A biomolecule is any molecule produced by a living organism that is necessary for life processes. These organic substances are broadly classified into four major groups: carbohydrates, lipids, proteins, and nucleic acids, each performing a vast array of functions within the cell. Starch belongs to the carbohydrate class, a group of compounds made up of carbon, hydrogen, and oxygen. Starch is categorized as a polysaccharide, positioning it as a complex form of energy material produced by plants.
Starch: A Complex Carbohydrate
Starch is a polysaccharide, signifying a complex molecular structure built from many smaller sugar units. Simpler carbohydrates include monosaccharides, which are single sugar units like glucose, and disaccharides, such as maltose, which are two units linked together. Polysaccharides like starch are long, polymeric chains formed by linking hundreds or even thousands of these single-sugar monomers.
The primary biological role of starch is energy storage in plants, serving as their long-term fuel reserve. Plants produce glucose during photosynthesis, and any excess is converted into starch for later use. This stored energy is typically found in specialized structures like seeds, roots, and tubers, such as corn, wheat, and potatoes. When the plant requires energy, it breaks the starch down into glucose.
The Chemical Architecture of Starch
The fundamental building block, or monomer, of starch is the single sugar molecule D-glucose. Starch is described as a homopolymer because, upon complete breakdown, it yields only glucose molecules. These glucose units are linked together by covalent bonds known as alpha glycosidic linkages. Starch is not a single uniform molecule but rather a mixture of two different types of polysaccharides: amylose and amylopectin.
Amylose
Amylose is the simpler component, forming a long, mostly linear chain of glucose units joined primarily by alpha-1,4 glycosidic bonds. This linear structure allows amylose to coil into a helical shape, which aids in compact energy storage.
Amylopectin
Amylopectin is a much larger and highly branched molecule. While its main chains are also connected by alpha-1,4 bonds, branches occur at intervals through alpha-1,6 glycosidic bonds. Natural starches typically contain about 20% to 30% amylose and 70% to 80% amylopectin. The highly branched structure of amylopectin provides numerous endpoints for enzymes to access, which influences the speed at which the starch is broken down.
Starch’s Role in Human Metabolism
As the most common carbohydrate in human diets, starch functions as a major source of metabolic energy. The process of breaking down this complex molecule begins in the mouth with the enzyme salivary amylase. The majority of starch digestion takes place in the small intestine, where pancreatic amylase is secreted.
Amylase enzymes cleave the alpha-1,4 glycosidic bonds in the starch chain. This action converts the large starch molecules first into smaller fragments, such as dextrins and the disaccharide maltose. Further enzymes located on the intestinal wall then break these fragments down completely into individual glucose molecules. Once released into the small intestine, the glucose is absorbed into the bloodstream for transport throughout the body. Glucose is the body’s preferred fuel source, especially for the brain and muscles. Any glucose not immediately required for energy is converted into a storage polysaccharide called glycogen, which is primarily deposited in the liver and muscle cells for future use.