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Unicellular Protists Synthesis Essay

Respiration and nutrition

At the cellular level, the metabolic pathways known for protists are essentially no different from those found among cells and tissues of other eukaryotes. Thus, the plastids of algal protists function like the chloroplasts of plants with respect to photosynthesis, and, when present, the mitochondria function as the site where molecules are broken down to release chemical energy, carbon dioxide, and water. The basic difference between the unicellular protists and the tissue- and organ-dependent cells of other eukaryotes lies in the fact that the former are simultaneously cells and complete organisms. Such microorganisms, then, must carry out the life-sustaining functions that are generally served by organ systems within the complex multicellular or multitissued bodies of the other eukaryotes. Many such functions in the protists are dependent on relatively elaborate architectural adaptations in the cell. Phagotrophic feeding, for example, requires more complicated processes at the protist’s cellular level, where no combination of tissues and cells is available to carry out the ingestion, digestion, and egestion of particulate food matter. On the other hand, obtaining oxygen in the case of free-living, free-swimming protozoan protists is simpler than for multicellular eukaryotes because the process requires only the direct diffusion of oxygen from the surrounding medium.

Although most protists require oxygen (obligate aerobes), there are some that may or must rely on anaerobic metabolism—for example, parasitic forms inhabiting sites without free oxygen and some bottom-dwelling (benthic) ciliates that live in the sulfide zone of certain marine and freshwater sediments. Mitochondria typically are not found in the cytoplasm of these anaerobes; rather, microbodies called hydrogenosomes or specialized symbiotic bacteria act as respiratory organelles.

The major modes of nutrition among protists are autotrophy (involving plastids, photosynthesis, and the organism’s manufacture of its own nutrients from the milieu) and heterotrophy (the taking in of nutrients). Obligate autotrophy, which requires only a few inorganic materials and light energy for survival and growth, is characteristic of algal protists (e.g., Chlamydomonas). Heterotrophy may occur as one of at least two types: phagotrophy, which is essentially the engulfment of particulate food, and osmotrophy, the taking in of dissolved nutrients from the medium, often by the method of pinocytosis. Phagotrophic heterotrophy is seen in many ciliates that seem to require live prey as organic sources of energy, carbon, nitrogen, vitamins, and growth factors. The food of free-living phagotrophic protists ranges from other protists to bacteria to plant and animal material, living or dead. Scavengers are numerous, especially among the ciliated protozoans; indeed, species of some groups prefer moribund prey. Organisms that can utilize either or both autotrophy and heterotrophy are said to exhibit mixotrophy. Many dinoflagellates, for example, exhibit mixotrophy.

Feeding mechanisms and their use are diverse among protists. They include the capture of living prey by the use of encircling pseudopodial extensions (in certain amoeboids), the trapping of particles of food in water currents by filters formed of specialized compound buccal organelles (in ciliates), and the simple diffusion of dissolved organic material through the cell membrane, as well as the sucking out of the cytoplasm of certain host cells (as in many parasitic protists). In the case of many symbiotic protists, methods for survival, such as the invasion of the host and transfer to fresh hosts, have developed through long associations and often the coevolution of both partners.

A Biological and Military (Army) Organizational Hierarchy Compared:

Biological Organization

Military Organization

   Kingdom (one or more phyla)  Brigade (two or more regiments)
   Phylum (one or more classes)  Regiment (two or more battalions)
   Class (one or more orders)  Battalion (two or more companies) 
   Order (one or more families)  Company (two or more platoons)
   Family (one or more genera)  Platoon (two or more squads)
   Genus (one or more species)  Squad (a group of 12 soldiers)
   Species (a distinct kind or unit)     Soldier (a distinct kind or unit)

The following table compares the complete taxonomic hierarchy of a marine lichen of the rocky Pacific coast Verrucaria maura with the minute aquatic flowering plant Wolffia borealis:

