Living organisms show an extraordinary range of structures and ways of life. Bacteria, single-celled protists, fungi, plants and animals cannot all be understood through a simple “plant or animal” division. Biological classification uses important characteristics to organise these organisms into meaningful groups.
Class 11 Biology Chapter 2 Biological Classification Notes
In this chapter, you will study the five-kingdom system, the main features of Monera, Protista and Fungi, and the special nature of viruses, viroids and lichens.
Table of Contents
Why Was a New Classification System Needed?
An older two-kingdom system placed organisms under Plantae or Animalia. It could not clearly separate organisms with different types of cells or methods of nutrition. For example, bacteria lack a membrane-bound nucleus, while plants have one. Fungi absorb nutrients from other sources, while most green plants make food by photosynthesis.
Scientists therefore considered characteristics such as:
- cell structure;
- whether an organism is unicellular or multicellular;
- presence and composition of the cell wall;
- mode of nutrition;
- methods of reproduction; and
- evolutionary relationships.
These characteristics helped produce a more useful classification.
The Five-Kingdom Classification
R. H. Whittaker proposed a system with five kingdoms:
- Monera — prokaryotic organisms such as bacteria.
- Protista — mainly unicellular eukaryotic organisms.
- Fungi — eukaryotes that absorb nutrients from their surroundings.
- Plantae — mainly photosynthetic multicellular eukaryotes.
- Animalia — multicellular eukaryotes that ingest food.
The five-kingdom system is a useful framework for this chapter. Biological classification continues to develop as scientists learn more about organisms and their relationships.
Kingdom Monera
Monera contains prokaryotic organisms. Their cells do not have a membrane-bound nucleus. Bacteria occur in soil, water, air and the bodies of other organisms.
Bacteria have several common shapes:
- Cocci: approximately spherical
- Bacilli: rod-shaped
- Vibrios: comma-shaped
- Spirilla: spiral-shaped
Shape is useful for describing bacteria, but it is not the only feature used to study them.
Nutrition in Bacteria
Bacteria show diverse methods of nutrition. Autotrophic bacteria make their own organic food. Some use light energy, while others obtain energy through chemical reactions involving inorganic substances.
Heterotrophic bacteria obtain nutrients from other organisms or organic material. Many help decompose dead matter. Some are useful in food production and nutrient cycling; others can cause disease.
Archaebacteria and Eubacteria
In the traditional NCERT five-kingdom treatment, archaebacteria and eubacteria are discussed under Monera.
Archaebacteria differ from other bacteria in important cellular features. Examples include methanogens, which can occur in the digestive systems of ruminants, and forms adapted to highly salty or hot environments.
Eubacteria include many familiar bacteria. Cyanobacteria are photosynthetic members of this group. Some cyanobacteria, such as Nostoc, can fix atmospheric nitrogen.
Important distinction: The three-domain system discussed in Chapter 1 separates Archaea and Bacteria into different domains. The five-kingdom system is an earlier classification framework, so the two systems should not be mistaken for identical hierarchies.
Mycoplasma
Mycoplasmas are very small organisms that lack a cell wall. Remembering this exception helps avoid the incorrect statement that every member discussed under Monera has a cell wall.
Kingdom Protista
Protista includes mainly unicellular eukaryotes. Their cells have a membrane-bound nucleus. Many live in water, and the kingdom contains organisms with varied forms of nutrition and movement.
The main protist groups discussed in this chapter are chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.
Chrysophytes
Chrysophytes include diatoms and golden algae. Many are microscopic aquatic organisms. Diatoms have distinctive walls containing silica. They are important photosynthetic producers in aquatic environments.
Dinoflagellates
Dinoflagellates are mostly aquatic. Many have two flagella and are photosynthetic. Under certain conditions, rapid growth of some species can produce a red tide.
Euglenoids
Euglenoids such as Euglena have a flexible outer covering called a pellicle rather than a typical cell wall. They can photosynthesise when light is available and can obtain nutrition in other ways when it is not.
Slime Moulds
Slime moulds are protists associated with decaying organic material. Under suitable conditions, some form a spreading mass. They can produce spores that survive unfavourable conditions.
Protozoans
Protozoans are heterotrophic protists. Their groups include amoeboid, flagellated, ciliated and sporozoan forms. Their methods of movement and ways of life differ. Some are free-living; others are parasites.
Remember: Protists are eukaryotic. Do not place them in Monera simply because many are microscopic and unicellular.
Kingdom Fungi
Fungi are eukaryotic organisms that obtain nutrients by absorption. Many act as decomposers, breaking down organic material. Others live as parasites or in mutually beneficial associations.
Most fungi have cell walls containing chitin. Their bodies commonly consist of fine filaments called hyphae. A network of hyphae forms a mycelium. Yeast is an example of a unicellular fungus.
Fungi can reproduce through spores. The traditional groups covered in the NCERT chapter are described below.
