Starch is made up of thousands of glucose molecules. As they are not dissolved in the cytoplasm, they do not influence the internal osmotic pressure and constitute a large energy reserve that occupies little volume.
Starch is found in seeds and tubers, such as potatoes and sweet potatoes. With this energy reserve, plants can obtain energy without the need for light.
- Amylose It is made up of a polymer of glucoses linked by α (1 → 4) bonds in an unbranched chain . It has a helical structure with six glucose molecules (three maltose ) per turn. It is made up of between 200 and 300 glucose molecules (α-D-glucopyranose). It is soluble in water, giving colloidal dispersions . With iodine it is stained blackish blue.
- Amylopectin. It is made up of a polymer of α-D-glucopyranose linked by α bonds (1 → 4), with branches with α bonds (1 → 6). Its structure is also helical, similar to that of amylose, but with a lateral branching caused by an α (1-6) bond every 25-30 glucose molecules. It is less soluble in water than amylose. With iodine it is stained dark red.
- The amylases, enzymes hydrolases present in the salivary and the pancreatic juice, hydrolyze α (1 → 4) to obtain molecules of glucose and maltose as final products.
- The branches with α (1→6) bonds presented by amylopectin have to be degraded by the debranching enzyme α (1→6) glucosidase, which completes the hydrolysis giving rise to maltose and glucose.
- Ultimately, the enzyme maltase will hydrolyze the maltose molecules and glucose will be obtained .