Biological Classification: common doubts

Question 1

Why was the two kingdom system of classification replaced?

Accepted Answer

It was replaced because it forced every organism into either plants or animals and ignored major differences. It did not separate prokaryotes from eukaryotes, unicellular from multicellular forms, or photosynthetic green algae from non-photosynthetic fungi. Many organisms fitted neither group well, so a system with more kingdoms was proposed.

Question 2

Who proposed the five kingdom classification and what are the kingdoms?

Accepted Answer

R.H. Whittaker proposed it in 1969. The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia. He grouped organisms using cell structure, body organisation, mode of nutrition, mode of reproduction and evolutionary relationships. Fungi were given their own kingdom, and all prokaryotes were collected in Monera.

Question 3

Why are fungi not classified as plants?

Accepted Answer

Fungi are heterotrophs with cell walls made of chitin, while plants are autotrophs with cellulose walls. Older systems put both under plants simply because both have a wall, but that single shared feature hides large differences in nutrition and wall chemistry. Whittaker therefore placed fungi in a separate kingdom.

Question 4

Why are Chlamydomonas and Amoeba placed in the same kingdom?

Accepted Answer

Both are unicellular eukaryotes, which is the criterion for Kingdom Protista. Chlamydomonas has a cell wall and was once treated as an alga among plants, while Amoeba has no wall and was treated as an animal. Once cell type and body organisation became the basis, they moved together.

Question 5

What is the three-domain system?

Accepted Answer

It is a proposed scheme that splits the prokaryotes of Monera into two domains and places all the eukaryotic kingdoms in a third domain. Counting the two bacterial domains separately gives six kingdoms in total. It shows that classification keeps being revised as knowledge of characters and evolutionary relationships improves.

Question 6

Why is the cell wall of bacteria called non-cellulosic?

Accepted Answer

Bacterial walls are built from a polysaccharide combined with amino acids, not from cellulose. Cellulose is the wall material of plants, and chitin is that of fungi, so the bacterial wall is described as non-cellulosic to set it apart. Mycoplasma is the exception, since it has no cell wall at all.

Question 7

How are bacteria classified on the basis of shape?

Accepted Answer

Bacteria fall into four shapes: spherical coccus, rod-shaped bacillus, comma-shaped vibrio and spiral spirillum. The plural forms are cocci, bacilli and spirilla. Despite their simple structure, bacteria show the widest metabolic diversity of any group, with some making their own food and most feeding on other organisms.

Question 8

How do archaebacteria differ from eubacteria?

Accepted Answer

Archaebacteria live in extreme habitats such as salty places, hot springs and marshes, and their cell wall is built differently from that of other bacteria, which helps them withstand harsh conditions. Eubacteria are the true bacteria, with a rigid cell wall and a flagellum if motile. They include cyanobacteria and many other familiar types.

Question 9

What are halophiles, thermoacidophiles and methanogens?

Accepted Answer

They are three kinds of archaebacteria named after their habitat. Halophiles live in very salty places, thermoacidophiles live in hot acidic springs, and methanogens live in marshy areas and in the gut of ruminants like cows and buffaloes. Methanogens produce methane, which is why they are involved in biogas production from dung.

Question 10

What is the function of heterocysts in cyanobacteria?

Accepted Answer

Heterocysts are specialised cells in which some cyanobacteria fix atmospheric nitrogen. They occur in filamentous forms such as Nostoc and Anabaena. Cyanobacteria are photosynthetic autotrophs with chlorophyll a, like green plants, so heterocysts let the same filament make its own food and also obtain nitrogen from the air, which is why these cyanobacteria enrich the soil and paddy fields with nitrogen.

Question 11

Why are mycoplasmas special among bacteria?

Accepted Answer

Mycoplasmas have no cell wall at all. Among living cells they are the tiniest known, and they can live without oxygen. Many are pathogenic in animals and plants. Because they have no wall, they differ from all other bacteria, and this fact is often asked as a point of exception.

