Unicellular organism

A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. These tiny living things are very important because scientists believe they might be the oldest form of life on Earth, with early organisms appearing between 3.5 and 3.8 billion years ago.

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, such as gametes, which are reproductive 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. Primitive protocells are believed to have been the first steps toward today's single-celled organisms.

One theory, called the RNA world hypothesis, suggests that early RNA molecules could have helped create chemical reactions and copy themselves. For reactions to happen more easily and to stay separate from the outside world, early cells needed to form compartments. Simple membranes made from fats likely came before other important molecules in cells. These membranes could gather in water to create 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. Instead, they keep their DNA in a special area called the nucleoid. Most prokaryotes have one round piece of DNA, which is different from the long DNA found in other types of cells.

Bacteria are very old and can be found almost everywhere on Earth. They can share small pieces of DNA called plasmids, which can give them new abilities, like resisting medicines called antibiotics. Bacteria usually make new ones by splitting into two, but some can share DNA in special ways. Some bacteria, called cyanobacteria, made 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. Some archaea can even live where there is no oxygen or where it is very salty. They are very strong and can survive in places where most other living things cannot.

Eukaryotes

Eukaryotic cells have special parts inside them that are surrounded by membranes. These parts, called organelles, include things like mitochondria, a nucleus, and the Golgi apparatus. Scientists believe that prokaryotic cells changed into eukaryotic cells between 2.0 and 1.4 billion years ago, which was a big step in how life evolved. Unlike prokaryotic cells, eukaryotic cells reproduce using processes called mitosis and meiosis.

The endosymbiotic theory suggests that mitochondria and chloroplasts in these cells came from bacteria. Mitochondria likely came from a type of 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 two processes—photosynthesis in chloroplasts and respiration in mitochondria—help cells get more energy than just fermentation.

Protozoa

Protozoa move around in different ways, such as using flagella, cilia, or pseudopodia. They can make their own food or eat other things. Some can even reproduce in a way that involves combining genetic material from two parents.

Ciliophora, or ciliates, use cilia to move. Examples include Paramecium, Stentors, and Vorticella. These organisms have special structures called trichocysts that they can use to catch food or defend themselves. Ciliates can also reproduce in a way that combines 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 contain 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 scientific 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.