CBSE Class 7 Science Advanced Biology Basis of Classification Notes Set 01

Basis of Classification

Introduction

Earth is home to millions of kinds of living things, and many are still waiting to be found. They differ in body structure, in the place where they live (habitat), in the way they get food, and in many other features. They range from microscopic bacteria to large plants and animals. Studying each one separately is impossible, so we need an orderly method of grouping them. This is why classification is needed.

Need for Classification

Biological classification means sorting living organisms into groups on the basis of how much they resemble one another. Studying organisms group by group is far easier than studying them one at a time. The main benefits are given below.
1. It makes the study of the huge variety of organisms on earth manageable.
2. It shows the similarities and differences between organisms.
3. It helps us find out whether two different organisms share a common origin.

Taxonomy

Taxonomy is the branch of biology that deals with identifying organisms, grouping them and giving them proper scientific names. Scientists who work in this field are called taxonomists. Carolus Linnaeus is called the Father of Taxonomy.

Binomial Nomenclature

Nomenclature means the naming of organisms. In binomial nomenclature every organism gets a two-part name: the genus name comes first and the species name comes second. For example, the scientific name of human beings is Homo sapiens, in which Homo is the genus name and sapiens is the species name.

Table 1.1 Scientific names of some common plants and animals

Universal Rules of Nomenclature

1. The genus name begins with a capital letter.
2. The species name begins with a small letter.
3. In handwriting, the genus name and the species name are underlined separately.
4. In print (typed text), the whole name is written in italics.

Hierarchy of Classification

Arranging organisms in a series of steps, from small groups to large groups, is called the hierarchy of classification. There are seven levels (taxa), described here from the lowest to the highest.
1. Species: the lowest level. It contains organisms that are alike in their characters and can breed with one another. All domestic cats, for example, belong to one species, catus.
2. Genus: a group of species that share some common characters. The lion and the tiger belong to the same genus, Panthera.
3. Family: a group of related genera. The lion, tiger and cat belong to the family Felidae.
4. Order: a group of related families. The lion, tiger, cat and wolf belong to the order Carnivora.
5. Class: a group of related orders. The orders Carnivora and Primates (which includes humans) both fall in the class Mammalia.
6. Phylum: a group of related classes. The phylum Chordata includes the classes Mammalia (humans), Aves (birds), Reptilia (reptiles) and Pisces (fishes).
7. Kingdom: the highest level, made of related phyla. All animals belong to the kingdom Animalia.

Table 1.2 Examples of each level of classification

History of Classification

One of the oldest classification systems was given by the Greek scientist Aristotle. He divided living things into two groups, plants and animals, and then subdivided each group.
1. Plants were grouped by their stem into herbs (no woody stem), shrubs (small woody stem) and trees (big woody stem).
2. Animals were grouped by the place where they live into aerial animals (live in air), aquatic animals (live in water) and terrestrial animals (live on land).

Two-kingdom Classification

Linnaeus put forward the two-kingdom system, which placed all organisms in either kingdom Plantae or kingdom Animalia. The two groups were separated on the basis of how the organisms get their food and whether they can move.

Five-kingdom Classification

R.H. Whittaker proposed the five-kingdom system. Its kingdoms are Monera, Protista, Fungi, Plantae and Animalia.

Whittaker used three characters to place organisms in these kingdoms.
1. Cellular organization (the type of cell)
2. Body organization (how many cells make up the body)
3. Mode of nutrition (how the organism gets food)

Cellular Organization

The cell is the basic structural and functional unit of life, that is, the building block of every organism. The nucleus is the control centre of the cell. It holds the DNA (genes), which decides the characters of an organism. Depending on whether a true nucleus is present, organisms fall into two groups.
1. Prokaryotes
2. Eukaryotes

Fig. 1.5 Structure of a typical prokaryotic (bacterial) cell and a eukaryotic (animal) cell

Prokaryotes and Eukaryotes

Prokaryotes. The word comes from the Greek 'pro' (before) and 'karyon' (nucleus). Prokaryotes are mostly single-celled and microscopic, for example bacteria. Their genetic material is not enclosed in a nucleus; it lies free in a region of the cell called the nucleoid. Some prokaryotes survive very harsh conditions. The thermophiles, for example, live in hot water springs, where water heated by the earth's interior comes out at temperatures of roughly 40 °C to 120 °C. Prokaryotes are believed to be the first form of life on earth.

Eukaryotes. The word comes from the Greek 'eu' (true) and 'karyon' (nucleus). Their cells have a clearly defined nucleus, and the genetic material is kept inside it, surrounded by a nuclear membrane. Fungi, humans, cats, whales, mosses and flowering plants are all eukaryotes.

Viruses are neither prokaryotes nor eukaryotes.

Table 1.3 Differences between prokaryotic and eukaryotic cells

Body Organization

Some organisms have a body of just one cell, while others have millions of cells. On this basis organisms are placed in two groups, unicellular and multicellular.

Table 1.4 Differences between unicellular and multicellular organisms

Mode of Nutrition

Organisms can also be grouped by the way they obtain food, as autotrophs or heterotrophs.

Table 1.5 Differences between autotrophs and heterotrophs

Five Kingdoms and Their Characteristics

Kingdom Monera
1. Contains all the bacteria.
2. Unicellular.
3. Prokaryotic.
4. Some bacteria make their own food (autotrophic) and others depend on ready-made food (heterotrophic).

Kingdom Protista
1. Contains organisms such as Amoeba and Paramoecium.
2. Unicellular.
3. Eukaryotic.
4. Some members are autotrophic and some are heterotrophic.

Kingdom Fungi
1. Contains yeast, mushrooms, bread mould and similar organisms.
2. Some are unicellular and some are multicellular.
3. Eukaryotic.
4. All are heterotrophic. They are of two kinds: saprophytes, which feed on dead and decaying matter, and parasites, which take food from the body of a host organism.
5. Yeast is a fungus, but it is single-celled and eukaryotic.

Kingdom Plantae
1. Includes all plants that can carry out photosynthesis.
2. Multicellular.
3. Eukaryotic.
4. Autotrophic.

Kingdom Animalia
1. Includes multicellular animals.
2. Eukaryotic.
3. Heterotrophic.

Table 1.6 Characteristic features of the five kingdoms

Fig. 1.12 Classification of living organisms

Key Points to Remember

1. Classification groups organisms by similarity and shows whether they share an origin.
2. Taxonomy deals with identifying, grouping and naming organisms; Linnaeus is the Father of Taxonomy.
3. Binomial nomenclature gives each organism a genus name followed by a species name.
4. Linnaeus proposed two kingdoms (Animalia and Plantae); Whittaker proposed five kingdoms.
5. The cell is the basic unit of life, and DNA in the nucleus decides the characters of an organism.
6. The seven levels of classification, from lowest to highest, are species, genus, family, order, class, phylum and kingdom.