Lesson: Chapter - 8

Classifying Life

The diversity of life on Earth is staggering. The science of identifying, describing, naming, and classifying all of these organisms is called taxonomy. Carolus Linnaeus, an eighteenth-century Swedish botanist, is considered the father of modern taxonomy. He carefully observed and compared different species, grouping them according to the similarities and differences he found. Taxonomists today still use his system of organization, though they classify organisms based on their evolutionary relationships, or phylogeny, rather than on simple physical characteristics. The classification system used in taxonomy is hierarchical and contains seven levels. The seven levels of taxonomic classification, from broadest to most specific, are:

Each kingdom contains numerous phyla; each phylum contains numerous classes; each class contains numerous orders; etc. It is more accurate to draw the diagram of the taxonomic categories in a tree structure, with each level of the hierarchy branching into the next:

As one moves through the hierarchy from species to kingdom, the common ancestor of all the species at a certain level dates further back in evolutionary history than the common ancestor of organisms in more specific levels. For example, the common ancestor of humans and chimpanzees (which are both in the order Primates) was alive more recently than the common ancestor of humans and dogs (which are both in the class Mammalia). Much in the same way, members of the same genus are more closely related than members of the same family; members of the same family are more closely related than members of the same order.

Each species is placed into the classification system with a two-part name. The first half of the name is the species’ genus, while the second is the species’ own specific name. The genus name is capitalized, and the species name is lowercase. Humans belong to the genus Homo and the species sapiens, so the name for humans is Homo sapiens.

The Five Kingdoms

Taxonomy splits all living things into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. For the SAT II Biology, you should know the basic characteristics of the organisms that belong in each of these kingdoms, and you should also be familiar with the names and features of the major phyla within each kingdom.

Kingdom Monera

Monerans are prokaryotic: they are single-celled organisms that lack a nucleus and membrane-bound organelles. Of the four kingdoms, monerans are the simplest, and they generally evolved the earliest. Of all the kingdoms, only monerans are prokaryotic.

Monerans are characterized by a single circular chromosome of DNA, a single cell membrane that controls the transport of substances into and out of the cell, and a process of asexual reproduction called binary fission that involves dividing into two identical clones. Some monerans have a cell wall made of a sugar-protein complex called peptidoglycan, which can be determined by Gram staining. A Gram-positive moneran has a thick peptidoglycan cell wall, while a Gram-negative moneran has a much thinner one. Monerans are broken down into phyla according to their means of procuring food.

We cover the structure and function of monerans in more detail in the section on microorganisms in the Organismal Biology chapter.

Phylum Bacteria

Bacteria are heterotrophic and can act as symbionts, parasites, or decomposers.

Phylum Cyanobacteria (blue-green algae)

Cyanobacteria are autotrophs that can perform photosynthesis.

Kingdom Protista

Protists are eukaryotic. In general, protists are less complex than the other eukaryotes and originated earlier in evolutionary history. Most are unicellular, though some are organized in colonies and some others are multicellular. The kingdom Protista can be separated into three primary divisions: animal-like, plantlike, and funguslike.

The animal-like protists are heterotrophic and motile. The most important protozoa for the SAT II Biology are the amoebas, sporozoa, and ciliates:

Phylum Rhizopoda

The members of phylum Rhizopoda are amoebas, known for their constantly changing body structure. Amoebas use membrane extensions called pseudopods (“false feet”) to move and to surround food particles, which they then engulf into their cytoplasm via phagocytosis. Amoebas generally live in fresh water, but some are found in soil or salt water. If an amoeba finds its way inside a human through contaminated drinking water, it can cause severe dysentery.

Phylum ApicomplexaPhylum Apicomplexav

The phylum Apicomplexa consists of spore-forming parasitic organisms, also known assporozoa . The adult form lives inside the cells of animals. The spores are transmitted to other host animals, usually by a carrier animal. For example, a mosquito bite transmits plasmodium, an apicomplexan that lives in red blood cells and causes malaria.

