This article series will cover the seven basic characteristics all living organisms have in common. This is a common topic in introductory biology courses and provides a solid foundation for other core biology concepts.
What makes something “alive?” What characteristics must an object possess to be classified as “living?” In this article series we’ll discuss the seven main attributes all living things, called biological organisms, have in common:
Wait a Minute…
The first characteristic of life is organization. “Wait!” you may say, “My sock drawer is organized, but it’s not alive…or is it? You’re right, there are things that may be organized, but not alive. This illustrates an important point; while some objects may possess one, or perhaps two, of the characteristics of life, to be classified as living, that object must possess most, if not all of these seven characteristics.
Looking at leaves of a tree or feathers on a bird we can see, with the unaided eye, there’s a high level of order. But “zooming” in closer with a microscope we find that order still exists at this magnified level. That’s because all organisms are made of cells, the basic units of life. Those organisms which only have a single cell are called unicellular (uni – which means “one”) organisms. Examples include all bacterial species and many protista such as amoeba, paramecium and other microscopic “critters” found in pond water. Those organisms with more than one cell are called multicellular (multi – which means “many”). Examples of these organisms include just about any “creature” you can see without a microscope: insects, fish, plants, yaks, and your uncle.
My Pet Is Organized?
Multicellular organisms obviously have a higher level of biological organization than do unicellular organisms. For example, let’s take a look at your pet dog, Fido. He is a living organism, therefore he must be organized…even though he doesn’t always put things back where he got them from. But, biologically speaking, Fido is organized. So as a biologist, we find that Fido is not just a furry, cute, bundle of friendship, but actually a furry, cute, bundle of systems (digestive system, nervous system, circulatory system, etc.). Looking further we find each system is made of organs (tongue, stomach, intestines, etc.) and each organ is made up of specific tissues (muscle, epithelium, etc.). Finally, each tissue is made up of a specific type of cells (epithelial cells, etc.).
Can we venture deeper than the cell? Sure we can, but those levels of organization below the cellular level are no longer considered “alive.” This is why it is the cell that is the most basic unit of life. So what’s “lower” than the cell? Proteins, carbohydrates, lipids, DNA, etc. but we’ll save those molecules for another discussion.
Again, there are many things in our world that may have some level of organization to them (cars, cell phones, sock drawers, etc.), but organization is not the only characteristic of life. In the following articles we’ll take a look at the rest of these characteristics.
Articles in this series, “Is It Alive?”
- Part 1 – Organization
- Part 2 – Metabolism
- Part 3 – Homeostasis
- Part 4 – Response
- Part 5 – Growth
- Part 6 – Reproduction
- Part 7 – Adaptation
Please visit my BioBlog @ SantiagoRLopez.com for other biology-related material.