Biology: Unit-wise Subjective Question Bank

Sixty-four of the seventy-five marks in the Biology paper are written answers. Group B asks eight short questions of four marks each, four from Botany and four from Zoology; Group C asks four long questions of eight marks each, two from each section. That is where the paper is won, and it is also where most candidates lose marks they already know the biology for — by writing a paragraph where the examiner wanted five labelled points, by describing a process without naming it, or by leaving out the diagram that carries two of the eight marks on its own.

This chapter gives you, for every unit of the CDC syllabus, at least eight four-mark questions and three eight-mark questions, each with a complete model answer written the way you should write it in the hall — headed, numbered, using the exact words an examiner ticks. Read the answer only after you have attempted the question in writing and against the clock.

9.1 Biomolecules and Cell Biology

Short-answer questions (4 marks each)

1. Define enzymes. Write any four properties of enzymes. [4]

Answer. Enzymes are biological catalysts — globular proteins produced by living cells which accelerate a biochemical reaction by lowering its activation energy, without themselves being consumed or altered at the end of the reaction.

Four properties:

i. Protein nature. All enzymes except ribozymes are proteins, so they are denatured by heat, extremes of pH and heavy metals. ii. Specificity. One enzyme acts on one substrate or one type of bond; urease hydrolyses only urea. This follows from the shape of the active site. iii. Catalytic efficiency. A very small quantity converts a very large quantity of substrate; one molecule of catalase decomposes about 5 million molecules of H₂O₂ per minute. iv. Sensitivity to temperature and pH. Each enzyme has an optimum, usually around 37 °C and a pH characteristic of its site — pepsin at pH 2, trypsin at pH 8.

2. Differentiate between prokaryotic and eukaryotic cells under any four heads. [4]

Answer.

Prokaryotic cell	Eukaryotic cell
Nucleus is not membrane-bound; the DNA lies naked in a nucleoid	A true nucleus with a nuclear envelope and nucleolus is present
Membrane-bound organelles (mitochondria, plastids, ER, golgi, lysosome) are absent	All these membrane-bound organelles are present
DNA is a single circular molecule without histones	DNA is linear, in several chromosomes, wound on histone proteins
Ribosomes are 70S	Ribosomes are 80S in the cytoplasm (70S inside mitochondria and plastids)
Cell division is amitosis or binary fission; no spindle is formed	Cell division is mitosis or meiosis with a spindle
Size usually 1–10 µm; e.g. bacteria, cyanobacteria	Size usually 10–100 µm; e.g. Amoeba, plant and animal cells

3. Why is the mitochondrion called the powerhouse of the cell? [4]

Answer. The mitochondrion is called the powerhouse because it is the site where the chemical energy of food is converted into the usable energy currency of the cell, ATP.

i. The Krebs cycle (citric acid cycle) takes place in the mitochondrial matrix, oxidising pyruvate completely to CO₂ and releasing NADH and FADH₂. ii. The electron transport chain and oxidative phosphorylation take place on the inner membrane, where the energy of those electrons is used to pump protons and drive ATP synthase. iii. Of the total ATP produced from one glucose molecule, about thirty-four of the thirty-eight are produced inside the mitochondrion; glycolysis in the cytoplasm gives only a net two. iv. Its structure is adapted for this: the inner membrane is thrown into cristae to increase the surface area, and it carries the F₁ particles (oxysomes) on which ATP synthesis occurs.

Because every energy-requiring activity of the cell — active transport, muscle contraction, synthesis — is ultimately paid for by this ATP, the organelle is the power house of the cell.

4. What is the fluid mosaic model of the plasma membrane? Draw a labelled diagram. [4]

Answer. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a fluid bilayer of phospholipids in which protein molecules float like icebergs in a sea, giving a mosaic appearance.

i. The phospholipid bilayer has hydrophilic heads outward and hydrophobic fatty-acid tails facing each other inward, so the membrane is a self-sealing barrier to water-soluble substances. ii. The bilayer is fluid — the lipid molecules move laterally, and cholesterol between them regulates that fluidity. iii. Proteins are of two kinds: extrinsic (peripheral) proteins on the surfaces and intrinsic (integral) proteins that span the bilayer and act as channels, carriers, pumps, enzymes and receptors. iv. Short carbohydrate chains attached to lipids and proteins form glycolipids and glycoproteins on the outer face, which act as recognition and antigen sites.

The model explains selective permeability, membrane growth, cell fusion and the movement of substances, which the earlier rigid sandwich model of Danielli and Davson could not.

5. Distinguish between mitosis and meiosis under any four heads. [4]

Answer.