Phycomycetes
In the textbook classification, this group includes forms such as Rhizopus, a bread mould. Their hyphae generally lack cross walls. They can form asexual spores.
Ascomycetes
Ascomycetes are often called sac fungi because their sexual spores develop in sac-like structures called asci. Examples include yeast and Penicillium.
Basidiomycetes
This group includes familiar mushrooms and some rust and smut fungi. Their sexual spores are produced on structures called basidia.
Deuteromycetes
This is a traditional group used for fungi whose sexual stage was not known when they were classified. A fungus may be reassigned when more information about it becomes available.
Exam tip: These four names describe the groups used in the textbook treatment. Learn the distinguishing features and an example for each.
A Brief Look at Plantae and Animalia
Plantae contains mainly multicellular eukaryotes that photosynthesise and generally have cellulose-containing cell walls. You will study plant groups in Chapter 3.
Animalia contains multicellular eukaryotes that ingest food. Animal cells lack cell walls. You will study the major animal groups in Chapter 4.
Viruses
A virus is not a cell. It contains genetic material—DNA or RNA—surrounded by a protein coat. It can multiply only inside a suitable host cell.
Viruses are not placed in one of Whittaker’s five kingdoms. Their dependence on host cells and their lack of cellular structure make them different from the cellular organisms discussed above.
Viroids
Viroids are infectious agents made of small RNA molecules. Unlike viruses, they do not have a protein coat. Some viroids cause diseases in plants.
A simple distinction to remember is:
- Virus: genetic material enclosed by a protein coat.
- Viroid: infectious RNA without a protein coat.
Lichens
A lichen is a close association between a fungus and a photosynthetic partner, usually a green alga or cyanobacterium. The photosynthetic partner produces organic food. The fungal partner provides a supporting environment and helps obtain water and minerals.
Lichens illustrate how two different organisms can live together in a mutually beneficial association.
Quick Comparison
- Monera: prokaryotic; no membrane-bound nucleus.
- Protista: mainly unicellular and eukaryotic.
- Fungi: eukaryotic; absorb nutrients; usually have chitin-containing cell walls.
- Plantae: mainly photosynthetic eukaryotes with cellulose-containing cell walls.
- Animalia: multicellular eukaryotes that ingest food and lack cell walls.
- Viruses and viroids: infectious agents outside the five cellular kingdoms.
- Lichens: an association involving a fungus and a photosynthetic partner.
Check Your Understanding
1. Why was the two-kingdom system inadequate?
It placed organisms with very different cell structures and methods of nutrition together and could not clearly accommodate many microorganisms and fungi.
2. What is the main cellular difference between Monera and Protista?
Members of Monera are prokaryotic; protists are eukaryotic and have a membrane-bound nucleus.
3. Name the five kingdoms.
Monera, Protista, Fungi, Plantae and Animalia.
4. What is a lichen?
It is a close association between a fungus and a photosynthetic partner such as an alga or cyanobacterium.
5. How does a viroid differ from a virus?
A viroid is infectious RNA without a protein coat, whereas a virus has genetic material enclosed by a protein coat.
Continue Learning
Return to the CBSE Class 11 Biology – Complete Study Guide to browse the chapters. Continue with Chapter 3: Plant Kingdom when its notes are available.
Frequently Asked Questions
What are the five kingdoms in Whittaker’s classification?
They are Monera, Protista, Fungi, Plantae and Animalia.
Which criteria are used in five-kingdom classification?
Cell structure, body organisation, mode of nutrition, reproduction and evolutionary relationships are considered.
Which organisms belong to Monera?
Monera includes prokaryotic organisms such as bacteria and cyanobacteria.
What is a prokaryotic cell?
It is a cell without a membrane-bound nucleus or membrane-bound organelles.
What are archaebacteria?
They are prokaryotes with distinctive cell chemistry; many are adapted to environments such as high salt or heat.
What are cyanobacteria?
They are photosynthetic prokaryotes, sometimes called blue-green algae.
What are mycoplasmas?
They are very small, wall-less prokaryotes that can change shape.
Which organisms belong to Protista?
Protista includes diverse, mostly unicellular eukaryotes, including diatoms, euglenoids and protozoans.
What are diatoms known for?
Diatoms are photosynthetic protists with silica-rich cell walls.
What are euglenoids?
Euglenoids are flagellated protists; many can photosynthesise in light and obtain nutrients differently in darkness.
How do fungi obtain food?
Fungi absorb nutrients from organic material; many are saprophytes or parasites.
What is the main component of a fungal cell wall?
Chitin is a major component of fungal cell walls.
What is a lichen?
A lichen is a close association between a fungus and a photosynthetic alga or cyanobacterium.
Why are viruses considered acellular?
They lack cellular structure and reproduce only using the machinery of a host cell.
How are viroids different from viruses?
Viroids are infectious RNA molecules without a protein coat; viruses have genetic material enclosed in a protein coat.