RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Placing a mushroom, a euglena and a cyanobacterium into one bin labelled 'plants' clearly does not work, and this chapter explains the reasoning that broke the old two-kingdom system apart. Whittaker's five-kingdom scheme sorts organisms using cell structure, body organisation, mode of nutrition and reproduction, giving Monera, Protista, Fungi, Plantae and Animalia. The chapter closes with viruses, viroids, prions and lichens — entities that sit awkwardly outside every kingdom.
The cell wall of fungi is composed mainly of:
Answer
Chitin is correct — fungal walls are built of chitin and polysaccharides, unlike plant walls of cellulose, bacterial walls containing peptidoglycan, or diatom walls of silica.
Nitrogen fixation in filamentous cyanobacteria such as Anabaena takes place in specialised cells called:
Answer
Heterocysts are correct — these thick-walled cells provide the oxygen-free interior that the nitrogen-fixing enzyme requires; akinetes are resting spores and conidia are asexual fungal spores.
Which protist group has cell walls of indestructible silica that accumulate as diatomaceous earth?
Answer
Chrysophytes are correct — the diatoms in this group have two overlapping silica shells fitting like a soap box, and these walls survive after death to form thick diatomaceous earth deposits used in filtration and polishing.
An infectious agent consisting of a free RNA molecule with no protein coat at all is called a:
Answer
Viroid is correct — it is a naked, low-molecular-weight RNA; a virus always has a protein capsid, while a prion is an abnormally folded protein containing no nucleic acid.
Assertion (A): Euglena cannot be placed comfortably in either Kingdom Plantae or Kingdom Animalia. Reason (R): Euglena is photosynthetic in sunlight but feeds heterotrophically when kept in the dark, and it has a flexible protein pellicle rather than a cell wall.
Answer
Both A and R are true and R is the correct explanation of A — because Euglena combines plant-like photosynthesis with animal-like ingestion and lacks a cell wall, it fits neither of the two traditional kingdoms and is placed in Protista.
Give two reasons why the earlier two-kingdom classification into plants and animals had to be abandoned.
Answer
First, the two-kingdom system did not distinguish between prokaryotes and eukaryotes, so bacteria with no true nucleus were grouped with flowering plants simply because both have a cell wall. Second, it ignored fundamental differences in nutrition and structure — fungi, which have a chitin wall and absorb ready-made food, were placed with photosynthetic green plants, and unicellular organisms like Euglena that show both plant-like and animal-like features could not be assigned at all. A system based on cell structure, thallus organisation, nutrition and reproduction was therefore needed.
What is a lichen? Describe the role played by each partner and explain why lichens are used as pollution indicators.
Answer
A lichen is a symbiotic association between an alga and a fungus living together as a single body. The algal partner, called the phycobiont, is autotrophic and prepares food by photosynthesis for both partners. The fungal partner, called the mycobiont, forms the bulk of the body, anchors it to the substratum, and absorbs water and mineral nutrients while protecting the alga from drying out. Lichens are sensitive to sulphur dioxide and other pollutants, which they absorb readily from the air along with moisture because they have no protective cuticle or means of excretion; they therefore fail to grow in polluted industrial areas, and their presence or absence is used to judge air quality.
Describe the four classes of Kingdom Fungi, mentioning the nature of the mycelium, the type of spores produced and one example of each.
