Biology is the scientific study of life and living organisms, a discipline that examines everything from the smallest cellular structures to the vast interactions among species across the planet. To set the boundaries of what counts as "living," biologists rely on seven defining characteristics of life: organization at the cellular level, metabolism, homeostasis, growth, reproduction, response to stimuli, and adaptation through evolution. Any entity that displays all seven of these traits is recognized as a living organism, while entities that fail to meet even one criterion are generally classified as non-living.
To make sense of the enormous complexity of life, biologists organize it into a hierarchy of levels, each building on the one below it. At the foundation are atoms, which combine into molecules and then into organelles, the specialized subunits within cells. Cells assemble into tissues, which form organs, which work together as organ systems within individual organisms. Beyond the individual, organisms of the same species living together form populations, multiple populations form communities, communities together with their physical environment make up ecosystems, and all of Earth's ecosystems combined constitute the biosphere.
Understanding this hierarchy is essential because principles discovered at one level often illuminate behavior at another. For example, studying molecules helps explain how cells function, and studying populations reveals how species evolve. This layered view also helps place new topics—like genetics, ecology, or physiology—within a structured framework, giving learners a roadmap for navigating the rest of biology.