Grade 8 Chapter 1: Living World and Classification of Microbes

Introduction

The living world is incredibly diverse, with millions of species inhabiting every corner of our planet. From the smallest bacteria to the largest whales, each organism plays a unique role in the ecosystem. To understand this vast diversity, scientists have developed systems to classify living things. This chapter explores the concept of biodiversity, the history and importance of classification, and delves into the fascinating world of microorganisms.

Biodiversity and Need for Classification

What is Biodiversity?

Biodiversity refers to the variety of living organisms in a specific area or on Earth. It encompasses the diversity of genes within a species, the diversity of species in an ecosystem, and the diversity of ecosystems themselves.

Why Classification is Necessary?

With millions of known species and many more yet to be discovered, studying each organism individually is impossible. Classification provides a systematic way to:

• Organize and group organisms based on their similarities and differences.
• Simplify the study of living things.
• Identify new organisms easily.
• Understand the relationships and evolutionary history among different groups.
• Predict characteristics of newly discovered organisms based on their group.

History of Classification

Carl Linnaeus (1707-1778) - Father of Taxonomy

Carl Linnaeus, a Swedish botanist, zoologist, and physician, is considered the 'Father of Taxonomy'. He developed:

• Binomial Nomenclature: A system of giving each organism a two-part scientific name (Genus species), e.g., *Homo sapiens*.
• A hierarchical system of classification (Kingdom, Phylum, Class, Order, Family, Genus, Species).

Robert Whittaker's Five Kingdom Classification (1969)

The American ecologist Robert Whittaker proposed the Five Kingdom Classification system in 1969. This system is widely accepted and classifies all living organisms into five major kingdoms:

1. Monera
2. Protista
3. Fungi
4. Plantae
5. Animalia

Criteria for Whittaker's Classification

Whittaker based his classification on the following five criteria:

1. Complexity of Cell Structure: Whether the cell is prokaryotic (lacking a true nucleus and membrane-bound organelles) or eukaryotic (having a true nucleus and membrane-bound organelles).
2. Complexity of Organism: Whether the organism is unicellular (single-celled) or multicellular (many-celled).
3. Mode of Nutrition: How the organism obtains its food (autotrophic - produces its own food; heterotrophic - obtains food from others; saprotrophic - absorbs nutrients from dead organic matter).
4. Life Style: The organism's role in the ecosystem (Producers - make food; Consumers - eat food; Decomposers - break down dead matter).
5. Phylogenetic Relationship: The evolutionary history and relationships among different groups of organisms.

The Five Kingdoms

1. Kingdom Monera

Characteristics:

• All organisms are unicellular.
• They are prokaryotic, meaning they do not have a true nucleus or membrane-bound organelles.
• They may be autotrophic (e.g., cyanobacteria performing photosynthesis) or heterotrophic (e.g., most bacteria absorbing food).
• They can live independently or as parasites.
• Most reproduce by simple binary fission.

Examples: Various types of bacteria (*E. coli*, *Clostridium*, *Streptococcus pneumoniae*) and cyanobacteria (blue-green algae).

2. Kingdom Protista

Characteristics:

• All organisms are unicellular.
• They are eukaryotic, possessing a well-defined nucleus and membrane-bound organelles.
• They exhibit diverse modes of nutrition: some are autotrophic (e.g., Euglena, Volvox), while others are heterotrophic (e.g., Amoeba, Paramoecium).
• Many have structures for locomotion, such as pseudopodia (false feet), cilia, or flagella.
• Reproduction is primarily asexual (binary fission).

Examples: Amoeba, Paramoecium, Euglena, Volvox.

3. Kingdom Fungi

Characteristics:

• These organisms are eukaryotic.
• They are mostly multicellular, except for yeast, which is unicellular.
• They are heterotrophic and primarily saprotrophic, meaning they obtain nutrition by absorbing organic matter from dead and decaying organisms.
• Their cell wall is made of a complex carbohydrate called chitin.
• They reproduce by budding (yeast) or by forming spores.

Examples: Yeast (*Saccharomyces cerevisiae*), Mushrooms, *Penicillium*, *Aspergillus*, *Mucor*, *Candida*.

4. Kingdom Plantae

Characteristics:

• All organisms are multicellular and eukaryotic.
• They are primarily autotrophic, performing photosynthesis using chlorophyll.
• They have a rigid cell wall made of cellulose.
• They are producers in most ecosystems.

Examples: Trees, shrubs, herbs, ferns, mosses, flowering plants.

