STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION B4A
Reflect Classifying and naming organisms is a practice that dates back to ancient Greece. Aristotle was one of the first to group and categorize living things based on their characteristics. After Aristotle, scientists and academics continued his work by adding to classification systems and creating new ones as discoveries were made. By the 18th century, there were multiple classification systems in place, each with its own way of categorizing and naming species. In order for collaboration in the scientific community to advance, changes had to be made. What changes to classification were necessary? What tools could scientists use to organize organisms consistently? What characteristics represent different groups of organisms?
A Standardized System of Classification and Naming In the past, scientists were unable to properly communicate about living organisms. Newly discovered species were randomly named. In fact, some may have been discovered multiple times due to the lack of ability to distinguish the classification systems. A standardized system of grouping and naming life was necessary in order to allow scientists to communicate and maintain organization of the wide diversity of life on Earth. Carolus Linnaeus was an 18th-century scientist who focused his studies on plants. However, he is known best as the father of taxonomy. Taxonomy is a systematic process of classifying living organisms into different groups based on their physical traits and genetic relationships. Over the years, Linnaeus’ original system has been modified as new discoveries were made, but the basic system is still intact. The groupings of living things begin as broad classifications and become narrower and more specific as they continue. The highest and broadest level of classification is called the domain. It is followed by kingdom, phylum, class, order, family, genus, and species. The table below shows the classification of the domestic dog from domain to species. Domain Eukarya
Kingdom Phylum Animalia Chordata
Class Order Family Mammalia Carnivora Canidae
Genus Species Canis Familiaris
Organisms are commonly referred to according to the two most specific taxonomic levels: genus and species, which are often Latin. This is called binomial nomenclature. The taxonomic name of modern humans is Homo sapiens. The genus is always capitalized, the species is lowercase, and the whole name is written in italics. By using this same system, scientists around the globe can freely communicate with certainty that they are referring to the same organisms.
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STEMscopedia: STRUCTURE, FUNCTION, AND What Do You Think?
CLASSIFICATION
Imagine that, before the establishment of taxonomy, a scientist working in Africa writes a letter to a biologist in England claiming that he has discovered a new organism. It is unique, and he has never seen one before. He has named the animal glotchbot. The British biologist spreads the word about the glotchbot through the scientific community, and everyone in the community becomes excited about this new discovery. The scientist returns from Africa with a picture of the glotchbot shown on the right. Suddenly, the scientific community loses all interest, and both the scientist and the biologist lose respect. How would a standard system of taxonomy have changed the outcome of this scenario? Career Corner: Taxonomist Scientists who study taxonomy and use the classification system to identify and name organisms are taxonomists. Taxonomists are first and foremost scientists. They have a fundamental knowledge of biology or other related fields. They often have advanced degrees in zoology, animal physiology, botany, or other life sciences. Museums, zoos, aquariums, and universities are common places of employment for taxonomists. Here they can study DNA, environments, and other influences that have contributed to characteristics of life. Taxonomists’ knowledge is often used to educate others through lectures and publications about conservation of endangered or threatened species. Classification Into Domains: Bacteria, Archaea, and Eukarya The three domains differ fundamentally in their cellular structures and genetic makeup. The domains are so broad that all life can be separated into just three different categories. Let’s examine the basic differences between these three categories of life. Patterns in Structures of Clades Domain Bacteria: This domain consists of unicellular prokaryotes. They lack a cell nucleus and membrane-bound organelles, but they are surrounded by a thick cell wall. Bacteria can be found nearly everywhere on Earth, including living inside human beings’ mouths and stomachs. Bacteria are incredibly diverse. Some are free living, while others rely on a host to survive. Many use oxygen, while others are killed by the presence of oxygen. Like plants, some bacteria are photosynthetic. Many bacteria cause infections, such as strep throat (Streptococcal pharyngitis), food poisoning (Escherichia coli and Salmonella enterica), and plant wilt in sweet corn (Erwinia stewartii). Most bacteria are beneficial and serve a necessary role in their environment. There are a wide variety of characteristics and functions among the members of domain Bacteria. Domain Archaea: Like domain Bacteria, the members of domain Archaea are unicellular prokaryotes. They also have a cell wall, but it differs in composition from those of bacteria. Archaean cell walls lack the substance peptidoglycan found in bacteria. Their cell membranes also differ, containing unusual lipids that are not found in any other organisms on Earth. (A lipid is a type of biomolecule; fats, oils, and waxes are examples of lipids.)
