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SubjectFree lesson

Classification of Living Things

ClassNotes Team 5 MIN READUPDATED 4 JUL 2026

Biology SSS 1 First Term

WEEK 2

Classification of Living Things

Performance Objectives

Student should be able to:

  1. Classify organisms into the kingdoms monera and protista
  2. Give the characteristics of Kingdom monera and protista

Content

All living organisms are classified into groups based on very basic, shared characteristics. Organisms within each group are then further divided into smaller groups. These smaller groups are based on more detailed similarities within each larger group. This grouping system makes it easier for scientists to study certain groups of organisms. Characteristics such as appearance, reproduction, mobility, and functionality are just a few ways in which living organisms are grouped together. The classification of living things includes 7 levels: kingdom, phylum, classes, order, families, genus, and species.

Kingdoms

The most basic classification of living things is kingdoms. Currently there are five kingdoms. Living things are placed into certain kingdoms based on how they obtain their food, the types of cells that make up their body, and the number of cells they contain.

Carolus Linneaus classified all living organisms into two major kingdoms namely plant kingdom and animal kingdom. Under this arrangement, lots of one-celled organisms could not fit in properly. Many biologists then decided to place all living things into five kingdoms. These kingdoms are:

1. Monera

2. Protista

3. Fungi

4. Plantae

5. Animalia

Kingdom Monera

Kingdom Monera belongs to the prokaryote family. The organisms belonging to this kingdom do not contain a true nucleus. These are the oldest known microorganisms on earth. Their DNA is not enclosed within the nucleus. They are unicellular organisms found mostly in a moist environment. They are found in hot springs, snow, deep oceans or as parasites in other organisms. The monerans do not possess any membrane-bound organelles.

Characteristics of Kingdom Monera

  1. They are unicellular or single-celled organisms.
  2. The DNA is naked and is not bound by a nuclear membrane i.e. the cells have no definite nucleus.
  3. The cells lack organelles like mitochondria, lysosomes, chloroplasts, golgi bodies, plastids, endoplasmic reticulum etc.
  4. There is no sexual reproduction.
  5. The produce asexually by binary fission or budding.
  6. They show different modes of nutrition such as autotrophic, parasitic, heterotrophic, or saprophytic.
  7. They may be motile or non-motile in nature.
  8. Flagella serve as the locomotory organ.
  9.  The cell wall does not contain cellulose, but polysaccharides and amino acids i.e peptidoglycan.
  10. They do not have complex chromosomes.

Example of organisms in this kingdom is bacteria and blue-green alga.

Bacteria

Bacteria are microscopic organisms that can survive in diverse environments. They can be beneficial as well as harmful. They possess a simple structure without a nucleus and a few cell organelles.

The bacteria are surrounded by two protective coverings – the outer cell wall and the inner cell membrane. Some bacteria are also covered by a capsule. Few bacteria like Mycoplasma do not have cell wall. Short whip-like extensions known as pili surround the surface of the bacteria. The long whip-like structures are known as flagella. They exhibit autotrophic and heterotrophic modes of nutrition.

Kingdom Protista

Protists are simple eukaryotic organisms that are neither animals, plants nor fungi. Protists are unicellular in nature, or they can be found as a colony of cells. Most protists live in water, damp terrestrial environments, or even as parasites. These organisms are usually unicellular and the cells of these organisms contain a nucleus which is bound to organelles. Some of them even possess structures like flagella or cilia.

Protists are traditionally considered as the first eukaryotic forms of life and a predecessor to plant, animals, and fungi.

Characteristics of Kingdom Protista

  1. Most protist species are unicellular organisms, however, there are a few multicellular protists such as kelp.
  2. They are usually aquatic, present in the soil or in areas with moisture.
  3. They are eukaryotes i.e. cells with membrane.
  4. Some protista are heterotrophic (including parasitic forms) while some are both heterotrophic and photosynthetic.
  5. Parasitism is observed in protists such as Trypanosoma protozoa which can cause sleeping sickness in humans.
  6. Protists exhibit locomotion through cilia and flagella. A few organisms belonging to kingdom Protista have pseudopodia that help them move.
  7. Mode of reproduction is usually by mitosis while some have sexual reproduction by fusion of gametes. The sexual method of reproduction is extremely rare and occurs only during time of stress.

Example of organisms in this kingdom include

  1. Protozoa e.g. Amoeba, Paramecium, Trapenosoma and Plasmodium.
  2. Moulds
  3. Algae e.g. Volvox, Netrium, Spirogyra etc.

Protozoa

Protozoans are unicellular organisms. These are also called animal protists. All protozoans are heterotrophic i.e. they feed on other organisms. There are also parasitic protozoans which live in the cells of larger organisms. Most of the members do not have a predefined shape. For instance, amoeba can change its shape indefinitely but a paramecium has a definite slipper-like shape. The protozoans can be divided into four major groups: amoeboid (amoeba), flagellated (euglena), ciliated (paramecium), and sporozoans (plasmodium).

Moulds

Moulds are saprophytic organisms (they feed on the dead and decaying matter). They are basically divided into two types viz. Slime moulds and the water moulds. These are tiny organisms that have many nuclei. Usually moulds are characterized by the presence of spores and are even visible to the naked eye.

Algae

These form another category under Kingdom Protista. These are generally single-celled or multicellular organisms. These are photosynthetic, found mostly in freshwater sources or marine lakes. They are characterized by a stiff cell wall.

The algae are divided into six types, namely, green algae, brown algae, red algae, diatoms, pyrrophytes, and euglenoids.