The question of what constitutes life is answered by a set of universally recognized characteristics. Biologists require an entity to satisfy all of these properties to be classified as a living organism. These criteria collectively provide a clear boundary distinguishing the animate world from the inanimate, offering a framework for studying life’s vast diversity.
Fundamental Structure: The Cellular Basis of Life
All known living things are composed of one or more cells, establishing the cell as the fundamental unit of structure and function. Organisms range from single-celled bacteria to complex multicellular animals or plants. This cellular organization provides a necessary boundary, separating the organism’s internal environment from the outside world.
Cells possess a high degree of order, with specialized structures and molecules arranged to carry out life processes. Prokaryotic cells, like bacteria, lack a membrane-bound nucleus, while eukaryotic cells contain organelles such as mitochondria and a true nucleus. The presence of this organized, self-contained unit is the foundational requirement for life.
Internal Operations: Energy, Metabolism, and Regulation
Living organisms must constantly manage energy and materials through metabolism, a collection of chemical reactions. Metabolism is divided into two interconnected activities: anabolism and catabolism. Anabolism involves constructive processes that consume energy to build complex molecules, such as plants using photosynthesis to build sugars.
Catabolism involves breaking down larger molecules to release stored energy, fueling all other activities. Cellular respiration is an example of catabolism, generating Adenosine Triphosphate (ATP), the primary energy currency of the cell. The energy released by catabolism powers the building work of anabolism.
Living systems also exhibit regulation, maintaining stable internal conditions through homeostasis. Organisms use feedback mechanisms to keep variables like internal temperature, blood sugar levels, or pH within a narrow, suitable range. This internal stability is necessary for efficient biochemical reactions.
Organisms must also display sensitivity, responding to external stimuli or changes in their environment. This can be as simple as a single-celled organism moving away from a chemical (chemotaxis) or a plant bending toward light (phototropism). This ability to react and adjust to surroundings is required for survival.
Long-Term Requirements: Growth, Reproduction, and Evolution
Life requires mechanisms for continuity beyond the lifespan of a single individual. A living organism must exhibit growth and development, increasing in size and complexity according to instructions encoded in its genes. This growth is an internal increase in cellular components and the formation of new structures, not simply the accumulation of external matter.
Organisms must possess the capacity for reproduction, creating new individuals of the same species. Reproduction can be asexual, where a single organism divides, or sexual, involving the combination of genetic material from two parents. This transfer of genetic material passes traits on to the next generation.
The long-term requirement for life is the capacity for evolution, occurring over successive generations within a population. Evolution involves a change in the genetic makeup of a population over time. This process, driven by natural selection acting on genetic variations, allows species to adapt to changing environmental conditions.
Challenging the Definition: Where the Lines Blur
The necessity of meeting all criteria becomes clear when considering entities that possess some, but not all, of the properties of life. A common example is a crystal, which appears to “grow” by accumulating molecules, or a natural fire, which consumes fuel and responds to oxygen. Neither of these examples possesses a cellular structure, nor can they independently pass on a complex genetic blueprint to offspring.
Viruses represent the most significant challenge to the definition, as they possess genetic material and can evolve and replicate. A virus is an acellular particle, lacking the internal machinery for independent metabolism. They must hijack a living host cell, using its energy and parts to force the creation of new viral particles. Because they cannot independently process energy or maintain homeostasis without a host, viruses are classified as being on the border between the living and the non-living.