Zoology प्राणीशास्त्र

Quick summary

This topic explains how characters pass from parents to offspring. It covers Mendel's laws with monohybrid and dihybrid crosses, deviations like incomplete dominance and codominance, multiple alleles (ABO blood groups), and then linkage and crossing over, which explain why some genes do not assort independently. Examples come mainly from animals and humans, especially Drosophila.

Key points

• Gregor Mendel (father of genetics) worked on garden pea; his laws were rediscovered in 1900 by de Vries, Correns and Tschermak.
• Monohybrid ratio: F₂ phenotype 3 : 1, genotype 1 : 2 : 1. Dihybrid ratio: 9 : 3 : 3 : 1.
• Laws: dominance, segregation (purity of gametes), independent assortment.
• Incomplete dominance: heterozygote is intermediate; F₂ 1 : 2 : 1 (e.g. flower colour in Mirabilis; in animals, Andalusian fowl: black × white → blue).
• Codominance: both alleles expressed fully, e.g. AB blood group, roan cattle (red and white hairs together).
• Multiple alleles: more than two alleles at one locus in a population. ABO blood groups are controlled by three alleles: Iᴬ, Iᴮ and i. Iᴬ and Iᴮ are codominant; both are dominant over i. There are 6 genotypes and 4 phenotypes.
• Lethal genes: e.g. yellow coat colour in mice gives a 2 : 1 ratio, as homozygous yellow die (Cuénot).
• Drosophila melanogaster (fruit fly) is ideal for genetics: short life cycle (about 2 weeks), many offspring, easy to culture, clear mutant characters, only 4 pairs of chromosomes.
• T. H. Morgan (Nobel Prize 1933) studied linkage, crossing over and sex-linked inheritance (white-eye) in Drosophila.
• Linkage: genes on the same chromosome tend to be inherited together. Complete linkage – no recombinants (seen in male Drosophila); incomplete linkage – some recombinants.
• Crossing over occurs in pachytene of meiosis I between non-sister chromatids; chiasmata are the visible cross points.
• Linkage groups = haploid chromosome number (4 in Drosophila, 23 in humans).

Important terms

• Allele – alternative form of a gene.
• Locus – fixed position of a gene on a chromosome.
• Coupling phase (cis) – two dominant alleles on one chromosome (AB/ab).
• Repulsion phase (trans) – dominant alleles on different homologues (Ab/aB).
• Recombinant – offspring with a new combination of parental characters.
• Genetic (linkage) map – diagram showing the order and relative distances of genes on a chromosome (Sturtevant, 1913).
• Sex-linked inheritance – inheritance of genes on the X chromosome, e.g. colour blindness and haemophilia in humans.

Formulas

• Recombination frequency (RF %) = (recombinants ÷ total progeny) × 100.
• 1% RF = 1 map unit = 1 centimorgan (cM).
• Gametes from n heterozygous pairs = 2ⁿ.
• Example: test cross progeny 420 parental-type A, 400 parental-type B, 90 recombinant-type C, 90 recombinant-type D. Total = 1000. RF = (90 + 90)/1000 × 100 = 18%. Map distance = 18 cM.

5 important questions

Q1. Why is Drosophila a good material for genetic studies?

It has a short life cycle of about two weeks, gives many offspring, is easy and cheap to culture, has easily visible mutations and has only four pairs of chromosomes, including giant polytene chromosomes.

Q2. A woman of blood group A (IᴬIᴬ) marries a man of blood group B (Iᴮi). Give the possible blood groups of their children.

Gametes: woman Iᴬ only; man Iᴮ and i. Children: IᴬIᴮ (group AB) and Iᴬi (group A) in a 1 : 1 ratio.

Q3. Explain incomplete linkage with Morgan's experiment.

• Grey body, long wing (BV, dominant) × black body, vestigial wing (bv).
• F₁ female (BbVv) test-crossed with bbvv male.
• Result: about 83% parental types and 17% recombinant types (not 1 : 1 : 1 : 1).
• Shows genes are linked but crossing over gives some recombinants.
• RF about 17% → distance about 17 map units.

Q4. Describe the ABO blood group system as an example of multiple alleles.

• Gene I on chromosome 9 with alleles Iᴬ, Iᴮ, i.
• Genotypes: IᴬIᴬ, Iᴬi (A); IᴮIᴮ, Iᴮi (B); IᴬIᴮ (AB); ii (O).
• Iᴬ and Iᴮ codominant; i recessive.
• Antigens on RBC and antibodies in plasma.
• Uses: transfusion, paternity disputes.

