NCERT Class 9 Science Chapter 7: Diversity in Living Organisms - A Detailed Guide

Introduction to Diversity in Living Organisms

Have you ever looked around and marvelled at the sheer variety of life on Earth? From the tiniest bacterium to the giant blue whale, from the moss growing on a damp wall to the towering redwood trees, our planet is home to an astonishing array of living beings. This variety of life is what we call biodiversity. Scientists estimate that there are millions of species, many of which are yet to be discovered! To study and understand this vast diversity, we need a system to organize it. This is where the science of classification comes in. Chapter 7 of the NCERT Class 9 Science textbook, "Diversity in Living Organisms," takes us on a fascinating journey to understand how and why scientists classify living things. This process not only helps us identify and study organisms but also reveals the evolutionary relationships between them.

The Basis of Classification

For centuries, thinkers have tried to classify living organisms. The ancient Greek philosopher Aristotle classified animals based on where they lived – on land, in water, or in the air. However, this was a very simplistic approach. For example, whales, octopuses, and sharks all live in water but are fundamentally very different. Scientific classification needs to be based on more fundamental characteristics.

Modern classification is based on a hierarchy of characteristics, moving from broad to specific. The primary characteristics considered are:

• Type of Cell: Is the cell prokaryotic (lacking a well-defined nucleus and membrane-bound organelles) or eukaryotic (with a true nucleus and organelles)? This is the most fundamental level of division.
• Number of Cells: Is the organism unicellular (made of a single cell) or multicellular (made of many cells)? In multicellular organisms, are the cells simply a cluster, or are they organized into tissues, organs, and organ systems?
• Mode of Nutrition: Is the organism autotrophic (makes its own food, like plants through photosynthesis) or heterotrophic (gets food from other organisms, like animals and fungi)?
• Body Organization: This looks at the level of complexity in multicellular organisms. Do they have a basic body design? Are there specialized tissues and organs for different functions? The complexity of the body plan is a key factor in classification.

These characteristics help build a system that reflects the evolutionary history of organisms. Organisms with more characteristics in common are considered to be more closely related and are placed in the same groups.

Hierarchy of Classification

To organize the millions of species, biologists use a system of hierarchical classification developed by Carolus Linnaeus. It involves arranging organisms into a series of successive levels or ranks, called taxa (singular: taxon). The main ranks, from broadest to most specific, are:

• Kingdom: The highest and most inclusive rank.
• Phylum (for animals) / Division (for plants)
• Class
• Order
• Family
• Genus
• Species: The most basic unit of classification, representing a group of individuals that can interbreed to produce fertile offspring.

A helpful mnemonic to remember this order is: "King Philip Came Over For Good Soup". As we move down the hierarchy from Kingdom to Species, the number of organisms in each group decreases, but the similarity between them increases.

The Five Kingdoms: A Grand Overview

The most widely accepted system of classification was proposed by Robert H. Whittaker in 1969. He divided all living organisms into five broad kingdoms based on the key characteristics we discussed above (cell structure, mode of nutrition, and body organization).

The Five Kingdoms are:

1. Monera: Unicellular prokaryotes.
2. Protista: Unicellular eukaryotes.
3. Fungi: Multicellular, non-photosynthetic eukaryotes with cell walls made of chitin.
4. Plantae: Multicellular, photosynthetic eukaryotes with cell walls made of cellulose.
5. Animalia: Multicellular eukaryotes without cell walls.

Let's delve deeper into each of these kingdoms.

Kingdom Monera: The Simplest Life Forms

This kingdom includes the oldest, simplest, and most abundant organisms on Earth. They are found everywhere – in hot springs, deep oceans, snow, and even inside other organisms.

Characteristics of Monera:

• Cell Structure: They are unicellular and prokaryotic. They lack a true nucleus and other membrane-bound organelles like mitochondria or chloroplasts. Their genetic material (DNA) is a single circular chromosome that floats in the cytoplasm in a region called the nucleoid.
• Cell Wall: Most have a rigid cell wall made of peptidoglycan (a combination of sugars and amino acids), which is different from the cell walls of plants and fungi.
• Mode of Nutrition: Their nutritional modes are diverse. Some are autotrophic (e.g., cyanobacteria, which perform photosynthesis) while others are heterotrophic (e.g., most bacteria, which obtain nutrients from their environment).
• Examples: Bacteria, Cyanobacteria (also known as blue-green algae), and Mycoplasma.

