Introduction to Organisms of Life
|Biology - The Kingdoms of Life|
Plants, animals, and fungi are the organisms we notice most often. They comprise three of the five kingdoms of life on earth. Organisms of the two remaining kingdoms, the monerans and the protists, live all around us, but they tend to be less conspicuous. More often than not, they're microscopic.
Taxonomy, or systematics, is the science of organising the earth's living things into categories. The categories have changed through history as biological knowledge accumulated. Needless to say, there have been and still are disagreements, as there are when people try to organise anything. Three people may be asked to organise the items on this table into two categories. One may group them this way: defining the categories according to appearance, in this case, color. Another may also base the distinction on appearance, in this case, shape. The third person may consider these distinctions superficial and let differences in function and origin override them. In a similar way, taxonomists try to determine the relative significance of differences and similarities between living organisms.
Years ago, conspicuous similarities or differences were the only distinguishing features; for instance, a mushroom was considered a plant because it grew from the ground and didn't move from place to place. After the acceptance of Charles Darwin's evolutionary theory, taxonomy took on new meaning. The degree of relatedness of living things, their origins, became the most important consideration in organising taxonomic groups.
An organism's family history is its phylogeny. Phylogenetic trees are often used to illustrate taxonomy. Major categories branch into smaller divisions that diverged from each other in the more recent past. This partial phylogenetic tree of mammals indicates that bats are more closely related to monkeys than to whales. It also indicates that bats are a little more closely related to whales than to kangaroos.
We've seen that items can be cataloged according to various patterns imposed by the person cataloging. But biological taxonomists try to reveal a natural pattern that already exists. How do they determine these evolutionary relationships and reach decisions about how to classify organisms?
Taxonomists rely heavily on the work of palaeontologists, who expose the earth's fossil history. By analysing fossils and piecing together evolutionary lines, taxonomists can get an idea of the origins of various groups of organisms. This famous fossil is of Archaeopteryx, the earliest known bird. Why is it a bird? It has feathers, but it also has teeth, and birds don't have teeth.
Several skeletal features are different from those of most reptiles of its time. They are more similar to those of modern birds. Also, the scaly legs and clawed feet are typically birdlike. Overall, the birdlike features outnumber the reptilian features, so Archaeopteryx is generally seen as an ancient bird. Identifying features, such as scaly feet, egg laying, milk production, number of appendages, or leaf arrangement, and so on, are compiled by some taxonomists into huge lists. Organisms that possess the most features in common fall into the same category.
Other taxonomists have criticised this method as too cut and dried, but in general these lists probably reflect evolutionary relationships. After all, in most cases organisms that are related have similar features. Increasingly, taxonomists use the work of biochemists and geneticists to determine ancestral relationships.
Biochemical analysis is especially important in the classification of bacteria. Similar chemical traits often indicate common ancestry. For example, the Gram stain indicates protein differences in the cell walls of bacteria. The staining process turns some bacteria purple and some pink. The outcome of the Gram stain is one factor used in bacterial taxonomy.
Geneticists can now analyse the nucleotide sequence in an organism's DNA, and taxonomists are using this information. They reason that the more sequences organisms have in common in genes coding for the same protein, the more recently those organisms diverged from a common ancestor. Ribosomes, seen here as the specks in this bacterial cell, are found in almost all cells. They are partially composed of RNA that has mutated so little during the course of evolution that ribosomal RNA sequences are often compared to determine the relationships among widely divergent groups, such as kingdoms, subkingdoms, and phyla of organisms.
A species is the smallest major taxonomic grouping. A species is a group of organisms with the natural potential to interbreed and produce fertile offspring. People make up the species Homo sapiens. We are alone in the genus Homo; our nearest relatives died out. We're in the family Hominidae, the order Primates, the class Mammalia, the phylum Chordata, and the kingdom Animalia. Each larger division encompasses a greater variety of organisms that diverged from common ancestors longer ago in the past. Kingdoms are the largest divisions.
Most taxonomists currently follow a five-kingdom system. Living things are classified as either monerans, protists, fungi, animals, or plants. Taxonomists often disagree about the placement of organisms, sometimes even at the kingdom level. Most do agree on the validity of this phylogenetic tree, even though they may differ about where to draw dividing lines on it.
All the procaryotes make up the kingdom Monera. The unicellular eucaryotes are all in the protist kingdom, which also includes multicellular algae. This tree reflects the idea that monerans, or bacteria, were the first life forms, and that other forms developed from ancestors among them. Monerans lived as long ago as three and a half billion years. These ancient rocks, called stromatolites, are fossilised mats of photosynthetic bacteria, similar to present-day colonies that live in such peculiar environments as salt ponds. Stromatolites are among the oldest fossils known.
Monerans are distinguished from other organisms by their lack of a membrane-bounded cell nucleus. Instead of chromosomes in a nucleus, a bacterium has a long, loosely tangled clump of DNA, called a nucleoid, in the cytoplasm. Such cells are called procaryotic, meaning "before kernel" or "before nucleus." Like other unicellular organisms, monerans reproduce by cell division. These bacilli sometimes stick together in chains as they divide, but each cell is still an individual organism. This bacterium produces a useful chemical, an antibiotic, as a product of its metabolism.
Metabolic products and pathways can provide clues for many taxonomic divisions within the moneran kingdom. Bacteria are well-known agents of many diseases of animals and plants, but many more live in or on other organisms without causing any harm. Thousands live in our own mouths and intestines, and grazing animals absolutely depend on the large populations of bacteria in their stomachs to digest their food.