Mitosis	Meiosis
One division; one cycle of DNA replication followed by one nuclear division	Two successive divisions, meiosis I and II, after one replication
Two daughter cells are produced	Four daughter cells are produced
Chromosome number remains the same, 2n → 2n; it is an equational division	Chromosome number is halved, 2n → n; it is a reductional division
Daughter cells are genetically identical to the parent	Daughter cells are genetically different; crossing over and independent assortment occur
No synapsis, no chiasmata, no crossing over	Homologous chromosomes pair in zygotene and cross over in pachytene
Occurs in somatic cells — root and shoot apices, cambium	Occurs in reproductive cells — spore mother cells, gamete mother cells
Function: growth, repair, vegetative reproduction	Function: gamete formation, maintenance of chromosome number, variation

6. Write the significance of meiosis. [4]

Answer.

i. It maintains a constant chromosome number in a species. Fertilisation doubles the chromosome number; meiosis halves it in the formation of gametes or spores, so the number is restored generation after generation. ii. It produces variation. Crossing over in pachytene recombines linked genes, and the independent assortment of maternal and paternal chromosomes in metaphase I produces 2ⁿ different gametes; both provide the raw material on which natural selection acts. iii. It is the basis of sexual reproduction, since haploid gametes or spores can be formed in no other way. iv. It removes harmful combinations and produces new ones, which is why meiosis is essential for evolution and for plant and animal breeding.

7. Describe the structure and functions of the nucleus. [4]

Answer. The nucleus, discovered by Robert Brown in 1831, is the largest organelle and the control centre of the eukaryotic cell.

Structure. It has four parts.

i. Nuclear envelope — a double membrane with a perinuclear space between, perforated by nuclear pores which allow RNA and ribosomal subunits out and proteins in. The outer membrane is continuous with the rough ER. ii. Nucleoplasm — the ground substance, containing nucleotides, enzymes and ions. iii. Chromatin — a network of DNA wound on histones; it condenses into chromosomes during division. iv. Nucleolus — a naked, spherical, dense body, the site of rRNA synthesis and ribosome assembly.

Functions. It stores the genetic information; it controls all metabolic activities of the cell through the synthesis of mRNA; it directs cell division; and through the nucleolus it manufactures ribosomes.

8. What are lysosomes? Why are they called suicidal bags? [4]

Answer. Lysosomes are single membrane-bound vesicles, about 0.1–1.2 µm across, discovered by Christian de Duve (1955), containing about forty hydrolytic enzymes — proteases, lipases, nucleases and acid phosphatase — which work best at an acid pH of about 5.

Functions: intracellular digestion of food taken in by phagocytosis; autophagy, the digestion of the cell's own worn-out organelles; and the destruction of bacteria engulfed by white blood cells.

They are called suicidal bags because if the lysosomal membrane ruptures — as it does in an injured, starved or dying cell — the enzymes are released into the cytoplasm and digest the cell's own contents, killing the cell. This self-destruction, called autolysis, is necessary in normal development: it is how the tail of a tadpole is removed at metamorphosis.

9. Write short notes on:(a) endoplasmic reticulum(b) golgi apparatus. [4]

Answer.

(a) Endoplasmic reticulum. A network of membrane-bound tubules, vesicles and flattened cisternae running through the cytoplasm and continuous with the nuclear envelope; described by Porter, Claude and Fullam. Rough ER bears 80S ribosomes on its outer face and synthesises and transports proteins for export; smooth ER has no ribosomes and synthesises lipids and steroids, detoxifies drugs in the liver, and stores calcium in muscle. The ER also gives mechanical support to the cell — it is its endoskeleton.

(b) Golgi apparatus. Discovered by Camillo Golgi (1898); a stack of four to eight flattened cisternae with associated tubules and vesicles, called dictyosomes in plant cells. Its cis face receives vesicles from the ER and its trans face buds them off. It processes, packages and secretes: it glycosylates proteins, forms lysosomes and secretory vesicles, synthesises cell-wall polysaccharides in plants, and forms the acrosome of the sperm. It is therefore the "export house" or traffic police of the cell.

10. Define carbohydrates. Classify them with one example of each. [4]

Answer. Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis, with the general formula Cₓ(H₂O)ᵧ. They are the chief source of energy and the commonest organic molecules on earth.

i. Monosaccharides — the simplest sugars, which cannot be hydrolysed further; sweet, soluble and reducing. Trioses (glyceraldehyde), pentoses (ribose, deoxyribose) and hexoses (glucose, fructose, galactose). ii. Oligosaccharides — two to ten monosaccharide units joined by glycosidic bonds. Disaccharides: sucrose (glucose + fructose), maltose, lactose. iii. Polysaccharides — many units, non-sweet and insoluble. Storage: starch in plants and glycogen in animals. Structural: cellulose in plant cell walls and chitin in fungal walls and arthropod cuticle.