Kingdom Protista: The Unicellular Eukaryotes

This kingdom is a diverse group of eukaryotic organisms. It's often considered a 'catch-all' kingdom for eukaryotes that don't fit into the other three eukaryotic kingdoms (Fungi, Plantae, or Animalia).

Characteristics of Protista:

• Cell Structure: They are unicellular and eukaryotic, meaning they have a well-defined nucleus and membrane-bound organelles.
• Locomotion: Many protists are motile and use specialized structures for movement, such as flagella (whip-like tails), cilia (short hair-like structures), or pseudopods ('false feet').
• Mode of Nutrition: Like Monera, they show diverse modes of nutrition. Some are autotrophic (e.g., algae like Diatoms), while others are heterotrophic (e.g., protozoans like Amoeba and Paramecium).
• Examples: Amoeba, Paramecium, Euglena, Diatoms, and various forms of algae.

Kingdom Fungi: The Great Decomposers

Fungi are a unique group of organisms that were once considered plants but are now placed in their own kingdom due to fundamental differences.

Characteristics of Fungi:

• Cell Structure: They are eukaryotic. Most are multicellular (e.g., mushrooms), but some are unicellular (e.g., yeast).
• Mode of Nutrition: Fungi are heterotrophic. They are primarily saprophytes, meaning they obtain nutrients by absorbing organic matter from dead and decaying organisms. This makes them crucial decomposers in ecosystems. Some are parasites, living on other living organisms.
• Cell Wall: Their cell walls are made of a tough, complex sugar called chitin, the same material found in the exoskeletons of insects.
• Symbiotic Relationships: Many fungi live in mutually beneficial symbiotic relationships. For example, Lichens are a composite organism arising from algae or cyanobacteria living among filaments of a fungus. Mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant.
• Examples: Yeast, Molds (like Penicillium), Mushrooms (like Agaricus), and Aspergillus.

Kingdom Plantae: The Producers

This kingdom includes all the multicellular, eukaryotic organisms that perform photosynthesis. They are the primary producers in most terrestrial ecosystems.

The first level of classification within Kingdom Plantae depends on whether the plant body is well-differentiated into distinct parts (stem, root, leaves). The next level depends on the presence or absence of specialized vascular tissues (xylem and phloem) for transporting water and food. Further classification is based on the ability to bear seeds and whether the seeds are enclosed within a fruit.

Division Thallophyta (Algae)

These are the simplest plants.

• Body Structure: The plant body is not differentiated into true roots, stems, and leaves. This undifferentiated body is called a thallus.
• Habitat: They are predominantly aquatic.
• Examples: Spirogyra, Ulothrix, Cladophora, Chara.

Division Bryophyta: Amphibians of the Plant Kingdom

Bryophytes are a step up in complexity from algae.

• Body Structure: The body is differentiated into stem-like and leaf-like structures, but they lack true roots. They have rhizoids for anchorage.
• Vascular Tissue: They do not have specialized vascular tissues (xylem and phloem).
• Habitat: They are called the 'amphibians of the plant kingdom' because they live on land but require water for sexual reproduction.
• Examples: Mosses (Funaria) and Liverworts (Marchantia, Riccia).

Division Pteridophyta: The First Vascular Plants

These are the first terrestrial plants to possess vascular tissues.

• Body Structure: The plant body is well-differentiated into true roots, stems, and leaves.
• Vascular Tissue: They have well-developed vascular tissues (xylem and phloem).
• Reproduction: They reproduce through spores produced in sporangia, not seeds. They are often called 'cryptogams' (hidden reproductive organs) along with Thallophytes and Bryophytes.
• Examples: Ferns, Marsilea, Equisetum (horsetail).

Plants that produce seeds are called Phanerogams. They are further divided into two groups based on whether the seeds are naked or enclosed.

Gymnosperms: The Naked Seed Bearers

• The term 'Gymnosperm' comes from the Greek words 'gymnos' (naked) and 'sperma' (seed).
• Seeds: They bear naked seeds, meaning the seeds are not enclosed within a fruit. The seeds develop on the surface of scales or leaves, often modified to form cones.
• Characteristics: They are usually perennial, evergreen, and woody.
• Examples: Pines (Pinus), Deodar, Cycas.

