Chapter 2 Solutions — Biological Classification

Diatoms belong to Chrysophytes in Kingdom Protista.

Nature of cell walls in diatoms:

1. The cell walls of diatoms form two thin overlapping shells (called frustules) that fit together like the two halves of a soap box (one half fits inside the other).

2. The cell walls are embedded with silica, making them extremely hard and indestructible.

3. These silica-impregnated walls do not decay easily. Over billions of years, the accumulated deposits of diatom cell walls form diatomaceous earth (also called kieselguhr).

4. Diatomaceous earth is gritty in texture and is commercially used in polishing, and for filtration of oils and syrups.

Viroids differ from viruses in the following ways:

1. Genetic material: Viroids consist of only a free, naked RNA molecule of very low molecular weight. Viruses contain either RNA or DNA enclosed within a protein coat (capsid).

2. Protein coat: Viroids lack a protein coat (capsid) entirely. Viruses possess a protein coat (capsid) made of capsomeres that protects the nucleic acid.

3. Size: Viroids are smaller than viruses.

4. Discovery: Viroids were discovered by T.O. Diener in 1971. Viruses were identified earlier (Ivanowsky, 1892; Beijerinck, 1898; Stanley, 1935).

5. Example of disease caused: Viroids cause potato spindle tuber disease. Viruses cause diseases like influenza, mumps, smallpox, AIDS (in animals/humans) and mosaic disease (in plants).

Although most plants are autotrophic (they prepare their own food through photosynthesis), some plants are partially heterotrophic. These are of two types:

1. Insectivorous (carnivorous) plants: These plants grow in nitrogen-poor soils and supplement their nutrition by trapping and digesting insects and other small organisms.

- Examples: Bladderwort (Utricularia), Venus fly trap (Dionaea muscipula), Sundew (Drosera), Pitcher plant (Nepenthes).

- They are autotrophic for carbon (via photosynthesis) but obtain nitrogen and minerals heterotrophically from insects.

2. Parasitic plants: These plants depend on other living plants (hosts) for water, minerals, and sometimes organic food.

- Example: Cuscuta (dodder) is a total stem parasite that lacks chlorophyll and entirely depends on the host.

- Partial parasites like Mistletoe (Viscum album) have chlorophyll and can photosynthesize but obtain water and minerals from the host.

Thus, insectivorous plants and parasitic plants represent plants that are partially or fully heterotrophic.