Viruses

Viruses resemble cells in that they contain nucleic acid and protein molecules, and some are surrounded by membranous envelopes of lipid and protein. Unlike cells, viruses lack the metabolic machinery to synthesize proteins and to generate energy, and many viruses can be crystallized. Cells reproduce by dividing in two, but hundreds of viruses may be produced in a host cell after infection by a single virus.

Viral disruption of host cells is responsible for many diseases, including some tumors, in plants and animals. Some plant diseases are caused by viroids, which are short, naked strands of RNA that do not code for protein.

Bacteria

Include the prokaryotic, simple cells. Genes are carried in a single strand of DNA with little protein. Lack membrane bound organelles, but some have internal membranes, such as mesosomes or systems of membranes for respiration or photosynthesis. Some bacteria have flagella and under adverse conditions can produce spores to continue life.

Most bacterial cells reproduce by binary fission. Mutation and rapid reproduction, coupled with small size and metabolic diversity, are believed to account for the evolutionary success of bacteria.

Three billion years ago, bacteria were the earth’s only inhabitants, living in communities of many different, interdependent species. Some were autotrophs, absorbing raw materials from their environments and making their own food; photoautotrophs used solar energy, whereas chemoautotrophs obtained energy from chemical reactions, mostly of sulfur or nitrogen compounds. Other bacteria evolved ways of living heterotrophically, exploiting their neighbors for food. When a cell died or eliminated its wastes, it provided raw materials for other bacteria, and so nutrients were recycled in the bacterial community. Today, bacteria are still the most numerous and ubiquitous organisms on earth, and they are still playing all of theses roles, although the eukaryotes (which may themselves be the descendants of cooperative prokaryote symbiosis) have become the predominant photosynthesizers and the most conspicuous heterotrophs. Nitrogen fixing bacteria support eukaryotes by converting nitrogen into a form that can be used by higher plants to make amino acids, and saprobic bacteria play the vital ecological role of decomposers, breaking down organic material and permitting its constituents to be recycled. Although many bacteria are free living, numerous others form symbiotic relationships—parasitic, commensal, or mutualistic—with other bacteria or with eukaryotes. Some bacteria are pathogens, producing toxins that cause disease and some are part of the normal flora of animals. The chloroplasts and possibly mitochondria of eukaryotes are also thought to have evolved from intracellular symbiotic bacteria. Chloroplasts show many similarities to the cyanobacteria which differ from other photosynthetic bacteria in possessing chlorophyll a and in using water as a hydrogen donor, releasing molecular oxygen as a photosynthetic by product.

Bacteria significant to the human economy include cyanobacteria, which are often members of "blooms" produced by an excess of nutrients in bodies of fresh water; actinomycetes, common soil organisms from which many antibiotics were first procured; and nitrogen fixing symbionts in the root nodules of legumes. We use some bacteria to produce foods such as yogurt, cheeses, and vinegar. Food poisoning is caused by ingesting exotoxins or pathogenic bacteria in food. Bacterial diseases have played major, often devastating roles in human history. Immunization, improved hygeine and antibiotics have reduced human and animal deaths from bacterial disease. One disadvantage of antibiotics is that their use selects from mutant bacteria and transferable plasmids that are resistant to the antibiotic in question.

Protists

Protists are complex organisms within a cell.

Autotrophic protists live wither free in the phytoplankton, or attached to moist surfaces or as symbionts inside larger organisms. The clues to their phylogenetic include such conservative characters as photosynthetic pigments; form of food storage; chemistry and form of the cell wall or other cell covering; and presence, type, and arrangement of flagella.

Heterotrophic protists may be free living predators, eating other protists, bacteria, and small multicellular organisms, or may be symbionts or parasites of larger life. Three major phyla are characterized by their locomotion. Some parasitic protists have an enormous impact on human health, which in turn may place tremendous burdens on the economy in the form of health care expenses and lost labor capacity.

Fungi

Members of the kingdom Fungi are eukaryotic saprobes and parasites that obtain nutrients by absorption and have a cells wall in at least some stages of the life cycle. Spores are involved in both sexual and asexual reproduction.

The most important role of fungi is that of decomposer, an organism which breaks down dead plants and animals and absorbs the resulting small food molecules. Molecules "uneaten" by the fungi then become important to green plants in the way of minerals.

Fungi are also important as parasites, causing diseases of both plants and animals. Most, however, attack plants. Once established, fungal diseases of both plants and animals are almost impossible to combat. Prevention is much more important in combating fungal disease.

Lichens are a symbiotic association between fungi and green algae. Lichens are important food producers in cold or barren areas; they may also help to form soil from bare rock. Other fungi form other associations with the roots of plants and benefit them by helping with their mineral supply.

Many fungi produce toxic compounds that serve to deter predators, reduce competition from bacteria and other fungi, or even catch animals as dietary supplements.

Fungi are economically harmful because they destroy seed, standing crops, and harvested food, as well as clothing, homes and other possessions. Fungi also cause disease and death to humans, livestock and pets. On the positive side, fungi are useful in the production of many edible fruiting bodies, of fermented foods, and of drugs, antibiotics, and various chemical compounds.