Diversity in Living Organisms — Study Notes

1. Introduction

The Earth is home to an enormous variety of living organisms - from tiny bacteria to giant trees, from microscopic amoebas to elephants and whales. This variety of life is called biodiversity. It is estimated that there are millions of different species of living organisms on our planet, and scientists have described and classified only a small fraction of them.

Classification is the process of grouping organisms based on their similarities and differences. The science of classifying organisms is called taxonomy. Classification helps us study the huge diversity of life in an organised way, makes it easier to identify organisms, and reveals the evolutionary relationships between different groups.

In this chapter we study the hierarchy of classification, the five-kingdom system proposed by R.H. Whittaker, the basis of classification, and the major groups of the plant and animal kingdoms.

2. Need for Classification

The number of living organisms is so large that it is impossible to study them individually. Classification provides a systematic way of studying them. When organisms are grouped together, we can study a few members of a group and understand the features common to the whole group.

Classification also helps to: 1. Identify organisms accurately. 2. Establish relationships between organisms. 3. Trace the evolutionary history of organisms. 4. Study the variety of life in a logical and organised manner.

3. The Hierarchy of Classification

Classification is done in a graded order called the taxonomic hierarchy. The main categories from the broadest to the most specific are:

Kingdom > Phylum (for animals) or Division (for plants) > Class > Order > Family > Genus > Species

Species is the smallest and most specific category. Organisms of the same species can interbreed among themselves. The genus is a group of closely related species. For example, a lion (Panthera leo) and a tiger (Panthera tigris) belong to the same genus Panthera but are different species.

Carolus Linnaeus, the father of taxonomy, gave the system of naming organisms called binomial nomenclature. In this system, each organism is given a two-word scientific name - the first word is the genus and the second word is the species. For example, the scientific name of humans is Homo sapiens, and of mango it is Mangifera indica. The first letter of the genus is written in capitals and the species name in small letters; both are written in italics.

4. Basis of Classification

The classification of organisms is based on several criteria:

1. Presence or absence of a nucleus: Organisms in which the nuclear material is not enclosed by a nuclear membrane are called prokaryotes (bacteria and blue-green algae), while those with a membrane-bound nucleus are called eukaryotes (plants, animals, fungi).
2. Number of cells: Organisms can be unicellular (a single cell, like amoeba) or multicellular (many cells, like a mango tree).
3. Mode of nutrition: Organisms that prepare their own food are called autotrophs (green plants), while those that depend on others for food are called heterotrophs (animals and fungi).
4. Level of body organisation: Organisms may have a cellular level, tissue level, organ level or organ-system level of organisation.

5. The Five Kingdom Classification

R.H. Whittaker (1969) classified living organisms into five kingdoms:

1. Kingdom Monera: These are prokaryotic, unicellular organisms. They have no true nucleus and no membrane-bound organelles. Examples are bacteria, blue-green algae (cyanobacteria) and mycoplasma. Nutrition may be autotrophic or heterotrophic.

2. Kingdom Protista: These are eukaryotic, unicellular organisms. They have a true nucleus and membrane-bound organelles. Examples are Amoeba, Paramecium and Euglena. Some are autotrophic (Euglena) and some are heterotrophic (Amoeba, Paramecium).

3. Kingdom Fungi: These are eukaryotic, multicellular organisms (except yeast which is unicellular). They have a cell wall made of chitin, are heterotrophic and obtain nutrition by absorbing dissolved organic matter from dead and decaying matter (saprophytic nutrition). They have thread-like structures called hyphae. Examples are mushroom, yeast and moulds.

4. Kingdom Plantae: These are eukaryotic, multicellular, autotrophic organisms with a cell wall made of cellulose. They contain chlorophyll and prepare their own food by photosynthesis.

5. Kingdom Animalia: These are eukaryotic, multicellular, heterotrophic organisms without a cell wall. They have a well-developed organ-system level of organisation and are usually capable of locomotion.

6. Classification of the Plant Kingdom

The plant kingdom is divided into major groups based on the presence or absence of differentiated body parts, vascular tissue and seeds.

Thallophyta: These are simple plants whose body is not differentiated into root, stem and leaves; the body is called a thallus. They are mostly aquatic and have no vascular tissue. Examples are algae like Spirogyra, Ulothrix and Chlamydomonas.

Bryophyta: These are called the amphibians of the plant kingdom because they grow in moist shady places, though they live on land. The plant body has root-like, stem-like and leaf-like structures, but there are no true roots, stems or leaves, and no vascular tissue. The main plant body is a gametophyte. Examples are moss and liverwort (Marchantia).

Pteridophyta: These are the first plants to have a vascular system (xylem and phloem) and differentiated body parts, but they do not produce seeds; they reproduce by spores. Examples are ferns, horsetails and Marsilea.

Gymnosperms: These are plants with well-differentiated body parts, vascular tissue and seeds, but the seeds are naked, that is, not enclosed in fruits. They are mostly perennial, evergreen and woody. Examples are pine, cedar and cycas.

Angiosperms: These are flowering plants in which the seeds are enclosed within fruits. They have well-differentiated body parts, vascular tissue and seeds. Angiosperms are classified into monocotyledons (one cotyledon, parallel venation, fibrous roots - like wheat and maize) and dicotyledons (two cotyledons, reticulate venation, tap roots - like sunflower and gram). Angiosperms are the most diverse group of plants on Earth.

