Classification of Living Beings

Nepal alone has more than 6,000 kinds of flowering plants, about 880 kinds of birds and thousands of insects. The whole Earth has millions of kinds of living things. To study such variety, scientists sort living things into groups according to their likeness. This sorting is called classification. In this unit you learn why we classify, how every organism gets a two-part scientific name, and the five great kingdoms into which all living things are placed.

2.1 Taxonomy and classification

Why classify?

Classification is the arrangement of living things into groups and subgroups on the basis of their similarities and differences. We classify because it:

• makes the study of millions of organisms easy;
• shows how organisms are related to one another;
• gives each organism a name known to scientists everywhere;
• helps us to identify an unknown organism;
• helps in the study of evolution.

Early classification

The Greek scientist Aristotle (384–322 BC) first grouped organisms as plants and animals, and plants as herbs, shrubs and trees. Carolus Linnaeus (1707–1778), a Swedish botanist, is called the father of taxonomy. He introduced the system of naming organisms with two names and arranged them in ranks. R. H. Whittaker (1969) proposed the five-kingdom system that this unit follows.

Taxonomic hierarchy

The ranks in classification, from the largest group to the smallest, are called the taxonomic hierarchy. Each step down holds fewer organisms that are more alike.

The species is the basic unit of classification. A species is a group of organisms that look alike, can breed with each other and produce fertile offspring.

Binomial nomenclature

Common names change from place to place. A mango is aamba in Nepali, mango in English and aam in Hindi, and the same common name can mean different plants in different places. To avoid confusion, Linnaeus gave each organism a two-word scientific name.

Rules for writing a scientific name:

1. The name is in Latin and is written in italics (when handwritten, each word is underlined separately).
2. The genus name begins with a capital letter; the species name begins with a small letter.
3. The first word is the genus and the second the species: Homo sapiens, not sapiens Homo.

Organism	Scientific name
Human being	Homo sapiens
Mango	Mangifera indica
Rice	Oryza sativa
Pea	Pisum sativum
Cat	Felis catus
Dog	Canis familiaris
Tiger	Panthera tigris
Rhododendron (national flower of Nepal)	Rhododendron arboreum

2.2 Five kingdoms

Earlier, all organisms were placed in two kingdoms, plants and animals. This did not suit bacteria, fungi or organisms like Euglena (which has green colour but also moves). In 1969 R. H. Whittaker therefore proposed five kingdoms, based on four main points:

1. type of cell: prokaryotic or eukaryotic;
2. number of cells: unicellular or multicellular;
3. mode of nutrition: autotrophic or heterotrophic;
4. body organisation and way of reproduction.

Prokaryotic and eukaryotic cells

Prokaryotic cell	Eukaryotic cell
No true nucleus; genetic material lies free in the cytoplasm	Has a true nucleus bounded by a nuclear membrane
Membrane-bound organelles absent	Membrane-bound organelles (mitochondria etc.) present
Very small (1–10 μm)	Larger (10–100 μm)
Found in bacteria	Found in protists, fungi, plants and animals

The five kingdoms

The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.

Kingdom	Cell type	Body	Nutrition	Examples
Monera	Prokaryotic	Unicellular	Autotrophic or heterotrophic	Bacteria, cyanobacteria (Nostoc)
Protista	Eukaryotic	Mostly unicellular	Autotrophic or heterotrophic	Amoeba, Paramecium, Euglena
Fungi	Eukaryotic	Mostly multicellular (yeast is unicellular)	Heterotrophic (absorptive)	Yeast, mushroom, Rhizopus, Penicillium
Plantae	Eukaryotic, cell wall of cellulose	Multicellular	Autotrophic (photosynthesis)	Moss, fern, pine, mango, rice
Animalia	Eukaryotic, no cell wall	Multicellular	Heterotrophic (ingestion)	Hydra, earthworm, fish, human

2.3 Characteristics of Monera, Protista, Fungi, Plantae and Animalia

Kingdom Monera

Monera includes the simplest and oldest living things, the bacteria and the blue-green algae (cyanobacteria).

• Microscopic, unicellular, prokaryotic: the DNA lies free in the cytoplasm without a nuclear membrane.
• The cell has a cell wall (of peptidoglycan in bacteria), a cell membrane, cytoplasm and ribosomes. Mitochondria and chloroplasts are absent.
• Nutrition may be autotrophic (cyanobacteria carry out photosynthesis) or heterotrophic (most bacteria absorb food from dead matter or living hosts).
• Reproduce mostly by binary fission: one cell divides into two. They can multiply very fast, as quickly as once every 20 minutes in good conditions.
• Found everywhere: soil, water, air, inside our bodies.

Bacteria are classified by shape into cocci (spherical), bacilli (rod-shaped), spirilla (spiral) and vibrios (comma-shaped).

Importance. Lactobacillus turns milk into curd; Rhizobium fixes nitrogen in the root nodules of legumes; bacteria decompose dead bodies. Harmful bacteria cause cholera (Vibrio cholerae), typhoid, tuberculosis and tetanus.

