General Microbiology - Schlegel, H. G. 1987

The position of microorganisms in nature
Three kingdoms: animals, plants, and protists

The differences in appearance and Structure between animals and plants, which served as the basis for classifying living organisms until the last century, are obvious at first glance. These differences stem from a fundamental divergence in their modes of Nutrition. Animals feed on preformed organic matter (C-heterotrophically), which is digested and absorbed within their bodies inside the digestive tract. During embryonic development, the digestive cavity is formed by the invagination of the embryo wall at the gastrula stage—a process designed to create internal absorptive surfaces. This structural principle is characteristic of the entire animal kingdom, ranging from Coelenterates (Hydrozoa; e.g., the hydra) to higher vertebrates.

Plants, with their entirely different C-autotrophic mode of nutrition, are structured quite differently. They synthesize the substances necessary for body construction directly from Inorganic Compounds, utilizing sunlight as an energy source. Photosynthetically active Cells and Tissues containing light-absorbing pigments (chlorophylls and carotenoids) are oriented toward the external environment in plants, forming large outer surfaces. Other major differences between animals and plants involve Cell walls, the capacity for active locomotion, and The ability to synthesize specific substances.

The animal and plant kingdoms could be delineated quite clearly as long as little was known about microorganisms. Even Fungi shared so many features with plants that, despite their heterotrophy, they could be classified as plants. It was more difficult to decide to which kingdom Bacteria, slime Molds, and other unicellular organisms should be assigned. A collective name, protists (Haeckel, 1866), was proposed for a third kingdom of living beings.

The Kingdom of Protists encompasses organisms that differ from animals and plants by a low degree of morphological differentiation; they are predominantly unicellular. Based on their cellular structure, protists can be divided into two clearly distinct groups. In higher protists, the cells are similar to animal and plant cells; these are eukaryotes. They include Algae, fungi, and Protozoa. The group of lower protists comprises bacteria, including cyanobacteria (blue-green algae); these are prokaryotes, which differ significantly in cellular structure from all other organisms. Rickettsiae—Obligate Intracellular Parasites—also belong to the bacteria. The term "microorganisms" merely indicates The small size of all these entities, which are actually protists. Viruses, as acellular particles, can be contrasted with all organisms; they are incapable of independent reproduction and can only replicate inside living cells.