Long-answer questions (8 marks each)

1. Describe the different stages of mitosis with suitable labelled diagrams. Add a note on its significance. [8]

Answer. Mitosis is the equational division of a somatic cell in which one parent cell divides into two daughter cells, each with the same chromosome number and the same genetic constitution as the parent. It was described by Walther Flemming (1882). It occupies the M phase of the cell cycle and has two parts: karyokinesis, the division of the nucleus, and cytokinesis, the division of the cytoplasm.

Karyokinesis passes through four stages.

(i) Prophase. The chromatin threads shorten and thicken into distinct chromosomes, each already made of two sister chromatids joined at the centromere, because DNA was replicated in the preceding S phase. The nucleolus and the nuclear envelope begin to disappear. In animal cells the centrioles move to opposite poles and the spindle begins to form; plant cells form an anastral spindle without centrioles. Prophase is the longest stage of mitosis.

(ii) Metaphase. The nuclear envelope and nucleolus have completely disappeared. The chromosomes, now at their most condensed and therefore most easily counted and photographed, move to the equator of the spindle and arrange themselves in a single plane, the metaphase plate. Each centromere is attached by a kinetochore to spindle fibres from both poles. This is the stage used for karyotyping.

(iii) Anaphase. The centromere of each chromosome divides longitudinally, and the two sister chromatids — now daughter chromosomes — are pulled to opposite poles by the shortening of the chromosomal fibres, appearing V-, L-, J- or I-shaped according to the position of the centromere. Anaphase is the shortest stage. At its end each pole has a complete diploid set.

(iv) Telophase. The chromosomes at each pole uncoil and lengthen back into chromatin; the nuclear envelope and the nucleolus re-form; the spindle disappears. Two daughter nuclei now lie in one cytoplasm.

Cytokinesis follows. In animal cells a cleavage furrow constricts the cell from the outside inwards; in plant cells, because of the rigid wall, a cell plate is laid down from the centre outwards by golgi vesicles and becomes the middle lamella.

Significance.

i. It brings about the growth of a multicellular organism from the zygote, every cell receiving an identical set of genes. ii. It repairs and replaces worn-out cells — skin, blood, the lining of the gut. iii. It is the basis of vegetative and asexual reproduction and of the regeneration of lost parts. iv. It maintains a constant nucleo-cytoplasmic ratio and keeps the chromosome number of the species constant in the body cells. v. Its failure of control produces uncontrolled division — cancer.

2. Give an account of the ultrastructure and functions of the chloroplast. How is it adapted to its work? [8]

Answer. The chloroplast is the green, double-membrane-bound plastid in which photosynthesis occurs. It is the organelle through which almost all the energy in the living world enters it. In higher plants it is discoid, 4–10 µm long and 2–4 µm thick, and a palisade cell may contain twenty to forty of them.

Ultrastructure.

i. Envelope. Two smooth membranes separated by a narrow intermembrane space. The outer is freely permeable, the inner selectively permeable and without chlorophyll. ii. Stroma. The colourless proteinaceous ground substance enclosed by the envelope, containing the enzymes of the Calvin cycle (notably RuBisCO), circular naked DNA, 70S ribosomes, starch grains and oil droplets. Because it has its own DNA and ribosomes the chloroplast is a semi-autonomous organelle, which supports the endosymbiotic theory. iii. Thylakoids. A system of flattened membranous sacs suspended in the stroma. Where they are stacked like a pile of coins they form a granum (40–60 per chloroplast, each of 5–30 thylakoids); the grana are joined by unstacked stroma lamellae or intergranal thylakoids. iv. Quantasomes. The thylakoid membrane carries the photosynthetic units — chlorophyll a and b, carotenoids, the two photosystems PS I and PS II, the electron carriers and ATP synthase.

Functions.

i. The light reaction occurs on the thylakoid membranes: absorption of light by the photosystems, photolysis of water releasing O₂, non-cyclic and cyclic photophosphorylation producing ATP and NADPH₂. ii. The dark reaction (Calvin cycle) occurs in the stroma, using that ATP and NADPH₂ to fix CO₂ into carbohydrate. iii. It stores the product temporarily as starch grains, and it synthesises some of its own proteins and lipids.

Adaptations to its work.

i. The stacking of thylakoids into grana packs an enormous pigment-bearing surface into a small volume, so that the maximum light is absorbed. ii. The separation of the two reactions — light reaction on a membrane, dark reaction in the fluid stroma — allows a proton gradient to be built across the thylakoid membrane and used for ATP synthesis. iii. The biconvex discoid shape presents a large surface to the light and shortens the diffusion path for CO₂. iv. Chloroplasts can move within the cell (cyclosis), turning their flat face to weak light and their edge to strong light. v. Its own DNA and ribosomes allow it to make key subunits of RuBisCO on the spot.

