Biological Classification

Classification systems have changed over time because scientists kept improving their understanding of organisms. Earlier classifications were based mainly on simple morphological characters and grouped organisms as plants or animals. Later, it was realised that this was not enough, because many organisms did not fit neatly into either group.

For example:
- Aristotle classified plants into trees, shrubs and herbs and animals into those with red blood and those without.
- Linnaeus proposed a two-kingdom system: Plantae and Animalia.
- This system was found inadequate because it did not separate:
- prokaryotes and eukaryotes,
- unicellular and multicellular organisms,
- photosynthetic and non-photosynthetic organisms.
- So, other features were added, such as cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, and evolutionary relationships.
- Finally, R.H. Whittaker proposed the five-kingdom classification: Monera, Protista, Fungi, Plantae and Animalia.
- Later, a three-domain system was also proposed, which divided Monera into two domains and placed the rest in a third domain.

Thus, classification keeps changing as our knowledge of organisms and their evolutionary relationships improves.

The cell walls of diatoms are made of two thin overlapping shells that fit together like a soap box. These walls are embedded with silica, so they are indestructible.

Viroids are different from viruses in that viroids are only free RNA molecules and do not have a protein coat. Viruses, on the other hand, contain nucleic acid plus a protein coat called the capsid.

So:
- Viroid = infectious RNA only
- Virus = nucleic acid + protein coat

Also, viroids are smaller than viruses and were first discovered as the cause of potato spindle tuber disease.

The four major groups of protozoans are:

1. Amoeboid protozoans – They live in fresh water, sea water or moist soil and move with pseudopodia. Example: Amoeba. Some marine forms have silica shells, and some like Entamoeba are parasitic.
2. Flagellated protozoans – They are free-living or parasitic and move with flagella. Parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.
3. Ciliated protozoans – They are aquatic and move actively by cilia. The cilia help sweep food into the gullet. Example: Paramoecium.
4. Sporozoans – They have an infectious spore-like stage in their life cycle. Example: Plasmodium, which causes malaria.

Yes. The chapter names Bladderwort, Venus fly trap, and Cuscuta as examples of plants that are partially heterotrophic. Bladderwort and Venus fly trap are insectivorous plants, while Cuscuta is a parasite.

In lichens, the two partners are:
- Phycobiont = the algal component
- Mycobiont = the fungal component

The algae prepare food, and the fungi provide shelter, water and mineral nutrients.