Kingdom Protista

• Mostly unicellular, eukaryotic organisms with a true nucleus and organelles.
• Most live in water or moist places.
• Nutrition: autotrophic (Euglena, diatoms), heterotrophic (Amoeba, Paramecium) or both (Euglena).
• Locomotion by pseudopodia (Amoeba), cilia (Paramecium) or flagella (Euglena).
• Reproduction mainly by binary fission; sexual reproduction (conjugation) also occurs in some.
• Some are harmful: Plasmodium causes malaria, Entamoeba histolytica causes amoebic dysentery, Giardia causes giardiasis.

Protista is often called the "kingdom of leftovers", because it contains eukaryotes that are neither plants, animals nor fungi.

Protist	Locomotion	Nutrition
Amoeba	Pseudopodia	Heterotrophic
Paramecium	Cilia	Heterotrophic
Euglena	Flagellum	Autotrophic and heterotrophic
Plasmodium	None (parasite)	Heterotrophic

Kingdom Fungi

• Mostly multicellular (yeast is unicellular), eukaryotic.
• The body is made of thread-like hyphae, which together form a mycelium.
• The cell wall is made of chitin (not cellulose).
• No chlorophyll, so no photosynthesis. Fungi are heterotrophic: saprophytes feed on dead matter (mushroom, bread mould), parasites on living hosts (the rust fungus on wheat).
• They secrete enzymes outside the body to digest food and then absorb the digested food (absorptive nutrition).
• Reproduce by spores, by budding (yeast) and by fragmentation.
• Store food as glycogen, as animals do.

Importance. Yeast is used in making bread and alcohol; Penicillium gives the antibiotic penicillin; some mushrooms are food (Agaricus, the button mushroom); other fungi cause athlete's foot, ringworm and food spoilage. Fungi, along with bacteria, are decomposers.

Kingdom Plantae

• Multicellular, eukaryotic, with a cell wall of cellulose.
• Contain chlorophyll in chloroplasts and make food by photosynthesis, so they are autotrophic (producers).
• Mostly fixed in one place; they do not move from place to place.
• Reproduction is vegetative, asexual (spores) or sexual (seeds).
• Store food as starch.

Plants are divided into five groups:

Group	Main features	Examples
Algae (Thallophyta)	No true root, stem or leaf; mostly aquatic	Spirogyra, Ulothrix
Bryophyta	Small, grow in moist places, no vascular tissue, root-like rhizoids	Moss (Funaria), liverwort (Marchantia)
Pteridophyta	Have root, stem, leaves and vascular tissue; no flowers or seeds; reproduce by spores	Ferns (Nephrolepis), horsetail
Gymnosperms	Seeds are naked (no fruit), cones present	Pine (Pinus), cycas
Angiosperms	Flowering plants; seeds enclosed in fruits	Mango, rice, pea

Angiosperms are further divided into monocots (one seed leaf, parallel leaf veins, fibrous roots: maize, rice, wheat) and dicots (two seed leaves, net-like veins, tap root: pea, mustard, mango).

Monocotyledon	Dicotyledon
One cotyledon in the seed	Two cotyledons
Parallel venation	Reticulate (net) venation
Fibrous roots	Tap root
Flower parts in threes	Flower parts in fours or fives

Kingdom Animalia

• Multicellular, eukaryotic, cells without cell wall.
• Heterotrophic: they take food in (ingestion) and digest it inside the body.
• Most can move from place to place at some stage of life.
• Have nervous system and sense organs (except in the simplest ones), so they respond quickly to stimuli.
• Reproduce mostly sexually; store food as glycogen and fat.

Animals are divided into invertebrates (no backbone) and vertebrates (backbone present).

Phylum	Main features	Examples
Porifera	Pores on the body, sessile, no organs	Sponge
Coelenterata (Cnidaria)	Tentacles with stinging cells; sac-like body	Hydra, jellyfish
Platyhelminthes	Flat body, no body cavity	Tapeworm, liver fluke
Aschelminthes (Nematoda)	Round, unsegmented worm	Ascaris
Annelida	Body divided into segments	Earthworm, leech
Arthropoda	Jointed legs, hard outer skeleton; the largest phylum	Honey bee, spider, prawn
Mollusca	Soft body, usually a shell	Snail, octopus
Echinodermata	Spiny skin	Starfish
Chordata	Notochord; vertebrates are included	Fish, frog, lizard, bird, human

Vertebrates are placed in five classes:

Class	Body covering	Respiration	Body temperature	Examples
Pisces (fish)	Scales	Gills	Cold-blooded	Rohu, shark
Amphibia	Moist skin	Gills (larva), lungs and skin (adult)	Cold-blooded	Frog, toad
Reptilia	Dry scales	Lungs	Cold-blooded	Lizard, snake, crocodile
Aves (birds)	Feathers	Lungs	Warm-blooded	Pigeon, Himalayan monal (Danphe)
Mammalia	Hair or fur	Lungs	Warm-blooded	Cow, tiger, human