3. What is the cell cycle? Describe its phases and explain how it is controlled. [8]

Answer. The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides into two. In a typical human cell in culture it takes about 24 hours, of which mitosis occupies barely an hour. It has two main phases: a long interphase and a short M phase.

Interphase (about 95 % of the cycle). Once wrongly called the resting stage, it is the most active period of the cell's life. It has three sub-phases.

i. G₁ phase (first gap). The longest and most variable. The cell grows, synthesises RNA, proteins and organelles, and prepares the enzymes of DNA replication. Its chromosome content is 2n with one chromatid each. ii. S phase (synthesis). DNA replicates, so each chromosome now consists of two identical sister chromatids joined at the centromere. Histones are synthesised. The DNA amount doubles but the chromosome number does not — this is the single point most often missed. iii. G₂ phase (second gap). The cell continues to grow, synthesises the tubulin for the spindle and the proteins needed for division, and checks the replicated DNA for errors.

M phase. Karyokinesis (prophase, metaphase, anaphase, telophase) followed by cytokinesis, giving two daughter cells that enter G₁ again.

G₀ phase. Cells that will not divide again — neurons, mature red blood cells, cardiac muscle — leave the cycle at G₁ and enter a quiescent G₀ state. Liver cells stay in G₀ but can be recalled into the cycle after injury.

Control of the cycle. The cycle is driven by cyclins and cyclin-dependent kinases (CDKs): a cyclin accumulates, binds its CDK, activates it by phosphorylating target proteins, and is then destroyed, so the cycle advances in one direction only. Progress is policed at three checkpoints:

i. G₁ (restriction) checkpoint — is the cell big enough, are nutrients and growth signals present, is the DNA undamaged? This is the main decision point. ii. G₂ checkpoint — has all the DNA been replicated correctly? iii. M (spindle) checkpoint — is every chromosome attached to the spindle from both poles before anaphase begins?

If a checkpoint detects a fault the cycle is arrested while repair takes place, and if the damage cannot be repaired the cell is destroyed by programmed cell death (apoptosis). Tumour-suppressor genes such as p53 operate here; p53 is called the guardian of the genome. When these controls fail the cell divides without restraint, and that uncontrolled division is cancer.

9.2 Floral Diversity

Short-answer questions (4 marks each)

1. What is binomial nomenclature? Write any four rules of it. [4]

Answer. Binomial nomenclature is the system, introduced by Carolus Linnaeus in Species Plantarum (1753), of giving every organism a scientific name of two words — the generic name followed by the specific epithet, for example Mangifera indica.

Four rules (from the ICN and ICZN):

i. The name is Latin or latinised, and is printed in italics; when handwritten each word is separately underlined. ii. The generic name begins with a capital letter and the specific epithet with a small letter, even if it is derived from a person's name — Rana tigrina. iii. The name of the author who first published the name is written after the specific epithet, in roman and not underlined — Mangifera indica Linn. iv. The name must be based on a type specimen preserved in a recognised herbarium or museum, and the earliest validly published name has priority.

2. Write any four economic importances of fungi. [4]

Answer.

i. Food and food processing. Agaricus (mushroom) and Morchella (guchhi chyau, a valuable export of Nepal) are eaten; Saccharomyces (yeast) leavens bread and ferments sugar to ethanol in brewing. ii. Medicine. Penicillium notatum yields penicillin, the first antibiotic, discovered by Alexander Fleming (1928); Claviceps yields ergotamine and other fungi yield griseofulvin and statins. iii. Decomposition and soil fertility. As saprophytes, fungi mineralise dead organic matter and return carbon and nitrogen to the soil; mycorrhizal fungi greatly increase the absorbing surface of tree roots. iv. Harmful effects. They cause plant diseases of great economic loss — late blight of potato (Phytophthora), rusts and smuts of wheat — spoil stored grain, and cause human diseases such as ringworm and athlete's foot.

3. What is a lichen? Describe the nature of the association between its partners. [4]

Answer. A lichen is a composite, self-supporting organism formed by the symbiotic association of a fungus (the mycobiont, usually an ascomycete) and an alga or cyanobacterium (the phycobiont). The fungus makes up most of the thallus; the alga lies as a layer within it.

