The six characteristics of living things are cellular organization, metabolism, homeostasis, growth and development, reproduction, and response to stimuli. Every known organism, from single-celled bacteria to blue whales, shares all six of these traits. Together, they form the biological checklist scientists use to distinguish living things from nonliving matter.
Cellular Organization
The cell is the basic organizational unit of life. Every living organism is made of at least one cell, and all cells arise from preexisting cells. Some organisms, like bacteria and amoebas, consist of a single cell that handles everything the organism needs to survive. Others, like humans, are built from trillions of specialized cells organized into tissues, organs, and systems.
Cells fall into two broad categories based on whether they contain a nucleus. Bacteria and similar microorganisms have simpler cells without a defined nucleus. Plants, animals, and fungi have more complex cells with a membrane-bound nucleus that houses their DNA. Regardless of type, the cell is the smallest structure that can independently carry out all the functions of life.
Metabolism
Every living thing needs a constant supply of energy, and metabolism is how it gets that energy and puts it to use. Metabolism refers to all the chemical reactions happening inside an organism’s cells to sustain life. These reactions fall into two broad categories: breaking things down and building things up.
Breaking down food or other energy sources into simpler molecules releases energy. Your body does this when it digests a meal, converting complex carbohydrates and fats into usable fuel. Building up is the opposite process: using that released energy to construct the proteins, fats, and other large molecules your cells need to function, grow, and repair themselves. These two halves of metabolism work in constant balance. Without them, no cell could maintain itself, and the organism would die.
Homeostasis
Living things maintain a stable internal environment even when conditions outside change. This balancing act is called homeostasis. Your body temperature stays near 37°C (98.6°F) whether you’re in a snowstorm or a desert. Your blood sugar, pH, oxygen levels, and ion concentrations are all tightly regulated so that the chemical reactions inside your cells can function properly.
Homeostasis works through feedback loops. When your body gets too hot, you sweat. When carbon dioxide levels rise in your blood, sensors near your heart and brain signal your lungs to breathe faster. When calcium or sodium levels drift out of range, glands and organs adjust how much of those minerals get absorbed or excreted. No single organ system handles this alone. Temperature regulation, for example, requires cooperation between your skin, nervous system, muscles, and cardiovascular system at a minimum. When homeostasis breaks down significantly, whether through hypothermia, heatstroke, or chemical imbalance, cells stop functioning and survival is at risk.
Growth and Development
All living things grow, and growth means more than just getting bigger. Biology distinguishes between growth and development. Growth is the quantitative, measurable increase in size: more cells, greater height, added weight. It happens through cell division, where one cell splits into two, and those two split again. Growth eventually stops when an organism reaches its full size.
Development is qualitative. It’s the process by which an organism becomes more complex and gains new abilities over time. A fertilized egg develops into an embryo with specialized tissues and organs. A seedling develops roots, stems, and leaves. Development includes the maturation of skills and systems, not just the accumulation of mass. In many organisms, development continues long after physical growth has stopped.
Reproduction
Perhaps the most fundamental property of all living things is the ability to reproduce. All organisms inherit the genetic information specifying their structure and function from their parents, and that information is carried by DNA. Every time a cell divides, its DNA is replicated and passed to the new cell.
Reproduction takes two forms. In asexual reproduction, a single organism copies itself, producing offspring that are genetically identical. Bacteria do this when they split in two. In sexual reproduction, two parents each contribute half of their genetic material. Specialized cells (sperm and egg in animals) each carry only one copy of each chromosome. When they fuse at fertilization, the resulting offspring has a full set of chromosomes with a unique combination of genes from both parents. This genetic mixing is a major driver of the variation that allows populations to adapt over time.
Response to Stimuli
All living things detect and respond to changes in their environment. These responses can be immediate and dramatic, or slow and subtle.
Animals tend to respond quickly. A squirrel flees at the sound of footsteps. A hen produces an alarm call when she spots an eagle, and her chicks run to hide under her feathers. Your own body responds to a hot stove before you consciously register the pain, and to cold air by raising the fine hairs on your skin.
Plants respond too, just on different timescales and through different mechanisms. A plant’s stems grow toward sunlight, a response called phototropism. Its roots grow downward in response to gravity. The sensitive plant, sometimes called touch-me-not, folds its leaves within seconds of being touched. Many trees shed their leaves in winter in response to shorter days and colder temperatures. Whether fast or slow, the ability to sense the environment and react to it is universal among living things.
Why Viruses Complicate the Picture
Viruses are the classic edge case in this framework. They have genetic material (DNA or RNA) and they can reproduce, but only by hijacking the machinery inside a living cell. Outside a host, a virus is essentially inert. It has no metabolism, no ability to maintain homeostasis, and no capacity to grow or respond to stimuli on its own. For these reasons, viruses have traditionally been classified as nonliving.
The debate isn’t fully settled. Some scientists argue that since viruses evolve, carry genetic programs, and have complex structures, they blur the boundary between living and nonliving. NASA’s working definition of life, “a self-sustaining chemical system capable of Darwinian evolution,” captures this tension. Viruses are capable of evolution but are not self-sustaining. For most biology courses and textbooks, the six characteristics provide a practical line: if something doesn’t meet all six, it isn’t considered alive.
How Adaptation Ties It All Together
Some textbooks list adaptation or evolution as a seventh characteristic rather than limiting the list to six. Adaptation isn’t something an individual organism does in a single lifetime. It’s a population-level process that unfolds over generations. When a random genetic mutation gives an organism a survival advantage, that organism is more likely to reproduce and pass the mutation to its offspring. Over time, the beneficial trait becomes more common in the population.
This process, natural selection, is the mechanism behind every adaptation that suits organisms to their environments. It depends on two of the six core characteristics: reproduction (to pass traits along) and DNA-based heredity (to ensure those traits are heritable). Whether your course lists six characteristics or seven, adaptation through evolution is the background process that has shaped every living thing on Earth into its current form.