Ch 2Biological Classification — Class 11 Biology, concept-first.

Since the earliest days of civilisation, people have tried to sort the living world into groups. At first this was not done scientifically at all -- it grew out of the practical need to use organisms…

Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms on the planet and occur almost everywhere.

Archaebacteria are special bacteria because they live in some of the most harsh habitats on Earth. On the basis of the extreme conditions they tolerate, they are described as:

There are thousands of different eubacteria, or 'true bacteria'. They are characterised by the presence of a rigid cell wall, and, if motile, a flagellum for movement.

All single-celled eukaryotes are placed under the Kingdom Protista, though the boundaries of this kingdom are not well defined.

The Chrysophytes include the diatoms and the golden algae (desmids). They are found in fresh water as well as in marine environments.

Dinoflagellates are mostly marine and photosynthetic organisms, and, like other protists, they are primarily aquatic.

The majority of Euglenoids are fresh water organisms found in stagnant water. A distinctive feature is that, instead of a cell wall, they have a protein-rich layer called the pellicle, which makes the…

Slime moulds are saprophytic protists, meaning they feed on dead and decaying organic matter. Their body moves along decaying twigs and leaves, engulfing organic material as it creeps forward.

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans:

The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat.

Members of the Phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places, or as obligate parasites on plants — that is, parasites that cannot survive without their host.

Commonly known as sac-fungi, the Ascomycetes are mostly multicellular (for example, Penicillium) or rarely unicellular (for example, yeast, Saccharomyces).

Commonly known forms of Basidiomycetes are mushrooms, bracket fungi and puffballs. They grow in soil, on logs and tree stumps, and in living plant bodies as parasites — for example, rusts and smuts.

The Deuteromycetes are commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.

The Kingdom Plantae includes all eukaryotic, chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic, such as the insectivorous plants or parasites.

The Kingdom Animalia is characterised by heterotrophic, eukaryotic organisms that are multicellular and whose cells lack cell walls. They directly or indirectly depend on plants for food.

In the five kingdom classification of Whittaker there is no mention of lichens and some acellular organisms like viruses, viroids and prions. These are briefly introduced here.

Biological classification of plants and animals was first proposed by Aristotle on the basis of simple morphological characters.

Higher-order thinking problems from the NCERT Exemplar.