The plant kingdom includes nonvascular and vascular plants. Nonvascular plants lack a water-conducting system of tubular cells (called xylem tissue), and do not have true roots, stems and leaves. Like algae and fungi, the plant body of some nonvascular plants is often called a thallus. Nonvascular plants are all placed in the Division Bryophyta, including the mosses and liverworts. The vast majority of the plant kingdom are vascular, with tubular, water-conducting cells called xylem tissue. Like a microscopic pipeline system, they are arranged end-to-end from the roots to the leaves. Unlike nonvascular plants, they have true roots, stems and leaves. Some references place all the vascular plants in a separate phylum or division called the Tracheophyta. Most botanists now subdivide vascular plants into 9 divisions. More primitive vascular plants that reproduce by spores, but without seeds, are called pteridophytes, and include the 4 divisions Psilophyta (whisk ferns), Lycophyta (club mosses), Sphenophyta (horsetails), and Pterophyta (ferns). Seed-bearing vascular plants are called spermatophytes and include the primitive gymnosperms (with immature seeds or ovules naked and exposed directly to pollen) and the more advanced angiosperms (with ovules enclosed in an ovary that ripens into a fruit). Gymnosperms include the 4 divisions Cycadophyta (cycads), Ginkgophyta (maidenhair tree), Gnetophyta (mormon tea & the bizarre South African Welwitschia), and the Coniferophyta (conifers). The angiosperms are placed in the single division Anthophyta which includes all the flowering plants and 90 percent of all the plant kingdom.

Twenty of the more than 100 species of Pinus on earth. All of these pines are native to the state of California, USA. 1. Monterey Pine (P. radiata), 2. Bishop Pine (P. muricata), 3. Santa Cruz Island Pine (P. remorata), 4. Whitebark Pine (P. albicaulis), 5. Limber Pine (P. flexilis), 6. Beach Pine (P. contorta), 7. Lodgepole Pine (P. murrayana), 8. Western White Pine (P. monticola), 9. Knobcone Pine (P. attenuata), 10. Bristlecone Pine (P. longaeva), 11. Foxtail Pine (P. balfouriana), 12. Four-Leaf Pinyon (P. quadrifolia), 13. Two-Leaf Pinyon (P. edulis), 14. One-Leaf Pinyon (P. monophylla), 15. Ponderosa Pine (P. ponderosa), 16. Coulter Pine (P. coulteri), 17. Digger Pine (P. sabiniana), 18. Torrey Pine (P. torreyana), 19. Jeffrey Pine (P. jeffreyi), 20. Sugar Pine (P. lambertiana).

Note: In the Jepson Flora of California (1993), Pinus remorata is now considered a synonym of P. muricata. Another species (left image) called the Washoe Pine (P. washoensis), with cones similar to a miniature Jeffrey Pine, is now recognized for California. In addition, the Beach and Lodgepole Pines are now recognized as subspecies of P. contorta, rather than separate species.

According to R.M. Lanner (Conifers of California, 1999), there may be other significant changes in the pines of California. Allozyme studies in two-leaf pinyons (Pinus edulis) of the New York Mountains indicate that these populations are biochemically (and genetically) consistent with nearby one-leaf pinyon (Pinus monophylla), and that P. edulis may not occur in California. The unusual New York Mountains population appears to be a 2-needle variant of P. monophylla. The four-leaf or Parry pinyon of San Diego County (P. quadrifolia) may be a hybrid between P. monophylla and Sierra Juárez pinyon (P. juarezensis) of northern Baja California. According to Lanner, the latter species has five needles per fascicle and occurs in San Diego County. The hybrid hypothesis might explain the perplexing variation in needle number for P. quadrifolia, with clusters of three, four and five.

Foxtail pines (Pinus balfouriana) on the 11,000 ft (3353 m) slopes of Alta Peak. The 13,000 ft. (3962 m) crest of the Great Western Divide of the Sierra Nevada can be seen in the distance.

Selection & Genetic Drift In California Cypress

Millions of years ago, cypress woodlands containing one or more ancestral species of the cone-bearing genus Cupressus once dominated vast areas of California. During the past 20 million years, as mountains were uplifted and the climate became increasingly more arid, most of these extensive cypress woodlands vanished from the landscape. In some areas, the cypress were probably unable to compete with more drought resistant, aggressive species, such as impenetrable chaparral shrubs and desert scrub. Although cypress are fire-adapted with serotinous seed cones that open after a fire, they are vulnerable if the fire interval occurs too frequently, before the trees are old enough to produce a sufficient cone crop. Chaparral shrubs quickly resprout after a fast-moving brush fire from well-established subterranean lignotubers. This may explain why some cypress groves occur in very rocky, sterile sites with poor soils where the chaparral shrubs can't compete as well.

