Taxonomy - Online Tutor, Practice Problems & Exam Prep

Taxonomy classifies all living organisms into eight categories: domain, kingdom, phylum, class, order, family, genus, and species. The three domains of life are bacteria, archaea, and eukarya. Prokaryotic cells (bacteria and archaea) lack a membrane-bound nucleus, while eukaryotic cells do. Prokaryotic cells are smaller and simpler, dividing by binary fission, whereas eukaryotic cells are larger, more complex, and divide by mitosis. Both cell types share essential characteristics, including cell membranes and ribosomes, but differ in DNA structure and organelle presence.

Biological Classification Categories

In this video, we're going to recap the classification categories of life. Taxonomy is the name of the branch of science that classifies, identifies, and properly names all living organisms. There are actually 8 biological classification categories used to identify all life. You guys have talked about these categories in your previous biology courses, so we're just going to do a quick review here. In our example, we have a vertical representation and a horizontal representation of the 8 categories. To refresh your memories, these 8 categories include domain, kingdom, phylum, class, order, family, genus, and species. In parentheses, we have the plural version of these categories. Notice that the domain at the very top has the largest, broadest bar, which means it is the most inclusive, and all living organisms fall under it. Now, as you move down in categories, the bars get smaller, which means the categories get less and less inclusive, and fewer organisms fall under them until you get to the least inclusive category, species, which only includes one type of organism. You can see that over here on the right where the domain is the most inclusive category and species is the least inclusive category.

There's an acronym to help you guys remember the categories in order from most inclusive to least inclusive, and that acronym is "Dear King Philip Came Over for Great Soup." We have an image of King Philip here and the delicious-looking soup he came over for. So if you can remember that "Dear King Philip came over for great soup," you can remember the order of these categories from most inclusive to least inclusive. In our next video, we're going to talk about a particular category, the domains of life. So I'll see you guys in that video.

Domains of Life

So we already know from our previous biology courses that there are three domains of life: bacteria, archaea, and eukarya. Bacteria and archaea are grouped together as single-celled prokaryotic organisms because they lack a membrane-bound nucleus. Bacteria are theorized to most closely resemble the very first cells because there are fewer divergences in the evolutionary tree, which is built by comparing the DNA sequences of many different organisms. Now, archaea include organisms such as halophiles and thermophiles, which live under intense conditions. Halophiles live under high salt conditions, whereas thermophiles live under high temperature conditions. We already learned that eukarya are cells that contain a membrane-bound nucleus, and this includes all multicellular organisms as well as the single-celled protists. In our example below, we have an evolutionary tree. Notice that on our timescale over here on the far right, in the present, we have the three domains of life which represent all living organisms in our time. Bacteria encounter fewer divergences than archaea and eukarya do. And for that reason, bacteria are theorized to most closely resemble the very first cells. Bacteria and archaea are grouped together as single-celled prokaryotic organisms, which lack a membrane-bound nucleus, whereas eukarya includes the eukaryotic organisms that do have a membrane-bound nucleus. Therefore, humans and ostriches are both multicellular organisms, so they both fall under the eukarya domain. Notice that when we compare the biological classifications, class, they start to fall under different categories of life. Now, in our biochemistry class, we're going to be mainly focusing on eukaryotic organisms. However, recall from our previous videos that the biochemical unity of life states that all living organisms have a resemblance at the biochemical level. Therefore, things that we learn about eukaryotic organisms are also going to be applicable to prokaryotic organisms. In our next video, we're going to directly compare prokaryotic organisms with eukaryotic organisms, and I'll see you guys in that video.

Prokaryotic vs. Eukaryotic Cells

So as you guys already know, prokaryotic and eukaryotic cells are the broadest and the most distinct groupings of all life, so it's important to be able to compare and contrast. In our example below, we have a Venn diagram, where on the left we have prokaryotic cells, on the right we have eukaryotic cells, and in the middle, we have characteristics of both. We'll start with the characteristics of both.

Both prokaryotic and eukaryotic cells contain cell membranes, and recall that all cells have cell membranes. They both also have structures known as ribosomes, which we'll do a refresher on in some of our later videos. They display all eight characteristics of life and contain all 4 major macromolecules that we talked about in our previous videos, which are proteins, nucleic acids, carbohydrates, and lipids.

