An organism is any living thing. Organisms can be as small as a single-celled bacterium or as large as a blue whale. All organisms share certain characteristics: they are made of cells, they grow and develop, they respond to their environment, they reproduce, they use energy, and they maintain a stable internal balance (called homeostasis).
Made of cells: Every organism is made of one or more cells, which are the basic units of life.
Example: A human is made of trillions of cells, while a bacterium is made of just one.
Grow and develop: Organisms increase in size and may change form as they mature.
Example: A caterpillar grows and changes into a butterfly through metamorphosis.
Respond to the environment: Organisms react to changes, such as temperature, light, or danger.
Example: A sunflower turns toward the sunlight during the day.
Reproduce: Living things make more of their own kind, either sexually or asexually.
Example: Rabbits reproduce sexually, while some plants like strawberries can reproduce asexually by sending out runners.
Use energy: Organisms need energy to move, grow, and carry out life processes.
Example: Humans get energy from food, while plants get energy from sunlight through photosynthesis.
Maintain homeostasis: Organisms keep their internal conditions stable, even if the environment changes.
Example: Humans sweat to cool down when they get too hot.
Organisms are grouped into categories based on how complex they are. Unicellular organisms, like bacteria and some protists, are made of just one cell. Multicellular organisms, like humans, trees, and dogs, are made of many cells that work together.
Unicellular Organisms: These are made of only one cell and carry out all life functions in that single cell.
Example: Amoebas and bacteria are unicellular.
Multicellular Organisms: These have many cells that work together, often with specialized functions.
Example: Humans, dogs, and oak trees are multicellular.
All organisms belong to one of the six major kingdoms of life: Animals, Plants, Fungi, Protists, Archaea, and Bacteria. Scientists use these groups to help classify and study living things based on their traits and how they are related to one another.
Animals: Multicellular, move on their own, and eat other organisms for energy.
Example: Lions, birds, and insects.
Plants: Multicellular, make their own food through photosynthesis, and don’t move.
Example: Grass, trees, and flowers.
Fungi: Can be unicellular or multicellular; they absorb nutrients from other organisms.
Example: Mushrooms, molds, and yeast.
Protists: Mostly unicellular, but some are multicellular; very diverse group.
Example: Amoeba and algae.
Bacteria (Eubacteria): Unicellular, simple cells without a nucleus, found almost everywhere.
Example: E. coli, found in your intestines.
Archaea (Archaebacteria): Unicellular, live in extreme environments like hot springs or salty lakes.
Example: Thermophiles that live in volcanic vents.
Organisms live in a wide variety of environments—from deep in the ocean to the tops of mountains, and even in extreme places like hot springs or polar ice. Each organism is adapted to survive in its specific habitat.
Studying organisms helps us understand how life works, how living things interact with each other, and how humans fit into the natural world. Whether big or small, every organism plays a role in Earth’s ecosystems.
Organisms Free Reading Passage
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Classification of Living Things
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Classification of Living Things Flashcards
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Middle School Science