Angiosperms: The Flowering Plants

• The term 'Angiosperm' comes from the Greek words 'angion' (vessel) and 'sperma' (seed).
• Seeds: They bear seeds that are enclosed within a fruit. The fruit develops from the ovary of the flower.
• Characteristics: They are the most advanced and diverse group of plants. Their reproductive organs are aggregated in a structure called a flower.
• Angiosperms are further divided into two groups based on the number of cotyledons (seed leaves) in their seeds:
• Monocotyledons (Monocots):
  • Seeds have a single cotyledon.
  • Roots are fibrous.
  • Leaves have parallel venation.
  • Flowers are trimerous (floral parts in multiples of three).
  • Examples: Maize, Wheat, Rice, Sugarcane, Bamboo.
• Dicotyledons (Dicots):
  • Seeds have two cotyledons.
  • Roots have a main taproot.
  • Leaves have reticulate (net-like) venation.
  • Flowers are tetramerous or pentamerous (floral parts in multiples of four or five).
  • Examples: Pea, Gram, Mango, Rose, Sunflower.

Kingdom Animalia: The Consumers

This kingdom includes all multicellular, eukaryotic, heterotrophic organisms that lack a cell wall. Most animals are motile. Their classification is based on the complexity of their body design and organization.

Key features used for animal classification:

• Level of Organization: Cellular level (e.g., Porifera) vs. Tissue level (e.g., Coelenterata) vs. Organ level.
• Symmetry: Asymmetrical, Radial symmetry (can be divided into two identical halves along any plane passing through the central axis), or Bilateral symmetry (can be divided into identical left and right halves in only one plane).
• Body Cavity (Coelom): The presence or absence of a body cavity between the body wall and the gut. Acoelomates (no coelom), Pseudocoelomates (false coelom), and Coelomates (true coelom).
• Germ Layers: Diploblastic (two germ layers: ectoderm and endoderm) vs. Triploblastic (three germ layers: ectoderm, mesoderm, and endoderm).
• Notochord: Presence or absence of a notochord, a flexible rod-like structure that supports the body.

Phylum Porifera: The Pore Bearers

These are the simplest multicellular animals, commonly known as sponges.

• Organization: Cellular level of organization.
• Symmetry: Mostly asymmetrical.
• Key Feature: Body is covered with pores called ostia. Water enters through ostia into a central cavity (spongocoel) and exits through a larger opening called the osculum. This is the canal system used for feeding, respiration, and excretion.
• Skeleton: They have a hard outside layer or skeleton made of spicules or spongin fibers.
• Habitat: They are non-motile (sessile) and attached to some solid support, mostly in marine environments.
• Examples: Sycon, Spongilla (freshwater sponge), Euplectella (Venus' flower basket).

Phylum Coelenterata (Cnidaria): The Stinging Ones

• Organization: Tissue level of organization and are diploblastic.
• Symmetry: Radial symmetry.
• Key Feature: They have a central body cavity called the coelenteron (gastrovascular cavity). Their body has tentacles armed with stinging cells called cnidoblasts or nematocysts, which they use for defense and to capture prey.
• Habitat: They are aquatic, mostly marine.
• Examples: Hydra, Jellyfish, Sea Anemone, Corals.

Phylum Platyhelminthes: The Flatworms

• Body Form: Their body is bilaterally symmetrical and dorsoventrally flattened (flattened from top to bottom), hence the name flatworms.
• Organization: They are triploblastic. This is the first phylum to have three germ layers.
• Coelom: They are acoelomate (do not have a true body cavity).
• Characteristics: Most are parasitic, but some are free-living (e.g., Planaria).
• Examples: Planaria (free-living), Liver fluke (parasite), Tapeworm (parasite).

Phylum Nematoda (Aschelminthes): The Roundworms

• Body Form: The body is bilaterally symmetrical and cylindrical in cross-section.
• Organization: They are triploblastic.
• Coelom: They are pseudocoelomate, meaning they have a false body cavity that is not lined by mesoderm.
• Characteristics: Many are parasitic worms that cause diseases, such as elephantiasis (caused by Filarial worms) and ascariasis (caused by roundworms).
• Examples: Ascaris (roundworm), Wuchereria (filarial worm).