To contrast prokaryotic and eukaryotic cells, we already know that prokaryotic cells do not have a nucleus whereas eukaryotic cells contain a nucleus. Prokaryotic cells tend to be smaller, less complex, and only unicellular organisms, and their diameters only range from 1 to about 5 micrometers. Now, in comparison, eukaryotic cells are larger, more complex, and they can be either unicellular or multicellular with their diameters ranging from 10 to about 100 micrometers. So eukaryotic cells are double the size all the way up to 100 times the size of a prokaryotic cell.

Prokaryotic cells tend to have circular DNA and do not contain membrane-bound organelles, whereas eukaryotic cells have linear DNA and do contain membrane-bound organelles. Prokaryotic cells divide by a process known as binary fission, whereas eukaryotic cells divide by a process known as mitosis.

Although both prokaryotic cells and eukaryotic cells contain ribosomes, their ribosomes differ from one another. Eukaryotic cells have larger 80S ribosomes, whereas prokaryotic cells have smaller 70S ribosomes. We'll talk more about that in some of our later videos.

There are many more similarities and differences between prokaryotic and eukaryotic cells. However, this is a good summary that I want you guys to know as we move forward in our course. This concludes our compare and contrast, and I'll see you guys in the practice videos.

Which of the following are correctly listed from least inclusive to most inclusive?

• A. Domain → Kingdom →Phylum →Class →Order
• B. Domain → Kingdom → Class →Phylum →Order
• C. Species → Genus → Family → Order →Class
• D. Species → Genus → Family → Class →Order

Which of the following statements is generally considered true for both prokaryotes and eukaryotes?

• A. Both have cells with nuclei.
• B. Both have cell walls.
• C. Both have circular DNA
• D. Both have ribosomes & plasma membranes

Here’s what students ask on this topic:

What are the eight biological classification categories in taxonomy?

The eight biological classification categories in taxonomy are domain, kingdom, phylum, class, order, family, genus, and species. These categories range from the most inclusive (domain) to the least inclusive (species). The acronym 'Dear King Philip Came Over For Great Soup' can help you remember the order. Domains are the broadest category and include all living organisms, while species are the most specific, including only one type of organism. This hierarchical system helps scientists organize and identify organisms based on shared characteristics.

What are the three domains of life and their characteristics?

The three domains of life are Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotic, meaning they lack a membrane-bound nucleus. Bacteria are thought to closely resemble the first cells and include many common microorganisms. Archaea include extremophiles like halophiles and thermophiles, which thrive in extreme conditions. Eukarya are eukaryotic, meaning they have a membrane-bound nucleus, and include all multicellular organisms and some unicellular ones, such as protists. This domain encompasses a wide variety of life forms, from plants and animals to fungi and protists.

How do prokaryotic and eukaryotic cells differ?

Prokaryotic cells, which include Bacteria and Archaea, lack a membrane-bound nucleus and are generally smaller and simpler than eukaryotic cells. They have circular DNA and divide by binary fission. Eukaryotic cells, found in the domain Eukarya, have a membrane-bound nucleus, are larger and more complex, and can be either unicellular or multicellular. They have linear DNA and divide by mitosis. Both cell types have cell membranes and ribosomes, but eukaryotic cells contain membrane-bound organelles, which prokaryotic cells do not.

What is the significance of the evolutionary tree in understanding the domains of life?

The evolutionary tree helps illustrate the relationships and divergences among the three domains of life: Bacteria, Archaea, and Eukarya. By comparing DNA sequences, scientists can trace the evolutionary history and determine how closely related different organisms are. Bacteria are thought to closely resemble the first cells due to fewer divergences in their evolutionary path. Understanding these relationships helps in studying the biochemical unity of life, as many fundamental processes are conserved across different domains.

What are some examples of organisms in the domain Archaea?

Organisms in the domain Archaea include extremophiles such as halophiles and thermophiles. Halophiles thrive in high-salt environments, like salt flats and saline lakes. Thermophiles live in high-temperature conditions, such as hot springs and hydrothermal vents. These organisms have unique adaptations that allow them to survive in extreme environments, making them of particular interest in studies of life's resilience and the potential for life in extreme conditions on other planets.