Despite the vast diversity of life on Earth, all living things share fundamental characteristics that define them. These universal properties allow organisms to sustain themselves, interact with their environment, and perpetuate their species. Understanding these commonalities provides a framework for comprehending biological existence.
Cellular Organization
All known living things are composed of one or more cells, which serve as the basic structural and functional units of life. This concept forms the foundation of cell theory. Organisms can be unicellular, consisting of a single cell like bacteria, or multicellular, formed from many specialized cells that work together, such as plants and animals. Within every cell, a high degree of organization is present, with internal structures called organelles performing specific functions. These cells contain genetic material, primarily deoxyribonucleic acid (DNA), which carries the instructions for cell growth, function, and reproduction.
Energy and Metabolism
Living organisms require and process energy to maintain their complex structures and carry out life processes. This energy flow is managed through metabolism, which encompasses all chemical reactions within an organism. Metabolism is divided into two processes: anabolism and catabolism. Catabolism involves breaking down complex molecules and releasing energy (e.g., digestion), while anabolism uses this energy to build complex molecules (e.g., synthesizing proteins). Cellular respiration, for instance, breaks down glucose to produce adenosine triphosphate (ATP), the primary energy currency for cellular activities.
Growth, Reproduction, and Response
Living organisms exhibit growth, an increase in size and complexity, through cell division and differentiation. In multicellular organisms, growth involves an increase in cell number, while for unicellular organisms, cell division serves as reproduction and increases population. Reproduction is a process by which living things produce offspring, ensuring species continuation, and can occur through asexual (single parent, genetically identical offspring) or sexual (two parents, unique offspring) means. Organisms also detect and react to changes in their environment, known as response to stimuli. Responses range from a plant growing towards light to animals seeking shelter from cold.
Homeostasis and Adaptation
Living systems maintain a stable internal environment despite external fluctuations, a process called homeostasis. This self-regulating ability keeps conditions like body temperature, pH, and blood glucose levels within a narrow, life-sustaining range. For example, the human body regulates temperature around 37 °C (98.6 °F) through sweating or shivering. Living populations also exhibit adaptation, a long-term process where traits change over generations in response to environmental pressures. This evolutionary process, driven by natural selection, allows species to become better suited to their habitats, enhancing their survival and reproductive success.