NEET Biological Classification Previous Year Questions with Solutions

Lichens represent a classic mutualistic symbiosis between two distinct kingdoms of organisms: an autotrophic photosynthetic partner (alga or cyanobacterium) and a heterotrophic fungal partner.

• • Phycobiont: Green alga (Chlorophyceae) or cyanobacterium; prepares food for fungi.
• • Mycobiont: Ascomycete or Basidiomycete fungus; provides shelter and absorbs water and mineral nutrients.

1. Sponges: Multicellular diploblastic aquatic organisms belonging to animal Phylum Porifera. 2. Mycorrhiza: Symbiotic association between fungi and the roots of higher gymnosperms/angiosperms (e.g., Pinus). 3. Chrysophytes: Diatoms and golden algae (desmids) belonging to Kingdom Protista.

• • Option (1) is the only valid term.

Lichens are pioneer colonizers on bare rock during ecological succession (lithosere) and serve as sensitive bioindicators of air pollution (especially SO₂).

• • Matches Option (1).

Prions are proteinaceous infectious agents consisting entirely of an abnormally folded version of a normal cellular prion protein (PrP^Sc). Unlike viruses and viroids, prions contain no nucleic acids (neither DNA nor RNA). When an abnormal prion enters a host, it induces normal helical proteins to misfold into beta-sheet-rich aggregates, destroying brain tissue and giving it a spongy appearance (spongiform encephalopathy).

• • Size: Similar in size to small viruses.

1. Bovine Spongiform Encephalopathy (BSE / Mad Cow Disease): Chronic neurodegenerative disorder in cattle caused by prions. 2. Creutzfeldt-Jakob Disease (CJD): Human analog of mad cow disease, also caused by prions. 3. Potato Spindle Tuber Disease: Caused by viroids (T.O. Diener, 1971). 4. Aspergillosis: Fungal infection caused by Aspergillus. 5. Pleuropneumonia / Atypical pneumonia: Caused by Mycoplasma.

• • Prions are the definitive causative agents.

Mad cow disease is caused by prions.

• • Matches Option (1).

Bacteria with a spherical or oval cell shape are termed Coccus (plural: Cocci). They can exist as single cells, diplococci, streptococci (chains), or staphylococci (clusters).

• • A correlates with II.

Bacteria that are elongated and rod-shaped are termed Bacillus (plural: Bacilli). Examples include Lactobacillus and Bacillus anthracis.

• • B correlates with IV.

Bacteria that possess a single curve, resembling a comma, are classified as Vibrio. A well-known medical example is Vibrio cholerae.

• • C correlates with I.

Rigid helical, spiral-shaped bacteria are termed Spirillum (plural: Spirilla).

• • D correlates with III.

Combining all pairs gives: A-II, B-IV, C-I, D-III. This matches Option (4).

• • Matches official NTA final answer key Option D / (4).

The sexual cycle in fungi involves three distinct sequential steps. The first step is the fusion of naked or motile/non-motile protoplasms between two compatible gametes or hyphae without immediate fusion of nuclei. This is termed Plasmogamy.

• • A correlates with II.

Following plasmogamy (either immediately in lower fungi or after a prolonged dikaryotic phase $n+n$ in Ascomycetes and Basidiomycetes), the two haploid nuclei fuse to form a diploid zygotic nucleus ($2n$). This nuclear fusion is termed Karyogamy.

• • B correlates with III.

The diploid nucleus undergoes reductional division (meiosis) within the zygote or fruiting body to regenerate haploid spores (ascospores or basidiospores).

• • C correlates with I.

The complete pairing is: A-II, B-III, C-I. This corresponds exactly to Option (1).

• • Matches official NTA final answer key Option A / (1).

Reduction of atmospheric dinitrogen ($N_2$) to ammonia ($NH_3$) requires the enzyme nitrogenase:

• • The nitrogenase enzyme complex is found exclusively in prokaryotes (bacteria and cyanobacteria).
• • No eukaryotic organism (plants, animals, fungi, protists, eukaryotic algae) natively encodes or possesses functional nitrogenase.

