Multicellular eukaryotic life forms characterized by photosynthesis, in which chemical energy is produced from water, minerals, and carbon dioxide with the aid of pigments and the radiant energy of the Sun. However, also included in the kingdom are plants that do not produce their food by photosynthesis, but rather are parasitic and live off of other living plants. Other plants are saprophytic, obtaining their food from dead organic matter. Plants have cells that contain cellulose in their walls and are therefore to some extent rigid. There is also an absence of organs of locomotion, resulting in a more or less stationary existence as well as no sensory and nervous systems. There are over 300,000 living species of plants.