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An organism is a single living thing that is made up of cells, feeds, grows, responds to stimuli, reproduces and can adapt to its environment. Organisms can be extremely complex, such as humans and other multicellular animals and plants, or they can be so simple that they consist of only a single cell, such as bacteria and many types of algae.
The complexity of an organism varies greatly in the animal and plant kingdoms and among microorganisms. Multicellular organisms consist of many cells that are differentiated into tissues, organs and organ systems that perform specialized functions. Single-celled organisms perform all vital functions within a single cell.
All organisms share certain characteristic features that define them as living. These include the ability to self-organize, metabolism (energy uptake and conversion), growth, adaptation to environmental changes, response to stimuli, and in many species, the ability to reproduce. Organisms are also able to store genetic information through DNA or RNA and pass this information on to their offspring, thereby inheriting the characteristics of the parent organisms.
Biology is the science that deals with the study of organisms and their interactions both with each other and with their environment. The study of organisms ranges from the investigation of basic life processes at the molecular and cellular level to the study of complex ecosystems and the evolution of life on earth.
