Why do storage tissues contain insoluble compounds such as starch?
Starches are insoluble compounds with a complex nature and formula (C6H10O6)n. Starch is the most common type of reserve food in green plants and is the key to their metabolic process. As you suggested, since starch’s nature is insoluble it must be digested–made soluble–before it can be used to fuel metabolic processes. This is done through the aid of enzymes present in cells. Were the compound easily soluble the organism could not store adequate amounts of it to run their metabolic processes.
