Living things are organisms that possess specific characteristics differentiating them from non-living matter. Scientists use these criteria to classify something as living, which helps in understanding life’s fundamental nature. These distinct traits provide a framework for biological study, highlighting the processes that sustain an organism.
The Seven Characteristics of Life
Living organisms display several shared characteristics. These traits encompass how organisms interact with their environment, process energy, and perpetuate their species.
Movement is a fundamental characteristic of living things, encompassing internal and external actions. Animals exhibit obvious locomotion, while plants demonstrate movement through processes like phototropism. Microorganisms like amoebas move via protoplasmic streaming. This ability to change position or place allows organisms to seek resources or avoid threats.
Respiration is the metabolic process by which living cells obtain energy, typically by breaking down organic substances. This process releases energy in the form of adenosine triphosphate (ATP), powering cellular activities. Respiration can occur with oxygen (aerobic), common in humans, animals, and plants, or without oxygen (anaerobic), as seen in some bacteria and yeast. Organisms have specialized structures for gas exchange, such as lungs, gills, or stomata.
Sensitivity is the ability of living organisms to detect and respond to environmental changes, known as stimuli. Responses can be rapid, like an octopus squirting ink, or slower, such as plants bending towards light. At a cellular level, sensitivity allows cells to react to nutrient levels or toxins, coordinating responses through signaling pathways. This responsiveness enables organisms to find food, avoid danger, and maintain internal stability.
Growth involves an irreversible increase in an organism’s size, mass, or volume, primarily through cell division and enlargement. In multicellular organisms, new cells are produced through mitosis, increasing size and repairing damaged tissues. Plants exhibit continuous growth, with cells at root and shoot tips dividing rapidly. Development, often accompanying growth, includes all changes an organism undergoes from early stages to maturity.
Reproduction is the process by which living organisms create new individuals, ensuring the continuation of their species. This can occur through sexual reproduction, involving the fusion of male and female gametes from two parents, or asexual reproduction, where offspring arise from a single parent. Asexual methods include binary fission in bacteria or budding. Reproduction is essential for passing on genetic traits.
Excretion is the process of removing metabolic waste products from an organism’s body. These wastes, generated from biochemical reactions, can be harmful if they accumulate. In humans, kidneys filter blood to produce urine, and lungs expel carbon dioxide. Plants excrete waste products like oxygen during photosynthesis and remove excess water through transpiration.
Nutrition is how living things obtain and use food for energy, growth, and tissue repair. Organisms can be autotrophs, like plants, producing their own food through photosynthesis, or heterotrophs, like animals, consuming other organisms for energy. This acquired energy fuels life activities.
Distinguishing Living from Non-Living
Determining whether something is living or non-living relies on the collective presence of these fundamental characteristics. Non-living things, such as rocks or water, do not possess cells, a life cycle, or internal processes to sustain themselves; their changes are due to external forces.
Viruses present a unique classification challenge, often considered to exist in a “grey area.” They possess genetic material and can reproduce, but only by infecting a host cell and hijacking its machinery, as they lack independent cellular structures and metabolic processes. Outside a host, viruses are inert, unable to carry out metabolic functions or grow, which differentiates them from fully living organisms. This highlights that the complete set of characteristics is generally required for something to be definitively considered alive.