The Living World: Class 11 Biology NCERT

What Defines 'Living'?

Defining what is 'living' might seem straightforward, but it's a complex task for biologists. Living organisms exhibit several unique characteristics that set them apart from non-living matter. These include growth, reproduction, metabolism, consciousness, and cellular organization. However, it's crucial to differentiate between defining characteristics (present in all living organisms without exception) and non-defining characteristics (which might show exceptions or be present in non-living things as well).

Growth is an increase in mass and number of individuals. In multicellular organisms, growth occurs by cell division. Plants grow continuously throughout their life, while animals grow only up to a certain age. Non-living objects like mountains or sand dunes also grow, but this growth is by accumulation of material on their surface, not from within. Hence, growth cannot be a defining property.

Reproduction is the process by which organisms produce offspring. While a characteristic of living beings, there are exceptions like mules, sterile worker bees, and infertile human couples, who cannot reproduce. Therefore, reproduction also cannot be a defining characteristic of all living organisms.

Metabolism refers to the sum total of all chemical reactions occurring inside a living body. These reactions involve thousands of biochemical processes, from breaking down food to synthesizing complex molecules. All living organisms, from bacteria to humans, exhibit metabolism. Metabolic reactions can occur in vitro (outside the body in test tubes) but are not considered living reactions until they occur within a living system. Thus, metabolism, without exception, is a defining characteristic of living organisms.

Cellular Organisation is another defining characteristic. All living organisms are made up of cells, whether single-celled or multicellular. The cell is the fundamental unit of life.

Consciousness is the ability of an organism to sense its surroundings and respond to environmental stimuli. This includes physical, chemical, and biological factors. All organisms, from the simplest prokaryotes to the most complex eukaryotes, show consciousness. Even a patient in a coma on life support is considered living because they respond to stimuli, even if they don't have self-consciousness. Therefore, consciousness is also a defining property of living organisms.

Biodiversity, Identification, Nomenclature, and Classification

Biodiversity

The total number and types of organisms present on Earth. This refers to the variety of life forms, from microscopic bacteria to giant whales, and their ecosystems. Currently, there are 1.7-1.8 million species known and described.

Identification

The process of determining if a collected organism is new or already known, and if known, establishing its correct identity. This involves comparing its characteristics with known organisms.

Nomenclature

The process of giving a scientific name to an organism. This ensures that a particular organism is known by the same name all over the world, avoiding confusion caused by local names.

Binomial Nomenclature

A system of naming organisms established by Carolus Linnaeus, where each organism has two parts to its scientific name: the generic name (genus) and the specific epithet (species). For example, Mangifera indica for mango.

Classification

The process by which organisms are grouped into convenient categories based on some easily observable characteristics. This helps in studying vast numbers of organisms systematically.

Taxonomy

The branch of biology dealing with the identification, nomenclature, and classification of organisms. It's the scientific study of classifying living organisms.

Systematics

The study of relationships among different kinds of organisms and their diversity, along with all evolutionary relationships. The term 'systematics' was coined by Linnaeus.

Understanding Taxonomic Categories: The Hierarchy of Classification

1. Species (Basic Unit) — The lowest and most specific taxonomic category. A group of individual organisms with fundamental similarities, capable of interbreeding to produce fertile offspring. For example, indica (mango), leo (lion), tuberosum (potato) are specific epithets within different genera.
2. Genus — A group of related species which has more characters in common as compared to species of other genera. For example, Panthera includes lion (Panthera leo), tiger (Panthera tigris), and leopard (Panthera pardus).
3. Family — A group of related genera, sharing even fewer similarities compared to genus and species. For example, the genus Panthera (lion, tiger) is placed in Family Felidae (cats), along with genus Felis (domestic cat).
4. Order — An assemblage of families exhibiting a few similar characters. For example, the families Felidae (cats) and Canidae (dogs) are included in the Order Carnivora.
5. Class — Comprises related orders. For example, Order Carnivora (mammals that eat meat) and Order Primates (monkeys, gorillas, humans) are placed in Class Mammalia.
6. Phylum (for Animals) / Division (for Plants) — Includes various classes with some common features. For example, Class Mammalia along with Pisces, Amphibia, Reptilia, and Aves are placed in Phylum Chordata (animals with a notochord).
7. Kingdom (Highest Category) — The highest taxonomic category, representing the largest group of organisms. All animals are grouped under Kingdom Animalia, and all plants under Kingdom Plantae. As we go higher from species to kingdom, the number of common characteristics decreases.

Taxonomic Aids: Tools for Study

Taxonomic aids are essential tools and techniques used by biologists for the identification, naming, and classification of organisms. They provide a systematic way to study the vast diversity of life. These aids are crucial for agricultural, forestry, industry, and for generally knowing our bio-resources and their diversity. Understanding these aids is vital for practical biology.

Herbarium: This is a storehouse of collected plant specimens that are dried, pressed, and preserved on sheets. These sheets are arranged according to a universally accepted system of classification. Herbaria serve as quick reference systems in taxonomical studies. They provide information about the specimen, like date and place of collection, English, local, and botanical names, family, collector's name, etc.

