Three of the major characteristics used to classify organisms are cell structure, mode of nutrition and cellularity. These characteristics help scientists determine how organisms are similar to each other as well as how they are different from each other.
Classifying an organism according to its cell structure means determining if it is a prokaryote or eukaryote and then determining if the organism has a cell wall or other organelles, according to Dave Krupp of the University of Hawaii .
Mode of nutrition refers to how an organism gets its food. Organisms can be classified as autotrophs or heterotrophs. Autotrophs make their own food, while heterotrophs must eat other organisms to survive. If an organism is an autotroph, it can also be classified as a photoautotroph or a chemoautotroph. Stephen T. Abedon of Ohio State explains that photoautotrophs produce their own food using energy from sunlight. Chemoautotrophs produce food using the energy from electron-donating compounds.
Cellularity refers to how many cells an organism has and how those cells are arranged. Some organisms are single-celled, while others are multicellular. Advanced organisms have cells organized into tissues.