Phylum Ciliophora

All members of the phylum Ciliophora propel themselves by waving many short, hairlike structures called cilia in a coordinated fashion; cilia also help draw food particles into the oral groove. Unlike other protozoa, ciliates have two nuclei: the smaller micronucleus is involved in reproduction, while the macronucleus controls the organism’s metabolic processes. A paramecium is the classic example of a ciliate protozoan. The plantlike protists include euglenoids and various kinds of algae. They are all photo-synthetic autotrophs, transforming light energy into food. Some are unicellular, but many are multicellular, forming fibrous seaweed structures.

Phylum Euglenophyta

Euglenoids are classified with the plantlike protists because many of them photosynthesize. But these unicellular organisms have flagella that allow them to move.

Phylum Phaeophyta

Brown algae of phylum Phaeophyta are all multicellular seaweeds, ranging in size from an inch to almost the length of a football field (the large varieties are called kelp). Brown algae provide both food and shelter to many animals in the coastal marine ecosystem.

Phylum Chlorophyta

Green algae of phylum Chlorophyta have the same photo-synthetic pigments and the same cell wall structure as plants. In fact, they are believed to be the ancestors of modern plants. Some are unicellular, and some are multicellular; however, none have specialized tissues like plants, and therefore they remain classified with the simpler organisms in kingdom Protista. The funguslike protists are called slime molds, which belong to the phyla Myxomycota and Acrasiomycota. All slime molds are heterotrophs.

Phylum Myxomycota

This phylum includes the plasmodial (acellular) slime molds. A plasmodium consists of a single cell with multiple nuclei. Plasmodial slime molds creep slowly along the decaying vegetation they digest; when food or water is scarce, they produce small tough spores that germinate when environmental conditions improve.

Phylum Acrasiomycota

The cellular slime molds belong to. The mold is really a large collection of individual amoebalike protists which congregate into a “pseudo-plasmodium” or “slug” only when food is scarce. In this cooperative form, they produce a single stalk that releases spores.

Kingdom Fungi

Fungi are typically nonmotile and, like plants, have cell walls. Unlike plants, fungi are heterotrophic and have cell walls made of chitin rather than cellulose. Fungi secrete enzymes to digest their food externally and then absorb the nutrients. They usually live as decomposers, living off dead and decaying organisms, or as parasites, growing on or in other living organisms. With the exception of yeast, most fungi are multicellular. Structurally, multicellular fungi are composed of filaments called hyphae; some have hyphae that are segmented by divisions called septa, while others have a continuous cytoplasm with many nuclei in each hyphae. Many fungi exist as a tangle of hyphae, called a mycelium. Examples of fungi are yeast and mushrooms.

tangle of hyphae, called a mycelium. Examples of fungi are yeast and mushrooms. Most fungi can also exist in the form of a spore, a microscopic reproductive structure that is much more resistant to lack of food or water. Unlike most plants and animals, which exist predominantly in a diploid state, fungi spend most of their time in a haploid state, with only a brief diploid phase during the reproductive cycle.

Some fungi grow in a mutually beneficial relationship with a photosynthetic algae or plant. Lichen is an example of such a partnership between a fungus and an algae. The benefits of the merger are apparent: lichen can grow in a wider range of temperatures than any individual plant or fungus, and lichen can often colonize rocks that will not support any other multicellular life forms.

Kingdom Plantae

Plants are complex multicellular photosynthetic autotrophs, with cellulose in their cell walls and a waxy cuticle covering their aboveground parts. They are easily distinguishable from members of all other kingdoms, with the possible exception of their simpler ancestors in the Protista kingdom, the green algae. Over evolutionary time, plants improved their ability to live on land by developing a variety of important features. Plants can be divided into four major groups, displaying a progressively greater degree of adaptation to the terrestrial environment.