Nature of the association.

i. The alga photosynthesises and supplies carbohydrate to the fungus; if the partner is a cyanobacterium such as Nostoc, it also fixes atmospheric nitrogen. ii. The fungus absorbs and retains water and mineral salts, anchors the thallus and shields the alga from strong light and desiccation. iii. The relationship is often described as mutualism, but because the fungus sends haustoria into the algal cells and the alga can live independently while the fungus usually cannot, many workers call it controlled parasitism or helotism. iv. Because they absorb minerals directly from rainwater they are extremely sensitive to SO₂ and are used as bio-indicators of air pollution; they are also the pioneer colonisers of bare rock in a xerosere.

4. Why are bryophytes called the amphibians of the plant kingdom? [4]

Answer. Bryophytes are called the amphibians of the plant kingdom because, although they have moved onto the land, they cannot complete their life cycle without water.

i. They grow on land, usually in moist shady places, and have a plant body differentiated into a thallus or into stem-like and leaf-like structures with rhizoids. ii. But the antherozoids are flagellate and must swim through a film of external water to reach the archegonium; without free water there is no fertilisation. iii. They have no true vascular tissue — no xylem and phloem — so water moves slowly by diffusion and capillarity and the plant must stay small and close to a damp surface. iv. They have no true roots, and the thin body lacks a thick cuticle, so they lose water readily and are poikilohydric.

They are therefore land plants that remain dependent on water for reproduction, exactly as an amphibian is a land animal that must return to water to breed.

5. Differentiate between gymnosperms and angiosperms under any four heads. [4]

Answer.

Gymnosperms	Angiosperms
Ovules are naked, borne on open megasporophylls; there is no ovary	Ovules are enclosed in an ovary formed by folded carpels
No true flower; reproductive structures are borne in cones or strobili	A true flower with calyx, corolla, androecium and gynoecium
No fruit is formed; the seed is naked	The ovary ripens into a fruit enclosing the seed
No double fertilisation; the endosperm is haploid and formed before fertilisation	Double fertilisation occurs; the endosperm is triploid and formed after fertilisation
Xylem lacks vessels (except Gnetales); phloem lacks companion cells	Xylem has vessels; phloem has companion cells
Mostly wind-pollinated, perennial woody trees; about 1,000 species	Pollination by wind, water, insects and animals; herbs to trees; about 300,000 species

6. Write the floral characters and any two economic importances of the family Brassicaceae. [4]

Answer. Floral characters (Brassicaceae or Cruciferae, the mustard family).

• Inflorescence — racemose, typically a corymbose raceme.
• Flower — bracteate or ebracteate, pedicellate, complete, bisexual, actinomorphic, hypogynous, tetramerous.
• Calyx — 4 sepals, polysepalous, in two whorls of two (2 + 2).
• Corolla — 4 petals, polypetalous, arranged in the form of a cross, cruciform; valvate.
• Androecium — 6 stamens in two whorls, the outer of two short and the inner of four long — tetradynamous; polyandrous, epipetalous absent.
• Gynoecium — bicarpellary, syncarpous, superior ovary, at first unilocular but becoming bilocular by a false septum (replum); parietal placentation.
• Fruit — siliqua or silicula.
• Floral formula — ⊕ ⚥ K 2+2 C 4 A 2+4 G (2).

Economic importance. (i) Oil and food: Brassica campestris (mustard) and B. napus yield edible oil and the seeds are a condiment; B. oleracea gives cabbage, cauliflower, knol-khol and broccoli, and Raphanus sativus the radish. (ii) Ornamentals and medicine: Iberis (candytuft) and Matthiola are grown in gardens, and Brassica seed poultice is used as a counter-irritant.

7. Write the floral formula of Fabaceae and explain, whorl by whorl, how you would recognise a member of the family in the field. [4]

Answer. Floral formula (Fabaceae, subfamily Papilionoideae): % ⚥ K (5) C 1+2+(2) A (9)+1 G 1.

Recognition in the field, whorl by whorl:

i. Habit and leaf. A herb, shrub or twiner with pinnately compound leaves and a pulvinus at the leaf base, and often root nodules containing Rhizobium. ii. Calyx — 5 sepals united into a tube, slightly two-lipped. iii. Corolla — the diagnostic whorl: a papilionaceous (butterfly-shaped) corolla of five unequal petals — one large posterior standard (vexillum), two lateral wings (alae) and two anterior petals fused into a boat-shaped keel (carina), with descending imbricate aestivation in which the standard overlaps the wings. This makes the flower zygomorphic, marked %. iv. Androecium — 10 stamens, nine united by their filaments into a sheath and the tenth free — diadelphous, hidden in the keel. v. Gynoecium — monocarpellary, superior, unilocular with marginal placentation, ripening into the unmistakable legume or pod that splits along both sutures.

