Life

Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy transformation, and reproduction. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Biology is the science that studies life. The gene is the unit of heredity, whereas the cell is the structural and functional unit of life. There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells and passes its genes onto a new generation, sometimes producing genetic variation. Organisms, or the individual entities of life, are generally thought to be open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve over multiple generations. Other definitions sometimes include non-cellular life forms such as viruses and viroids, but they are usually excluded because they do not function on their own; rather, they exploit the biological processes of hosts. Abiogenesis, also known as the origin of life, is the natural process of life arising from non-living matter, such as simple organic compounds. Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has also adapted to survive in most ecosystems and conditions. New lifeforms have evolved from common ancestors through hereditary variation and natural selection, and today, estimates of the number of distinct species range anywhere from 3 million to over 100 million. Death is the permanent termination of all biological processes which sustain an organism, and as such, is the end of its life.