7. Classification of the Animal Kingdom

The animal kingdom is divided into several phyla based on features such as body symmetry, presence of a notochord, segmentation and body cavity.

Porifera: These are the simplest multicellular animals, with pores all over the body. They are non-mobile, mostly marine, and have a cellular level of body organisation. Examples are Sycon and Spongilla.

Coelenterata (Cnidaria): These are aquatic animals with radial symmetry and a central body cavity. They have tentacles with stinging cells (cnidoblasts) used to capture food. Examples are Hydra and jellyfish.

Platyhelminthes: These are flatworms with bilateral symmetry, a flat body and a soft body. They may be free-living or parasitic. Examples are planaria and liver fluke (tapeworm).

Nematoda: These are roundworms with a cylindrical body, no segmentation and bilateral symmetry. Many are parasitic. Examples are Ascaris and Wuchereria (which causes elephantiasis).

Annelida: These are segmented worms with bilateral symmetry, true body cavity (coelom) and a segmented body. Examples are earthworm and leech.

Arthropoda: This is the largest phylum of animals. These organisms have jointed legs, an open circulatory system and a body divided into head, thorax and abdomen. They have an exoskeleton made of chitin. Examples are insects, spiders, crabs, prawns and butterflies. Some arthropods like butterflies undergo metamorphosis.

Mollusca: These have a soft body, a muscular foot and usually a calcareous shell. The body is unsegmented and has bilateral symmetry. Examples are snails, octopus, cuttlefish and bivalves like mussels.

Echinodermata: These are marine animals with radial symmetry and spiny skins. They have a water vascular system for locomotion. Examples are starfish and sea urchins.

Protochordata: These are marine animals with a notochord at some stage of life. Examples are Amphioxus and Balanoglossus.

Vertebrata: These have a notochord replaced by a vertebral column (backbone) in adults. They have a closed circulatory system, well-developed brain and paired appendages. Vertebrates are classified into five classes:

1. Pisces (Fish): Aquatic animals with scales, fins, gills for respiration and a streamlined body. They are cold-blooded. Examples are shark, rohu and Labeo.
2. Amphibia: These live in water and on land. They have moist skin, breathe through gills and lungs or skin, and lay eggs in water. They are cold-blooded. Examples are frogs and toads.
3. Reptilia: These are terrestrial, cold-blooded animals with dry scaly skin. They breathe with lungs and lay eggs. Examples are lizards, snakes, turtles and crocodiles.
4. Aves (Birds): These are warm-blooded animals with feathers, wings, a beak, a four-chambered heart and air-breathing lungs. Examples are pigeons, crows and sparrows.
5. Mammalia: These are warm-blooded animals with mammary glands for feeding milk to young ones. They have hair on the body, a four-chambered heart and give birth to young ones (except platypus and echidna which lay eggs). Examples are humans, whales, bats and elephants.

Quick Revision Tables

Mind Map

Important Diagrams (SVG)

Diagram 1: Hierarchy of Classification

Diagram 2: Five Kingdoms of Classification

Common Mistakes

1. Confusing the order of the taxonomic hierarchy; it is Kingdom, Phylum, Class, Order, Family, Genus, Species.
2. Thinking that fungi have cellulose cell walls; fungi have a cell wall made of chitin, while plants have cellulose.
3. Saying that bryophytes are aquatic; they are called amphibians of the plant kingdom because they live on land but need water for reproduction.
4. Believing that pteridophytes produce seeds; they reproduce by spores, while gymnosperms and angiosperms produce seeds.
5. Confusing gymnosperms and angiosperms; gymnosperms have naked seeds while angiosperms have seeds enclosed in fruits.
6. Forgetting that a whale is a mammal, not a fish, and that a bat is a mammal too.
7. Writing binomial names incorrectly; the genus begins with a capital letter, the species with a small letter, and both are italicised.

Exam Tips

1. Memorise the taxonomic hierarchy in order and give the scientific name of humans (Homo sapiens) and mango (Mangifera indica).
2. Learn the basis of classification: prokaryote/eukaryote, unicellular/multicellular, autotroph/heterotroph.
3. List the five kingdoms of Whittaker with one feature and one example of each.
4. Know the characteristics of thallophyta, bryophyta, pteridophyta, gymnosperms and angiosperms, especially the presence of vascular tissue and seeds.
5. Compare the animal phyla with respect to symmetry, body cavity and examples.
6. Learn the five classes of vertebrates and one example of each: Pisces (shark), Amphibia (frog), Reptilia (lizard), Aves (pigeon), Mammalia (human).
7. Remember that Arthropoda is the largest phylum and that Angiosperms are the most diverse plant group.

Conclusion

In this chapter we studied the enormous diversity of living organisms and understood the need for classification. We learned the taxonomic hierarchy from kingdom down to species, and the rules of binomial nomenclature given by Carolus Linnaeus. Whittaker's five-kingdom classification helped us organise life into Monera, Protista, Fungi, Plantae and Animalia based on the presence of a nucleus, number of cells, mode of nutrition and level of body organisation. We then explored the major groups of plants - from simple thallophytes to advanced angiosperms - and the many phyla of animals, ending with the five classes of vertebrates. Classification not only organises biodiversity but also reflects the evolutionary relationships between organisms, making the study of life both systematic and meaningful.