Phylum Annelida: The Segmented Worms

• Body Form: The body is bilaterally symmetrical and metamerically segmented (divided into segments or metameres \texternally and internally).
• Organization: They are triploblastic.
• Coelom: They are true coelomates, being the first phylum with a true body cavity.
• Characteristics: They have well-developed organ systems. They have special organs for excretion called nephridia.
• Examples: Earthworm, Leech, Nereis.

Phylum Arthropoda: The Jointed-Legged Animals

This is the largest phylum in the animal kingdom, with over two-thirds of all named species on Earth.

• Key Feature: The name Arthropoda means 'jointed appendages' or 'jointed legs'.
• Body Form: Bilaterally symmetrical and segmented.
• Exoskeleton: The body is covered by a tough, non-living exoskeleton made of chitin.
• Circulatory System: They have an open circulatory system, where blood does not flow in well-defined blood vessels but flows in the body cavity (haemocoel).
• Examples: Insects (cockroach, butterfly), arachnids (spider, scorpion), crustaceans (prawn, crab), centipedes, and millipedes.

Phylum Mollusca: The Soft-Bodied Animals

This is the second-largest animal phylum.

• Body Form: Bilaterally symmetrical with little segmentation. The body is soft and generally divided into a visceral mass, a muscular foot, and a mantle.
• Shell: Most molluscs secrete a hard, calcareous shell for protection.
• Circulatory System: They have an open circulatory system and kidney-like organs for excretion.
• Examples: Snails, Mussels, Octopus, Pila, Unio.

Phylum Echinodermata: The Spiny-Skinned Animals

• Key Feature: The name Echinodermata means 'spiny-skinned' (from Greek 'echinos' meaning hedgehog and 'derma' meaning skin). Their body is covered with calcareous spines.
• Symmetry: Adult echinoderms are radially symmetrical, but their larvae are bilaterally symmetrical.
• Organization: They are triploblastic and coelomate.
• Water Vascular System: They possess a unique water-driven tube system (water vascular system) which they use for locomotion, capturing food, and respiration.
• Habitat: They are exclusively free-living marine animals.
• Examples: Starfish, Sea urchin, Sea cucumber, Brittle star.

Phylum Protochordata: The Early Chordates

These are considered the link between invertebrates and vertebrates.

• Characteristics: They are bilaterally symmetrical, triploblastic, and coelomate.
• Notochord: They show the presence of a notochord at some stage in their life cycle. The notochord is a flexible rod that provides skeletal support.
• Habitat: They are exclusively marine.
• Examples: Balanoglossus, Herdmania, Amphioxus.

Phylum Vertebrata: The Backbone Animals

These are the most advanced group of animals. The defining feature is that the notochord present in the embryonic stage gets replaced by a cartilaginous or bony vertebral column (backbone) in the adult.

Vertebrates are further divided into five classes:

Class Pisces (Fish)

• Habitat: Exclusively aquatic.
• Body: Streamlined body covered with scales.
• Respiration: Breathe through gills.
• Heart: Two-chambered heart.
• Body Temperature: Cold-blooded (poikilothermic).
• Reproduction: They lay eggs.
• Examples: Rohu, Tuna (bony fish); Shark, Rays (cartilaginous fish).

Class Amphibia

• Habitat: They can live both on land and in water ('amphi' means dual, 'bios' means life).
• Skin: Smooth or rough, moist, slimy skin without scales.
• Respiration: Through gills (in larvae), lungs, and skin (in adults).
• Heart: Three-chambered heart (two auricles, one ventricle).
• Body Temperature: Cold-blooded.
• Reproduction: They lay eggs in water.
• Examples: Frogs, Toads, Salamanders.

Class Reptilia

• Habitat: Mostly terrestrial.
• Skin: Body is covered with dry, cornified skin with scales or scutes.
• Respiration: Breathe through lungs.
• Heart: Mostly three-chambered heart (Crocodiles are an exception with a four-chambered heart).
• Body Temperature: Cold-blooded.
• Reproduction: Lay eggs with a tough, protective shell on land.
• Examples: Snakes, Lizards, Turtles, Crocodiles.

