Exercises
Discuss how classification systems have undergone several changes over a period of time?
Answer
Classification systems have evolved significantly over time:
(i) Two-Kingdom system (Linnaeus): The earliest formal system divided all living organisms into Kingdom Plantae (plants) and Kingdom Animalia (animals). This was simple but failed to accommodate bacteria, fungi, and unicellular organisms properly.
(ii) Three-Kingdom system: Ernst Haeckel proposed adding Kingdom Protista for unicellular organisms.
(iii) Four-Kingdom system: Copeland added Kingdom Monera for prokaryotes.
(iv) Five-Kingdom system (Whittaker, 1969): Currently the most widely accepted system. Kingdoms are: Monera, Protista, Fungi, Plantae, and Animalia. The criteria used include cell type (prokaryotic/eukaryotic), body organisation (unicellular/multicellular), mode of nutrition (autotrophic/heterotrophic), and phylogenetic relationships.
(v) More recently, Six-Kingdom or Three-Domain systems have been proposed, incorporating molecular data. Carl Woese proposed Archaea as a separate domain from Bacteria.
Q2. State two economically important uses of
(a) heterotrophic bacteria (b) archaebacteria
Ans.
(a) Heterotrophic bacteria:
(i) Decomposers like Pseudomonas and Bacillus break down dead organic matter, recycling nutrients in the soil, which is vital for agriculture.
(ii) Lactobacillus is used in food industries for making curd, cheese, vinegar, and fermented foods.
(b) Archaebacteria:
(i) Methanogens (e.g., Methanobacterium) produce biogas (methane) from organic waste in biogas plants, serving as an alternative energy source.
(ii) Thermophilic archaebacteria produce heat-stable enzymes (e.g., Taq polymerase from Thermus aquaticus, used in PCR technology).
What is the nature of cell-walls in diatoms?
Answer
Diatoms have cell walls made of silica (silicon dioxide, SiO₂). Their cell walls are called frustules and consist of two overlapping halves (like a soap box with a lid), the upper half called the epitheca and the lower half called the hypotheca. These silica walls are extremely hard and do not decay easily. Deposits of diatom cell walls form diatomaceous earth, which is used as a filtering agent, in polishing materials, and as an insulating material.
Find out what do the terms 'algal bloom' and 'red-tides' signify.
Answer
Algal bloom: This refers to a rapid increase or accumulation of algae (mainly cyanobacteria or green algae) in a water body, often giving the water a green, blue-green, or scum-like appearance. It is usually caused by excess nutrients (especially nitrogen and phosphorus) in the water – a process called eutrophication. Algal blooms deplete oxygen in the water and produce toxins that are harmful to aquatic life and humans.
Red tides: These are caused by the explosive multiplication of certain dinoflagellates (red or brown pigmented protists like Gonyaulax) in coastal waters. The dense population colours the water red or reddish-brown. Red tides release powerful neurotoxins that kill fish, shellfish, and marine animals, and can be harmful to humans who consume contaminated shellfish.
How are viroids different from viruses?
Answer
Viroids: Discovered by T.O. Diener, viroids are the smallest known pathogens – they are free, naked, circular RNA molecules without a protein coat (no capsid). They infect only plants and replicate using the host's enzymes. Example: Potato spindle tuber viroid.
Viruses: Viruses are nucleoproteins – they have genetic material (DNA or RNA) enclosed within a protein coat called a capsid. They can infect plants, animals, and bacteria (bacteriophages). They are larger than viroids and more structurally complex.
Key difference: Viroids lack a protein coat entirely, whereas viruses always have a protein coat. Viroids contain only RNA; viruses contain either DNA or RNA but not both.
Describe briefly the four major groups of Protozoa.
Answer
The four major groups of Protozoa are:
(i) Amoeboid protozoans (Amoeboids): Move and capture food using pseudopodia (false feet). They are found in fresh water, sea water, and moist soil. Some are parasitic. Example: Amoeba, Entamoeba (causes amoebic dysentery).
(ii) Flagellated protozoans (Flagellates): Move using flagella. They may be free-living or parasitic. Example: Trypanosoma (causes sleeping sickness), Euglena (has chloroplasts in some forms).
(iii) Ciliated protozoans (Ciliates): Move using cilia. They are aquatic, actively moving organisms with a gullet for ingesting food. Example: Paramecium. They have two types of nuclei: macronucleus and micronucleus.
(iv) Sporozoans: Have an infectious spore-like stage in their life cycle. They are endoparasites. Example: Plasmodium (causes malaria). They lack locomotory structures in adult stage.
Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Answer
Yes, some plants are partially heterotrophic (they supplement their autotrophic nutrition by capturing and digesting prey or are partially parasitic):
(i) Insectivorous plants: Drosera (sundew), Nepenthes (pitcher plant), Dionaea (Venus flytrap). These plants grow in nitrogen-poor soils and supplement their nutrition by trapping and digesting insects.
(ii) Partial parasites: Viscum album (mistletoe) is a partial parasite – it has chlorophyll and can photosynthesise, but obtains water and minerals from its host plant.
(iii) Cuscuta is a total parasite with no chlorophyll, but some related species may be semi-parasitic.
What do the terms phycobiont and mycobiont signify?
Answer
These terms refer to the two partners in the symbiotic association found in lichens:
Phycobiont: The photosynthetic partner in a lichen, which is either a green alga or a cyanobacterium. It provides organic food through photosynthesis.