Nonvascular Plants—Bryophytes

Bryophyta is the only phylum in the group of nonvascular seedless plants. These mosses and worts are the most primitive true plants. Because they lack a vascular system (vascular systems are discussed in much more detail in the section on Structure and Function of Plants, which is part of the Organismal Biology chapter), bryophytes do not have a stem, leaves, or roots; they must distribute water and nutrients throughout their bodies by absorption and diffusion. As a result, they cannot grow beyond a small size and must keep their bodies close to moist earth. Bryophytes reproduce by spores and need water in order to bring about fertilization. Because the male gamete is a flagellated sperm, reproduction requires water in which the sperm can swim. Unlike all other plants, which have a diploid adult stage, adult bryophytes are haploid, passing only briefly through a diploid phase during the reproductive cycle.

Seedless Vascular Plants

There are three phyla of seedless vascular plants: Lycophyta (club mosses), Sphenophyta (horsetails), and, most likely to appear on the SAT II Biology, Pterophyta (ferns). Vascular plants have a dual fluid transport system: xylem transports water and inorganic minerals from the roots upward, and phloem transports sugars and other organic nutrients up and down. This vascular system represents a major evolutionary step in the adaptation to life on land. The ability to transport water and nutrients across long distances allows plants to grow much larger, sending specialized photosynthetic structures (leaves) upward toward sunlight and specialized root structures downward toward the water and minerals in the ground. Like bryophytes, seedless vascular phyla reproduce by spores and have flagellated sperm that require water in which to swim, limiting these plants to relatively moist environments.

Flowerless Seed Plants—Gymnosperms

The evolution of seeds provided plants with another advantage in their prolonged pilgrimage onto land. Unlike the spores of more primitive plants, seeds are multicellular, containing both a complete diploid embryo and a food supply. Having a food supply inside the seed provides the newborn plant with a period of growth that is independent of food resources in the environment. This independence allows seed plants to grow in a greater variety of environments. Further freeing seed plants, the male gametes of the seed plants take the form of pollen, making reproduction independent of water.

The seed plants that evolved first, called gymnosperms (“naked seeds”), do not produce flowers. Their seeds are exposed directly to the air, without any capsule or fruit enclosing them. The most important group of gymnosperms is phylum Coniferophyta; these plants, commonly called conifers, produce cones that carry seeds on their scales. Examples of gymnosperms are pines, firs, cedars, and sequoias.

Flowering Seed Plants—Angiosperms

Flowering plants, called angiosperms (“covered seeds”), are vascular seed plants with specialized reproductive structures, which include both flowers and fruit. Instead of depending on currents of wind or water for the dispersal of their gametes and seeds, plants with flowers and fruit provide protection and attract animals that then serve as the means of fertilization.

Flowering plants are divided into two classes, monocots and dicots. Monocot seeds have a single cotyledon, while dicots have two cotyledons in each seed. Monocots and dicots are covered in more detail in the section on the Structure and Function of Plants.

Kingdom Animalia

Animals are eukaryotic, multicellular, and heterotrophic. Animals also have specialized tissues to perform various functions. Most animals are motile, at least during part of their life cycle, reproduce sexually, and have nervous systems that allow them to respond rapidly to changes in their environment.

Taxonomists use several observable features to classify animals into groups according to their evolutionary relationships. One of the most important of these features is body symmetry. In bilateral symmetry, the left half of the organism is the mirror image of the right half, but the top does not resemble the bottom, and the front is dissimilar to the back. In radial symmetry, the organism has a circular body plan, with similar structures arranged like spokes on a wheel, such as a starfish. Most animals have three layers of cells: the ectoderm, mesoderm, and endoderm. Almost all animals have a hollow tube inside, which acts as a digestive tract; the opening where food enters is called the mouth, and the opening where digested material exists is called the anus.

Animals are the most diverse of the kingdoms. Any of their various phyla may come up on the SAT II Biology, though the vertebrates come up most often.

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