The Living World Question Answers: NCERT Class 11 Biology

G. Bentham and Joseph Dalton Hooker are known for their contribution to the field of plant taxonomy. Natural system of classification for the higher plants was forwardly given by Bentham and Hooker in their book ‘Plantarum’ in which they gave the natural classification of spermatophytes.

For any organism, the population cannot remain same or constant. Population can be more because of favorable condition and can be less because of unfavorable condition in some generation. The kind of organism cannot be constant because evolution is never ending process. 

International Code of Zoological Nomenclature (ICZN) which is used to promote the stability, uniqueness and universality in the scientific names of animals.

Couplet is defined as the pair of contrasting characters. These are used for the identification of specimens and these are shown in the bracketed key.

A monograph is a comprehensive or detailed systematic study of a particular taxonomic group of organisms. It provides work on primary research.

After mitotic division in amoeba, there is no growth in their size and two new organisms formed. Thus, formation of new cells in amoeba is Reproduction.

Metabolism is the chemical reaction that happens in the body’s cells which converts the food into energy; and is also defined as the exchange of materials and energy within the organisms and between the organism and environment. It consists of two process:

The breakdown of organic nutrients into the simple molecules to obtain energy called catabolism and the synthesis of complex substances from the simpler ones which are needed by the cells called as Anabolism. It is a term which is used to define all chemical reaction involved in maintaining the living state of the cells and the organism.

Botanical gardens conserves genetic diversity and also rare and endangered species. In Kew (London) the Royal Botanical Garden is the largest botanical garden in the world. Some of well- known Botanical garden in India are:

1. Indian Botanical Garden, Shibpur, Kolkata
2. Botanical garden of India Republic, Noida
3. Botanical garden of FRI, Dehradun
4. National Botanical garden, Lucknow

Phyllotaxy, venation, inflorescence etc. are some characters in these plants which make them a member of same genus. Capability of interbreeding is the most defining criterion for members of that particular species and breeding between Brinjal and Potato is not possible. Hence, they are kept under two different species. 

All the cell organelles are made up of more or less similar molecules. All the molecules come together to build up cell organelle. The constituents remains same but the function of product changes. Thus, we can say that properties of cell organelles are not always found in the molecular constituents of cell organelles.

A large variety of plants, animals, and microbes found on earth and they differ in size, shape, colour, habitat, and many other characteristics. So, studying each of them is impossible. Therefore, scientists have devised mechanisms to classify all living organisms on certain rules and principles that allow identification, nomenclature, and finally classification of an organism.

For example, based on presence of three pairs of legs and 2 pairs of wings, an organism is identified as an insect, it will be given a scientific name and then grouped with other similar organisms. Thus, various groups or taxon include organisms based on their similarity and differences.

Therefore, the biological classification helps in revealing the relationship between various organisms. It also helps in making study of organisms easy and organized.

Key is a taxonomical aid that helps in identification of plant and animal species. These keys are based on similarities and dissimilarities in characters, generally in a pair called couplet.

Each statement in a taxonomic key is referred to as a lead. For categorizing each taxonomic rank, such as family, genus, species, etc., different keys are used. It is also useful in identification of unknown organisms.

Keys are of two types- indented and bracketed keys. Indented key provides a sequence of choices between two or more statements while in bracketed key, a pair of contrasting characters are used.

(i) Indented key to identify different species of Rhododendron.

1. Leaves evergreen

2. leaves densely hairy below, orange or white hair; flower

appears to have separate petals

………………………. Rhododendron groenlandicum

2. hair absent on leaves, flower has five petals fused in a shallow

tube

………………………. Rhododendron maximus

1. Leaves deciduous

2. pink flowers with two free petals and three fused petals

………………………. Rhododendron canadense

3. white to pink flowers with all petals fused together

(ii) Bracketed key to identify different species of Rhododendron.

1.      Leaves evergreen-----------------------------------------------2

1. Leaves deciduous-----------------------------------------------3

2. Leaves densely hairy below, orange or white hair; flower

appears to have separate petals

……….………………………. Rhododendron groenlandicum

2. Hair absent on leaves, flower has five petals fused in shallow

tube

………………………………..Rhododendron maximus

3. Pink flowers with two free petals and three fused petals

……………………………… Rhododendron canadense

3. White to pink flowers with all petals fused together-----4

The arrangement of various taxa in a hierarchical order is called taxonomic hierarchy.

In this hierarchy, species is present at the lowest level whereas kingdom is present at the highest level.

Classification of a plant

As an example, let us classify Solanum melongen a (Brinjal).

Kingdom – Plantae

Division – Angiospermae

Class – Dicotyledonae

Order – Solanales

Family – Solanaceae

Genus – Solanum

Species – melongen a

Classification of an animal

As an example, let us classify MAN

Kingdom – Animalia

Phylum – Chordata

Class – Mammalia

Order – Primate

Family – Hominidae

Genus – Homo

Species – Sapiens

Millions of plants, animals, and microorganisms found on earth, have been identified by the scientists while many new species are still being discovered around the world. Therefore, to classify these newly discovered species, with new characters, new systems of classification have to be devised every now and then. This creates the requirement to change the existing systems of classification.

To classify a class of forty students, let us start the classification on the basis of sexes of the students. This classification will result in the formation of two major groups- boys and girls. Each of these two groups can be further classified on the basis of the names of the students falling in these groups.

Since it is possible that more than one student can have a particular name, these names can be further suffixed with the surnames.

Since there is still some chance that more than one student can have the same surname, the final level of classification will be based on the roll numbers of each student.

The knowledge of characteristics of an individual or its entire population helps in the identification of similarities and dissimilarities among the individuals of same kind or between different types of organisms. It helps the scientists to classify organisms in various categories depending upon these similarities and dissimilarities.

In binomial system of nomenclature, the generic name of a species always starts with a capital letter whereas the specific name starts with a small letter. Therefore, the correct scientific name of Mango is Mangifera indica.

Each unit or category of classification is termed as a taxon. It represents a rank. For example, the basic level of classification is species, followed by genus, family, order, class, phylum or division, in ascending order. The highest level of classification is known as kingdom. So each of these categories can be called as a taxon.

Both (a) and (c) represent correct sequences of taxonomic categories as the correct hierarchical arrangement of taxonomic categories in ascending order is

Species → Genus → Family → Order → Class → Phylum → Kingdom

In sequence (b), species should have been followed by genus. Therefore, it does not represent the correct sequence.

In biological terms, species is the basic unit of taxonomy. It can be defined as a group of similar organisms capable of interbreeding freely among themselves under natural conditions to produce fertile off springs.

Therefore, a group of similar individuals that are reproductively isolated from other group of individuals form a species.

Species can also be defined as group of individuals that share the same gene pool.

(i) Phylum

Phylum is second highest unit of classification after Kingdom. It includes one or more related classes of animals. In plants, instead of phylum, the term ‘division’ is used.

(ii) Class

Class is a taxonomic group consisting of one or more related orders. For example, the class, Mammalia, includes many orders like Primata (Man), Carnivora(Lim, tiger) etc.

(iii) Family

Family is a taxonomic group containing one or more related genera eg. Family hominidae contains Apes, Monkeys and Man. In plants, families are categorized on the basis of vegetative and reproductive features.

(iv) Order

Order is a taxonomic group containing one or more families. For example, the order, carnivora, includes many families.

(v) Genus

Genus is a taxonomic group including closely related species. For example, the genus, Solanum, includes many species such as nigrum, melongen a, tuberosum, etc.