The Living World
1. Characteristics of Living Organisms
Biology is the scientific study of life forms and their processes. The Earth’s living world showcases an astonishing variety of organisms. Historically, humans easily distinguished between inanimate matter and living organisms. Early humans often revered inanimate objects like wind, sea, and fire, as well as certain animals and plants, due to the awe or fear they inspired. The systematic description of living organisms, including humans, developed much later in history. Societies that indulged in an anthropocentric (human-centered) view of biology registered limited progress in biological knowledge. The necessity to describe life forms systematically led to the development of detailed systems for identification, nomenclature (naming), and classification. The most significant outcome of these studies was the recognition of shared similarities among living organisms, both horizontally (among contemporary species) and vertically (across generations and evolutionary lineages). The revelation that all present-day living organisms are related to each other, and also to all organisms that ever lived on this Earth, humbled humanity and led to cultural movements advocating for the conservation of biodiversity. In the following sections, we will delve into a description, including classification, of animals and plants from a taxonomist’s perspective.
The living world is truly wonderful and encompasses an amazing wide range of living types. The extraordinary habitats in which we find living organisms, such as cold mountains, deciduous forests, oceans, freshwater lakes, deserts, or hot springs, leave us speechless. The beauty of a galloping horse, the sight of migrating birds, a valley of flowers, or an attacking shark evokes awe and a deep sense of wonder. The ecological conflict and cooperation among members of a population, and among populations of a community, or even the molecular traffic inside a cell, prompt us to deeply reflect on what life truly is. This fundamental question contains two implicit aspects: a technical one seeking to define ‘living’ versus ‘non-living’, and a philosophical one concerning the purpose of life. As scientists, we will focus on the technical aspect and try to understand what constitutes ‘living’.
1.1. Growth
All living organisms exhibit growth. Growth is characterized by two fundamental aspects, often called the “twin characters of growth”:
- Increase in Mass
- Increase in Number of Individuals (or Cells)
Types of Growth:
- Intrinsic Growth (Growth From Inside):
- Also known as Protoplasmic Growth.
- Occurs due to the increase in protoplasmic material within the organism.
- This is a Defining Character of Living organisms.
- Example: A watermelon cell increases its initial size by 3,50,000 times during growth.
- Extrinsic Growth (Growth From Outside):
- Occurs due to the accumulation of material on the surface.
- Observed in non-living objects like mountains, boulders, and sand mounds.
- This is NOT a Defining Character of Living, as non-living things can also grow extrinsically.
Measuring Growth:
- In Multicellular Organisms: Growth primarily occurs through cell division.
- Plants: Continuous growth by cell division throughout their lifespan (e.g., Maize root tip cells form 17,500 cells per hour).
- Animals: Growth seen only up to a certain age, though cell division continues for tissue replacement.
- In Unicellular Organisms: Grow by cell division. Easily observed in in vitro cultures by counting cell numbers.
- Indirect Ways to Measure Protoplasmic Growth:
- Length (e.g., Pollen tube growth)
- Mass / Size (e.g., Watermelon cell increase)
- Area (e.g., Surface area of a leaf)
Key Takeaway: Growth as a Defining Property
While increase in body mass is a characteristic of growth, it is only a defining property when it occurs intrinsically (from inside). A dead organism does not grow.
1.2. Reproduction
Reproduction is a characteristic feature of living organisms, involving the production of progeny similar to parents. Critically, No non-living thing reproduces.
Forms of Reproduction:
- Primarily refers to Sexual Reproduction.
- Organisms also reproduce by Asexual Means (often interchangeably called Vegetative Reproduction in plants).
Examples of Asexual/Vegetative Reproduction:
- Fragmentation: Seen in Spirogyra (algae), fungi, filamentous algae, and the protonema of mosses. Plant body breaks, detaches, and forms new organisms.
- Budding: Observed in lower organisms like yeast and Hydra.
- Asexual Spores: Fungi multiply easily by producing millions of asexual spores.
- Regeneration: High power of true regeneration in Planaria (flatworms), where a fragmented organism regenerates lost parts into a new organism.
Relationship between Growth and Reproduction:
- In Unicellular Organisms (Prokaryotes & some Eukaryotes): Growth and Reproduction are mutually inclusive.