Answer
Phycomycetes are the lower fungi found in damp places, on decaying wood and as obligate parasites on plants. Their mycelium is aseptate and coenocytic, meaning the hyphae are continuous tubes containing many nuclei with no cross walls. Asexual reproduction occurs by motile zoospores or non-motile aplanospores produced inside a sporangium, and sexual reproduction produces a zygospore by the fusion of two gametes; examples are Rhizopus, the bread mould, and Albugo, a parasite on mustard. Ascomycetes, the sac fungi, have a septate and branched mycelium and are saprophytic, parasitic or coprophilous. They reproduce asexually by conidia borne on conidiophores, and sexually by ascospores produced in sac-like asci that are usually arranged inside a fruiting body called an ascocarp; examples are Penicillium, Aspergillus, Neurospora and the edible morels. Basidiomycetes, the club fungi, include mushrooms, bracket fungi, puffballs, rusts and smuts, and have a well developed septate mycelium. Asexual spores are generally absent, vegetative reproduction occurring by fragmentation, while sexual reproduction produces four basidiospores externally on a club-shaped basidium, the basidia being packed inside a fruiting body called a basidiocarp; examples are Agaricus, Ustilago and Puccinia. Deuteromycetes are called the imperfect fungi because only their asexual or vegetative phase is known — reproduction is by conidia and the mycelium is septate. Many are decomposers of litter and mineralisers of organic matter, and when their sexual stage is discovered they are transferred to one of the other classes; examples are Alternaria, Colletotrichum and Trichoderma.
Explain the general structure of a virus and describe how viruses differ from viroids and prions.
Answer
A virus is a nucleoprotein particle and an obligate intracellular parasite that shows no sign of life outside a living host cell, where it neither respires nor grows nor reproduces. Its body has two essential parts: an inner genetic core of a single kind of nucleic acid — either DNA or RNA, never both — and an outer protective protein coat called the capsid, built of many identical subunits called capsomeres arranged in a helical or polyhedral pattern. Some animal viruses additionally have a lipid envelope derived from the host membrane, and bacteriophages have an elaborate head-and-tail structure with tail fibres used to attach to the bacterial wall. In general, plant viruses carry single-stranded RNA while bacteriophages carry double-stranded DNA. Once inside a host, the viral nucleic acid takes over the host machinery and directs it to make copies of the viral genome and coat protein, so the virus multiplies only at the cost of the host and often kills it, causing diseases such as mosaic and leaf curl in plants and mumps, influenza and AIDS in humans. Viroids differ from viruses in that they consist of free, low-molecular-weight RNA with no protein coat whatsoever, and they cause plant diseases such as tuber spindle disease. Prions differ still more radically: they contain no nucleic acid at all and are simply abnormally folded versions of a normal cellular protein, which force normal protein molecules to misfold on contact, producing degenerative disorders of the nervous system in cattle and humans. Thus a virus has both nucleic acid and protein, a viroid has only nucleic acid, and a prion has only protein.
A water sample drawn from a warm coastal lagoon during an algal bloom is examined. Under the microscope the technician records: (i) large numbers of a brown, armour-plated single-celled organism with two flagella, (ii) smaller cells with two neatly fitting silica shells, and (iii) filaments of a blue-green photosynthetic organism carrying occasional pale, thick-walled cells. Fish in the lagoon are dying. (a) To which protist group does organism (i) belong, and what phenomenon is it causing? (b) Identify group (ii) and state one commercial use of the material its walls leave behind. (c) Name the pale thick-walled cells in filament (iii) and state their function. (d) Which of the three organisms is prokaryotic, and give one structural reason for your answer.
Answer
(a) Organism (i) is a dinoflagellate. Its rapid multiplication in warm coastal water produces a red tide, and the toxins released by such blooms kill fish and other marine animals. (b) Group (ii) are the chrysophytes, specifically diatoms. Their silica walls are indestructible and pile up on the sea bed as diatomaceous earth, which is used commercially for filtering oils and syrups and as a polishing agent. (c) The pale, thick-walled cells are heterocysts. They are the sites of atmospheric nitrogen fixation in filamentous cyanobacteria, their thick walls keeping oxygen out so that the nitrogen-fixing enzyme can work. (d) The blue-green filament (iii) is prokaryotic. It belongs to Kingdom Monera and has no membrane-bound nucleus or membrane-bound organelles — its photosynthetic pigments lie on membranes in the cytoplasm rather than inside chloroplasts, and its ribosomes are of the 70S type.
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