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STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION
One of the most distinct features of domain Archaea is that they are able to survive in some of the most extreme environments on Earth. Archaea have been found in the hot springs of Yellowstone National Park in Wyoming and in deep oceanic hydrothermal vents measuring over 100 degrees Celsius (212 degrees Fahrenheit). Others live in environments with extremely high salinity and acidity. Domain Eukarya: This domain differs from the others because its members’ cells contain a nucleus and membrane-bound organelles. Most eukaryotic species are multicellular, but some are unicellular. Domain Eukarya is quite diverse and contains the most well-known organisms. Eukaryotes are found all over the world in a variety of environments. The domain is so diverse that it is best to study the organisms of domain Eukarya in their narrower classification groups.
Archaea are thermophiles because they thrive in hot environments like this geothermal pool.
Suppose a group of scientists discovered a new prokaryotic organism in a highly acidic sulfur vent in Antarctica. Which domain does the organism most likely belong to? The Four Kingdoms of Eukaryotes Domain Eukarya is incredibly diverse. it includes organisms from daffodils to dragonflies and orangutans to oak trees. It is divided into four kingdoms based on the most general characteristics. The kingdoms are Protista, Plantae, Fungi, and Animalia. Each kingdom is further divided into phyla, then classes, orders, and so on. The members of each kingdom have distinct enough characteristics to allow us to begin identifying organisms. Protista: These ancient eukaryotes have some characteristics not shared by many other members of the domain, including the fact that many are unicellular. Even within the kingdom there is great diversity. In fact, many protists are classified in this kingdom just because they do not fit in any of the others. They vary greatly in their appearance, mobility, reproduction, and methods for obtaining food. Some protists are even photosynthetic. Examples include many phytoplankton, red and brown algae, and dinoflagellates. Plantae: Plants are very common eukaryotes. They include a wide variety of organisms with unique characteristics and functions, as well. But there are some properties of kingdom Plantae that they all share. Plants are multicellular organisms that are able to photosynthesize. Since plants can use energy from the Sun to produce food, they are considered autotrophs. Plants lack mobility and often must rely on the wind or animals to help them reproduce through cross pollination. All plants have the same basic parts, including roots, stems, and leaves. Their cells are unique from other eukaryotes because they are surrounded by a rigid cell wall made mostly of cellulose. The cell wall gives plants structure and support, allowing them to grow tall and expose their green leaves to the Sun for photosynthesis.
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STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION
Fungi: Fungi, such as mushrooms, are often confused for plants. They do share some similarities. Most, for example, are multicellular, although yeasts, a type of fungi, are unicellular. Like plants, the cells of fungi have a cell wall, which is usually made of chitin instead of cellulose. Fungi, however, cannot produce their own food through photosynthesis, so they are called heterotrophs. This kingdom has some characteristics that differ from any of the other eukaryotes. A primary difference is that fungi grow long filaments called hyphae.
A mushroom is actually the fruiting body, or reproductive organ, of a fungus.
Many fungi feed by releasing enzymes outside of their bodies. The enzymes break down and digest nearby leaves, fruits, and other substances. Once digested, the molecules of food are absorbed into the fungal body. These enzymes are also important to decomposition. Fungi break down dead, organic matter and return nutrients to the soil. They help maintain the balance with organisms like plants that take nutrients from the soil. Animalia: The animal kingdom is undoubtedly the most well-known because it includes humans. Like all the kingdoms, Animalia is quite diverse. In addition to humans, it includes birds, fish, insects, and a wealth of other animals. What they all have in common is that animals are multicellular, are heterotrophic, and have cells lacking a cell wall. Also, animals are motile at least at some point in their lives. Beyond these characteristics, animals vary greatly in their body plans, reproduction, methods for obtaining food, and many other factors. Scientists in the Spotlight: Lynn Margulis In the 1960s, biologist Lynn Margulis proposed the endosymbiotic theory. This sea sponge is in the same kingdom as humans. She suggested that mitochondria, which convert food into energy in cells, and chloroplasts, which convert sunlight into energy in plant cells, have more in common with prokaryotes than eukaryotes. In fact, she suggests that mitochondria and chloroplasts were once free-living bacteria that evolved a mutualistic relationship with early eukaryotic cells. Over time, the reproductive cycle of the endosymbiont became completely tied to that of the host, and the endosymbionts lost the ability to live outside of the host cell. Support for the endosymbiotic theory is found in the DNA and ribosomes of mitochondria—both of which are similar to that found in Rickettsia, a parasitic endosymbiont. Like mitochondrial DNA, chloroplast DNA is also similar to prokaryotic photosynthetic cyanobacteria.
