The iodine test is used to test for the presence of starch. The iodine reacts with starch giving a blue-black color. This reaction is the consequence of the production of polyiodide chains from the reaction of starch and iodine.
The carbohydrates one consumes as foods have their source in the plants generated by the process of photosynthesis. These carbohydrates take the form of starches, sugars, and cellulose. The plants produce all the three, while the animals cannot perform the process of photosynthesis due to the absence of chloroplasts, which are found in plants.
Thus, an animal cell does not react with iodine due to the absence of starch, while the plant cell will give a reaction with iodine forming black-blue color due to the presence of starch.
