All known types of organisms are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. An organism consists of one or more cells; when it has one cell is a unicellular organism; and when it has more than one it is known as a multicellular organism.ost unicellular organisms are of microscopic size and are thus classified as microorganisms. Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs. An organism may be either a prokaryote or a eukaryote.