Imagine that you are looking at a pond. Many organisms live in and
around the pond. You might see birds, fish, frogs, and water plants.
Do you think you could fit any living things in a single drop of
water? You can!
A drop of pond water can have many living things that are too small
for your eyes to see. The smallest living things on Earth are made of
only one cell. They are called single-celled organisms. Tiny
organisms called bacteria are made of only one cell. Many living
things, called protists, are single-celled organisms. Protists usually
live in the water. Euglenas, diatoms, amoebas, and paramecia are
all single-celled protists. You can find all of these living things in
one drop of pond water.
Euglenas are similar to plants
because they have green
chloroplasts. Chloroplasts let the
euglenas make their own food. Like
plants, euglenas can make their
food using energy from the sun.
Not all protists have green
chloroplasts. Some single-celled
organisms have yellow or brown
chloroplasts.
Some single-celled protists cannot make their own food. But they
still need food to stay alive. These organisms must move to find
their food. Their bodies move in several different ways. Amoebas
move by pushing part of their body forward. Then they pull the rest
of their body toward this part. Paramecia have hundreds of short
hairs called cilia. They move these hair-like paddles to travel
through the water.
All living things must get food and remove wastes. And every kind
of living thing can make more of itself. Single-celled organisms do
all these jobs with only one cell. Some single-celled organisms make
their own food. Others bring food into themselves. They also get rid
of the wastes that they produce. They also divide in half to make