Lesson 1: Alive and Kickin'

Quick. Grab a pencil and a blank piece of paper. Without thinking too much, take thirty seconds to list as many things as you can that are alive. Ready? GO!

Here is my list:

Ants…firemen…plants…ivy…people…cats…insects…flowers…dogs…giraffes…cells…amoeba…children…octopi…fish…beavers.

Not bad for thirty seconds, huh?

So what do all of these things have in common? They have vastly different sizes, shapes, and structures. What common characteristics make all of the things on my list alive?

Over the next few months we are going to carefully examine many life forms in order to help us understand the answers to this question – and not only such everyday organisms as made my brainstorming list. From viruses and vampire bats to yellow jackets and yeasts, from mosses and molds to birds and bacteria, from crayfish and chrysali to mealworms and mushrooms– the complex and intriguing world of biology is all about life. Let’s talk about a few of the features common to all living creatures.

Do you know what the word “symmetric” means? It means “regular” or “even”. One way we can classify living things is by determining how even or regular the arrangement of their parts is.

Some organisms have spherical symmetry. A sphere shape is round like a ball. We say living things that have a shape similar to a ball have spherical symmetry. Most organisms with spherical symmetry are tiny water creatures that move by rolling about in response to currents in the water.

Some other organisms, like fish, have bilateral symmetry. Fish have matching fins on each side. They have corresponding sets of gills, eyes, and scales on opposite side as well. If you were to slice a fish longways from head through the torso and through the tail, each half would be a mirror image of the other. But if you sliced the fish into top and bottom halves, the top half and the bottom half would NOT be mirror images. This characteristic is typical of organisms with bilateral symmetry. Can you think of other bilaterally symmetrical living things? It is an ideal structure for creatures that move by walking, swimming, or flying, because these types of locomotion work best when there are paired legs, fins, or wings on either side.

Mushrooms have radial symmetry. If you slice a muchroom from top to bottom you get mirror images no matter at which angle you slice. There is a definite top and bottom to radially symmetrical organisms, but the sides are basically circular like a cylinder. Most organisms with radial symmetry are sessile, meaning they are attached to the ground, or they move by floating in water like jellyfish.

Some organisms, like snails, have no symmetry. There is no place to make a slice through the center of a snail that would give you mirror image halves.

Look at each of these photographs and see if you can figure out what kind of symmetry each organism possesses. Send me your answers by clicking the envelope icon on the right and I will let you know if you are right.