An amoeba is a single-celled organism that moves by pushing out parts of its body. The most familiar kind, Amoeba proteus, lives in fresh water and is about 0.5 millimeters, or 500 micrometers, across. A micrometer is a millionth of a meter.

Size

An amoeba has no fixed shape. It changes shape as it moves, so its size is given as its greatest length or width. The biology textbook published by OpenStax describes Amoeba proteus as about 500 micrometers across. Richard Kudo's reference book Protozoology records it as 600 micrometers or more in its largest dimension.

Size also differs between species. Amoeba borokensis, another species in the same genus, is 210–545 micrometers long when it moves, and averages about 370 micrometers in laboratory cultures.

Size comparisons

About 60–70 red blood cells, each about 7–8 micrometers across, would fit side by side across an Amoeba proteus.
An Amoeba proteus is about five times as wide as a human hair, taking a hair as 0.1 millimeters wide.
It is about as wide as a grain of sand 0.5 millimeters across, the size at the border between medium and coarse sand.
It is about 1.6 times the width of a pixel on a 27-inch full-HD computer monitor.
If an amoeba were enlarged to the height of a person 1.7 meters (5 feet 7 inches) tall, a red blood cell would be about 2.5 centimeters (1 inch) across.

How an amoeba moves and feeds

An amoeba moves by pushing out pseudopodia, lobes of the cell that can grow from anywhere on its surface. A pseudopod anchors itself to the surface below. The rest of the cell's contents then flow into it, and the whole amoeba moves forward.

To feed, an amoeba wraps its outer membrane around a food particle, such as a smaller organism. The membrane pinches off around the food, leaving it inside a small sac called a food vacuole. There the food is broken down into small molecules the cell can use. This way of eating is called phagocytosis.

Giant amoebae

Some amoebae are much larger. The giant amoeba Chaos carolinense has many nuclei, the structures that hold a cell's DNA. Amoeba proteus usually has only one.

The giant's moving cells are 0.3–3 millimeters long, so the largest are about six times the size of an Amoeba proteus. Another giant with many nuclei, Pelomyxa, can be 10 times the size of Amoeba proteus.

Sources

Groups of Protists. In Biology 2e. OpenStax, 2018.
Characteristics of Protists. In Biology 2e. OpenStax, 2018.
Eukaryotic Cells. In Biology 2e. OpenStax, 2018.
Amoeba. In The World of Protozoa, Rotifera, Nematoda and Oligochaeta. National Institute for Environmental Studies, Japan.
Erythrocytes. In Anatomy and Physiology 2e. OpenStax, 2022.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Beaches and Sand. In Exploring Our Fluid Earth. University of Hawaii.
Dell 27 Monitor - E2724HS. Dell.

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