Six Kingdom System of Classification

Study Notes

The classification of living things into animals and plants is an ancient practice.
Aristotle (384-322 BC) classified animal species.
In 1866, Ernst Haeckel proposed a third kingdom: Protista, for "neutral organisms" that were neither animal nor plant.
The five-kingdom system, popularized in 1969 by Whittaker, still forms the basis for many biological classifications.
In 1977, Carl Woese and colleagues proposed dividing prokaryotes into Eubacteria (Bacteria) and Archaebacteria (Archaea), based on ribosomal RNA structure.
Archaea and bacteria are prokaryotes, thriving in diverse environments (e.g., acidic, salty, cold, hot). There are more prokaryotes in a handful of fertile soil than the number of people who have ever lived. Prokaryotes show astonishing genetic diversity.

Prokaryotes thrive in various extreme environments.
Bacteria and Archaea are classified into two domains based on ribosomal RNA structure.
Prokaryotes have diverse shapes (spherical, rod-shaped, spiral) and cell wall compositions.

Protists, mostly unicellular eukaryotes, display a wide variety of structural and functional adaptations.
This group shows greater diversity than other eukaryote groups.
Protists include photoautotrophs (containing chloroplasts), heterotrophs (absorbing or ingesting organic molecules), and mixotrophs (combining photosynthesis and heterotrophic nutrition).

Plants diversified onto land at least 475 million years ago.
Plants are the primary source of oxygen and are vital for food production within terrestrial ecosystems.
Green algae are the closest relatives of land plants.
Plants show roughly 290,000 living species.
Nonvascular plants, known as bryophytes, are commonly called mosses, liverworts, and hornworts.
Most other plants are vascular plants, which feature vascular tissue enabling them to support themselves.
Seeds are a significant adaptation allowing for dissemination of plants and nutrient storage.
The two major clades within seed plants are the gymnosperms ("naked seed" plants) and angiosperms (flowering plants). Note that gymnosperms have cone-like structures while angiosperms use flowers as reproductive components.

Fungi are widespread and essential for terrestrial ecosystems. They break down organic material returning nutrients.
Fungi are heterotrophs absorbing nutrients from outside their bodies.
Fungi interact with other organisms (plants, algae, cyanobacteria, and animals) forming critical mutualistic relationships.
Fungi form mutualistic relationships with plant roots creating "mycorrhizae".
Fungi are important decomposers.
Fungi are used for food, medicine, and biotechnology.

Animals are multicellular heterotrophs developing from embryonic layers.
Animals display remarkable diversity through their ecological niches.
Animals exhibit different body plans such as radial symmetry and bilateral symmetry. These symmetries are used for categorizing and grouping animal phyla.
Three key body plans for animals are acoelmates, pseudocoelomates, and coelomates.
Animals lack a cell wall unlike plants.

Habitat destruction is a primary driver of plant extinctions.
Loss of plant habitats often accompanies declines in associated animal populations.
Current rates of habitat loss indicate 50% of Earth's species could become extinct in the next 100-200 years.

Humans utilize fungi for food, alcoholic beverages, cheese production, and antibiotics.
Fungi play a role in biotechnology like manufacturing certain proteins.