An organism is a living entity that has an organized structure and functions as a whole. The term can refer to either a single cell or a multi-cellular creature. Organisms are classified by their level of organization, from simple bacteria to complex plants and animals. All organisms must take in energy and matter from their environment in order to grow and reproduce.
Organisms are made up of one or more cells, which are the basic units of life. Cells are enclosed by a plasma membrane and contain organelles, such as the nucleus, that perform specific functions within the cell. Multicellular organisms also have tissues, which are groups of cells that work together to perform a particular function, such as digestion or reproduction. In addition, multicellular organisms have organs, which are collections of tissues that work together to carry out a specific function, such as the heart or lungs.
All organisms must obtain energy from their environment in order to fuel their metabolic processes. Plants produce their own food through photosynthesis, while animals obtain food by eating other organisms. Both plants and animals use cellular respiration to convert this food into usable energy in the form of ATP molecules.
Organisms must also take in matter from their environment in order to grow and reproduce. Plants absorb water and minerals through their roots, while animals ingest food through their mouths. This matter is used to build new cells and tissues, as well as providing energy for growth and reproduction. Organisms excrete waste products back into the environment through various means, such as sweating or defecation.
Organisms can be categorized based on their level of organization: unicellular vs multicellular; prokaryotic vs eukaryotic; autotrophic vs heterotrophic; etc. Unicellular organisms are single-celled creatures, while multicellular organisms have multiple cells working cooperatively. Prokaryotic cells lack a nucleus or other organelles enclosed within membranes (eukaryotic), while eukaryotic cells do have these structures. Autotrophs produce their own food (plants), while heterotrophs must consume other organisms for sustenance (animals).