Biologists use a standardized set of characteristics to distinguish living organisms from inanimate matter. While an absolute, single-sentence definition of life remains elusive, identifying these shared traits is necessary for scientific classification and study. By examining these features collectively, scientists can determine whether an entity, such as a rock or a virus, meets the criteria to be considered a living thing. These characteristics provide a framework for understanding the diversity and underlying unity of all organisms on Earth.
The Fundamental Unit: Cellular Organization
All known forms of life are structurally based on one or more cells, which represent the smallest unit that can independently perform all life functions. This cellular organization is a hallmark of life, whether the organism is a simple, single-celled bacterium or a complex animal. Living matter is highly structured, exhibiting a trait known as order.
Within a cell, atoms are organized into complex molecules that assemble into specialized structures called organelles. In multicellular organisms, this order extends to higher levels of organization, where similar cells group together to form tissues. Tissues combine to create organs, and multiple organs work in concert to form organ systems. This hierarchical arrangement, from the molecular level up to the entire organism, separates living things from non-living collections of matter.
Energy Processing and Internal Stability
Living organisms must continuously process energy to fuel their complex activities, a process known as metabolism. Metabolism is divided into two main categories: catabolism and anabolism. Catabolism involves breaking down complex molecules, such as sugars, to release energy, while anabolism uses that energy to build new cellular components.
This constant energy conversion is necessary to maintain an organism’s internal environment within a narrow, functional range, a state called homeostasis. For example, humans regulate body temperature through mechanisms like shivering or sweating despite external fluctuations. Homeostasis also involves regulating internal conditions such as blood sugar levels, pH, and water balance, which are controlled through intricate feedback loops. These regulatory mechanisms are powered by the energy derived from metabolic activities.
Interaction, Growth, and Reproduction
Living things actively interact with their surroundings by sensing and responding to environmental stimuli. This responsiveness allows organisms to adjust their behavior or physiology to survive. For instance, plants exhibit phototropism by growing toward a light source, while a single-celled organism may move away from a harmful chemical.
In addition to responding to external cues, organisms undergo an ordered increase in size and complexity, which is described as growth and development. Growth is the increase in mass and cell number, while development encompasses the changes that occur over an organism’s lifespan, resulting in a specialized form and function. For instance, a caterpillar not only grows larger but also develops into a butterfly, representing a dramatic change in structure and capabilities.
The continuity of life is ensured by reproduction, the process of creating new individuals. Reproduction can occur asexually, where a single parent produces genetically identical offspring. Sexual reproduction involves two parents combining genetic material to produce offspring that are genetically unique. This capacity to produce a new generation is fundamental to the long-term persistence of a species.
Genetic Basis and Evolutionary Change
Every living organism carries a universal genetic blueprint, encoded in molecules like Deoxyribonucleic Acid (DNA) or Ribonucleic Acid (RNA). This genetic material contains the instructions for building and operating the organism, and it is faithfully passed down from parent to offspring through heredity. This chemical language governs the structure, function, and development of all life.
While individual organisms grow and respond to their environment, populations of organisms exhibit a long-term characteristic of life called evolution. Evolution is the change in the heritable traits of a population over successive generations, driven by processes like natural selection. When a random change, or mutation, in the genetic code provides an advantage in a specific environment, the organism is better suited to survive and reproduce, passing the beneficial trait to the next generation. This process results in adaptations, which are features that enhance an organism’s fitness in a changing environment.