Today we are looking at the following learning objective:
3.2.8 Classification is a means of organising the variety of life based on relationships between organisms and is built round the concept of species.
Read the article below then answer the comprehension questions underneat the article.
The current system owes it genesis to the father of modern classification or “taxonomy”, a Swede named Carl von Linné (1707-1778). He is better known by the Latinized name he adopted, Carolus Linnaeus. Linnaeus introduced the hierarchy from broadest to most specific which today with some amendment consists of a “kingdom”, “phylum” or “division”, “class”, “order”, “family”, “genus”, and “species”.
From Aristotle’s time until at least the mid-twentieth century, there were two kingdoms of life recognized—Plants and Animals (or Plantae and Animalia in Latin). However, starting in the late nineteenth century, this number began to be questioned with regard to the simpler life forms which were often not easily classified into one of the two kingdoms. A current approach now argued for is for five kingdoms—Prokarya or Bacteria, Animalia, Plantae, Fungi and Protoctista.
The Kingdom Prokarya or Bacteria are distinguished from the life forms in all other kingdoms in that they do not have a membrane bound nucleus containing the genetic material of the cell. They are called “prokaryotes”. The genetic material is simply found in strands (“plasmids”) within the cell’s cytoplasm. Note that what was previously called blue green algae are now classified as cyanobacteria because they are prokaryotes. Since they are so different from all other life, under the five kingdom system, Bacteria also comprise the Superkingdom Prokarya.
Kingdom Protoctista is the catch-all kingdom for everything that does not fit into the other four. It is comprised of many microscope organisms that are of great interest to this group (as well as some macroscopic organisms). These include protozoa (or protista under the more modern name) and algae but also such diverse organisms as slime molds and slime nets. Although we often think of this group from its microscopic members, it is also comprised of some large organisms such as giant kelps that can grow as much as 10 meters (over 30 feet).
The cells of life forms in the other four kingdoms are classified as “eukaryotes” and have a nucleus in which the genetic material is organized on “chromosomes” within a cellular nucleus. These four kingdoms comprise the Superkingdom Eukarya. Besides the presence or absence of a nucleus, there are other major differences between prokaryotes and eukaryotes. For example, Bacteria are all over the map in whether they utilize oxygen or another gas such as nitrogen or methane. Some cannot even tolerate oxygen—for these “anaerobic” Bacteria, oxygen is a poison. Almost all eukaryotes are aerobes—they need oxygen to live. That some Bacteria require an oxygen-free environment harkens back to the earth’s earliest times and suggest their ancient origin.
The Kingdom Animalia is comprised of multi-celled organisms which develop
from an embryo resulting from the fertilization of an egg by a much smaller
sperm. However, even among the vertebrate animals, there is an exception to
sexual reproduction that makes the definition slightly less than a 100%
accurate. A species of lizard of the genus Cnemidophorus reproduces by
parthenogenesis—no males or sperm required. Yet I think everyone would
accept that this lizard is an animal (this lizard being one exception that
proves the rule—there are other a few other parthenogenetic animals).
Animals also share the characteristic that most must ingest or eat other living
or decayed organic matter as food to live (or live as parasites or symbionts
off of the nutrients provided by other living things) (although this trait is
also shared with some of the members of the Kingdom Protoctista).
The Kingdom Plantae is composed of multi-celled organisms that grow from embryos that are usually the result of sexual fusion of a male and female cell. Again there are exceptions although somewhere in every plant’s past, there were sexual forbears. Most plants (but again not all) plants engage in photosynthesis—that complicated and almost miraculous process whereby the energy of sunlight is used by the plant to produce carbohydrates and gaseous O2from H2O and CO2. As a result, plants are the great producers of life. Plants generally have a rigid cell wall composed of cellulose. They are non-motile (the entire organism does not move about under its own energy) but some produce motile cells.
The Kingdom Fungi is comprised of non-motile cells that have cell walls made of chitin (the same hard stuff that the outer bodies of insects are made of) and not cellulose. Therefore, some argue that fungi are more closely related to animals than plants. Fungi develop from spores without any embryonic stage. They digest other living things outside their bodies by releasing enzymes and then absorbing the product.
How do you decide how to classify organisms into each of the narrowing categories? The classic method used was through visual (and later microscopic) identification of the form of the organism. At the higher or more general levels of classification, such distinctions are easier to make (but not always easy). They become increasingly more difficult as you become more specific. For example, how do you decide an individual organism is a member of one species and not another? The classic definition of a “species” is related organisms that share common characteristics and are capable of interbreeding. This works fine for “higher” (from our homocentric viewpoint) life forms such as Homo sapiens, the species to which you and I belong (or at least I do). We look significantly different than chimpanzees and cannot mate with them to produce offspring despite being closely related.
The problem is that this definition breaks down or is more difficult to implement in the “lower” life forms which microscopists are particularly interested in, depending on what one means by “interbreeding”. For example, all different types of bacteria, although they do not reproduce sexually, are notoriously “promiscuous” about exchanging genetic material between species (such as how to become resistant to antibiotics). Further, different species (and even genera) of microorganisms look confusingly similar.
Here are some comprehension questions that we will discuss:
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