- Increase in cell number is considered both growth and reproduction.
- Terms can be used synonymously (interchangeably).
- In Multicellular Organisms: Growth and Reproduction are mutually exclusive.
- Growth (increase in mass/size) and reproduction (producing offspring) are distinct processes.
- In Eukaryotes, they are related (e.g., reproductive growth in plants), but do not have the same meaning.
Key Takeaway: Reproduction as a Defining Property
Reproduction cannot be considered a defining property of living organisms because it is not universally true for all living beings. Examples include:
- Mules
- Sterile Worker Bees
- Infertile Human Couples (or Drones)
1.3. Metabolism and Cellular Organization
Metabolism is a Defining Character of Living organisms.
- Definition: It is the sum total of all chemical reactions occurring in the cell/body of an organism.
- All living organisms are composed of chemicals that are constantly being formed and converted into other biomolecules through thousands of metabolic reactions.
- Exhibited by all plants, animals, fungi, and microbes.
- No non-living object exhibits metabolism.
Metabolic Reactions in Test Tubes (In Vitro):
- Chemical reactions can occur in cell-free systems or in vitro (in a test tube).
- An isolated metabolic reaction in a test tube is:
- Neither a Living Being.
- Nor Non-Living.
- It is a Living Reaction, but not a Living Being.
- This is because for a reaction to be considered “living,” it must occur within a cellular boundary.
Key Takeaway: Cellular Organization as a Defining Property
Therefore, Cellular Organization of the body is the Defining Property and Feature of Living organisms.
1.4. Consciousness
Consciousness is the most obvious and technically complex feature of all living organisms.
- Definition: It is the ability to sense their surroundings or environment and respond to external stimuli (physical, chemical, or biological).
- Organisms sense their environment through sense organs or other mechanisms.
- Examples: Plants respond to light, water, temperature, other organisms, pollutants. All organisms, from prokaryotes to complex eukaryotes, respond to environmental cues.
- Photoperiod affects reproduction in seasonal breeders.
- All organisms are ‘aware’ of their surroundings.
- Therefore, all living creatures are conscious.
Self-Consciousness:
- Human beings are unique in having Self-Consciousness (awareness of themselves).
- Comatose Patients:
- Considered conscious (organs interact).
- But not self-conscious (brain-dead).
- This poses a complex dilemma in defining the living state.
Key Takeaway: Consciousness as a Defining Property
Consciousness, including self-consciousness in humans, is a Defining Property of Living Organisms.
1.5. Interaction and Emergence
- All living phenomena result from underlying interactions within the biological system.
- Emergent Properties: Properties at a higher level of organization arise from interactions among components at a lower level.
- Example: Properties of tissues are not present in individual cells but emerge from interactions among cells.
- Example: Properties of cellular organelles emerge from interactions among their molecular components.
- This phenomenon is true across all levels of organizational complexity.
1.6. Self-Evolution
Living organisms are:
- Self-replicating
- Evolving
- Self-regulating interactive systems
- Capable of responding to external stimuli.
Biology is the story of life’s evolution on Earth. All living organisms (past, present, future) are linked by shared genetic material to varying degrees.
2. Diversity in the Living World
- Earth harbors a vast diversity of living organisms (visible and microscopic).
- Each distinct kind of plant, animal, or organism represents a species.
- Number of known and described species: 1.7 to 1.8 million.
- This vast array is called Biodiversity (number and types of organisms).
- New organisms are continuously being identified as new/old areas are explored.
3. Nomenclature and Identification
Local names for organisms vary widely, causing confusion. Hence, a standardized naming system is essential.
- Nomenclature: The process of standardizing the naming of living organisms so a particular organism is known by the same name globally.
- Identification: The process of correctly describing an organism and knowing its identity before naming.
International Codes for Nomenclature:
- For Plants: International Code for Botanical Nomenclature (ICBN).
- For Animals: International Code of Zoological Nomenclature (ICZN).
- These codes ensure:
- Each organism has only one scientific name.
- A name is not used for any other known organism.
- Description allows universal identification to the same name.
3.1. Binomial Nomenclature
- System of using two components for scientific naming.
