Starch is a complex carbohydrate, a polysaccharide made of numerous glucose units. Produced by nearly all green plants, it serves as a major energy storage molecule. It is also a significant component of the human diet, found in staple foods such as potatoes, wheat, corn, rice, and cassava. Starch forms a white, tasteless powder that is insoluble in cold water.
Starch’s Function in Plants
Plants synthesize starch to store excess glucose produced during photosynthesis. This process converts light energy into chemical energy, primarily glucose, which is then polymerized into starch. Starch acts as the plant’s main energy reserve, providing fuel for future growth, metabolic activities, and survival during periods without sunlight or dormancy. It is stored in various plant parts, including leaves for temporary reserves, and more permanently in specialized storage organs like roots, seeds, and tubers.
Within plant cells, starch is stored in organelles called plastids, specifically in chloroplasts for temporary storage and in amyloplasts for long-term reserves. Starch molecules are arranged into semi-crystalline granules, with each plant species having a unique granule size. The structure of starch involves two main types of glucose polymers: amylose, a linear chain of glucose units, and amylopectin, a highly branched chain. Amylose constitutes 20-30% of plant starch, while amylopectin makes up the remaining 70-80%. This combination of linear and branched structures allows for dense packing, enabling plants to store a large amount of energy in a compact form.
Starch’s Function as a Nutrient
When consumed by humans and animals, starch serves as a primary dietary source of energy. Digestion begins in the mouth, where the enzyme salivary alpha-amylase starts breaking down the complex carbohydrate into smaller units like maltose. This initial breakdown is brief, as the enzyme’s activity is mostly halted by the acidic environment of the stomach. Most starch digestion occurs in the small intestine.
In the small intestine, pancreatic amylase, secreted by the pancreas, continues to break down remaining starch into disaccharides such as maltose, maltotriose, and alpha-dextrins. Enzymes on the brush border of small intestinal cells, including maltase, further hydrolyze these smaller sugars into individual glucose molecules. For instance, maltase breaks down maltose into two glucose units. This conversion into monosaccharides is essential because only single sugar units like glucose can be absorbed into the bloodstream.
Once starch is fully broken down into glucose, these molecules are absorbed through the intestinal wall into the bloodstream. Glucose is then transported to cells throughout the body, where it is used as the main fuel source for various metabolic activities, including brain function and muscle contraction. If not immediately needed for energy, glucose can be stored in the liver and muscles as glycogen for later use. The digestion of starch into glucose provides a sustained energy release compared to simple sugars. This is because starch, a complex carbohydrate, takes longer to break down, preventing rapid blood sugar spikes and offering a more gradual energy supply.