5. Kingdom Animalia

Characteristics:

• All organisms are multicellular and eukaryotic.
• They are heterotrophic, obtaining nutrition by ingesting food.
• They lack a cell wall.
• They are consumers in most ecosystems.
• Most are capable of locomotion.

Examples: Humans, mammals, birds, fish, insects, reptiles, amphibians.

Classification of Microbes

Microorganisms or microbes are living organisms that are too small to be seen with the naked eye. They can only be observed under a microscope. They are broadly classified into the following types:

1. Bacteria
2. Protozoa
3. Fungi (Microscopic)
4. Algae (Microscopic)
5. Viruses

Size of Microbes

Microbes are measured in micrometers (µm) and nanometers (nm).

Detailed Characteristics of Microorganisms

1. Bacteria

• Size: 1 µm to 10 µm.
• Structure: Unicellular, prokaryotic organisms. They can exist independently or as parasites.
• Shapes:
  • Cocci (spherical)
  • Bacilli (rod-shaped)
  • Spirilla (spiral-shaped)
  • Vibrio (comma-shaped)
• Reproduction: Primarily by simple binary fission (one cell divides into two). In unfavorable conditions, they form endospores to survive.
• Nutrition: Can be autotrophic (e.g., some sulfur bacteria) or heterotrophic.

2. Protozoa

• Size: Approximately 200 µm.
• Structure: Unicellular, eukaryotic organisms with great diversity in structure.
• Habitat: Typically found in soil, fresh water, sea water, or as parasites in the bodies of other organisms.
• Reproduction: Reproduce by simple cell division (binary fission).
• Nutrition: Heterotrophic.
• Locomotion: Move using pseudopodia (Amoeba), cilia (Paramoecium), or flagella (Euglena).

Examples: *Amoeba proteus*, *Paramoecium caudatum*, *Euglena*, *Entamoeba histolytica* (causes amoebiasis), *Plasmodium vivax* (causes malaria).

3. Fungi (Microscopic)

• Size: 10 µm to 500 µm.
• Structure: Non-green, eukaryotic organisms. Most are saprotrophic, growing on decaying organic matter.
• Cell Wall: Made of tough and complex carbohydrate called chitin.
• Reproduction: Reproduce sexually and asexually by budding (yeast) or by forming spores.

Examples: Yeast (*Saccharomyces cerevisiae*), *Candida*, *Mucor* (bread mould), *Aspergillus*.

4. Algae (Microscopic)

• Size: 10 µm to 100 µm.
• Structure: Unicellular or multicellular, eukaryotic organisms.
• Nutrition: Autotrophic, as they contain chlorophyll and perform photosynthesis.
• Habitat: Mostly aquatic (found in fresh and marine water).

Examples: *Chlamydomonas*, *Chlorella*, *Spirogyra* (multicellular filamentous algae).

5. Viruses

• Size: 10 nm to 100 nm.
• Structure: Extremely small, acellular (not made of cells) particles. They consist of genetic material (DNA or RNA) surrounded by a protein coat.
• Existence: They do not exist independently. They can only reproduce by living inside the cells of other organisms (plants, animals, bacteria). They are obligate intracellular parasites.
• Boundary Organisms: Viruses are considered at the boundary of living and non-living things because they behave like non-living outside a host cell but replicate and show life characteristics inside a host.

Examples: HIV (Human Immunodeficiency Virus), common cold virus, influenza virus, plant viruses (e.g., Tobacco Mosaic Virus), bacteriophages (viruses that infect bacteria).

Summary

• The living world exhibits vast biodiversity, necessitating classification for systematic study.
• Robert Whittaker's Five Kingdom Classification system categorizes organisms into Monera, Protista, Fungi, Plantae, and Animalia based on cell structure, organism complexity, nutrition, lifestyle, and phylogeny.
• Microorganisms are tiny living beings observed only under a microscope, including bacteria, protozoa, fungi, algae, and viruses.
• Bacteria are unicellular prokaryotes with diverse shapes, reproducing by binary fission.
• Protozoa are unicellular eukaryotes with varied locomotion, found in water or as parasites.
• Fungi are eukaryotic, mostly multicellular (except yeast), saprotrophic, with chitin cell walls.
• Algae are autotrophic, eukaryotic, and mostly aquatic, performing photosynthesis.
• Viruses are acellular, obligate intracellular parasites, considered at the boundary of living and non-living.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 1: Living World and Classification of Microbes.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.