- Proposed and popularized by Carolus Linnaeus.
- Components:
- Generic Name (Genus)
- Specific Epithet (Species)
- Example: Mango → Mangifera indica
- Mangifera: Genus
- indica: Specific Epithet (Species)
Key Contributions of Carolus Linnaeus:
- Proposed the Binomial Nomenclature system.
- Authored seminal works:
- Systema Naturae: 10th Edition (1758) is the starting point of Zoological Nomenclature.
- Species Plantarum: Publication date (May 1) is the starting point of Botanical Nomenclature.
- Genera Plantarum
Universal Rules of Binomial Nomenclature:
- Latin Origin: Biological names are in Latin and italicized (or Latinized). Latin is a “dead language,” ensuring stability. (e.g., Santalum album for white sandalwood).
- Two Components: First word is Genus, second is Specific Epithet.
- Formatting:
- Handwritten: Separately underlined.
- Printed: Italicized.
- Capitalization:
- Genus name: Starts with a capital letter.
- Specific epithet: Starts with a small letter.
- Author Citation: Abbreviated author name appears after the specific epithet (e.g., Mangifera indica Linn.).
Special Cases: Trinomial System & Tautonyms
- Trinomial System (e.g., Homo sapiens sapiens for subspecies):
- Generally acceptable in Zoology.
- Avoided in Botany (e.g., Brassica oleracea var. Capitata not generally accepted).
- Tautonyms (Generic name and specific epithet are identical, e.g., Naja naja, Gorilla gorilla):
- Not acceptable in Botany.
- Accepted in Zoological nomenclature.
4. Classification and Taxonomy
Studying all organisms individually is impossible, necessitating grouping.
- Classification: Process of grouping organisms into convenient categories based on easily observable characteristics.
- Taxa (singular: Taxon): The scientific term for these convenient categories.
- Can represent categories at different levels (e.g., ‘Plants’, ‘Wheat’, ‘Animals’, ‘Mammals’, ‘Dogs’ are all taxa at varying levels).
- A taxon is an actual group of organisms belonging to a particular category, defined by rules.
- Unit of classification is a Taxon (not merely an abstract ‘Category’).
The process of classification is called Taxonomy.
Basis of Modern Taxonomy:
Classification of organisms based on:
- External Structure
- Internal Structure
- Cellular Structure
- Ecological Role
- Developmental Processes
5. Systematics
Beyond classification, humans have been interested in the relationships among organisms.
- Systematics: The branch of study dealing with the systematic arrangement of organisms and their relationships.
- Word origin: Latin ‘systema’.
- Carolus Linnaeus’s publication title: “Systema Naturae”.
6. Taxonomic Categories and Hierarchy
Classification is a multi-step process involving a hierarchy of steps.
- Each step represents a Rank or Category.
- Taxonomic Category: Each category is part of an overall taxonomic arrangement.
- Taxonomic Hierarchy: All categories together form this arrangement.
- Each category is a unit of classification, commonly called a Taxon.
Seven Basic Taxonomic Categories (in Ascending Order):
- Species
- Genus
- Family
- Order
- Class
- Phylum (for animals) / Division (for plants)
- Kingdom
- Species is the lowest category for all organisms.
- As we move towards upper ranks (species → kingdom), the number of common characteristics shared by members decreases.
- Conversely, lower the taxa, more shared characteristics.
- Higher the category, more complex the classification problem.
- Species name is binomial; all other categories have uninominal names.
6.1. Species
- Group of individual organisms with fundamental similarities.
- Distinguishable from closely related species based on distinct morphological differences.
- Specific epithet represents the species.
- Examples:
- Mangifera indica
- Solanum tuberosum (potato)
- Panthera leo (lion)
- Homo sapiens (human)
6.2. Genus
- Comprises a group of related species.
- More characters in common compared to species of other genera.
- Aggregates of closely related species.
- Examples:
- Solanum: includes potato, tomato, brinjal.
- Panthera: includes lion (P. leo), leopard (P. pardus), tiger (P. tigris).
- Felis: includes cats; differs from Panthera.
6.3. Family
- Group of related genera.
- Fewer similarities compared to genus and species.
- Characterized by both vegetative and reproductive features (for plants).