8. Give an account of the taxonomic hierarchy with a suitable example. [4]

Answer. The taxonomic hierarchy is the arrangement of taxonomic categories in a descending series, each category containing one or more of the category below it. It was given by Linnaeus and the obligatory categories are seven.

Kingdom → Division (Phylum in animals) → Class → Order → Family → Genus → Species.

As we go down the hierarchy the number of organisms in a taxon decreases while the number of characters they share increases; the species is the basic unit and consists of individuals that resemble one another and interbreed freely.

Example — mustard:

Category	Taxon
Kingdom	Plantae
Division	Magnoliophyta (Angiospermae)
Class	Magnoliopsida (Dicotyledonae)
Order	Brassicales
Family	Brassicaceae
Genus	Brassica
Species	campestris

9. Distinguish between artificial, natural and phylogenetic systems of classification. [4]

Answer.

i. Artificial system. Based on one or a few easily observed characters chosen arbitrarily, such as habit or the number of stamens. Linnaeus's sexual system, which classified plants by the number and union of stamens and carpels, is the best example. It is convenient for identification but places unrelated plants together and separates related ones. ii. Natural system. Based on as many characters as possible — external morphology, anatomy, embryology and phytochemistry — taken together, so that the groups formed express real overall resemblance. The system of Bentham and Hooker (Genera Plantarum, 1862–83) is the classic example and is still used in herbaria. It does not, however, show the evolutionary sequence. iii. Phylogenetic system. Based on the evolutionary relationship and common ancestry of the groups, so that the classification is a picture of the course of evolution. Engler and Prantl and later Hutchinson and the modern APG system, which uses DNA sequence data, are examples.

10. Write any four differences between monocotyledonous and dicotyledonous plants. [4]

Answer.

Dicotyledon	Monocotyledon
Seed has two cotyledons	Seed has one cotyledon
Tap root system	Fibrous (adventitious) root system
Leaves with reticulate venation	Leaves with parallel venation
Flower parts in fours or fives (tetramerous or pentamerous)	Flower parts in threes (trimerous)
Vascular bundles in the stem are arranged in a ring, conjoint, collateral and open with cambium; secondary growth occurs	Vascular bundles scattered, conjoint, collateral and closed; no normal secondary growth
Example: Brassica, Pisum, Solanum	Example: Allium, rice, wheat, Lilium

Long-answer questions (8 marks each)

1. Describe the family Solanaceae under the heads of habit, vegetative characters, floral characters and economic importance. Give its floral formula. [8]

Answer. Solanaceae, the potato or nightshade family, contains about 2,500 species in 90 genera, distributed over the tropics and temperate regions; it is well represented in Nepal.

Habit. Mostly annual or perennial herbs (Solanum nigrum, Petunia), some shrubs (Cestrum, Withania) and a few small trees (Solanum grandiflorum); a few are climbers. Hairs are common and the plants frequently smell strongly because of alkaloids.

Vegetative characters.

• Root — branched tap root; in potato the underground stem forms tubers.
• Stem — herbaceous or woody, erect, cylindrical, branched, often hairy; underground in Solanum tuberosum; modified into thorns in some species.
• Leaf — alternate, exstipulate, simple (pinnately compound in tomato), entire or lobed, with reticulate venation.

Floral characters.

• Inflorescence — usually a cymose inflorescence, axillary or extra-axillary; solitary axillary in Datura.
• Flower — bracteate, pedicellate, complete, bisexual, actinomorphic, hypogynous, pentamerous.
• Calyx — 5 sepals, gamosepalous, persistent in tomato and Withania, valvate.
• Corolla — 5 petals, gamopetalous, rotate, campanulate or funnel-shaped, valvate.
• Androecium — 5 stamens, epipetalous, alternating with the petals, polyandrous; anthers usually basifixed and dithecous, dehiscing longitudinally or by apical pores (Solanum).
• Gynoecium — bicarpellary, syncarpous; ovary superior, bilocular, with the placentae swollen and bearing many ovules on axile placentation. The ovary is placed obliquely in the flower, a character found in no other family. A false septum makes the ovary four-chambered in Datura.
• Fruit — a berry (tomato, brinjal, Solanum nigrum) or a capsule (Datura, Petunia).
• Seed — endospermic, with a curved embryo.

Floral formula. ⊕ ⚥ K (5) C (5) A 5 G (2) — with the note that the ovary is obliquely placed.

