Glucose is a simple sugar molecular. Glucose molecules hooked together in a specific way creates starch molecules. Plants store excess glucose in the form of starch. When glucose is needed, plants break down the links between the glucose molecules in starch and obtain free glucose molecules, which are then available for energy. Starch is very similar to cellulose, except for the way in which the glucose molecules are linked together. This difference has significant biochemical implications.
Structure of Glucose
The glucose molecule is a simple monosaccharide sugar molecule. It has a chemical formula of C6H12O6, indicating that there are six carbon atoms, 12 hydrogen atoms and six oxygen atoms for every one atoms of glucose. In a straight chain configuration, the first carbon atom is part of an aldehyde or CHO group. This aldehyde group can reach with the hydroxyl OH group on the fifth carbon atom forming a ring structure for glucose. In human blood and inside cells of plants and animals, free glucose molecules exists as this hexagonal ring. The orientation of the hydrogen and hydroxyl groups relative to the plane of the ring is crucial for a variety of biochemical reactions.
For alpha-glucose, the hydrogen atom of carbon-1, the carbon originally part of the aldehyde group in the straight chain structure, is above the plane of the ring, whereas in beta-glucose, that hydrogen atom is below the plane of the ring.
Structure of Starch
Glucose molecules can link together to form a variety of different molecules. Starch is formed when the carbon-1 atom of one glucose molecule is linked to the carbon-4 atom of a second glucose molecule. When only two glucose molecules are connected in that manner, the result is a disaccharide called maltose. However, when many glucose molecules are connected in this 1-4 linkage, the resulting molecule is the polysaccharide called starch.
Starch has two components; amylose and amylopectin. Amylose occurs when there is a straight chain of carbon-1 to carbon-4 linkages. These chains can be long and fold into a three-dimensional helical shape. Glucose polymers can also be made when the carbon-6 atom of one glucose molecule links to the carbon-1 atom of another glucose atom. These 1-6 linkages are considered branches off of the straight chain of glucose 1-4 linkages. Branching is seen in amylopectin.
Starch Versus Cellulose
Starch is very similar to cellulose; both being composed of polymers of glucose. However, in cellulose, instead of the linkages between glucose molecules occurring between alpha-glucose, where the hydrogen atom on the carbon-1 atom is above the plane of the ring, the linkages occur on beta-glucose molecules, where the hydrogen atom on the carbon-1 atom is below the plane of the ring.
The difference between polymers made from alpha-linkages as in starch versus beta-linkages in cellulose has significant biochemical consequences. Because of this simple stereochemical difference, humans can digest starch, but not cellulose. Human enzymes are specifically designed to recognize the stereochemistry, or three-dimensional structure of alpha-linkages, but not beta-linkages. In addition, cellulose is stronger than starch, it does not dissolve in water the way starch does and does not break down easily.
Foods Containing Starch
Starch is in a variety of foods. For example, starch is found in significant quantities in breads and pastas, as well as potatoes and corn. It is used as a gravy, pudding and sauce thickener.
Non-Food Starch Items
Starch is also used in non-food items. Spray starch is used to keep clothes from wrinkling when ironing. It is also used to make packing biodegradable packing materials, as they dissolve in water.