Comparison: plants and animals

Plantae	Animalia
Cell wall present	Cell wall absent
Autotrophic	Heterotrophic
Chlorophyll present	Chlorophyll absent
Do not move from place to place	Move from place to place
Grow throughout life at tips	Growth stops after maturity
Store food as starch	Store food as glycogen and fat

Chapter summary

• Taxonomy is the identification, naming and classification of organisms. Linnaeus is the father of taxonomy; Whittaker proposed five kingdoms in 1969.
• The hierarchy is Kingdom → Phylum → Class → Order → Family → Genus → Species; the species is the basic unit.
• Binomial nomenclature: two Latin names, genus (capital) then species (small letter), in italics or underlined.
• Monera: prokaryotic, unicellular (bacteria, cyanobacteria); binary fission.
• Protista: unicellular eukaryotes (Amoeba, Paramecium, Euglena).
• Fungi: chitin cell wall, hyphae, saprophytic or parasitic, spores (yeast, mushroom).
• Plantae: multicellular, cellulose wall, photosynthesis; algae, bryophytes, pteridophytes, gymnosperms, angiosperms.
• Animalia: multicellular, heterotrophic, no cell wall; invertebrate phyla and five classes of vertebrates (fish, amphibians, reptiles, birds, mammals).

Practice questions

Group A — Objective questions (1 mark each)

1. Who is called the father of taxonomy? [1]aAristotlebLinnaeuscWhittakerdDarwin
2. Which of the following is a prokaryote? [1]aYeastbAmoebacBacteriadEuglena
3. The cell wall of fungi is made of [1]acellulosebchitincpeptidoglycandprotein
4. The correct way of writing the scientific name of man is [1]ahomo sapiensbHomo SapienscHomo sapiensdSapiens homo
5. Which kingdom includes Paramecium? [1]aMonerabProtistacFungidAnimalia
6. Which group of plants bears naked seeds? [1]aPteridophytesbBryophytescGymnospermsdAngiosperms

Answers to Group A

1. (b) — Linnaeus introduced binomial nomenclature and the ranks.

2. (c) — bacteria have no true nucleus.

3. (b) — chitin, a tough carbohydrate.

4. (c) — genus capital, species small, in italics.

5. (b) — Paramecium is a ciliated unicellular eukaryote.

6. (c) — gymnosperm means "naked seed".

Group B — Short answer (2 marks each)

1. Define taxonomy and binomial nomenclature. [2]
2. Write any two differences between prokaryotic and eukaryotic cells. [2]
3. Why is Whittaker's five-kingdom system better than the older two-kingdom system? [2]
4. Name the four shapes of bacteria with one example each. [2]
5. List any two characteristics of fungi. [2]

Answers to Group B

1. Taxonomy is the branch of biology that identifies, names and classifies organisms. Binomial nomenclature is the system of naming an organism with two words, the genus and the species.

2. Prokaryotic cells have no true nucleus and no membrane-bound organelles; eukaryotic cells have a true nucleus and membrane-bound organelles.

3. It uses cell type, number of cells, mode of nutrition and body organisation; so bacteria, fungi and unicellular organisms like Euglena get proper places which they did not have in the plant-animal system.

4. Cocci (Streptococcus), bacilli (Bacillus), spirilla (Spirillum), vibrios (Vibrio cholerae).

5. They have a chitin cell wall and no chlorophyll; they feed by absorption; they reproduce by spores.

Group C — Long answer (4 marks each)

1. Describe the characteristics of the kingdom Monera and Protista. [4]
2. Differentiate between plants and animals on the basis of four points. [4]
3. Write the taxonomic hierarchy with a suitable example and state the rules for writing scientific names. [4]
4. Classify the vertebrates into classes with two characteristics and an example of each. [4]

Answers to Group C

1. Monera: prokaryotic and unicellular; cell wall present; no mitochondria or chloroplast; nutrition autotrophic or heterotrophic; binary fission; examples bacteria and cyanobacteria. Protista: unicellular eukaryotes with a true nucleus; live in water or moist places; locomotion by pseudopodia, cilia or flagella; nutrition autotrophic or heterotrophic; binary fission; examples Amoeba, Paramecium, Euglena.

2. Plants have a cell wall, animals do not; plants are autotrophic, animals heterotrophic; plants have chlorophyll, animals do not; plants do not move from place to place, most animals do; plants store starch, animals glycogen.

3. Kingdom Animalia → Phylum Chordata → Class Mammalia → Order Primates → Family Hominidae → Genus Homo → Species Homo sapiens. Rules: Latin; italics (or underlined); genus name begins with a capital letter and species with a small letter; genus first, then species.

4. Pisces: scales and gills, rohu. Amphibia: moist skin, lungs and skin in adult, frog. Reptilia: dry scaly skin, lungs, lizard. Aves: feathers, warm-blooded, pigeon. Mammalia: hair, mammary glands, warm-blooded, human.