Today this fascinating genus is represented by 10 species (or 8 species and 2 subspecies), confined to isolated groves scattered throughout the coastal and inland mountains, from the Mexican border to Oregon. Because some of these populations became isolated into "arboreal islands," gradual genetic changes over millions of years resulted in the present-day species and subspecies. This is somewhat analogous to the evolution of Darwin's finches on the Galapagos Islands. It is quite likely that natural selection played a role in cypress speciation. Cypress of arid inland mountains and valleys (such as Piute cypress, Macnab cypress, Cuyamaca cypress, and Arizona cypress) have glandular (resinous) foliage and are more drought resistant. Coastal species (such as Monterey cypress, Gowen cypress, Santa Cruz cypress and Mendocino cypress) are generally nonglandular without resin glands on the leaf surfaces. Some phenotypic variability, particularly between different isolated groves of the same species may be due (in part) to genetic drift. These differences include slight variations in foliage, bark characteristics (exfoliating vs. persistent), and the general shape of seed cones. These differences attributed to genetic drift are analogous to racial differences in people, such as different blood type percentages and facial characteristics.

The relatively short period of isolation for Cupressus (cypress) species may be one of the reasons taxonomists disagree on the total number of species native to North America. In 1948, Carl B. Wolf published his "Taxonomic and Distributional Studies of the New World Cypresses" (El Aliso 1: 1-250). Dr. Wolf listed a total of 15 species, one in Baja California, one on Guadalupe Island off the coast of Baja California, one in Mexico and Central America, two in Arizona, and 10 in California. In 1953, the number of U.S. species was reduced to six by Dr. Elbert Little, Jr. in his Check List of Native and Naturalized Trees of the United States (USDA Agriculture Handbook No. 41). These numbers have fluctuated greatly in subsequent publications. In addition, the nursery trade has added several cultivated varieties, including at least four different cultivars for the Arizona cypress.

New evidence from DNA sequencing has further complicated the number of cypress species, including the transfer of other conifer genera into the genus Cupressus. For example, the Jepson Manual of California Plants lists ten species; however, two of these C. nootkatensis (Alaska cedar) and C. lawsoniana (Port Orford cedar) were formerly placed in the genus Chamaecyparis. It is possible that some of the isolated species of Cupressus in California and Arizona have not been isolated long enough to warrant the status of a species. In fact, this is why most modern floras have consolidated four species into subspecies of the Arizona cypress (C. arizonica). These species have been isolated long enough for genetic drift to occur, but perhaps not long enough for the development of distinct species populations.

Left: Seed cones of cypress (Cupressus) from groves in southern California. A. Tecate cypress (C. forbesii), B. Sargent cypress (C. sargentii), C. Piute cypress (C. nevadensis) [Syn. C. arizonica ssp. nevadensis], D. Cuyamaca cypress (C. stephensonii) [Syn. C. arizonica spp. stephensonii], E. Smooth-bark Arizona cypress (C. glabra) [Syn. C. arizonica ssp. glabra], F. Rough-bark Arizona cypress (C. arizonica) [Syn. C. arizonica ssp. arizonica]. Right: Seed cones of cypress from groves in central and northern California. G. Monterey cypress (C. macrocarpa), H. Gowen cypress (C. goveniana) [Syn. C. goveniana ssp. goveniana], I. Santa Cruz cypress (C. abramsiana), J. Sargent cypress (C. sargentii), K. Mendocino cypress (C. pygmaea) [Syn. C. goveniana ssp. pigmaea], L. Macnab cypress (C. macnabiana), M. Modoc cypress (C. bakeri).

Male (pollen) cones of the Piute cypress (Cupressus nevadensis) [syn. C. arizonica ssp. nevadensis). Each scalelike leaf bears a dorsal gland that exudes a resin droplet (red arrow). Interior cypress species such as this one typically have glaucous, resinous foliage, presumably an adaptation to dry, arid habitats.

A. Foliage and pollen cones of the Smooth-bark Arizona cypress (Cupressus glabra) [Syn. C. arizonica ssp. glabra]. B. Foliage of the Tecate cypress (C. forbesii). The scalelike leaves of Arizona cypress are glaucous and very glandular (sticky). The scalelike leaves of Tecate cypress are green and without dorsal resin glands.

Left: Monterey cypress (Cupressus macrocarpa) in Point Lobos State Park on the coast of central California. Right: Grove of Piute cypress (C. nevadensis) in the Piute Mountains, with Lake Isabella and the snow-covered Sierra Nevada in the distance. The Piute cypress are more drought resistant, with gray (glaucous), glandular (resinous) foliage similar to the Arizona cypress. In fact, some botanists now consider the Piute cypress to be a subspecies of the Arizona cypress and have named it C. arizonica ssp. nevadensis.

A grove of Sargent cypress (Cupressus sargentii) in the San Rafael Mountains of Santa Barbara County, California. This species typically grows on outcrops of serpentine in the Coast Ranges of central and northern California. Serpentine is a shiny rock with a waxy luster and feel. It varies in color from creamy white and shades of green to black. In California, many species of rare and endangered plants are endemic to serpentine outcrops. Genetic drift has undoubtedly occured in isolated cypress groves such as this one, which are often referred to as "arboreal islands."

