Early systems used a few observable characters, such as habit or morphology. Linnaeus used a two-kingdom system, Plantae and Animalia. Later, the discovery of microorganisms, cell structure, modes of nutrition, reproduction and evolutionary relationships led to Whittaker's five-kingdom system: Monera, Protista, Fungi, Plantae and Animalia. Modern systems increasingly use molecular and phylogenetic evidence.
Classification has changed from simple artificial systems to more natural and phylogenetic systems as biological knowledge increased.
Heterotrophic bacteria are useful in dairy fermentation, antibiotic production, decomposition and nitrogen fixation in legume roots. Methanogenic archaebacteria live in the gut of ruminants and in dung, where they produce methane used as biogas.
(a) Heterotrophic bacteria help in making curd and producing antibiotics; they also act as decomposers and nitrogen fixers. (b) Archaebacterial methanogens produce methane/biogas from dung and help digestion in ruminants.
The two wall halves fit together like a soap box. Because silica makes them indestructible, diatom walls accumulate over time as diatomaceous earth.
Diatom cell walls are made of two thin overlapping silica-embedded shells.
Cyanobacteria often form blooms in polluted water bodies. In marine habitats, red dinoflagellates may multiply so rapidly that the sea appears red; toxins released during such red tides may kill fishes and other marine animals.
Algal bloom is rapid growth of algae in water bodies; red tides are red-coloured blooms caused by rapid multiplication of red dinoflagellates such as Gonyaulax.
Viruses contain either DNA or RNA and a protein coat made of capsomeres. Viroids lack this protein coat; T.O. Diener discovered them as infectious RNA agents causing potato spindle tuber disease.
Viroids are smaller than viruses and consist of free low-molecular-weight RNA without a protein coat, whereas viruses have nucleic acid enclosed in a protein capsid.
Amoeboid protozoans move and capture prey using pseudopodia; Amoeba is an example, and Entamoeba is parasitic. Flagellated protozoans possess flagella; Trypanosoma causes sleeping sickness. Ciliated protozoans move by cilia and have a gullet; Paramoecium is an example. Sporozoans have an infectious spore-like stage; Plasmodium causes malaria.
The four major groups are amoeboid, flagellated, ciliated and sporozoan protozoans.
These plants contain chlorophyll or are grouped with plants, but obtain part or all of their nutrition from insects or host plants. Hence, they are not fully autotrophic in nutrition.
Yes. Insectivorous plants such as bladderwort and Venus fly trap, and parasitic plants such as Cuscuta, are partially heterotrophic.
The phycobiont is autotrophic and prepares food. The mycobiont is heterotrophic and provides shelter, water and mineral nutrients.
In lichens, the algal component is called phycobiont and the fungal component is called mycobiont.
Phycomycetes are aquatic, saprophytic or parasitic; they reproduce asexually by zoospores/aplanospores and sexually by zygospores. Ascomycetes are saprophytic, decomposer, parasitic or coprophilous; they produce conidia asexually and ascospores in asci sexually. Basidiomycetes grow in soil, logs or as parasites; vegetative fragmentation is common and basidiospores are produced on basidia. Deuteromycetes are saprophytes, parasites or decomposers; only asexual reproduction by conidia is known.
Phycomycetes, ascomycetes, basidiomycetes and deuteromycetes differ in habitat, hyphae and reproductive spores.
They occur mainly in stagnant freshwater. They lack a cell wall and have a protein-rich pellicle. They photosynthesise in sunlight but behave heterotrophically when sunlight is absent. Euglena is a common example.
Euglenoids are mostly freshwater protists with a flexible pellicle, two flagella and plant-like pigments; they can behave as autotrophs or heterotrophs.
A virus is a nucleoprotein. The capsid is made of capsomeres arranged in helical or polyhedral forms and protects the nucleic acid. Plant viruses commonly have single-stranded RNA, animal viruses may have RNA or DNA, and bacteriophages are usually double-stranded DNA viruses. Common viral diseases include mumps, smallpox, herpes and influenza; AIDS is also viral.
Viruses are non-cellular obligate parasites made of nucleic acid enclosed in a protein capsid. Their genetic material may be either DNA or RNA, never both.
Viruses are non-living outside a host because they are acellular, inert and can be crystallised. Inside a living host cell, they use host machinery to replicate and show infectivity. Therefore, a balanced discussion should note that they are not truly cellular organisms but show life-like properties only within host cells.
Viruses show both living and non-living characters, so they are often considered borderline biological entities.