Amoeba

An amoeba is a tiny living thing.

It can change its shape. It moves by stretching out parts of its body.

It uses these parts to catch food.

It wraps its body around the food. Then it pulls the food inside.

Some amoebae live in fresh water. They have a tiny part that pushes out extra water. This keeps the cell from bursting.

Some amoebae have a hard shell. Others are naked and have no shell at all.

Most are too small to see. But some are big enough to see with your eyes.

An amoeba is a tiny living thing made of just one cell.

These cells are special because they can change their shape. They do this by stretching out parts of their body. We call these parts pseudopods.

These bulges help the cell move and find food.

Amoebae do not have a mouth. Instead, they use phagocytosis to eat. This is a way of eating where the cell wraps its pseudopods around food. It then pulls the food inside the cell.

Some amoebae eat bacteria. Others eat dead bits of organic matter.

Many amoebae live in fresh water. These cells must manage water that leaks inside. They use a contractile vacuole to pump out extra water. This keeps the cell from bursting. Some amoebae also have hard shells. These shells can be made of sand or calcium. Others are naked and have no shell. Most amoebae are too small to see. However, some giant species are big enough to see with your eyes.

An amoeba is a tiny living thing made of only one cell.

These cells are special because they can change their shape. They do this by stretching out and pulling in parts of their body. Scientists call these stretchy parts pseudopods. These bulges help the cell move through its home. Because they lack a hard cell wall, they can move very freely.

Different types of amoebae have different kinds of pseudopods. Some have round, tube-like shapes called lobose pseudopods. Others have thin, thread-like shapes called filose pseudopods. Some even form net-like structures to catch things.

How do these tiny cells eat and stay safe?

Most amoebae do not have a mouth. Instead, they use a way of eating called phagocytosis. They stretch out their pseudopods to wrap around food like bacteria. Once they encircle the prey, they pull it inside the cell. Some amoebae also use pinocytosis to soak up tiny bits of nutrients. Living in fresh water can be hard because water leaks into the cell. To stop from bursting, many use a contractile vacuole. This part acts like a little pump to push extra water out.

People have been studying these creatures for a very long time.

In 1755, August Johann Rösel von Rosenhof made the first record of them. He called his discovery "the Little Proteus." Later, in 1822, a naturalist named Bory de Saint-Vincent used the name Amiba. This name comes from a Greek word meaning "change." By 1841, Félix Dujardin used the word "sarcode" to describe the thick substance inside them. For a long time, scientists grouped them all into a class called Sarcodina. Today, we know they belong to many different groups of life.

There is a huge variety in the size of these organisms.

Some marine amoebae, like Massisteria voersi, are only 2.3 to 3 micrometers wide. This makes them as small as some bacteria. On the other hand, some deep-sea creatures can reach 20 centimeters across. Many common pond amoebae are too small to see without help. However, giant species like Chaos carolinense can be seen with your eyes. These can grow to be 2000 micrometers in size. Other species, like Pelomyxa palustris, can reach up to 5000 micrometers.

Some amoebae can also affect the health of other living things.

For example, Entamoeba histolytica can cause a sickness called amoebic dysentery. There is also a freshwater species called Naegleria fowleri. This one is known as the "brain-eating amoeba" because it can be very dangerous to humans. Some amoebae can even host other tiny microbes inside them. These microbes use the amoeba as a safe place to grow. This shows how these single cells are deeply linked to the rest of the living world.

An amoeba is a single-celled organism known for its ability to change shape.

These organisms are often called amoeboid because they move by extending and retracting parts of their body. While people often use the terms "amoeba" and "amoeboid" interchangeably, they are not all part of one single group. Instead, amoeboid cells appear across many different lineages of eukaryotic life. You can find them in fungi, algae, animals, and protozoa. This ability to shift shape makes them highly adaptable in many environments.

The primary way an amoeba moves and eats is through pseudopods. These are temporary bulges of cytoplasm, the jelly-like substance inside the cell. Pseudopods form when actin microfilaments push against the plasma membrane, which is the thin outer layer of the cell. Because amoebae lack a rigid cell wall, they can stretch these parts freely. The specific shape of these pseudopods helps scientists tell different groups apart. For example, Amoebozoans have lobose pseudopods, which are rounded and tube-like. In contrast, Cercozoans have filose pseudopods, which are thin and thread-like. Some groups, like Foraminifera, use reticulose pseudopods that form a branching, net-like structure. Others, such as Radiolaria, have stiff axopodia supported by internal bundles of microtubules.

Amoebae use these same pseudopods to acquire nutrition through a process called phagocytosis. Since they do not have a mouth or a fixed feeding spot, they must surround their food. They extend their pseudopods to encircle prey, such as bacteria or other protists, and then engulf it. Some species are detritivores, meaning they eat dead organic material. Other amoebae use pinocytosis, which is a way of absorbing dissolved nutrients through small vesicles. To survive in freshwater, many amoebae must manage water levels using a contractile vacuole. Because the surrounding water has fewer solutes than the cell's cytosol, water enters the cell through osmosis. The contractile vacuole acts as a pump to expel this excess water so the cell does not burst.

The history of studying these organisms is quite long. In 1755, August Johann Rösel von Rosenhof made the first recorded observation of an amoeboid. He named his discovery "Der Kleine Proteus," or "the Little Proteus." In 1822, the naturalist Bory de Saint-Vincent created the genus name Amiba, derived from a Greek word meaning "change." Later, in 1841, Félix Dujardin described the thick substance inside these cells as "sarcode." For much of the 20th century, scientists used the class Sarcodina to group amoebae together based on their flexible bodies. However, modern molecular phylogenetic studies have shown that Sarcodina is not a single group sharing a common descent. Today, amoeboid organisms are classified into several different distinct groups.

Amoebae vary significantly in size. The marine species Massisteria voersi is extremely small, measuring only 2.3 to 3 micrometers in diameter. This puts it in the same size range as many bacteria. On the other end of the scale, some deep-sea xenophyophores can reach 20 centimeters in diameter. Most common freshwater amoebae are microscopic, but some "giant" species are visible to the naked eye. For instance, Chaos carolinense can reach 2000 micrometers, and Pelomyxa palustris can grow up to 5000 micrometers. This massive range shows how diverse single-celled life can be.

Some amoebae can also impact the health of other living things. Certain species are pathogenic, meaning they can cause disease. Entamoeba histolytica causes amoebic dysentery, while Naegleria fowleri can be fatal to humans if it enters the nose. Acanthamoeba can cause infections in the eyes or the brain. Interestingly, some amoebae can act as hosts for other microbes. Microorganisms can live and multiply inside an amoeba, using it as a shield from harsh outside conditions. This makes amoebae important players in the movement of bacteria and other pathogens.

Recent research into the genetics of amoebae has also revealed secrets about their reproduction. Scientists have found genes in Acanthamoeba that suggest it can undergo meiosis, a type of cell division used in sexual reproduction. In the species Dictyostelium discoideum, mating and meiosis occur when food becomes scarce. Because amoeboid lineages diverged so early in the history of life, these findings suggest that sexual reproduction may have been present very early in eukaryotic evolution. This connects the study of tiny single cells to the very beginnings of complex life on Earth.