NCERT-Class-11-Biology

Introduction to Diversity in the Living World

• Definition of Biology: Biology is the science of life forms and living processes. The living world comprises an amazing diversity of living organisms.

• Historical Perception: Early man perceived the difference between inanimate matter (wind, sea, fire) and living organisms. He deified some inanimate objects and some animals/plants. A common feature of such early forms of anthropocentric view of biology was the sense of awe or fear they evoked.

• Systematic and Monumental Description: This began out of necessity and led to detailed systems of identification, nomenclature, and classification.

• Evolutionary Perspective: The biggest spin-off of classification studies was the recognition of the sharing of similarities among living organisms both horizontally (contemporary) and vertically (evolutionary history). The realization that all present-day living organisms are related to each other and to all organisms that ever lived on earth was a revelation that led to conservation movements for biodiversity.

Ernst Mayr (1904–2005)

• Background: Born on July 5, 1904, in Kempten, Germany. He was the Harvard University evolutionary biologist who has been called ‘The Darwin of the 20th Century’.

• Scientific Standing: He was one of the 100 greatest scientists of all time. He joined Harvard’s Faculty of Arts and Sciences in 1953 and retired in 1975, assuming the title Alexander Agassiz Professor of Zoology Emeritus.

• Research Scope: His research spanned ornithology, taxonomy, zoogeography, evolution, systematics, and the history and philosophy of biology.

• Key Contributions: He almost single-handedly made the origin of species diversity the central question of evolutionary biology that it is today. He pioneered the currently accepted definition of a biological species.

• Awards: Mayr was awarded the three prizes widely regarded as the ‘Triple Crown of Biology’:     * The Balzan Prize in 1983.     * The International Prize for Biology in 1994.     * The Crafoord Prize in 1999.

• Death: He died at the age of 100 in the year 2004.

Defining Characteristics of Living Organisms

Growth

• Parameters: Increase in mass and increase in number of individuals are twin characteristics of growth.

• Multicellular Organisms: Grow by cell division. In plants, this growth by cell division occurs continuously throughout their life span. In animals, growth is seen only up to a certain age, though cell division occurs in certain tissues to replace lost cells.

• Unicellular Organisms: Grow by cell division. One can easily observe this in in-vitro cultures by simply counting the number of cells under the microscope.

• In Higher Organisms: Growth and reproduction are mutually exclusive events.

• Non-Living Growth: Non-living objects also grow if we take increase in body mass as a criterion (e.g., mountains, boulders, and sand mounds). However, this kind of growth in non-living objects is by accumulation of material on the surface. In living organisms, growth is from inside.

• Conclusion: Growth cannot be taken as a defining property of living organisms. Conditions under which it is observed in all living organisms must be explained.

Reproduction

• Multicellular Organisms: Refers to the production of progeny possessing features more or less similar to those of parents.

• Asexual Reproduction:     * Fungi multiply and spread easily due to the millions of asexual spores they produce.     * In lower organisms like yeast and hydra, we observe budding.     * In Planaria (flatworms), we observe true regeneration where a fragmented organism regenerates the lost part of its body and becomes a new organism.     * Fungi, filamentous algae, and protonema of mosses all easily multiply by fragmentation.

• Unicellular Organisms: In unicellular organisms like bacteria, unicellular algae, or Amoeba, reproduction is synonymous with growth (increase in numbering of cells).

• Exceptions: There are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc.).

• Conclusion: Reproduction cannot be an all-inclusive defining characteristic of living organisms.

Metabolism

• Chemical Context: All living organisms are made of chemicals. These chemicals, small and big, belonging to various classes, sizes, and functions, are constantly being made and changed into some other biomolecules.

• Definition: These conversions are chemical reactions or metabolic reactions. The sum total of all the chemical reactions occurring in our body is metabolism.

• Non-living Context: No non-living object exhibits metabolism.

• In-vitro Reactions: Metabolic reactions can be demonstrated outside the body in cell-free systems. An isolated metabolic reaction outside the body of an organism, performed in a test tube, is neither living nor non-living.

• Conclusion: While metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in-vitro are not living things but are surely living reactions.

Consciousness/Response to Stimuli

• Sensitivity: The ability of all living organisms to sense their surroundings or environment and respond to these environmental stimuli which could be physical, chemical, or biological.