Evaluating each candidate organism:

• • A. Azotobacter: Free-living, aerobic, heterotrophic eubacterium known as a potent nitrogen fixer in soils.
• • B. Oscillatoria: Filamentous cyanobacterium that fixes atmospheric nitrogen and enriches paddy fields.
• • C. Anabaena: Filamentous cyanobacterium possessing specialized thick-walled cells called heterocysts for N₂ fixation (symbiont in Azolla, Cycas coralloid roots).
• • D. Volvox: A colonial, flagellated eukaryotic green alga belonging to Division Chlorophyta (Plantae). Being a eukaryote, it lacks nitrogenase and cannot fix nitrogen.
• • E. Nostoc: Cyanobacterium possessing heterocysts for biological nitrogen fixation in terrestrial and aquatic habitats.

Among all five organisms, only Volvox (D) is a eukaryote and completely lacks the ability to fix atmospheric nitrogen.

R.H. Whittaker (1969) proposed the Five Kingdom Classification based on cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships:

• • Statement C: "Prokaryotes with cell wall made of polysaccharides and amino acids (peptidoglycan)" -> Kingdom Monera (Cellular level, prokaryotic).
• • Statement E: "Eukaryotes with cellular body organization" -> Kingdom Protista (Cellular level, eukaryotic).
• • Statement A: "Multicellular heterotrophs with cell wall made of chitin" -> Kingdom Fungi (Multicellular / loose tissue level).
• • Statement D: "Eukaryotic autotrophs with tissue/organ level of body organization" -> Kingdom Plantae (Tissue / organ level).
• • Statement B: "Heterotrophs with tissue/organ/organ system level of body organization" -> Kingdom Animalia (Tissue / organ / organ system level).

Tracing the progressive structural advancement according to NCERT Table 2.1:

• • 1. Monera (C): Cellular (simplest, lacking membrane-bound nucleus and organelles).
• • 2. Protista (E): Cellular (eukaryotic cell with defined compartmentalization).
• • 3. Fungi (A): Multicellular / loose tissue.
• • 4. Plantae (D): Tissue / organ.
• • 5. Animalia (B): Tissue / organ / organ system (highest structural complexity).

The unambiguous increasing sequence of body complexity is C -> E -> A -> D -> B, which is Option (2).

Rhizopus stolonifer is commonly known as the black bread mould, growing on stale, moist bread.

Agaricus campestris is the common edible mushroom. Ustilago maydis / nuda causes smut diseases with powdery black spore masses (smut fungus). Puccinia graminis causes black stem rust of wheat (rust fungus).

A-III, B-II, C-IV, D-I matches Option (3) uniquely and unambiguously.

Kingdom Fungi is divided into various classes: Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.

NCERT explicitly states: "The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the kingdom into various classes."

Fungi are uniformly heterotrophic absorptive organisms (either saprophytes, parasites, or symbionts). Hence, mode of nutrition does not serve as a criterion for class-level fungal taxonomy.

In Kingdom Fungi, sexual reproduction involves three fundamental sequential events:

- Statement (1): Karyogamy refers strictly to the fusion of two nuclei ($karyon$ = nucleus), not the fusion of entire vegetative cells. - Statement (3): Organisms that depend directly on living host plants are parasites; saprophytes feed strictly on dead, decaying organic matter. - Statement (4): Atmospheric nitrogen-fixing cyanobacteria (e.g., *Nostoc*, *Anabaena*) fix nitrogen in specialized thick-walled cells called heterocysts, not sheath cells (the gelatinous mucilaginous sheath simply protects the colony).

In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.

It was found to be a free RNA; it completely lacked the protein coat (capsid) that is found in viruses, hence the name viroid.

The RNA of the viroid was of low molecular weight. Viroids contain RNA, NEVER DNA.

Statement (1) describes a virus (RNA with protein coat). Statement (2) accurately describes a viroid (free RNA without protein coat). Statements (3) and (4) falsely claim DNA.