- Examples:
- Plants: Solanum, Petunia, Datura → Family Solanaceae.
- Animals: Genus Panthera + Genus Felis → Family Felidae.
- Cat (Felidae) and Dog (Canidae) are in different families.
6.4. Order
- Assemblage of families.
- Identified based on aggregates of characters.
- Fewer similar characters compared to genera in a family.
- Examples:
- Plants: Convolvulaceae, Solanaceae → Order Polymoniales (based on floral characters).
- Animals: Felidae, Canidae → Order Carnivora.
6.5. Class
- Includes related orders.
- Example: Order Primata (monkey, gorilla, gibbon) + Order Carnivora (tiger, cat, dog) → Class Mammalia.
6.6. Phylum (for animals) / Division (for plants)
- Next higher category after Class.
- Animals: Classes (fishes, amphibians, reptiles, birds, mammals) → Phylum.
- Example: Presence of notochord & dorsal hollow neural system → Phylum Chordata.
- Plants: Classes with few similar characters → Division (e.g., Angiospermae).
6.7. Kingdom
- Highest category in classification.
- Kingdom Animalia: All animals belonging to various phyla.
- Kingdom Plantae: All plants from various divisions.
Taxonomic Classification Examples:
| Common Name | Biological Name | Genus | Family | Order | Class | Phylum/Division | Kingdom |
|---|---|---|---|---|---|---|---|
| Man | Homo sapiens | Homo | Hominidae | Primata | Mammalia | Chordata | Animalia |
| Housefly | Musca domestica | Musca | Muscidae | Diptera | Insecta | Arthropoda | Animalia |
| Mango | Mangifera indica | Mangifera | Anacardiaceae | Sapindales | Dicotyledonae | Angiospermae | Plantae |
| Wheat | Triticum aestivum | Triticum | Poaceae | Poales | Monocotyledonae | Angiospermae | Plantae |
7. Taxonomical Aids
These are essential tools for correct classification and identification of organisms, crucial for agriculture, forestry, industry, and understanding bio-resources.
- Requires intensive laboratory and field studies.
- Actual specimens (plant/animal) are prime sources.
- Information gathered is stored with specimens for future studies.
- Biologists use specific procedures/techniques for storage and preservation.
7.1. Herbarium
- Definition: Storehouse of collected plant specimens that are dried, pressed, and preserved on sheets.
- Arranged according to a universally accepted classification system.
- Repository for future use.
- Label information: Date & place of collection, English name, local name, botanical/scientific name, family, collector’s name.
- Serves as a quick referral system.
7.2. Botanical Gardens
- Definition: Specialized gardens with collections of living plants for reference.
- Plants grown for identification purposes.
- Each plant labeled with botanical/scientific name and family.
- Famous examples: Kew (England), Indian Botanical Garden (Howrah, India), National Botanical Research Institute (Lucknow, India).
7.3. Museum
- Definition: Generally set up in educational institutes (schools, colleges).
- Collections of preserved plant and animal specimens for study and reference.
- Specimens preserved in containers/jars in preservative solutions.
- Dry specimens also used (e.g., insects in insect boxes after collecting, killing, pinning).
- Larger animals (birds, mammals) usually stuffed and preserved.
- Often contain animal skeletons.
7.4. Zoological Parks (Zoos)
- Definition: Places where wild animals are kept in protected environments under human care.
- Purpose: Learn about their food habits and behavior.
- Conditions provided are as similar as possible to natural habitats.
- Popular for children’s visits.
7.5. Key
- Definition: Taxonomical aid for identification based on similarities and dissimilarities.
- Based on contrasting characters in a pair called a couplet.
- Couplet: Choice between two opposite options (acceptance of one, rejection of other).
- Each statement in the key is called a lead.
- Separate keys required for each taxonomic category (family, genus, species).
- Generally analytical in nature.
7.6. Other Means of Recording Descriptions
These also help in correct identification and disseminating information:
- Flora: Contains actual account of habitat and distribution of plants of a given area. Provides an index to plant species.
- Manuals: Useful for providing information for identification of names of species found in an area.
- Monographs: Contain comprehensive information on any one taxon.
- Catalogues: Provide a list of species found in a particular area with brief descriptions.