Some monerans produce their own food. The most widespread of these are the various photosynthetic cyanobacteria occurring in water and soil. Until cyanobacteria became prevalent, most of the oxygen on earth was tied up in water molecules, so earth's primitive atmosphere did not support aerobic life. Monerans are the only procaryotes. Organisms of the other four kingdoms a real eucaryotes; their cells have nuclei, and usually mitochondria and other organelles.
The common features of eucaryotic cells indicate their common ancestry. One characteristic feature is the type of flagellum or cilium found in eukaryotic cells. Not all have them, but examples are found among protists, animals, and plants. Whereas a procaryote's flagellum is a relatively simple tail attached to the membrane and composed of several protein filaments, a eucaryote's flagellum is surrounded by the cell membrane and composed of a standard arrangement of microtubules, a circle of nine pairs with a pair in the centre. This arrangement is consistent, whether on a single-celled euglena or a human sperm cell.
Microtubules also form the spindle apparatus involved with the separation of chromosomes during mitosis, the division of eucaryotic cells. The protist kingdom consists of all the single-celled eucaryotic organisms, such as amoebas and diatoms. Protists also include many multicellular forms, such as the seaweeds, or colonial forms, such as Volvox. These were traditionally placed in the plant kingdom, but now most taxonomists place them with the protists. Even the largest seaweeds share ancestry more closely with the single-celled algae than with plants.
Many other protists, called protozoans, were once classed with the animals, based on their locomotion and heterotrophic nutrition. Paramecium is an example. Like a tiny animal, it manoeuvres rapidly and eats bacteria. Currently, however, no unicellular organisms are considered animals.
Relationships within the protist kingdom are not well understood, but it's clear that the three kingdoms of multicellular organisms, the animals, the plants, and the fungi - descended from various ancestors in the protist kingdom. The kingdom Fungi is most familiar to us as mould or as mushrooms. Usually, these visible growths are the reproductive structures of a larger organism. For instance, these mushrooms are attached to an underground network, called a mycelium, that may extend for several feet in every direction.
All fungi produce reproductive spores. Spores line the gills under the caps of these mushrooms. The fungi are divided into phyla based mostly on differences in reproduction and spore formation. One phylum, the Deuteromycota, is made up of mostly tiny fungi among which no sexual reproduction is known to occur. Even these fungi produce spores, however. Their spores are asexually, rather than sexually, produced.
Fungi are heterotrophic. For food, they depend on organic molecules from other organisms, either living or dead. Strings of cells form filaments called hyphae that penetrate a substrate, such as this log, and secrete enzymes that break down complex molecules. The simpler molecules of food are then absorbed through the fungal membrane. The animal kingdom also includes heterotrophic organisms, but these typically ingest food. The food is then broken down by enzymes inside the organism rather than outside it. This is called ingestive nutrition as opposed to the fungi's absorptive nutrition.
Like fungi, animals are all multicellular, but animals develop from the sexual union of a haploid egg and a smaller haploid sperm. The fertilised egg, called a zygote, undergoes repeated cell divisions until a hollow ball of cells, called a blastula, is formed. Some taxonomic divisions within the animal kingdom are based on differences in the development of the embryo after the blastula stage.
Also, basic anatomical differences are important indicators of kinship and origin. For example, some phyla have radial symmetry and others have bilateral symmetry. Animals seem very diverse; for instance, the insect phylum probably contains more than a million separate species, and our own phylum of chordates is also large and varied.
The plant kingdom is probably the most conspicuous manifestation of life on earth. Plants live in a variety of habitats and include the largest organisms known. Most plants are autotrophs, producing food in green organelles called chloroplasts through the process of photosynthesis. A common feature of plants is development from a sexually produced embryo. The embryo in this pine seed is surrounded by nourishing tissue. Other plants, such as ferns, disperse by spores rather than seeds. Their embryos develop in an entirely different way from those of seed plants.
Similarities and differences in life cycles are important criteria for classifying the plants. Plant divisions are also based on the presence or absence of a vascular system to carry water, minerals, and food through the plant's tissues. The veins in the leaf of this oak, an angiosperm, are branches of the vascular system. The bryophyte phylum, which includes mosses, has no such system.
The photosynthesis of plants is responsible for much of the food production on earth, as well as most of the current production of atmospheric oxygen. This food and oxygen not only supplies the growth needs of the plants themselves, but is vital for the metabolism of heterotrophs, like animals, fungi, and some protists and monerans.
Countless associations exist between organisms of different kingdoms. This non-photosynthetic Indian pipe plant depends on the action of tiny fungal hyphae attached to its roots for its food. The fungi secrete enzymes that help decay humus in the soil, and the plant's roots absorb some of the food taken in by the fungi. Even many photosynthetic plants depend on fungi attached to their roots for absorption of chemical compounds they need.
Many fungi also form tightly knit associations with algae or photosynthetic bacteria. All the lichens are such symbiotic partnerships. Fungal hyphae entwine algal cells; the fungi provide some minerals to the algae, and the algae provide some of their food to the fungi.
Many animals, ourselves included, depend on plants not only for food but for homes. Certain plants may depend on animals as much as animals depend on plants. Some plants, for instance, cannot reproduce without the actions of animals. Many groups of monerans and fungi are vital for all life because they recycle the materials that would otherwise be wasted by death. Through evolution, organisms of all five kingdoms are intricately associated.
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