Distinguishing between living and non-living things is a fundamental challenge in biology. Scientists identify a set of shared characteristics that define all known forms of life. These attributes provide a framework for understanding the immense diversity of organisms on Earth, from the smallest bacteria to the largest whales. By observing these common features, researchers can classify and study how organisms persist and thrive in various environments.
Organized Structure
All living things exhibit a highly organized structure, beginning with the cell as their fundamental unit. A cell is the smallest entity capable of independent existence and performing life processes. Organisms can be unicellular, like bacteria, or multicellular, comprising many cells.
In multicellular organisms, cells are organized hierarchically. Similar cells form tissues, which combine to create organs, each with a specific function. Organs then cooperate within organ systems to carry out complex activities, such as digestion or circulation. This organization allows for specialized functions and increased complexity.
Energy Use
All living organisms constantly require and utilize energy to sustain life processes. This complex network of chemical reactions is known as metabolism. Metabolism involves both anabolic reactions, which build complex molecules, and catabolic reactions, which break down molecules to release energy.
Organisms obtain energy in various ways. Plants, for instance, capture light energy from the sun and convert it into chemical energy through photosynthesis. Animals and fungi acquire energy by consuming other organisms or organic matter, breaking down these substances through cellular respiration to fuel their activities.
Growth and Change
All living things undergo growth and development throughout their lifespan. Growth refers to an increase in size or the number of cells within an organism. For example, a single-celled bacterium grows by increasing its cell volume, while a multicellular animal grows by producing more cells.
Development encompasses a broader series of changes from an organism’s beginning until maturity. This includes cellular differentiation, where cells specialize for particular functions, and the formation of tissues and organs. A tree growing from a seedling to an adult, or a human infant developing into a mature adult, illustrates these processes.
Passing On Life
Reproduction is a fundamental process that enables living organisms to produce offspring, ensuring the continuation of their species. This involves transferring genetic material from parent to offspring, carrying life’s instructions. Two primary modes of reproduction exist.
Asexual reproduction involves a single parent producing genetically identical offspring, common in bacteria, fungi, and some plants. Sexual reproduction typically involves two parents contributing genetic material to create offspring genetically distinct from either parent. This genetic variation, facilitated by DNA recombination, provides raw material for adaptation and evolution.
Reacting to Surroundings
Living things detect and respond to stimuli from their internal and external environments. A stimulus is any change that triggers a reaction. These responses are important for survival, enabling organisms to adapt to changing conditions and maintain stable internal states.
Examples include plants growing towards a light source (phototropism) to maximize energy absorption. Animals often withdraw from painful stimuli or seek shelter from extreme temperatures. Organisms also maintain stable internal conditions, such as body temperature or blood sugar levels, through the process called homeostasis.