• Mechanisms: We sense our environment through our sense organs. Plants respond to external factors like light, water, temperature, other organisms, pollutants, etc.

• Breadth: All organisms, from prokaryotes to the most complex eukaryotes, can sense and respond to environmental cues.

• Photoperiodism: This affects reproduction in seasonal breeders, both plants and animals.

• Human Specificity: Human beings are the only organisms who are aware of themselves (self-consciousness).

• Conclusion: Consciousness is the defining property of living organisms.

Diversity in the Living World and Taxonomy

• Biodiversity: The number and types of organisms present on earth. The number of species that are known and described range between $1.7-1.8\,\text{million}$.

• Nomenclature: There is a need to standardize the naming of living organisms such that a particular organism is known by the same name all over the world. Nomenclature is only possible when the organism is described correctly.

• Identification: Determining what organism is attached to a specific name.

• International Codes:     * ICBN: International Code for Botanical Nomenclature (for plants).     * ICZN: International Code of Zoological Nomenclature (for animals).

• Binomial Nomenclature:     * Introduced by Carolus Linnaeus.     * Each name has two components: the Generic name and the Specific epithet.     * Example: Scientific name of Mango is Mangifera indica.

• Universal Rules of Nomenclature:     1. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.     2. The first word represents the genus while the second component denotes the specific epithet.     3. Both the words in a biological name, when handwritten, are separately underlined, or printed in italics to indicate their Latin origin.     4. The first word denoting the genus starts with a capital letter while the specific epithet starts with a small letter (e.g., Mangifera indica).     * The name of the author appears after the specific epithet in an abbreviated form, e.g., Mangifera indica Linn. (indicated it was first described by Linnaeus).

Taxonomic Categories

• Classification: The process by which anything is grouped into convenient categories based on some easily observable characters. The scientific term for these categories is Taxa.

• Taxonomy: Based on characteristics, all living organisms can be classified into different taxa. Internal and external structure, along with the structure of cell, development process, and ecological information are essential for modern taxonomic studies.

• Characterisation, Identification, Classification, and Nomenclature: The processes basic to taxonomy.

• Systematics: Humans were interested in knowing more about different kinds of organisms and their diversities, but also the relationships among them. This branch of study is systematics. Derived from Latin "systema", meaning systematic arrangement of organisms. Linnaeus used Systema Naturae as the title of his publication. Systematics takes into account evolutionary relationships.

The Taxonomic Hierarchy

1. Kingdom: Highest category (e.g., Kingdom Animalia).

2. Phylum / Division: Phylum for animals, Division for plants (e.g., Chordata).

3. Class: (e.g., Mammalia).

4. Order: (e.g., Primata, Carnivora).

5. Family: Group of related genera with less similarity (e.g., Solanaceae which includes Solanum, Petunia, Datura; Felidae includes Felis and Panthera).

6. Genus: Group of related species which has more characters in common in comparison to species of other genera (e.g., Solanum includes potato, tomato, brinjal; Panthera includes lion, leopard, tiger).

7. Species: Group of individual organisms with fundamental similarities. The lowest category.

• Trend: As we go higher from species to kingdom, the number of common characteristics goes on decreasing. The higher the category, the greater is the difficulty of determining the relationship to other taxa at the same level.

Taxonomical Aids

Herbarium

• A storehouse of collected plant specimens that are dried, pressed, and preserved on sheets.

• Sheets are arranged according to a universally accepted system of classification.

• Labels include: Date and place of collection, English, local, and botanical names, family, collector’s name, etc.

• Serves as a quick referral system in taxonomical studies.

Botanical Gardens

• Specialized gardens that have collections of living plants for reference.

• Plants are grown for identification and labeled with botanical/scientific names and family.

• Famous gardens: Kew (England), Indian Botanical Garden (Howrah, India), National Botanical Research Institute (Lucknow, India).

Museums

• Biological museums are generally set up in educational institutes like schools and colleges.

• Collections of preserved plant and animal specimens for study and reference.

• Specimens are preserved in containers or jars in preservative solutions. Some are preserved as dry specimens.

• Insects are preserved in insect boxes after collecting, killing, and pinning.

• Larger animals (birds, mammals) are usually stuffed and preserved.

• Museums often have collections of skeletons as well.