Class Aves (Birds)

• Key Feature: Body is covered with feathers, and forelimbs are modified into wings for flight.
• Bones: Have light, hollow bones (pneumatic bones) to aid in flight.
• Respiration: Breathe through lungs.
• Heart: Four-chambered heart, ensuring no mixing of oxygenated and deoxygenated blood.
• Body Temperature: Warm-blooded (homeothermic).
• Reproduction: They lay eggs.
• Examples: Sparrow, Crow, Pigeon, Ostrich, Penguin.

Class Mammalia

• Key Feature: The most defining feature is the presence of mammary glands to produce milk for nourishing their young.
• Skin: Skin is covered with hair. They have \texternal ears (pinnae).
• Heart: Four-chambered heart.
• Body Temperature: Warm-blooded.
• Reproduction: Most mammals give birth to live young (viviparous). Some, like the platypus and echidna, lay eggs (oviparous).
• Examples: Humans, Cats, Dogs, Whales, Bats, Kangaroos.

Binomial Nomenclature: A Universal Naming System

With so many organisms and languages worldwide, a universal naming system is essential. Carolus Linnaeus introduced the system of Binomial Nomenclature in the 18th century. This system gives each species a unique, two-part scientific name.

Rules of Binomial Nomenclature:

1. The scientific name consists of two parts: the first name is the Genus, and the second is the species.
2. The Genus name always begins with a capital letter.
3. The species name always begins with a small letter.
4. When printed, the scientific name is written in italics.
5. When handwritten, the Genus and species names are underlined separately.
6. The name is usually derived from Latin or is Latinized.

Example: The scientific name for humans is Homo sapiens. Here, Homo is the genus and sapiens is the species. The scientific name for a mango tree is Mangifera indica.

Important Questions and Answers

Question 1: What is the primary characteristic on which the first division of organisms is made?

Answer: The primary and most fundamental characteristic for the first division of organisms is the nature of their cells. Specifically, it is the distinction between prokaryotic cells and eukaryotic cells. Prokaryotic organisms, which lack a defined nucleus and membrane-bound organelles, are placed in the Kingdom Monera. All other organisms, which have eukaryotic cells, are grouped separately and then further classified into Kingdoms Protista, Fungi, Plantae, and Animalia.

Question 2: In which phylum would you place an organism which is marine, radially symmetrical, triploblastic, and has a calcareous exoskeleton?

Answer: Based on the given characteristics, the organism would be placed in the Phylum Echinodermata. The key identifiers are:

• Marine: Echinoderms are exclusively marine.
• Radially symmetrical: Adult echinoderms exhibit radial symmetry.
• Triploblastic: They are triploblastic coelomates.
• Calcareous exoskeleton: Their skin is spiny and supported by a skeleton of calcareous plates (ossicles).

A classic example is a starfish.

Question 3: How are pteridophytes different from phanerogams?

Answer: Pteridophytes and phanerogams (Gymnosperms and Angiosperms) are both vascular plants, but they differ primarily in their mode of reproduction and the presence of seeds.

Question 4: What are the differences between amphibians and reptiles?

Answer: Amphibians and reptiles are both classes under Phylum Vertebrata, but they have several distinct differences.

Chapter Summary

Here are the key takeaways from this chapter on the diversity of living organisms:

• Classification is the systematic arrangement of organisms into groups based on their similarities and evolutionary relationships.
• The hierarchy of classification is: Kingdom > Phylum/Division > Class > Order > Family > Genus > Species.
• R.H. Whittaker's Five Kingdom Classification includes Monera, Protista, Fungi, Plantae, and Animalia.
• Monera are unicellular prokaryotes (e.g., bacteria).
• Protista are unicellular eukaryotes (e.g., Amoeba).
• Fungi are saprophytic eukaryotes with chitin in their cell walls (e.g., mushrooms).
• Plantae are multicellular, autotrophic eukaryotes. They are divided into Thallophyta, Bryophyta, Pteridophyta, Gymnosperms, and Angiosperms.
• Animalia are multicellular, heterotrophic eukaryotes without a cell wall. They are classified into several phyla, from Porifera to Chordata.
• Vertebrates are chordates with a backbone and are divided into Pisces, Amphibia, Reptilia, Aves, and Mammalia.
• Binomial Nomenclature, proposed by Carolus Linnaeus, provides a unique two-part scientific name (Genus and species) for every organism.