Organisms are traditionally classified into three domains and further subdivided into one of six kingdoms of life.
Organisms are placed into these categories based on similarities or common characteristics. Some of the characteristics that are used to determine placement are cell type, nutrient acquisition, and reproduction. The two main cell types are prokaryotic and eukaryotic cells. Common types of nutrient acquisition include photosynthesis, absorption, and ingestion. Types of reproduction include asexual reproduction and sexual reproduction. Some more modern classifications abandon the term "kingdom." These classifications are based on cladistics, which notes that kingdoms in the traditional sense are not monophyletic; that is, they do not all have a common ancestor. Moelyn Photos/Getty ImagesArchaebacteria are single-celled prokaryotes originally thought to be bacteria. They are in the Archaea domain and have a unique ribosomal RNA type. The cell wall composition of these extreme organisms allows them to live in some very inhospitable places, such as hot springs and hydrothermal vents. Archaea of the methanogen species can also be found in the guts of animals and humans.
These organisms are considered to be true bacteria and are classified under the Bacteria domain. Bacteria live in almost every type of environment and are often associated with disease. Most bacteria, however, do not cause disease. Bacteria are the main microscopic organisms that compose the human microbiota. There are more bacteria in the human gut, for instance, than there are body cells. Bacteria ensure that our bodies function normally. These microbes reproduce at an alarming rate under the right conditions. Most reproduce asexually by binary fission. Bacteria have varied and distinct bacterial cell shapes including round, spiral, and rod shapes.
The protista kingdom includes a very diverse group of organisms. Some have characteristics of animals (protozoa), while others resemble plants (algae) or fungi (slime molds). These eukaryotic organisms have a nucleus that is enclosed within a membrane. Some protists have organelles that are found in animal cells (mitochondria), while others have organelles that are found in plant cells (chloroplasts). Protists that are similar to plants are capable of photosynthesis. Many protists are parasitic pathogens that cause disease in animals and humans. Others exist in commensalistic or mutualistic relationships with their host.
Fungi include both unicellular (yeast and molds) and multicellular (mushrooms) organisms. Unlike plants, fungi are not capable of photosynthesis. Fungi are important for the recycling of nutrients back into the environment. They decompose organic matter and acquire nutrients through absorption. While some fungal species contain toxins that are deadly to animals and humans, others have beneficial uses, such as for the production of penicillin and related antibiotics.
Plants are extremely important to all life on earth as they provide oxygen, shelter, clothing, food, and medicine for other living organisms. This diverse group contains vascular and nonvascular plants, flowering and nonflowering plants, as well as seed-bearing and non-seed bearing plants. As is true of most photosynthetic organisms, plants are primary producers and support life for most food chains in the planet's major biomes. Doug Allan / Getty ImagesThis kingdom includes animal organisms. These multicellular eukaryotes depend on plants and other organisms for nutrition. Most animals live in aquatic environments and range in size from tiny tardigrades to the extremely large blue whale. Most animals reproduce by sexual reproduction, which involves fertilization (the union of male and female gametes).
In this section learners are introduced to the most common way of grouping living organisms based on simple distinctive characteristics. Classification systems are always changing as new information is made available. Modern technologies such as electron microscopy make it possible to observe microscopic organisms in greater detail. The current system was developed by Robert H. Whittaker in 1969 and was built on the work of previous biologists such as Carolus Linnaeus. The five kingdom system is the most common way of grouping living things based on simple distinctive characteristics. Classification systems are always changing as new information is made available. Modern technologies such as Genetics makes it possible to unravel evolutionary relationships to greater and greater detail. The five-kingdom system was developed by Robert H. Whittaker in 1969 and was built on the work of previous biologists such as Carolus Linnaeus. Living things can be classified into five major kingdoms:
A video showing a brief summary of the five kingdoms Video: 2CXG We will now identify the main distinctive features of each kingdom: The Kingdom Monera consists of prokaryotic, unicellular organisms. No nuclear membrane or membrane-bound organelles such as chloroplasts, Golgi complex, mitochondria or endoplasmic reticulum are present. Monera have a cell wall of protein plus polysaccharide compound, but not cellulose. They reproduce asexually by binary fission. Important examples of Monera include Archaea and Bacteria.
