Life comes in many shapes and sizes, but at its foundation, all living things share one defining feature: they are made of cells. Whether a tiny bacterium drifting in a pond or a towering oak tree in a forest, every organism is built from one or more cells. This leads us to two major categories of life—unicellular organisms, which are made of just a single cell, and multicellular organisms, which are built from many cells working together.
Let’s explore how these two forms of life function, survive, and thrive in our world.
What Are Unicellular and Multicellular Organisms?
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Unicellular organisms are made up of a single cell. That one cell must perform every task needed to keep the organism alive, from finding food and removing waste to reproducing and responding to the environment. Examples include bacteria, yeast, amoebae, and paramecia.
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Multicellular organisms are made up of more than one cell. In fact, many are composed of trillions of cells! Unlike unicellular organisms, these cells can specialize, forming tissues, organs, and systems to divide the work of survival. Examples include plants, trees, animals, and humans.
Unicellular Organisms
Unicellular organisms are often microscopic, meaning they can only be seen through a microscope. Despite their small size, they’re remarkably diverse in how they survive:
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Nutrition: Many unicellular organisms live in watery environments where they absorb or engulf food. Amoebae, for example, extend parts of their cell membrane (pseudopodia) to surround food particles. Others, like algae and some bacteria, can produce their own food using photosynthesis.
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Gas Exchange: Oxygen and carbon dioxide move in and out of their cells through diffusion across the cell membrane. Some unicellular organisms, like yeast, can even survive without oxygen.
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Responding to the Environment: While they don’t have eyes or noses, unicellular organisms can detect light and chemicals. A bacterium, for instance, will move toward nutrients or away from harmful substances.
Unicellular life is everywhere—even inside of us. In fact, the human body contains more bacterial cells than human cells, and many of them help with digestion and overall health.
Multicellular Organisms
Multicellular organisms are much more complex because their survival doesn’t depend on a single cell doing everything. Instead, their cells specialize. Muscle cells contract to help animals move, leaf cells capture sunlight for photosynthesis, and nerve cells carry messages across the body.
Plants
Plants are multicellular organisms with around 400,000 known species worldwide. They meet their needs in similar ways:
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Nutrition: Roots absorb water and minerals from the soil. Using sunlight, carbon dioxide, and these nutrients, plants perform photosynthesis, creating food and releasing oxygen.
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Gas Exchange: Plants exchange gases through tiny pores on their leaves called stomata, which open during the day to take in carbon dioxide and release oxygen.
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Responding to the Environment: Plants sense gravity, water, and light. Roots grow downward into the soil while stems and leaves grow upward toward the sun.
Animals
Animals are also multicellular, with nearly 2 million identified species. They can be divided into two groups:
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Invertebrates (no backbone): Includes jellyfish, insects, worms, snails, and octopi.
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Vertebrates (with a backbone): Includes mammals, reptiles, birds, amphibians, and fish.
Animals meet their needs in a variety of ways:
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Nutrition: Most animals eat other organisms. Vertebrates have digestive systems with separate openings for food intake and waste removal, while some invertebrates, like jellyfish, use the same opening for both.
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Gas Exchange: Land animals use lungs to breathe air, while many aquatic animals use gills to extract oxygen from water. Some invertebrates rely on direct diffusion or specialized tubes.
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Responding to the Environment: Animals have nervous systems that allow them to sense their surroundings. From owls hunting with sharp eyesight to spiders spinning webs, these responses are key to survival. Movement, too, varies widely—vertebrates use muscles and bones, while invertebrates may crawl, swim, or fly using specialized structures.
While unicellular and multicellular organisms differ greatly in complexity, they share the same basic goals:
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Obtain nutrition
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Exchange gases
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Respond to their environment
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Reproduce and survive
Whether it’s a single cell engulfing food or an owl hunting in the night, life has developed countless strategies to meet these needs.
While unicellular and multicellular organisms differ greatly in complexity, they share the same basic goals:
-
Obtain nutrition
-
Exchange gases
-
Respond to their environment
-
Reproduce and survive
Whether it’s a single cell engulfing food or an owl hunting in the night, life has developed countless strategies to meet these needs.