Tree of life
All Organisms Share Common Characteristics
In last month’s column, I wrote about what microbes are, where they are found, and why everyone should have some understanding of the microbial world. In this article, I will provide a superficial overview of how microbes live. I’ll start with a summary of the common characteristics of all living things. As illustrated in Figure 1, all life forms share at least six common properties (variations of this list that include additional properties):
I’ll spend this and the next two articles discussing each of these properties. This article will focus on order and growth.
All organisms are ordered. Each cell is bound by a membrane, wall, or both. The fluid (cytoplasm) within each cell contains the ingredients the cell needs to function. Bacteria and Archaea (prokaryotes) have no visible, membrane-bound internal structures. All protozoan, plant, animal, and fungal cells have membrane-bound, internal organelles and are classified as eukaryotes. Figure 2 compares prokaryotic (Figure 2a) and eukaryotic (Figure 2b) cell structures.
Fig 1. Common properties of all living things.
Fig 2. Typical cell structures – a) prokaryote (bacterial cell); b) eukaryote (yeast cell).
Growth is an organism’s increase in size, mass, or both. Often growth is conflated with proliferation. As I’ll discuss in a future article, proliferation is the increase in cell numbers as a result of reproduction. Thus, growth and proliferation (reproduction) are two different properties. Figure 3 illustrates growth. The size of the cell increases to a specific size, after which it begins to divide.
Fig 3. Cell growth – a) cell’s initial size; b) cell’s size as it begins to divide.
All living things – organisms – share a set of properties. The list of common properties varies among authors, but the list I provided at the beginning of this article is uncontroversial. The first two universal properties that define living beings are order and growth.
Order means that there is a consistent structure. Bacteria and archaea are protists – they have cell membranes and walls but no membrane-bound internal structures. Algae, fungi, and protozoans (like all plant and animal cells) are eukaryotes – their cells all have membrane-bound internal structures. I’ll return to these properties in a future article.
Growth means that the organism’s mass increases – cells become larger – to a certain point – as they mature. I’ll explain the difference between growth and reproduction in my June What’s New article.
Some have argued that viruses meet these two criteria. They are indeed ordered – i.e., have genetic and structural components – but individual virions do not grow.
If you have any questions about this article or microbial contamination-related issues, please contact me at firstname.lastname@example.org.