The Classification of the Kingdoms of Life

Students can learn about the evolution of science by discussing the classification of living things.

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Classification of the Kingdoms of Life

There is an amazing diversity of living things on Earth. About two million species have been discovered and described, but scientists estimate that anywhere between five and 30 million species are present on Earth. Strangely, the number of kingdoms also seems to be in question. Various textbooks and websites differ on the number of kingdoms identified. As a result, my students sometimes express confusion when we discuss the kingdoms of life. So, I take this as an opportunity to help them understand the nature of science and our evolving understanding of the world.

Understanding the Kingdoms of Living Things

The first two kingdoms ever recognized were plants and animals. Until the advent of the microscope, there was little evidence to show that life consisted of anything other than these two groups. Today, we distinguish plants as multicellular, eukaryotic autotrophs. Simply stated, this means that plants are made of more than one complex cell with membrane-bound organelles, including the chloroplast that it uses to conduct photosynthesis. When you look at a lush landscape, plants are the backdrop. Therefore, you might draw the conclusion that plants are the largest group of living organisms. However, there are about 250,000 known plants on earth today, and this group makes up only 15% of all known organisms.

It is actually animals that make up the majority of known organisms. Well over half of the identified organisms on Earth are animals. Over one million animal species have been discovered, though arthropods, such as insects, represent most of those species. Animals are remarkably similar to plants because they are also multicellular eukaryotes. The animal cell contains the same complex membrane-bound organelles, with the exception of the chloroplast. Unlike plants, animals cannot conduct photosynthesis to make their own food. They are heterotrophs that must consume chemical food energy from other organisms.

Another surprising group of heterotrophs are the fungi. Fungi were originally thought to be  members of the plant kingdom. However, the cellular and genetic structures of fungi are astonishing. We now know that fungi are actually more similar to animals than they are to plants. Like animals, the majority of fungi are also multicellular eukaryotic heterotrophs that must consume food energy from other organisms. Unlike animals, though, fungi digest their food before they ingest it. Most of these foods are dead organisms that are decomposed by the fungi. This is why the fungi are so essential to the process of nutrient cycling.

There are also many protists that resemble the plants, animals, and fungi. Protists also have eukaryotic cells, but most protists are unicellular or live in colonies. The plant-like protists conduct photosynthesis, the animal-like protists can often move and ingest other organisms, and the fungi-like protists can cause decomposition. Most protists were not discovered until after the microscope was developed.

Learning About Bacteria

The bacteria are also relatively new to our understanding. Like most protists, bacteria are microscopic. However, they are completely different from other forms of life. They do not have any complex membrane-bound organelles. Their cells are small and contain the bare essentials. Some are able to conduct photosynthesis, others are predators. Their primitive structures are remarkably well-adapted, though, since bacteria are all around us, even within us.

Of course, it’s with the bacteria that the number of kingdoms gets complicated. Some sources list all types of bacteria as members of one kingdom, Monera. Others now divide the bacteria into Eubacteria and Archeabacteria (now refered to as simply Archea). Eubacteria include the types that we tend to think of, Streptococcus and Lactobaccillus are examples. Archea includes the ‘extreme bacteria’ that have been found in areas where we did not think life could exist. They are also extremely different from regular bacteria. They can be found in hot springs, ocean floor vents, deep underground, and even in digestive tracts where they produce methane. Archea give us hope for life on other planets.

Students Create a Finished Product

After my students have done research on the characteristics of each kingdom, and have found examples of organisms that belong to each kingdom, I ask them to create a picture book. We also discuss the way that these groups came about. This is a great way to illustrate how our understanding of the world through science is ever changing because of technology and discoveries. The lesson ideas below can help your students understand the characteristics of the kingdoms of life and the nature of scientific classification.

Classification and Kingdoms of Life Lessons:

The Kingdoms

As a class, students brainstorm a list of living things. In groups of two, students work to determine a classification system for the organisms. The teacher gives instruction on the classification system used by scientists.

Kingdoms on the Farm

Students use examples of organisms on a farm to identify the characteristics of each kingdom. As a class, they create a bar graph of the organisms in each kingdom.

Five Kingdoms of Organisms

Students work in groups and use picture cards to identify the kingdom of various organisms.

The Five Kingdoms

Students make a graphic organizer and PowerPoint presentation about the kingdoms of organisms. Links to handouts are included.