When a plant that contains starch is kept in a dark room for a week, it is unable to carry out photosynthesis. Photosynthesis is the process by which plants convert light energy from the sun into chemical energy, producing glucose and oxygen from carbon dioxide and water. Starch is a polysaccharide that plants use as a form of stored energy, which is derived from glucose.
In the absence of light, the plant cannot produce new glucose. However, it still requires energy for various metabolic processes, such as respiration. During this time, the plant will use the stored starch as a source of energy. The starch is broken down into glucose, which is then utilized for cellular respiration to meet the plant's energy needs.
As a result, after a week in the dark, the starch reserves are depleted, explaining why you can no longer detect any starch in the plant. The absence of light leads to the consumption of the stored starch for energy, confirming that the starch was metabolized to sustain the plant's life processes.