Economic importance.

i. Food. Solanum tuberosum (potato) is the world's fourth staple crop; S. lycopersicum (tomato), S. melongena (brinjal) and Capsicum annuum (chilli, a major cash crop in Nepal) are vegetables. ii. Medicine. Atropa belladonna yields atropine, used to dilate the pupil; Datura yields hyoscyamine and stramonium, used in asthma; Withania somnifera (ashwagandha) is an important tonic in Ayurveda. iii. Narcotic. Nicotiana tabacum yields nicotine for tobacco, and also an insecticide. iv. Ornamental. Petunia, Cestrum nocturnum (raat ki rani) and Solanum pseudocapsicum are grown in gardens. v. Poisonous. Solanum nigrum berries when unripe, and all parts of Datura, are poisonous because of their alkaloids.

2. What is alternation of generations? Describe the life cycle of a moss (Funaria) to illustrate it. [8]

Answer. Alternation of generations is the regular alternation, in the life cycle of a plant, of two morphologically distinct phases: a haploid gamete-producing gametophyte and a diploid spore-producing sporophyte. The gametophyte produces gametes by mitosis; their fusion at fertilisation doubles the chromosome number and gives the sporophyte; the sporophyte produces spores by meiosis, which halves it again and restores the gametophyte. The cycle is therefore driven by two events, fertilisation and meiosis, and the plant is said to be diplohaplontic.

The life cycle of Funaria. In bryophytes the gametophyte is the dominant, independent, green phase and the sporophyte is small and permanently attached to it — the opposite of the situation in a fern or a flowering plant.

(i) The gametophyte. The leafy Funaria plant is haploid. It has a slender erect axis bearing spirally arranged sessile leaves and, at the base, multicellular branched rhizoids with oblique septa. It is autotrophic.

(ii) Sex organs. Funaria is monoecious but autoicous — the antheridia and archegonia are borne on separate branches of the same plant. The main shoot ends in a rosette of leaves (the perigonium) enclosing club-shaped antheridia, each producing many biflagellate antherozoids, mixed with sterile paraphyses. A lateral branch bears the flask-shaped archegonium, with a swollen venter containing a single egg and a long neck, mixed with paraphyses.

(iii) Fertilisation. In the presence of water, the antheridium bursts and the antherozoids swim to the archegonium, attracted chemotactically by the sucrose and organic acids released by the disintegrating neck canal cells. One antherozoid fuses with the egg to form the diploid zygote (oospore), which is retained in the venter.

(iv) The sporophyte. The zygote divides mitotically to form the sporophyte or sporogonium, which remains attached to and partly dependent on the gametophyte. It has three parts: a foot embedded in the gametophyte for anchorage and absorption; a long seta; and a capsule at the top, covered at first by the calyptra derived from the old archegonial wall. The capsule is complex, with an apophysis, a theca containing the spore sac and a central sterile columella, and a peristome of two rings of sixteen teeth beneath the operculum.

(v) Spore formation and dispersal. The spore mother cells in the spore sac divide by meiosis, so the spores are haploid — this is the point at which the diploid phase ends. The capsule dries, the operculum falls, and the hygroscopic peristome teeth bend outward in dry weather to scatter the spores.

(vi) Germination. A spore germinates into a green, branched, filamentous protonema, on which buds arise, and each bud grows into a new leafy gametophyte, completing the cycle.

Significance. The sporophyte enables the plant to produce and disperse large numbers of resistant spores, an adaptation to land; the gametophyte, being haploid, exposes every gene to selection; and the meiosis in the capsule provides variation. The retention of the embryo on the gametophyte is the feature that unites all land plants as embryophytes.

3. Describe the general characters of algae and give an account of their economic importance. [8]

Answer. Algae are simple, autotrophic, chlorophyll-bearing, thalloid plants whose sex organs are unicellular (or, if multicellular, have no sterile jacket) and which produce no embryo. Their study is phycology, and F. E. Fritsch's classification of 1935 is the standard.

General characters.

i. Habitat. Predominantly aquatic, both freshwater and marine; also terrestrial on damp soil and bark, cryophytic on snow (Chlamydomonas nivalis), thermophilic in hot springs, epiphytic, endophytic, and symbiotic in lichens and in the coralloid roots of Cycas. ii. Plant body. A thallus with no true root, stem or leaf and no vascular tissue. It ranges from unicellular motile (Chlamydomonas) and non-motile (Chlorella) forms through colonies (Volvox), filaments (Spirogyra, Ulothrix) and sheets (Ulva) to the giant parenchymatous kelps (Macrocystis, up to 60 m). iii. Cell structure. Eukaryotic, with a cell wall usually of cellulose with pectin, and sometimes algin or agar; the chloroplast has a characteristic shape — spiral in Spirogyra, cup-shaped in Chlamydomonas, girdle-shaped in Ulothrix — and contains pyrenoids that store starch. iv. Pigments. Always chlorophyll a, with other chlorophylls and accessory pigments that give the classes their names and their colours: Chlorophyceae green (chl a, b), Phaeophyceae brown (fucoxanthin), Rhodophyceae red (r-phycoerythrin). v. Reserve food. Starch in green algae, laminarin and mannitol in brown algae, floridean starch in red algae. vi. Reproduction. Vegetative by fragmentation, fission or hormogonia; asexual by zoospores, aplanospores, akinetes or hypnospores; sexual by isogamy, anisogamy or oogamy, and by conjugation in Spirogyra. Red algae have non-motile gametes and show complex post-fertilisation development.

