The purpose of this experiment is to measure fruit ripening caused by the plant hormone ethylene, by using an iodine indicator to detect the conversion of plant starch to sugar.
The ripening of an unripe fruit will be unaffected by storing it with a banana.
You've heard that 'one bad apple spoils the whole bushel', right? It's true. Bruised, damaged, or overripe fruit gives off a hormone that accelerates the ripening of the other fruit.
Plant tissues communicate by means of hormones. Hormones are chemicals that are produced in one location that have an effect on cells in a different location. Most plant hormones are transported through the plant vascular system, but some, like ethylene, are released into the gaseous phase, or air.
Ethylene is produced and released by rapidly-growing plant tissues. It is released by the growing tips of roots, flowers, damaged tissue, and ripening fruit. The hormone has multiple effects on plants. One is fruit ripening. When fruit ripens, the starch in the fleshy part of the fruit is converted to sugar. The sweeter fruit is more attractive to animals, so they will eat it and disperse the seeds. Ethylene initiates the reaction in which the starch is converted into sugar.
Iodine solution binds to starch, but not to sugar, forming a dark-colored complex. You can estimate how ripe a fruit is by whether or not is is darkened after painting it with an iodine solution. Unripe fruit is starchy, so it will be dark. The more ripe the fruit is, the more starch will have been converted to sugar. Less iodine complex will be formed, so the stained fruit will be lighter.