11th chapter 2

BIOLOGICAL CLASSIFICATION

1. Introduction to Classification of Living Organisms

• Throughout history, classification of organisms was largely instinctive and based on human utilization (food, shelter, clothing).

• Aristotle's Contribution:

  • Aristotle attempted a systematic classification using morphological features.

  • He categorized plants into trees, shrubs, and herbs, and animals into two groups (those with red blood and those without).

• Linnaeus' Contribution:

  • Introduced a Two Kingdom classification system:

  • Plantae and Animalia kingdoms.

  • This system failed to distinguish between prokaryotes and eukaryotes, unicellular and multicellular organisms, and various modes of nutrition in organisms.

  • Issues arose because many organisms did not fit neatly into these classifications.

2. Evolution of Classification Systems

• The inadequacies of the Two Kingdom system prompted further refinement.

• Criteria for classification were expanded beyond gross morphology to include:

  • Cell structure

  • Nature of the cell wall

  • Mode of nutrition

  • Habitat

  • Methods of reproduction

  • Evolutionary relationships.

3. Five Kingdom Classification by R. H. Whittaker (1969)

• Proposed five kingdoms:

  • Monera

  • Protista

  • Fungi

  • Plantae

  • Animalia

• Main criteria for classification used include:

  • Cell structure

  • Body organization

  • Mode of nutrition

  • Reproductive methods

  • Phylogenetic relationships.

Table 2.1: Characteristics of the Five Kingdoms

4. The Kingdom Monera

• Characteristics:

  • Only members are bacteria; they are the most abundant microorganisms.

  • Found in diverse habitats, including soil, extreme environments (hot springs, deserts, etc.).

  • Can be parasitic or symbiotic.

• Classification based on Shape:

  • Cocci: Spherical (pl. cocci)

  • Bacilli: Rod-shaped (pl. bacilli)

  • Vibrium: Comma-shaped (pl. vibrio)

  • Spirilla: Spiral-shaped (pl. spirilla)

4.1. Archaebacteria

• Thrive in extreme environments (e.g., halophiles, thermoacidophiles, methanogens).

• Different cell wall structure enables survival in harsh conditions.

• Methanogens in ruminants produce methane from dung.

4.2. Eubacteria

• Contain rigid cell walls; motile species have flagella.

• Cyanobacteria: Photosynthetic, contain chlorophyll a; form blooms in polluted waters, fix atmospheric nitrogen in heterocysts.

• Chemosynthetic bacteria play critical roles in nutrient recycling; significant in food production and antibiotic synthesis.

• Reproduce mainly through fission; capable of forming spores under stress.

• Mycoplasma: Smallest known cells without cell walls; can thrive without oxygen.

5. The Kingdom Protista

• Encompasses all single-celled eukaryotes; boundaries not well defined.

• Morphological characteristics can vary, leading to varying classifications amongst biologists.

• Included Groups:

  • Chrysophytes (Diatoms, golden algae)

  • Dinoflagellates

  • Euglenoids

  • Slime Moulds

  • Protozoans

• Reproduce both asexually and sexually; possess defined nuclei and membrane-bound organelles.

5.1. Chrysophytes

• Microscopic; photosynthetic, mainly aquatic.

• Diatom cell walls form two overlapping shells made of silica, leading to deposits known as diatomaceous earth.

5.2. Dinoflagellates

• Mainly marine, photosynthetic, exhibit color variations based on pigments.

• Have rigid cell walls and two flagella; red tides can occur with population explosions.

5.3. Euglenoids

• Freshwater organisms lacking a cell wall but possessing a flexible pellicle.

• Have both autotrophic (sunlight) and heterotrophic capabilities, depending on environmental conditions.

5.4. Slime Moulds

• Saprophytic protists; engulf organic material.

• In unfavorable conditions, form spores that can survive long periods.

5.5. Protozoans

• Heterotrophic and primarily aquatic.

  • Amoeboid Protozoans: Use pseudopodia for movement and capturing prey. Eg: Entamoeba are parasites

  • Flagellated Protozoans: These organisms have flagella, and some are parasitic. Eg: Trypanosoma (Sleeping sickness)

  • Ciliated Protozoans: Use thousands of cilia for movement and capturing food. Eg: Paramoecium

  • Sporozoans: Notable for having sporic stages; Plasmodium causes malaria.

6. The Kingdom Fungi

• Diverse morphology and habitat; primarily heterotrophic.

• Common Forms: Mushrooms, molds, and yeasts; they play important roles in ecosystem health and human industry.

• Fungi consist of hyphae (thread-like structures) and mycelium (a network formed by hyphae).

• They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.

