Question

what is starch?and how is formed?

Answer 1

starch is the internal energy reserve of plants

it is formed from excess glucose during photosynthesis

Answer 2

starch is glucosan homopolysaccharide which is major reserve food of plants.
Human depend upon this for there calories.

A molecule of the starch consist of two component, an inner core of amylose and an outer of amylopectin. these two differ by there solubility and reaction to iodine.
amylose give blue black color with iodine molecule. amylopectin give red-voilet color with iodine.

as starch is a polysaccharide it form by the dehydrate synthesis or polymerisation of the large no. of units called monosaccharides.