Botanical Gardens: These are specialised gardens that have collections of living plants for reference. Plant species in these gardens are grown for identification purposes and each plant is labeled indicating its botanical/scientific name and its family. Famous botanical gardens include the Royal Botanical Garden at Kew (England), Indian Botanical Garden at Howrah (India), and National Botanical Research Institute at Lucknow (India).

Museums: Biological museums are generally set up in educational institutes such as schools and colleges. They have collections of preserved plant and animal specimens for study and reference. Specimens are preserved in containers or jars in preservative solutions. Larger animals like birds and mammals are usually stuffed and preserved. Insect collections are pinned after collecting, killing, and pinning. Skeletons of animals are also found in museums.

Zoological Parks (Zoos): These are places where wild animals are kept in protected environments under human care, allowing us to learn about their food habits and behavior. All animals in a zoo are provided with conditions as similar as possible to their natural habitats.

Key: A taxonomic key is another aid used for identification of plants and animals based on similarities and dissimilarities. The keys are based on contrasting characters generally in a pair called a couplet. Each statement in the key is called a lead. By making a choice between two contrasting characters, the organism is identified. Keys are generally analytical in nature.

Other aids include Manuals (provide information for identification of species in an area), Monographs (contain comprehensive information on any one taxon), and Catalogues (alphabetical list of species with brief description).

Exam Tips & Common Mistakes

To ace this chapter, pay close attention to these points:

1. Defining vs. Non-Defining Characteristics: This is a frequently tested concept. Remember: Cellular organisation, Metabolism, and Consciousness are defining properties of living organisms. Growth and Reproduction are not defining properties due to exceptions. Be ready to justify why.
2. Binomial Nomenclature Rules: Understand and recall the rules: scientific name has two parts (genus and species), written in Latin and italicized when printed (underlined separately when handwritten), genus name starts with a capital letter, species name with a small letter. For example, Mangifera indica.
3. Taxonomic Hierarchy: Memorize the correct sequence: Kingdom → Phylum/Division → Class → Order → Family → Genus → Species. Practice examples for each category.
4. Purpose of Taxonomic Aids: Don't just list them; understand why each aid is used and what kind of information it provides. For instance, a Herbarium stores dried plant specimens, while a Botanical Garden has living ones. A Key helps in identification through contrasting characters.
5. Scientific Names: Be familiar with a few examples of scientific names for common organisms (e.g., Mango, Human, Housefly, Wheat) as they are often asked in multiple-choice or short answer questions.

Practice Questions with Solutions

• Q: Why is growth not considered a defining characteristic of living organisms? A: Step 1: Recall the definition of growth for living organisms and non-living objects. Step 2: Living organisms grow by internal cellular processes (increase in mass and number of cells). Non-living objects (like mountains, sand dunes) can also increase in size, but this is due to the accumulation of material externally. Step 3: Consider that not all living organisms exhibit continuous growth (e.g., animals grow only up to a certain age). Final answer: Growth is not a defining characteristic because non-living objects can also grow by external accumulation of material, and it is not an intrinsic property from within. Moreover, growth is not continuous in all living organisms.
• Q: State the two components of a scientific name according to binomial nomenclature and mention two rules for writing them. A: Step 1: Identify the components of a scientific name. Step 2: Recall the rules for binomial nomenclature. Step 3: The two components are the generic name (genus) and the specific epithet (species). Final answer: The two components are the Generic name (Genus) and the Specific epithet (Species). Two rules are: 1. Biological names are generally in Latin and are written in italics (or underlined separately when handwritten). 2. The first word (genus) starts with a capital letter, while the second word (species) starts with a small letter.
• Q: Arrange the following taxonomic categories in ascending order: Class, Family, Genus, Order, Species, Kingdom, Phylum. A: Step 1: Recall the hierarchical arrangement of taxonomic categories. Step 2: Start from the most specific (lowest) category and move towards the broadest (highest) category. Final answer: Species → Genus → Family → Order → Class → Phylum → Kingdom.
• Q: Differentiate between a Herbarium and a Botanical Garden, highlighting their primary purpose. A: Step 1: Define a Herbarium and its contents. Step 2: Define a Botanical Garden and its contents. Step 3: Compare their primary functions or the type of specimens they house. Final answer: A Herbarium is a storehouse of dried, pressed, and preserved plant specimens on sheets, serving as a quick reference system for taxonomical studies. Its primary purpose is to preserve dead specimens for scientific reference. A Botanical Garden is a specialized garden that has collections of living plants for reference. Its primary purpose is to grow living plant species for identification and study, often for public education and conservation.
• Q: Why is a taxonomic key considered analytical in nature? A: Step 1: Recall the structure and function of a taxonomic key. Step 2: Understand how identification is achieved using a key. Final answer: A taxonomic key is considered analytical because it involves making a choice between two contrasting characters at each step (a couplet). This process of systematically analysing and choosing between options, with each choice leading to the acceptance of one statement and rejection of another, ultimately helps in the identification of the organism. This step-by-step analysis makes it analytical.

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