Economic importance.

i. Primary production and oxygen. Marine phytoplankton fix a very large part of global CO₂ and release more than half the world's oxygen; they are the base of every aquatic food chain. ii. Food. Porphyra (nori), Laminaria (kombu), Ulva (sea lettuce) and Sargassum are eaten in East Asia; Chlorella and Spirulina are protein-rich single-cell foods used by astronauts and in famine relief. iii. Industrial products. Agar-agar, used to culture microorganisms and in food, comes from Gelidium and Gracilaria; algin from Laminaria and Macrocystis is used in ice cream, paint and textiles; carrageenan from Chondrus is an emulsifier; diatomite from diatom frustules is used in filters, polishes and insulation. iv. Fertiliser and fodder. Seaweeds are used as manure for their potash, and blue-green algae such as Nostoc and Anabaena fix nitrogen in paddy fields; Azolla with Anabaena is a standard green manure. v. Medicine. Antibiotic chlorellin from Chlorella; iodine from Laminaria; vermifuges from Digenea. vi. Harmful effects. Algal blooms in eutrophic water deoxygenate it and kill fish; Microcystis and the dinoflagellates of the red tide release toxins that poison fish and shellfish; some algae block water pipes and spoil the taste of drinking water.

4. Describe the general characters of pteridophytes and explain why they are considered the first true vascular land plants. [8]

Answer. Pteridophytes — the ferns and their allies — are seedless vascular cryptogams. About 12,000 species survive; in the Carboniferous they formed the great tree forests whose remains are our coal.

General characters.

i. Habitat. Chiefly moist, shady terrestrial habitats; some xerophytic (Selaginella rupestris), some aquatic (Marsilea, Salvinia, Azolla), some epiphytic. ii. Plant body. The dominant plant is the sporophyte, differentiated for the first time in the plant kingdom into true root, stem and leaf. The stem is usually an underground rhizome; the leaves are microphyllous (Selaginella, Equisetum) or megaphyllous and pinnately compound (Dryopteris), and the young leaf shows circinate vernation. iii. Vascular tissue. Xylem and phloem are present, arranged in a stele; the xylem consists of tracheids only, without vessels (except Selaginella and Pteridium) and the phloem of sieve cells without companion cells. Protostele, siphonostele, dictyostele and polystele all occur. iv. Reproduction. Spores are produced in sporangia, borne on sporophylls that may be aggregated into a strobilus or, in ferns, grouped into sori on the underside of the leaf and protected by an indusium. Most are homosporous; Selaginella, Marsilea and Salvinia are heterosporous, producing microspores and megaspores — a step towards the seed habit. v. Gametophyte. The spore germinates into a small, green, independent, heart-shaped prothallus bearing antheridia and archegonia. The antherozoids are multiflagellate and need external water to reach the archegonium. vi. Alternation of generations is heteromorphic, with a large independent sporophyte and a small independent gametophyte; after fertilisation the embryo develops on the prothallus and soon becomes independent.

Why they are the first true vascular land plants.

i. They are the first group to possess true vascular tissue. Xylem conducts water and mineral salts from the soil rapidly to the whole plant, and phloem translocates food. This is what frees a land plant from having to stay small and damp, and it is why tree ferns can be ten metres tall while no moss exceeds a few centimetres. ii. The lignified tracheids of the xylem give mechanical support, so the plant can hold its leaves erect in air and compete for light. iii. They are the first plants with a true root to anchor them and to absorb water from deep in the soil, instead of the rhizoids of a bryophyte. iv. The sporophyte is the dominant, independent generation; the aerial parts are covered with a cuticle and bear stomata, so water loss is controlled. v. Spores are produced in enormous numbers and are wind-dispersed, an adaptation to the terrestrial environment.

They remain, however, incompletely adapted to land, because the gametophyte is thin and must live in a damp place and the antherozoids must swim to the egg. They are therefore the first true vascular land plants but not the first fully terrestrial ones — that position belongs to the seed plants, in which the gametophyte is retained inside the sporophyte and the pollen tube replaces the swimming sperm.