Mycobiont: The fungal partner in a lichen. It provides shelter, water, and minerals to the alga. The fungus also protects the alga from desiccation.
Lichens are excellent examples of mutualistic symbiosis where both partners benefit from the association.
Give a comparative account of the classes of Kingdom Fungi under the following: (i) mode of nutrition (ii) mode of reproduction
Answer
Phycomycetes: (i) Saprophytic or parasitic. (ii) Asexual reproduction by sporangiospores or zoospores; sexual reproduction produces oospores or zygospores. Example: Rhizopus, Albugo.
Ascomycetes: (i) Saprophytic, coprophilous (growing on dung), or parasitic. (ii) Asexual by conidia; sexual reproduction produces ascospores in sac-like asci. Example: Aspergillus, Penicillium, Saccharomyces.
Basidiomycetes: (i) Saprophytic, parasitic, or symbiotic. (ii) Asexual reproduction uncommon (by fragmentation); sexual reproduction produces basidiospores on club-shaped basidia. Example: Agaricus (mushroom), Puccinia (rust).
Deuteromycetes: (i) Saprophytic or parasitic. (ii) Only asexual reproduction known (by conidia); sexual stage absent or unknown – hence called "Fungi Imperfecti". Example: Alternaria, Colletotrichum.
What are the characteristic features of Euglenoids?
Answer
Characteristic features of Euglenoids (e.g., Euglena):
(i) They are unicellular, freshwater, flagellated organisms found mostly in stagnant water.
(ii) They lack a true cell wall; instead, have a flexible, protein-rich layer called pellicle.
(iii) They have two flagella – one short and one long – for locomotion.
(iv) They are mixotrophic: in sunlight they are autotrophic (contain chlorophylls a and b); in darkness they become heterotrophic, absorbing organic matter from surroundings.
(v) They have a red eye-spot (stigma) that is photosensitive, helping them move towards light.
(vi) They reproduce only by asexual means (longitudinal binary fission).
Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Answer
Structure of viruses: Viruses are non-cellular, obligate intracellular parasites. They consist of:
(i) Genetic material: Either DNA or RNA (never both), which may be single-stranded (ss) or double-stranded (ds), linear or circular. DNA viruses: bacteriophages, poxvirus. RNA viruses: TMV, influenza virus, HIV.
(ii) Protein coat (capsid): Made of protein subunits called capsomeres, which protect the nucleic acid.
(iii) Envelope: Some animal viruses have a lipid-protein envelope surrounding the capsid (enveloped viruses).
Viruses that infect bacteria are called bacteriophages. Plant viruses generally have single-stranded RNA; animal viruses may have either DNA or RNA.
Four common viral diseases: Influenza (flu), Mumps, Smallpox (eradicated), Common cold (rhinovirus).
Organise a discussion in your class on the topic – Are viruses living or non-living?
Answer
This is a discussion-based question. Key points:
Viruses show characteristics of living organisms: They contain nucleic acid (genetic material), can reproduce inside host cells, mutate and evolve, and show host specificity.
Viruses show characteristics of non-living matter: They can be crystallised (like inert chemicals), do not have cellular organisation, cannot carry out metabolism on their own, do not grow or respond to stimuli independently, and are inert outside a host cell.
Conclusion: Viruses occupy a position at the borderline between living and non-living. They are best described as obligate intracellular parasites that are non-living outside the host but show life-like properties inside the host cell.
Important Extra Questions
Name the scientist who proposed the Five Kingdom classification and the criteria used.
Answer
R.H. Whittaker (1969) proposed the Five Kingdom classification. The main criteria used were: cell type (prokaryotic/eukaryotic), body organisation (unicellular/multicellular), cell wall composition, mode of nutrition (autotrophic/heterotrophic/absorptive), and ecological role (producers, consumers, decomposers).
What are mycorrhizae? Why are they important?
Answer
Mycorrhizae are mutualistic associations between fungi (mostly Basidiomycetes or Glomeromycetes) and roots of higher plants. The fungal hyphae greatly increase the absorptive surface of roots for water and minerals (especially phosphorus), while the plant provides carbohydrates to the fungus. Mycorrhizae are vital for plant growth, especially in nutrient-poor soils.
Differentiate between Eubacteria and Archaebacteria.
Answer
Eubacteria (true bacteria) have peptidoglycan in their cell walls and are found in common environments. Archaebacteria lack peptidoglycan and live in extreme environments (halophiles in salt pans, thermoacidophiles in hot springs, methanogens in marshy areas). Their cell membrane lipids have ether linkages (not ester linkages as in other organisms), making them more stable at extremes of temperature and pH.
What is the significance of lichens as indicators?
Answer
Lichens are excellent bioindicators of air pollution. They are extremely sensitive to sulphur dioxide (SO₂) in the atmosphere. The absence of lichens in an area typically indicates heavy air pollution. They also serve as pioneer colonisers in ecological succession, growing on bare rocks and preparing the substratum for other organisms.
What are the differences between Kingdom Monera and Kingdom Protista?
Answer
Kingdom Monera consists of prokaryotes – unicellular organisms lacking a defined nucleus or membrane-bound organelles (e.g., bacteria, cyanobacteria). Kingdom Protista consists of eukaryotes – unicellular organisms with a well-defined nucleus and membrane-bound organelles (e.g., Amoeba, Paramecium, diatoms, Euglena). Protists are evolutionarily more advanced than monerans.