life

Introduction
Definitions of life
- Metabolic
- Physiological
- Biochemical
- Genetic
- Thermodynamic
- Autopoietic
Life on Earth
- The biosphere
- Chemistry of life
  - DNA, RNA, and protein
  - Chemistry in common
- Modes of nutrition and energy generation
  - Energy, carbon, and electrons
  - ATP
- Diversity
  - Prokaryotes and eukaryotes
  - Multicellularity
  - Classification and microbiota
  - Sex
  - Viruses
- Limits to life
  - Temperature and desiccation
  - Radiation and nutrient deprivation
  - Sizes of organisms
  - Metabolites and water
- Sensory capabilities and awareness
  - Photosensitivity, audiosensitivity, thermosensitivity, chemosensitivity, and magnetosensitivity
  - Sensing with technology
- Evolution and the history of life on Earth
  - Heritability
  - Convergence
  - Spontaneous generation
  - Geologic record
The origin of life
- Hypotheses of origins
- Production of polymers
- The earliest living systems

References & Edit History Related Topics

Images, Videos & Interactives

A herd of African elephants stands at the edge of the Chobe River in Botswana, drinking water with a backdrop of dry hills and scattered green trees.

Bald eagle with wings partly spread perches on a bare tree branch near Kenai, Alaska, against a blurred green background.

A small speckled reed frog (Hyperolius species) sits inside the center of a pink and yellow lily flower.

Illustrated diagram of the carbon cycle showing how carbon moves as carbon dioxide through the atmosphere, water, living organisms, rocks, and soil. It depicts processes like photosynthesis, respiration, sedimentation, volcanic activity, fossil fuel use, and carbon sequestration, with arrows indicating the flow between air, water, plants, animals, rocks, and industrial sources.

Detailed diagram of a plasma membrane's lipid bilayer showing two layers of phospholipid molecules with hydrophilic heads facing outward and hydrophobic tails inward, embedded cholesterol, integral (intrinsic) proteins spanning the membrane, peripheral (extrinsic) proteins attached to the surface, and sugar side chains extending from the outer face.

At a Glance

life summary

Quizzes

Male muscle, man flexing arm, bicep curl.

Facts You Should Know: The Human Body Quiz

greylag. Flock of Greylag geese during their winter migration at Bosque del Apache National Refugee, New Mexico. greylag goose (Anser anser)

Biology Bonanza

Related Questions

How did Ernst Haeckel influence the classification of microorganisms?
How did Robert H. Whittaker’s five-kingdom system classify living organisms?
How did ribosomal RNA evidence change the classification of life?
How did the Miller-Urey experiment simulate conditions on early Earth?
How did the invention of the microscope change biological classification?
How do bacteria reproduce by fission?
How do bacterial flagella differ from eukaryotic flagella?
How do fungal cell walls differ from plant cell walls?
How do fungi obtain nutrients through osmotrophic nutrition?
How do heterotrophs obtain energy and carbon from food?
How do homologous structures provide evidence of evolutionary relationships?
How do living organisms maintain their individuality through metabolism?
How do mutations create variation in populations?
How do organisms survive in extreme environments such as Antarctica, hot springs, and deep-sea vents?
How do photosynthesis and cellular respiration cycle carbon and oxygen through living systems?
How do proteins support movement, transport, and chemical reactions in cells?
How do protists move with flagella, cilia, or pseudopodia?
How do taxonomic ranks from domain to species help classify an organism?
How do three-nucleotide codons determine amino acids in proteins?
How does biodiversity include genetic diversity and ecosystem diversity as well as species diversity?
How does ecology examine relationships between organisms and their environments?
How does the RNA world hypothesis explain a possible early stage of life?
Which branches of biology study the structure and functions of living things?
Which three domains are recognized in Carl Woese’s classification system?
Why are autotrophs important as primary producers in ecosystems?
Why are endemic species especially vulnerable when their habitats are damaged?
Why can a population decline harm an ecosystem even if the species does not become extinct?
Why can scientific classifications change when new genetic evidence is discovered?
Why did Carl Woese propose the three-domain system?
Why did scientists reject spontaneous generation?
Why did the two-kingdom system of plants and animals become inadequate?
Why do botanists use division while zoologists use phylum for equivalent ranks?
Why do some biologists no longer consider Protista a valid natural group?
Why do tropical regions generally have more species than regions near the poles?
Why is panspermia considered a possible explanation for life arriving on Earth?
Why is peptidoglycan in bacterial cell walls useful for classifying bacteria?
Why is the presence of a membrane-bound nucleus important in classification?
Why might hydrothermal vents have provided a possible habitat for abiogenesis?
Why was Euglena difficult to classify as either a plant or an animal?
How do fungi obtain nutrition?
What is a fungal spore?
Where do fungi grow?
How did most dinosaurs go extinct?
Did dinosaurs have feathers?
What is the significance of photosynthesis by plants?
What is chronic venous insufficiency?
What is homeostasis?
What is an example of homeostasis in a living thing?
What is an example of homeostasis in a mechanical system?

Science Biology

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External Websites

National Center for Biotechnology Information - PubMed Central - LifeÂ’s Mechanism
University of Minnesota Pressbooks - Introductory Biology - Definition of Life
University of Hawaii - Exploring Our Fluid Earth - Properties of Life
Stanford Encyclopedia of Philosophy - Life

life

biology

Written by

Lynn Margulis

Distinguished University Professor, University of Massachusetts, Amherst. Coauthor of Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth; What Is Life?; and others.

Lynn MargulisAll

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Nov. 12, 2026 •History

life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction. Although a noun, as with other defined entities, the word life might be better cast as a verb to reflect its essential status as a process. Life comprises individuals, living beings, assignable to groups (taxa). Each individual is composed of one or more minimal living units, called cells, and is capable of transformation of carbon-based...

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