Bacteria are found everywhere and are the most numerous organisms on Earth. In a single gram of soil, there are about 40 million bacterial cells. The human body also contains \(\text{10}\) times as many bacterial cells as human cells!
Protista are eukaryotic and can be unicellular or simple multicellular. They reproduce sexually or asexually. Important examples of protists include the organism known as Plasmodium (which causes malaria), Amoeba and Euglena. There are two major groups of protists which include the Protozoans, whose cells are similar to animal cells in that they do not have cell walls and the plant-like cells which do have cell walls and are similar to algae.
Fungi are eukaryotic organisms that can be multicellular or unicellular. Mushrooms and moulds are examples of multicellular fungi and yeast is an example of a unicellular fungi. All fungi have a cell wall made of chitin. They are non-motile (not capable of movement) and consist of threads called hyphae. Fungi are heterotrophic organisms which means they require organic compounds of carbon and nitrogen for nourishment. They are important as decomposers (saprophytes) and can be parasitic. They store carbon as glycogen, not in the form of starch. Fungi reproduce sexually and asexually by spore formation. An important example of a useful fungus is Penicillium (a fungus which was used to make penicillin, one of the most powerful antibiotics ever created).
A TED video on the many uses of Fungi Video: 2CXH
Sir Alexander Fleming discovered the first antibiotics in 1928, after observing that colonies of Staphylococcus aureus bacteria could be destroyed by the fungi Penicillium notatum. This observation that certain substances were deadly to microbial life lead to the discovery and development of medicines that could kill many types of disease-causing bacteria in the body. Organisms belonging to the plant kingdom are eukaryotic and multicellular organisms. They have a distinct cell wall made of cellulose. Cells are organised into true plant tissues. Plants contain plastids and photosynthetic pigments such as chlorophyll. They are non-motile. Plants make their own food by photosynthesis and are therefore said to be autotrophic. Plants undergo both sexual and asexual reproduction. They store food as starch. Important examples of plants are mosses, ferns, conifers and flowering plants. Examples of plant variety
Members of the animal kingdom are eukaryotic and multicellular but have no cell wall or photosynthetic pigments. They are mostly motile and they are heterotrophic, which means they must feed on other organisms and cannot make their own food. They reproduce sexually or asexually. Animals store carbon as glycogen and fat. Important examples of this kingdom include: Porifera (sponges), Cnidaria (jellyfish), Nematoda (nematode worms), Platyhelminthes (flatworms), Annelidas (segmented worms), Mollusca (Snails and Squid), Echinodermata (starfish), Arthropoda (Insects and Crustaceans), Chordata (includes all the vertebrates: fish, amphibians, reptiles, birds, mammals). Examples of animal variety: Animal Phyla
Classes of vertebrates
TEACHER RESOURCES: To investigate examples from each kingdom.
Activity: Investigate examples of life forms from each kingdom In this activity a learners are to research one beneficial and one harmful application of one member from each of the kingdoms including examples of their use in South Africa. Learners can be grouped into smaller groups and each one is given one kingdom to research. Learners can present their results in the form of a poster. Since this is an OPTIONAL activity, no detailed memorandum is provided. If teachers are keen to have learners do this, they will have to provide some form of guidance to learners about what level of detail is expected, e.g. a short paragraph on one beneficial and one harmful member of each kingdom. It is suggested that this be done in GROUPS of two or four, so each group gets only one kingdom to research (learner 1 and 2 find a beneficial plant and learner 3 and 4 find a harmful plant, for example. One can find the information and the other finds a picture.) Teachers should ensure that all 5 kingdoms are covered by the class, so it will be important to do some planning beforehand. A dichotomous key is a tool that taxonomists often use to classify organisms correctly. It is a form of hierarchical grouping that involves making decisions in a series of steps, from general differences to very specific differences. It is called a dichotomous key because there are always two choices. There is a very specific way to set up a dichotomous key. For instance, one must always move from the general to the specific, and one must always ensure that the two choices in the decision tree are mutually exclusive and jointly exhaustive. Mutually exclusive means that there cannot be overlap between the two options, as this would result in wanting to place an organism in two groups. Jointly exhaustive means that your two options must cover all possibilities, otherwise you won't be able to place an organism in either of the groups. To use a dichotomous key to identify arthropods.
Answers:
This website shows you an exercise with answers , using a dichotomous key: |
Which table correctly lists the characteristics of the organisms in the two kingdoms?
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