How Does an Amoeba Eat Without a Mouth?

Put a drop of pond water under a microscope and you may spot a grey blob slowly changing shape. That blob is an amoeba (Amoeba proteus), and it has a surprising trick: it has no mouth, because any part of its body can become one.

A body with no fixed shape

A large amoeba is about 250 to 750 micrometres across, and big ones come close to 1 mm, just about visible to the naked eye. It has no skeleton and no set outline. To move, it pushes part of itself forward as a pseudopod, a “false foot”, and the rest of the cell pours in after it.

What is happening inside is a little like a river that keeps freezing and melting:

• Runny cytoplasm flows forward to the tip of the pseudopod.
• At the front it sets into a stiffer gel that forms the wall.
• At the back, the gel melts again and streams forward.

Melt, set, melt, set. The result is a slow, flowing crawl of a few micrometres per second along the pond bottom, where the amoeba lives on organic debris.

The food cup

When an amoeba bumps into something tasty, such as a paramecium, a flagellate or a small alga, it does not bite. Instead it reaches around the prey with two pseudopods, one on each side, forming a food cup. When the tips of the cup meet, they fuse, and the prey is sealed inside a pouch in the amoeba’s body called a food vacuole.

Digestive enzymes then pour into the vacuole and slowly break the meal down. Whatever cannot be used is pushed out at the back of the cell.

A built-in pump

Because pond water is much less salty than the inside of a cell, water keeps seeping into the amoeba. If nothing stopped it, the cell would swell. A contractile vacuole solves the problem: it fills up with extra water and squeezes it out every few minutes, like a tiny bilge pump.

Splitting in two

An amoeba does not lay eggs. When it has grown enough, its middle pinches in and the cell divides into two new amoebae. This is called binary fission.

Be the amoeba

In Micro Pond, the amoeba is the classic way to begin a run. You are a soft, stretchy blob in a drop of pond water: food particles are eaten the moment you touch them, and once you are clearly bigger than another creature you can wrap around it and swallow it, just like a real food cup. Grow big enough and you can evolve into the next microbe on the ladder.