Question 12

Which organisms make up Kingdom Protista?

Accepted Answer

Kingdom Protista holds all single-celled eukaryotes: chrysophytes such as diatoms and golden algae, dinoflagellates, euglenoids, slime moulds and protozoans. Being eukaryotes, their cells have a well-defined nucleus and membrane-bound organelles, and some move with flagella or cilia. The boundaries of the kingdom are not sharply drawn, and protists form a link with the kingdoms of plants, animals and fungi.

Question 13

What is the difference between diatoms and dinoflagellates?

Accepted Answer

Diatoms have walls embedded with silica, built as two thin overlapping shells that fit together like a soap box, and their deposits form diatomaceous earth. Most dinoflagellates have stiff cellulose plates on the outer surface and two flagella, one lengthwise and one crosswise in a groove between the plates. Both are mostly photosynthetic, and red dinoflagellates such as Gonyaulax can multiply so fast that they cause red tides.

Question 14

Does Euglena have a cell wall?

Accepted Answer

No, Euglena has no cell wall. Instead it has a flexible pellicle, a protein-rich layer that lets the body change shape. Euglena is unusual because it is photosynthetic in sunlight but behaves as a heterotroph by feeding on small organisms when light is absent, which blurs the line between plants and animals.

Question 15

How do slime moulds survive unfavourable conditions?

Accepted Answer

Slime moulds survive bad conditions by forming fruiting bodies that bear spores at their tips. The spores have true walls, stay alive for many years even in harsh conditions and are carried by air currents. When conditions are favourable, the cells come together into a plasmodium that can spread over several feet while feeding saprophytically on decaying twigs and leaves.

Question 16

What are the four groups of protozoans and how do they move?

Accepted Answer

The four groups are amoeboid, flagellated, ciliated and sporozoan protozoans. Amoeboid forms such as Amoeba use pseudopodia. Flagellated forms such as Trypanosoma use flagella. Ciliated forms such as Paramoecium use cilia. Sporozoans such as Plasmodium have an infectious spore-like stage and no obvious locomotory structure.

Question 17

What is the difference between plasmogamy and karyogamy?

Accepted Answer

Plasmogamy is the fusion of the protoplasms of two cells, while karyogamy is the fusion of their two nuclei. In many fungi they do not happen together, so an intermediate stage with two nuclei in each cell, the dikaryon (n + n), occurs. This dikaryotic stage is seen in ascomycetes and basidiomycetes.

Question 18

What is the difference between a hypha and a mycelium?

Accepted Answer

A hypha is one thread-like filament of a fungal body, whereas the mycelium is the whole network of hyphae. Hyphae may be septate, divided by cross walls, or coenocytic, forming a continuous tube with many nuclei. The cell wall in most fungi is made of chitin and polysaccharides.

Question 19

What is the difference between ascospores and basidiospores?

Accepted Answer

Ascospores are formed inside a sac called the ascus, so they are endogenous spores, while basidiospores are formed outside on a club-shaped basidium, so they are exogenous. Ascomycetes such as Aspergillus, Claviceps and Neurospora make ascospores, whereas mushrooms, bracket fungi and puffballs, which are basidiomycetes, make basidiospores.

Question 20

Why are deuteromycetes called imperfect fungi?

Accepted Answer

They are called imperfect fungi because only their asexual or vegetative phase is known, since no sexual stage has been seen. They reproduce by asexual spores called conidia. Once the sexual stage of a deuteromycete is discovered, the fungus is moved into the ascomycetes or basidiomycetes. Alternaria and Trichoderma are examples.

Question 21

What is the difference between a virus and a viroid?

Accepted Answer

A virus has a nucleic acid genome, either DNA or RNA but never both, inside a protein coat called the capsid. A viroid is only a free, low-molecular-weight RNA with no protein coat. Viroids cause diseases such as potato spindle tuber disease, while prions are infectious agents made of abnormally folded protein alone.

Biological Classification: common doubts, answered

From Aristotle to two kingdoms