have been on earth for perhaps 3.8 billion years, so they have
had a long time in which to diversify.
Many of the tiny
simple shapes: they
may be little spheres,
a shape which resists
drying out: they may
be oblong, a shape
which provides the
maximum surface for
with the environment,
or they may be spiral
(spirochetes), a shape
which makes it easier
for them to
Although humans have studied
thousands of single
celled organisms, there are
millions more for aspiring biologists
to describe. They are interesting to study because each species
is unique, and many are beautiful
to look at or have interesting adaptations.
Let's look at some of these.
Adaptations for Movement
We have already looked at factors that limit cell
size and shape. These tiny organisms also have the power
cells have tiny hair-like fibers that are called flagella. These
fibers thrash in the water and drive the cell forward. (The word
flagellum means whip in Latin. Flagella is the plural form.)
Both prokaryotes and
eukaryotes can have
Although this is a feature of some self-sufficient little
organisms, we also see this same feature in sperm cells, which
propel themselves with flagella.
eukaryotes have groups or rows of tiny fibers that move together
like oars in a sort of rippling pattern. These are called
(pronounced silly-ah). Cells with
cilia move fast!
Cilia are also found in the oviducts of some multicellular
animals. Their beating causes a gentle current that moves an
egg cell from the ovary towards the womb.
Some cells, called amoebas, are able to change their shapes.
They can extend parts of themselves into what are called pseudopodia,
and then flow into the direction in which they wish to go. If
they find something to eat, they can engulf the particle with
their pseudopodia and pull themselves around it to digest it.
Pseudopodia means "false feet".
Can you tell if this
cell is a prokayrote
or a eukaryote?
Look at the picture
Some unicellular organisms are sensitive to light. They may
move toward it or away from it. They also respond to touch.
Although cells are small, they are complex. They have ancient
and effective survival mechanisms. The more that we learn about
them, the more amazing we find them to be.