Zoological Parks (Zoos)

• Places where wild animals are kept in protected environments under human care.

• Enables us to learn about their food habits and behavior.

• Conditions are provided that mimic their natural habitats as much as possible.

Key

• Used for identification of plants and animals based on similarities and dissimilarities.

• Based on contrasting characters, usually in a pair called a couplet.

• It represents the choice made between two opposite options. This results in acceptance of only one and rejection of the other.

• Each statement in the key is called a lead.

• Separate taxonomic keys are required for each taxonomic category such as family, genus, and species for identification purposes.

• Keys are generally analytical in nature.

Flora, Manuals, Monographs, and Catalogues

• Flora: Contains the actual account of habitat and distribution of plants of a given area. Provides the index to the plant species found in a particular area.

• Manuals: Useful in providing information for identification of names of species found in an area.

• Monographs: Contain information on any one taxon.

Biological Classification

Early Systems

• Aristotle was the earliest to attempt a more scientific basis for classification. He used simple morphological characters to classify plants into trees, shrubs, and herbs. He divided animals into two groups: those which had red blood and those that did not.

• Two Kingdom System: Linnaeus developed a system with Plantae and Animalia. It did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, or photosynthetic (green algae) and non-photosynthetic (fungi) organisms.

Five Kingdom System

• Proposed by R.H. Whittaker (1969).

• The kingdoms defined were: Monera, Protista, Fungi, Plantae, and Animalia.

• Main criteria: Cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships.

• Evolutionary perspective: It looks at morphological, physiological, and reproductive similarities, but also phylogenetic (evolutionary) relationships.

Kingdom Monera

• General Characteristics: Bacteria are the sole members. They are the most abundant micro-organisms. They occur almost everywhere (hundreds in a handful of soil). They live in extreme habitats: hot springs, deserts, snow, and deep oceans. Many are parasites.

• Shapes:     * Spherical: Coccus (pl.: cocci).     * Rod-shaped: Bacillus (pl.: bacilli).     * Comma-shaped: Vibrium (pl.: vibrio).     * Spiral: Spirillum (pl.: spirilla).

• Metabolism: As a group, they show the most extensive metabolic diversity. Most are heterotrophs (depend on other organisms or dead organic matter). Some are autotrophs (photosynthetic or chemosynthetic).

• Archaebacteria: Live in harsh habitats. Halophiles (salty areas), Thermoacidophiles (hot springs), and Methanogens (marshy areas). Methanogens are present in the gut of several ruminant animals (cows, buffaloes) and produce methane (biogas) from dung.

• Eubacteria (True Bacteria): Characterized by a rigid cell wall and, if motile, a flagellum.     * Cyanobacteria (Blue-green algae): Have chlorophyll a similar to green plants. Unicellular, colonial, or filamentous, freshwater/marine or terrestrial. Colonies are generally surrounded by a gelatinous sheath. Often form blooms in polluted water. Some can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc and Anabaena).     * Chemosynthetic Autotrophs: Oxidize various inorganic substances (nitrates, nitrites, ammonia) and use released energy for ATP production. Play a great role in recycling nutrients (nitrogen, phosphorus, iron, sulfur).     * Heterotrophic Bacteria: Most abundant in nature. Majority are decomposers. Helpful in making curd from milk, production of antibiotics, fixing nitrogen in legumes, etc. Some are pathogens causing cholera, typhoid, tetanus, citrus canker.

• Reproduction: Mainly by fission. Under unfavorable conditions, they produce spores. Also reproduce by a sort of sexual reproduction by adopting a primitive DNA transfer.

• Mycoplasma: Organisms that completely lack a cell wall. Smallest living cells known. Can survive without oxygen. Many are pathogenic in animals and plants.

Kingdom Protista

• General Characteristics: All single-celled eukaryotes. Primarily aquatic. Links with Plantae, Animalia, and Fungi. Reproduce asexually and sexually (cell fusion and zygote formation).

• Chrysophytes: Includes diatoms and golden algae (desmids). Found in freshwater and marine environments. Microscopic and float passively in water currents (plankton). Most are photosynthetic.     * Diatoms: Cell walls form two thin overlapping shells (like a soap box) embedded with silica, making them indestructible. Over billions of years, deposits of cell wall form "diatomaceous earth,"