Many of the tiny prokaryotes have 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 chemical exchanges with the environment, or they may be spiral (spirochetes), a shape which makes it easier for them to move.  

     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 of movement.

Flagella

Some 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 flagella.

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.

Cilia

Some eukaryotes have groups or rows of tiny fibers that move together like oars in a sort of rippling pattern. These are called cilia (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.

Pseudopodia

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 carefully!

Senses

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.