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STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION
Look Out Anyone who has visited tide pools or snorkeled in the ocean knows that some animals, such as sponges, barnacles, and coral, are fixed in place. However, these organisms all belong to kingdom Animalia, which is characterized by motility. The caveat is that animals are motile at some point in their lives, but not necessarily their entire lives. Adult sponges and coral, for example, are sessile— that is, immobile or fixed in one place. However, as zygotes they have cilia allowing movement through the water to find a preferred location. Barnacles are somewhat similar. They have two larval stages during which they are able to swim through the water using setae, which are hairlike bristles used for movement. In the second larval stage, barnacles cannot take in any food, so they have a limited amount of time to find the best place for their adult form to become fixed in place. Tools for Classifying Organisms Two methods scientists rely on to identify and classify organisms are dichotomous keys and cladograms. These tools help scientists determine how organisms are related through common ancestry. A dichotomous key is a type of flow chart made up of questions or paired statements about an organism. Following each of the steps of a dichotomous key helps scientists identify organisms based on their traits.
What Do You Think? Use the dichotomous key below to identify the fish shown on the right. 1. Is the fish’s body long Yes Go to step 2 and thin? No Go to step 3 2. Does the fish have pointed or rounded fins?
Pointed
Trumpet fish
Rounded
Moray eel
3. Are the eyes on top of the Yes fish’s head? No
Go to step 4
4. Does the fish have a long tail or a short tail?
Long tail
Spotted eagle ray
Short tail
Witch flounder
5. Does the fish have spots? Yes 6. Does the fish have whiskers?
Go to step 5
Go to step 6
No
Glassy sweeper
Yes
Spotted goat fish
No
Bandtail puffer
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STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION
A cladogram is a branched diagram resembling a tree that shows the evolutionary relationship among organisms. It is often used to show how similarities are derived from common ancestry. Places where a lineage branches off in a cladogram are called nodes. They represent speciation events. The fewer the number of nodes between organisms, the more closely they are related. Cladograms provide scientists with a visual summary of how organisms in any taxonomic grouping are related.
What Do You Think? Look at the cladogram below. Which two organisms are more closely related, a hagfish and a lizard or a pigeon and a chimp? How do you know?
What Do You Know? The characteristics used to classify organisms into taxonomic groups help scientists identify and organize living things. With the information gathered, they can construct tools such as cladograms that show the evolutionary relationships of living things, including common ancestry. Fill in the table with information about the three domains. Domain
Bacteria
Kingdom
Eubacteria
Archaebacteria
Cell type Number of Cells
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Plantae
Fungi
Eukaryote
Eukaryote
Eukaryote
Unicellular
Presence of Cell Wall Mode of Food Intake
Protista
Multicellular Yes
Autotroph & heterotroph
Autotroph & heterotroph
Eukaryote Multicellular
Yes
Heterotroph
STEMscopedia: STRUCTURE, FUNCTION, AND
CLASSIFICATION
Connecting With Your Child Taxonomy in the Real World To help students learn more about taxonomy, have them visit a zoo, botanical garden, plant nursery, or even a local pond or stream where they can observe the characteristics of many different organisms. Have students collect the binomial nomenclature of the species they observe where possible. These are commonly posted at zoos, botanical gardens, and some nurseries. Students should gather as much information about the different species’ physical appearances they encounter as possible. Encourage students to use a camera to take pictures of the different species or draw pictures of what they see. At home, have students identify the domain for each of the organisms they observed. Most likely every organism will belong to domain Eukarya. Then have students classify the organisms into kingdoms, phyla, and so on, as far as they can go with the information they gathered. Next, have students research the genus and species names for the organisms. Using this information and online resources, students can check their own classifications against the true taxonomy. Once complete, have students build either a cladogram or a dichotomous key for organisms they observed. Encourage students to analyze the cladogram to determine which characteristics evolved latest and which organisms are most closely related. Here are some questions to discuss with students: 1. How does a standard system of classification help you do research? 2. What characteristics can you use to classify organisms into domains? Into kingdoms? 3. What role would a cladogram play for a scientist who discovers a fossil of an extinct organism?
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