Other Members Of The Division Coniferophyta

Podocarpus gracilior, a member of the Podocarpaceae native to eastern Africa. Although it is sometimes called "fern pine" it does not belong to the genus (Pinus); however, like pines and other cone-bearing species, it does belong to the Division Coniferophyta. Minute female cones are composed of 2-4 reduced scales, but usually only one scale bears an ovule that matures into a seed. There is little resemblance to a cone in the mature seed. The seed has a hard coat surrounded by a fleshy outer layer (aril). The drupelike seed often sits on a fleshy red or purple base or cone axis that is called an aril in some references. The seeds are similar to the California nutmeg (Torreya californica) and Pacific yew (Taxus brevifolia), members of the closely-related Yew Family (Taxaceae). In the latter species, the naked seed sits partially exposed in a red, cup-shaped aril. Podocarpus seeds are often referred to as fleshy fruits called drupes, but this is incorrect because drupes develop from the ovaries of flowering plants. Another group of conifers with fleshy seed-bearing structures are the junipers (Juniperus) in the Cypress Family (Cupressaceae). Junipers actually produce small cones with fleshy, fused scales bearing one-several seeds. Podocarpus is a dioecious species, with separate male and female trees in the population. Podocarpus has an ancient lineage dating back to distant relatives that lived during the Jurassic Period 170 million years ago.

California nutmeg (Torreya californica), a member of the Division Coniferophyta, Order Taxales, Family Taxaceae. Like Podocarpus, the "naked" seed is enclosed in a fleshy, outer layer (called an aril) which superficially resembles a one-seeded fruit of an angiosperm. The name "nutmeg" is derived from its superficial resemblance to the fruit of the true nutmeg (Myristica fragrans).

Pacific yew (Taxus brevifolia), another member of the Division Coniferophyta, Order Taxales, Family Taxaceae that occurs in northern California, Oregon and Washington. Unlike the California nutmeg, the naked seed is not completely enclosed by the fleshy aril. Instead, the seed sits in a cup-shaped aril. Since this species is native to regions of the Pacific northwestern United States containing the timber tree Douglas fir (Pseudotsuga menziesii), it was once considered a weedy species when areas of the forest were logged. Luckily, the Pacific yew still survives because it is now considered to be an exceedingly valuable species. An extract from the bark (and needles) called taxol has been found to be a very effective treatment for ovarian and breast cancers. It is very important to preserve natural, old growth forests with a diversity of species, some of which may prove to be valuable medicines for the treatment of diseases.

Santa Lucia Fir (Abies bracteata)

The Santa Lucia or bristlecone fir (Abies bracteata) has a tall, slender, steeple-like crown. Seed cones are produced near the top of the slender spire, and they are some of the most unusual cones of all cone-bearing trees on earth. Long, spine-like bracts extend outwardly from between the cone scales, and resemble the antennae of a space satellite. This uncommon and remarkable fir tree is endemic to steep, rocky slopes in the Santa Lucia Range of California's Coast Ranges.

Santa Lucia fir (Abies bracteata), a remarkable California endemic.

Using fossil evidence and computerized cladistic analyses, it is generally concluded that evolution in the plant kingdom proceeded from nonvascular, spore-bearing ancestors to vascular, seed-bearing, flowering plants, as more and more advanced morphological and biochemical traits gradually appeared along the geologic time scale. This is somewhat analogous to the evolution of Microsoft; however, unlike Microsoft, the phenomenal success of flowering plants is based on natural selection rather than timely, strategic decisions by brilliant top level executives such as Bill Gates.


  1. Armstrong, W.P. 1978. "Southern California's Vanishing Cypresses." Fremontia 6 (2): 24-29.
  2. Armstrong, W.P. 1977. "The Close-Cone Pines and Cypresses" (Chapter 9, pp. 295-358). In: Terrestrial Vegetation of California, John Wiley & Sons.
  3. Hickman, J.C. (Editor). 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley.
  4. Lecointre, G. and H.L. Guyader. [Illustrated by D. Visset & Translated by K. McCoy.] 2006. The Tree of Life: A Phylogenetic Classification. Harvard University Press, Cambridge, Massachusetts.
  5. Margulis, L., K.V. Schwartz, and M. Dolan. 1994. The Illustrated Five Kingdoms: A Guide To The Diversity Of Life On Earth. HarperCollins College Publishers, New York.

All text material & images on these pages copyright © W.P. Armstrong