and ring forms.
Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. 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 Figure 4.
Glycosidic bonds also called glycosidic linkages can be of the alpha or the beta type. Figure 4. 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 Figure 5. 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. Figure 5.
Common disaccharides include maltose grain sugar , lactose milk sugar , and sucrose table sugar. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be , daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin both polymers of glucose.
The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. The numbers and refer to the carbon number of the two residues that have joined to form the bond. Notes: Though we have learned about the forming of bonds through the transfer of electrons, the majority of compounds that we come in contact with in our daily lives are formed in a completely different manner.
The atoms are covalently bonded. A covalent bond is one where the electrons are shared. A group of covalently bonded atoms is called a molecule. These molecular substances include DNA, sugar and carbon dioxide. The molecules can contain as few as 2 atoms and as many as a million.
What this means is that electrons are not evenly shared between the atoms. Bees gather nectar from flowers which contains sucrose. They then use an enzyme to hydrolyze or break apart the sucrose into its component parts of glucose and fructose. High Fructose Corn Syrup. The chair form of fructose follows a similar pattern as that for glucose with a few exceptions. Since fructose has a ketone functional group, the ring closure occurs at carbon 2.
In the case of fructose a five membered ring is formed. The -OH on carbon 5 is converted into the ether linkage to close the ring with carbon 2. This makes a 5 member ring - four carbons and one oxygen. The ring structure is written with the orientation depicted on the left for the monosaccharide and is consistent with the way the glucose is depicted.
The anomeric carbon is the center of a hemiketal functional group.
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