BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.2. The Five Kingdoms

Until relatively recently, All living organisms were universally classified into just two kingdoms: the animal kingdom and the plant kingdom. The primary distinction between animals and plants traditionally came down to their mode of Nutrition. Animals were defined as organisms that consume preformed organic matter (heterotrophic nutrition), whereas plants were those that synthesize their own organic molecules from Inorganic Compounds (autotrophic nutrition). More precisely, heterotrophic organisms must obtain carbon in the form of Organic compounds, whereas autotrophic organisms can utilize inorganic carbon, specifically in the form of carbon dioxide (CO₂). Because animals typically need to forage for food, they must be capable of locomotion. This, in turn, requires a Nervous system to coordinate movement in more complex animals. Plants, by contrast, lead a sedentary lifestyle; they are immobile and therefore have no need for a nervous system.

However, this traditional Classification overlooks the fundamental fact that all cellular life falls naturally into two distinct groups, now known as PROKARYOTES AND EUKARYOTES.

A profound distinction separates these two groups. The terms "prokaryote" and "eukaryote" reflect where DNA (genetic material) is localized within The Cell. In prokaryotes, the DNA is not enclosed by a nuclear membrane and floats freely in the Cytoplasm. In other words, these Cells lack a true Nucleus (pro — before; karyon — kernel or nucleus). Eukaryotic cells, however, possess a true nucleus (eu — well or truly). Eukaryotes evolved from prokaryotic ancestors.

Dividing all organisms strictly into plants and animals presents certain dilemmas. Fungi, for example, are heterotrophs, yet they are entirely non-motile. So where do they belong? To resolve such ambiguities, biologists concluded that more than two kingdoms were necessary. In 1982, Lynn Margulis and Karlene Schwartz proposed a five-kingdom system comprising one prokaryotic kingdom and four eukaryotic kingdoms (Fig. 2.4). The Margulis-Schwartz system has gained widespread acceptance and is now the recommended framework. Eukaryotes are considered to form the superkingdom Eukaryotae. The most contentious group is the protoctists, largely because they do not form a natural evolutionary group. This topic is examined in detail in Section 2.6.

Another group of "organisms" that defies all standard classification schemes is the Viruses. Viruses are extremely minute particles consisting solely of genetic material (DNA or RNA) encased in a protective protein coat. Unlike all true organisms, viruses are non-cellular and can only replicate by invading a living host cell. The Nature of Viruses is discussed in Section 2.4, and they are highlighted as a separate, distinct category in Fig. 2.4A.

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Fig. 2.4. A. The Margulis-Schwartz classification: all organisms are divided into five kingdoms. Viruses do not fit into any group within this CLASSIFICATION OF LIVING organisms because their Structure is extremely simple, non-cellular, and they cannot exist independently of other organisms. B. Evolutionary relationships among the five kingdoms. As the diagram illustrates, evolution progressed toward multicellularity starting from the protoctists.

Although microscopic organisms do not form a single natural taxonomic unit, they are frequently grouped together under the umbrella terms microorganisms or microbes. This group encompasses Bacteria (prokaryotes), viruses, fungi, and protoctists. Such a grouping is practically useful because the Methods employed to study these organisms are generally similar. For instance, visualizing them requires a Microscope, and they must be cultured under aseptic conditions. The branch of biology dedicated to studying microorganisms is known as microbiology. Microorganisms are playing an increasingly vital role in fields such as biochemistry, genetics, agricultural science, and medicine, while also forming the cornerstone of a major industrial sector known as biotechnology. This subject is explored further in Chapter 12. Furthermore, certain microorganisms, such as bacteria and fungi, play a crucial ecological role as decomposers (Section 10.3.2).