...................... The classification of living things. Viruses and bacteri

Introduction. When we visit well preserved natural areas we can see that the organisms belong to a large number of different species. It is said that these places have a high biodiversity. The opposite happens in a city or in a breeding, in which individuals are all of the same species or belong to a few species.

The creation of a new species, form an existing one is a process that requires hundreds of thousands of years of evolution. In an evolutionary perspective is believed that biodiversity is the response for living beings, through adaptive evolution of the species, to the multitude of environments that have appeared throughout the history of life on Earth. Each species has a large number of characteristics, often unique, that humans sometimes have taken advantage of for our feed or heal our sicknesses. Therefore it is worth maintaining large natural areas, where they all of the species can continue to live.

As there are over 3.000.000 of living species, to facilitate its study they were grouped into five different kingdoms. The following chapter describes these kingdoms and study the two most simple living structures viruses and bacteria.

Activities to do. Read the explanations on the classification of organisms and perform Multiple Choice Test 11.1. Then read the text on viruses and bacteria and perform Multiple Choice Test 11.2. Crossword 11 and Complete Sentences 11.

1. Concept of species. A species is a group of organisms that can interbreed and result in descandants that are also fertile.

2. Concept of biodiversity. Biodiversity is defined as the set of species that live or have lived on Earth. One way to measure the biodiversity of an area is to calculate the probability that two randomly picked organisms belong to different species. If the probability is high, it is said that the biodiversity of the area is high. It happens for example in a coral reef and in a virgin jungle. If the probability is low, it is said that the biodiversity of the area is low. It happens for example in a wheat fields or in an anthill.

3. History of the classifications of the organisms. Because of the high number of discovered species it has become necessary to group them for easier study. For example to find out if an individual is of a kind or another very similar, similar species should all be grouped together.

The main classifications, proposed throughout history are:

1. The classification of Aristotle. 2000 years ago, the Greek philosopher believed that there were only a few hundred of species. He classified the into two kingdoms: the Kingdom of animal and plant Kingdom, In addition, the animals were divided into two groups according together they had red blood or not.

2. The classification of Linnaeus. Linnaeus was a great Swedish botanist who lived between 1707 and 1778. He established a hierarchy of groups called taxa. In it, each top-level group includes one or several lower-level groups. He also divided the binominal nomenclature of the species, which is explained below.

• Taxon. By taxon is called any group of organisms within a hierarchical classification of living things.

• Types of taxa. From superior to inferior order:

• Kingdom
• Phylum (or Division)
• Class
• Order
• Family
• Genus
• Species

A Kingdom includes several phyla, a Phylum includes several Classes and so on.

• Taxonomy. It is a specialty in Biology that deals with establishing the different taxa.

• Systematic. It is a specialty in Biology that deals with the different taxa hierarchical-ly grouped in the manner considered most logical. It is currently considered, that the best thing is, to group together taxa that have common ancestors.

• Binomial nomenclature. It is a way of referring to each species by two Latin names: the first name is the genus name and the second name is the species name. The name of the genus and species have to be written in cursive if it is typed and underlined if it is a manuscript. These two names always have to be written in lower case except the first letter of the genus that has to be capitalize. For example, the scientific name is written as wolf: Canis lupus (the genus is Canis and species is lupus).

3 . The classification of the five kingdoms. Is the current classification. The approach taken has been to group together species with common ancestors (evolutionary relatedness).

• The first criteria that is considered is whether the cells have a nucleus (eukaryotes) or not (prokaryotes)
• The second criteria taken into account is whether they are unicellular or multicellular.
• The third criteria considered is whether if its cells form tissues (living tissue) or not (thallophytic beings).
• The quart criteria taken into account is whether if they feed upon inorganic matter (autotrophs) through photosynthesis or organic matter (heterotrophs beings) by digesting.
• The fifth criteria that is considered is whether the digestion is done by outside segregating digestive juices (external digestion) or a cavity or digestive tube interior (internal digestion).

(In green background appear only taxa that is nurtured by photosynthesis)

4. Viruses.

Viruses are living structures formed by one or more nucleic acidmolecules molecules (DNA or RNA, never both at once), surrounded by a protein coat called capsid. Some are hold also by a membranous envelope. Fail to have a cell complexity since the lack of a set of enzymes in its interior are able to perform the function of nutrition. Because of this they are considered a cellular lifestyle. Are obligate parasites of other living things, have to reproduce since entering their cells and use their ribosomes and their molecular reserves. They are very small, more than 10 times smaller than bacteria, therefore, in general, only visible under the electron microscope. We need to remember that eukaryotic cells usually measure 0.02 mm and are 10 times larger than prokaryotic cells and that these are usually 10 times larger than viruses.

Bacteria is an unicellular prokaryotic organism. They are formed of a single cell without a nucleus. Its deoxyribonucleic acid (DNA) is not surrounded by a membrane forming a core, but is more or less condensed in a region called the cell cytoplasm nucleoid or false core. They are very simple cells. From outside to inside we can distinguish the following structures: the bacterial capsule (outer mucus layer that may be missing), the bacterial wall (rigid layer that sometimes supports flagella which are very simple), the plasma membrane and the cytoplasm. Within this can be differentiate DNA, ribosomes, mesosomes (exclusive of these organelles cells) and inclusions.

Their size is very small, about 10 times less than the current of a eukaryotic cell. It only presents four types of shapes: coccid (spherical), bacilli (rods), vibrio (comma-shaped spelling) and spiral (spiral).

Multiple Choice Test

Crossword.