6.1. Types of Fungi

1. Phycomycetes:

   • Aquatic or on decaying wood; aseptate mycelium.

   • coenocytic (multiple nuclei)

   • Asexcual reproduce via zoospores (motile) or aplanospores (non-motile). These spores are endogenously produced in a sporangium.

   • A zygospore is formed by the fusion of two gametes. These gametes are similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).

   • Some common examples are Mucor (Figure 2.5a), Rhizopus (the bread mould mentioned earlier), and Albugo (the parasitic fungi on mustard).

2. Ascomycetes:

   • Commonly known as Sac fungi.

   • Sac fungi that produce ascospores in specialized structures; includes yeast and Penicillium.

   • They are saprophytic, decomposers, parasitic, or coprophilous (growing on dung)

   • Mycelium is branched and septate.

   • The asexual spores are conidia produced exogenously on the special mycelium called conidiophores

   • Sexual spores are called ascospores, which are produced endogenously in sac-like asci (singular ascus). These asci are arranged in different types of fruiting bodies called ascocarps.

   • Some examples are Aspergillus (Figure 2.5b), Claviceps, and Neurospora. Neurospora is used extensively in biochemical and genetic work. Many members like morels and truffles, which are edible and considered delicacies

3. Basidiomycetes:

   • Commonly known forms of basidiomycetes are mushrooms, bracket fungi, or puffballs.

   • The mycelium is branched and septate.

   • The asexual spores are generally not found, but vegetative reproduction by fragmentation is common

   • The sex organs are absent, but plasmogamy is brought about by the fusion of two vegetative or somatic cells of different strains or genotypes.

   • Include mushrooms; produce basidiospores in basidia and exist primarily as dikaryotic cells.

   • Some common members are Agaricus (mushroom) (Figure 2.5c), Ustilago (smut), and Puccinia (rust fungus).

4. Deuteromycetes:

   • Commonly known as imperfect fungi because only the asexual vegetative phase of these fungi is known

   • without known sexual reproduction; reproduces via asexual spores.

   • The deuteromycetes reproduce only by asexual spores known as conidia.

   • The fungi were correctly identified and moved out of deuteromycetes. Once perfect (sexual) stages of members of the dueteromycetes were discovered, they were often moved to the ascomycetes and basidiomycetes.

7. The Kingdom Plantae

• Includes all eukaryotic, chlorophyll-containing organisms (plants).

• Some exhibit partial heterotrophy (insectivorous or parasitic).

• Plant cells have distinct chloroplasts and cellulose cell walls.

• Major groups: algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

• Life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other

• Exhibit alternation of generations (haploid and diploid phases).

8. The Kingdom Animalia

• Multicellular, eukaryotic, heterotrophic organisms having no cell walls.

• Mode of nutrition is holozoic; develop through defined growth patterns.

• Store food reserves as glycogen or fat.

• Mode of nutrition is holozoic (organism ingest solid foods and injest)

• Reproduction is primarily sexual through copulation, leading to embryological development.

9. Viruses, Viroids, Prions, and Lichens

• Viruses do not fit into the traditional kingdoms as they are acellular and considered non-living.

  • Characteristics: Inert outside host cells, obligate parasites, consist of nucleic acid (RNA or DNA) and protein. No virus contains both RNA and DNA

  • Virus means venom or poisonous fluid.

  • viruses that infect plants have single stranded RNA and viruses that infect animals have either single-stranded or double-stranded RNA or double-stranded DNA.

  • The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.

• Viroids: Infectious agents consisting solely of RNA without a protein coat. caused- Potato spindle tuber

• Prions: Abnormal proteins causing neurological diseases.

• Lichens: Symbiotic relationships between fungi (mycobiont) and algae (phycobiont). Algae perform photosynthesis while fungi provide shelter and minerals.

10. Summary of Biological Classification

• Historical context from Aristotle to Linnaeus and then to Whittaker's five kingdom model.

• Characteristics of organisms based on structure, ecology and metabolism play crucial roles in classification.

• Classification systems have evolved alongside advances in biological understanding.

11. Exercises

• Discuss the changes in classification systems.

• Identify economically important uses of archaebacteria and heterotrophic bacteria.

• Describe the diatom cell wall structure, and the nature of algal blooms and red tides.

• Explain the differences between viroids and viruses.

• List the four major groups of protozoans and their characteristics.

• Explore the concept of partially heterotrophic plants and the terms phycobiont and mycobiont.

• Compare the different classes of Kingdom Fungi by modes of nutrition and reproduction.

• Investigate the structure of viruses and identify common viral diseases.

• Organize a discussion on living vs. non-living status of viruses.