Unicellular organism

A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell. This is different from a multicellular organism, which has many cells.

These tiny living things are very important. Scientists think they might be the oldest form of life on Earth.

Organisms can be divided into two main groups: prokaryotic and eukaryotic organisms. Most prokaryotes are unicellular and are called bacteria and archaea. Some eukaryotes, such as protozoa, unicellular algae, and unicellular fungi, are also single-celled.

Even though some prokaryotes live together in colonies, each cell must do everything itself to stay alive. In multicellular organisms, cells work together, but in unicellular organisms, one cell does all the work. Many multicellular organisms, like animals and plants, start their lives as single cells. Some interesting organisms, like Dictyostelium discoideum, are partly unicellular, and others, like Caulerpa, Plasmodium, and Myxogastria, can have many nuclei in one cell.

Evolutionary hypothesis

The origin of life is still a mystery. Simple early cells are thought to have been the first steps toward today's single-celled organisms.

One theory, called the RNA world hypothesis, says that early RNA molecules might have helped create chemical reactions and copy themselves. To make reactions happen more easily and stay separate, early cells needed to form compartments. Simple membranes made from fats probably came first. These membranes could gather in water to make tiny bubbles that kept reactions inside and the outside world out.

Prokaryotes

Prokaryotes are tiny living things that do not have special parts inside them called organelles, like a nucleus or mitochondria. They keep their DNA in a special area called the nucleoid. Most prokaryotes have one round piece of DNA.

Bacteria are very old and can be found almost everywhere on Earth. Some bacteria, called cyanobacteria, helped make Earth’s air have oxygen a long time ago.

Archaea look like bacteria but are different inside. They can live in very tough places, like very hot or very cold spots. They are very strong and can survive where most other living things cannot.

Eukaryotes

Eukaryotic cells have special parts inside them called organelles. These include mitochondria, a nucleus, and the Golgi apparatus. Scientists think prokaryotic cells changed into eukaryotic cells a very long time ago. Eukaryotic cells reproduce using mitosis and meiosis.

The endosymbiotic theory says that mitochondria and chloroplasts came from bacteria. Mitochondria likely came from bacteria similar to Rickettsia. Chloroplasts, found in plants and algae, probably also came from bacteria. Most eukaryotic cells have mitochondria, and plants and algae have chloroplasts. These help cells get more energy than just fermentation.

Protozoa

Protozoa move in different ways, like using flagella, cilia, or pseudopodia. They can make their own food or eat other things. Some can reproduce by combining genetic material from two parents.

Ciliophora, or ciliates, use cilia to move. Examples include Paramecium, Stentors, and Vorticella. These organisms have special structures to catch food or defend themselves. Ciliates can also reproduce by combining genetic material.

The Amebozoa move using pseudopodia. One example is Entamoeba histolytica.

Unicellular algae

Unicellular algae are like plants because they can make their own food and have chlorophyll. They live in many places, mostly in water.

• Euglenophyta are mostly unicellular algae in fresh water. An example is Euglena gracilis.
• Chlorophyta are mostly unicellular green algae in fresh water.
• Diatoms are unicellular algae with glass-like walls. They are very common in the ocean and help produce oxygen.
• Dinoflagellates are unicellular algae that can glow in the dark, like Pyrocystis fusiformis.

Unicellular fungi

Unicellular fungi include yeasts. Yeasts reproduce by dividing and sometimes by a process called budding. Saccharomyces cerevisiae is a type of yeast used to make bread and beer. It is also used in research to study how cells grow and divide.

Macroscopic unicellular organisms

Most single-celled organisms are so tiny that you need a microscope to see them. But some single-celled creatures are big enough to see without a microscope.

Some of these large single-celled organisms include Brefeldia maxima, a type of slime mold, which can be as thick as a finger and cover an area as big as a table. There are also very big creatures like Xenophyophores, which can be as wide as a dinner plate. Other examples are Valonia ventricosa, a green alga, that can be the size of a small ball, and Caulerpa, another alga that can grow as long as a guitar. Even some bacteria, like Thiomargarita magnifica, can be as long as a small ant.