Over time, new discoveries and improved methods for studying organisms have expanded on Linnaeus’s original classification system. Today, scientists group organisms based on evolutionary relationships and key characteristics such as cell type, ability to obtain or produce food, and the number of cells that make up their bodies.

The first domain is Bacteria. Bacteria are tiny, single-celled organisms that are found almost everywhere, including soil, water, and air. They are prokaryotic, meaning they lack a nucleus. Their cell walls contain a substance called peptidoglycan, which provides structure and support. Bacteria are very versatile and can be either heterotrophic (obtaining food from other organisms) or autotrophic (making their own food). An example of a bacterium is E. coli.

Another group of unicellular, prokaryotic organisms belongs to the domain Archaea. Archaea are often found in extreme environments, such as hot springs, deep-sea vents, or areas with little to no oxygen. Like bacteria, they can be heterotrophic or autotrophic. However, unlike bacteria, their cell walls do not contain peptidoglycan. Their unique cell structures allow them to survive in conditions that would be harmful to most other organisms.

The third domain is Eukarya, which includes all organisms with cells that contain a nucleus. This domain is made up of four kingdoms: Protista, Fungi, Plantae, and Animalia. Organisms in this domain can be unicellular or multicellular and are generally more complex than those in the other two domains.

The kingdom Protista includes a diverse group of mostly unicellular eukaryotic organisms. These organisms are often classified based on how they obtain food. Some, like algae, are plant-like and contain chloroplasts for photosynthesis. Others, known as protozoans, are animal-like and are heterotrophic, typically living in moist environments such as oceans, freshwater, or damp soil. Fungus-like protists, including slime molds, absorb nutrients from decaying matter. All protists share one key feature: their cells contain a nucleus.

Organisms in the kingdom Fungi, such as mushrooms, molds, and yeasts, are also eukaryotic. Their cell walls are made of chitin, a tough substance that provides structural support. Rather than making their own food, fungi are heterotrophic and absorb nutrients from dead or decomposing plant and animal material. Because of this, they play an essential role as decomposers in ecosystems.

Members of the kingdom Plantae are eukaryotic organisms that produce their own food through photosynthesis. Most are multicellular, though some, like certain green algae, can be unicellular. As autotrophs, plants use sunlight to make food and have cell walls composed of cellulose. This group includes mosses, ferns, and flowering plants.

The kingdom Animalia includes all animals, which are multicellular and heterotrophic. Unlike plants, their cells do not have cell walls or chloroplasts. Instead, animals obtain energy by consuming other organisms. This kingdom includes a wide range of life forms, from simple sponges and worms to more complex insects, fish, and mammals.

Review:

1. Identify the six kingdoms of life.
2. Describe characteristics of the domain Eukarya.
3. Describe characteristics of the Kingdom Fungi.