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NCERT Solutions - Class 11 Biology Chapter 2: Biological Classification

Exercises
Q1. Discuss how classification systems have undergone several changes over a period of time?
Answer - Classification systems have evolved to reflect better understanding of living organisms.

  • Initially, organisms were grouped as either plants or animals (two-kingdom system).
  • Later, the three-kingdom and five-kingdom systems were proposed based on complexity and nutrition.
  • Modern classification includes six-kingdom and three-domain systems based on molecular biology and genetic evidence.
Q2. State two economically important uses of:
(a) heterotrophic bacteria
(b) archaebacteria
Answer -

(a) Heterotrophic bacteria:
  • Used in making curd, cheese, and vinegar.
  • Help in decomposing organic matter and recycling nutrients in nature.

(b) Archaebacteria:
  • Methanogens are used in biogas production.
  • Play an important role in sewage treatment under anaerobic conditions.
Q3. What is the nature of cell-walls in diatoms?
Answer - The cell wall of diatoms is made of silica. It forms two overlapping shells that fit together like a soap box.

These walls are very strong and resist decay, which is why diatomaceous earth is formed from their deposits.
Q4. Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
Answer -
  • Algal bloom: A rapid increase in the population of algae in water bodies, often due to excess nutrients.
  • Red tides: Discoloration of seawater caused by harmful algal blooms (especially dinoflagellates) that release toxins and affect marine life.
Q5. How are viroids different from viruses?
Answer - Viroids are simpler than viruses.

  • Viroids are small, circular RNA molecules without a protein coat.
  • Viruses have both genetic material (DNA or RNA) and a protective protein coat.
Q6. Describe briefly the four major groups of Protozoa.
Answer -
  • Amoeboid protozoa: Found in fresh water, sea water or moist soil. They move and capture food by forming pseudopodia (false feet). Example: Amoeba.

  • Flagellated protozoa: Possess flagella for locomotion. Some are free-living, while others are parasitic and cause diseases. Example: Trypanosoma causes sleeping sickness.

  • Ciliated protozoa: Have thousands of cilia covering their body. Cilia help in movement and in feeding. These organisms show a definite shape. Example: Paramecium.

  • Sporozoans: These are non-motile and parasitic. They reproduce by spores and often cause serious diseases in hosts. Example: Plasmodium (causes malaria).
Q7. Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Answer - Yes, some plants are partially heterotrophic.

Examples include:
  • Pitcher plant and Venus flytrap: Trap insects to get nitrogen.
  • Cuscuta (Amarbel): A parasitic plant that absorbs food from host plants.
Q8. What do the terms phycobiont and mycobiont signify?
Answer -
  • Phycobiont: The algal component in a lichen that performs photosynthesis.

  • Mycobiont: The fungal component in a lichen that provides water, minerals, and shelter.
Q9. Give a comparative account of the classes of Kingdom Fungi under the following:
(i) mode of nutrition
(ii) mode of reproduction
Answer -
Mode of Nutrition Mode of Reproduction
Zygomycetes Saprophytic or parasitic Asexual by sporangiospores, sexual by zygospores
Ascomycetes Saprophytic, parasitic, or coprophilous Asexual by conidia, sexual by ascospores in asci
Basidiomycetes Mostly saprophytic Asexual reproduction is rare; sexual by basidiospores
Deuteromycetes Mostly saprophytic or parasitic Asexual by conidia; sexual stage is absent
Q10. What are the characteristic features of Euglenoids?
Answer -
  • They are unicellular protists with both plant and animal features.
  • They have no cell wall but have a flexible pellicle.
  • Can perform photosynthesis in sunlight and act heterotrophically in dark.
  • They have two flagella and an eye spot to detect light.
Q11. Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Answer - Viruses are acellular and require a host cell to reproduce.

  • Structure: Composed of nucleic acid (DNA or RNA) and a protein coat (capsid).
  • Genetic material: Can be either DNA or RNA, but not both.
  • Examples of viral diseases: Influenza, Dengue, Polio, AIDS
Q12. Organise a discussion in your class on the topic – Are viruses living or non-living?
Answer - Viruses show characteristics of both living and non-living things:

  • Living: Can reproduce and mutate inside host cells.
  • Non-living: Inactive outside the host, no cellular structure, can be crystallized.

So, they are considered at the boundary of living and non-living organisms.