Q5. Explain sex-linked inheritance with the example of colour blindness.

• Gene on X chromosome, recessive (Xᶜ).
• Males (XY) show the trait with one copy; females need two copies.
• Carrier mother (XXᶜ) × normal father (XY): half of sons colour-blind, half of daughters carriers.
• Criss-cross inheritance: father → daughter → grandson.
• Haemophilia follows the same pattern.

Exam tip

Show crosses with gametes and a Punnett square every time. Practise one map-distance numerical and one blood-group problem.

Quick summary

Evolution is the slow, gradual change in living organisms over many generations, leading to new species from older forms. This topic covers the origin of life, evidences of evolution, theories of Lamarck, Darwin and de Vries, Modern Synthetic Theory, Hardy–Weinberg principle, speciation and an outline of human evolution.

Key points

• Evolution is उत्क्रांती in Marathi.
• Origin of life: Oparin (1924) and Haldane (1929) proposed chemical evolution in a reducing atmosphere. Miller and Urey (1953) made amino acids by passing electric sparks through CH₄, NH₃, H₂ and water vapour.
• Evidences of evolution: palaeontology (fossils, e.g. horse evolution, Archaeopteryx), comparative anatomy (homologous and analogous organs, vestigial organs), embryology, biogeography, connecting links, molecular biology (DNA and protein similarity).
• Lamarckism (Philosophie Zoologique, 1809): use and disuse of organs, and inheritance of acquired characters (e.g. long neck of giraffe). Weismann disproved it by cutting tails of mice for many generations (germplasm theory).
• Darwinism (On the Origin of Species, 1859): over-production, struggle for existence, variations, survival of the fittest (term by Herbert Spencer) and natural selection. Alfred Russel Wallace reached the same idea independently.
• Mutation theory: Hugo de Vries (1901), working on Oenothera lamarckiana (evening primrose) – sudden large changes (mutations) form new species.
• Modern Synthetic Theory (Neo-Darwinism): combines Darwin with genetics; factors are gene mutation, genetic recombination, chromosomal changes, natural selection, reproductive isolation, genetic drift and gene flow.
• Industrial melanism in the peppered moth (Biston betularia) in England is a classic example of natural selection.

Important terms

• Homologous organs – same origin and structure, different function (forelimbs of man, whale, bat, cat) → divergent evolution.
• Analogous organs – different origin, same function (wings of insect and bird) → convergent evolution.
• Vestigial organs – reduced, non-functional organs (human appendix, wisdom teeth, nictitating membrane, tail bone/coccyx).
• Adaptive radiation – evolution of many forms from one ancestor in different habitats (Darwin's finches; Australian marsupials).
• Genetic drift – random change in gene frequency in small populations (founder effect, bottleneck).
• Speciation – formation of new species; allopatric (geographical separation) and sympatric (in the same area).
• Reproductive isolation – barriers that stop interbreeding between populations.

Formulas

• Hardy–Weinberg principle (1908): in a large, randomly mating population with no mutation, migration, selection or drift, allele and genotype frequencies remain constant.
• p + q = 1 and p² + 2pq + q² = 1 (p = frequency of dominant allele, q = recessive allele).
• Example: if 16% of a population shows a recessive trait, q² = 0.16, so q = 0.4 and p = 0.6. Heterozygotes 2pq = 2 × 0.6 × 0.4 = 0.48 (48%); homozygous dominant p² = 0.36 (36%).

5 important questions

Q1. What are homologous organs? Give an example.

Homologous organs have the same basic structure and embryonic origin but may do different jobs. Example: the forelimbs of human, whale, bat and cheetah. They show common ancestry (divergent evolution).

Q2. In a population, the frequency of the recessive allele is 0.3. Find the frequency of heterozygotes.

q = 0.3, so p = 1 − 0.3 = 0.7. Heterozygotes 2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42%.

Q3. Explain Darwin's theory of natural selection.

• Organisms produce more offspring than can survive.
• Struggle for existence (intraspecific, interspecific, environmental).
• Variations exist; some are favourable.
• Survival of the fittest; natural selection.
• Favourable variations inherited; accumulate over generations; new species arise.
• Criticism: could not explain the source of variation.

Q4. Describe the Modern Synthetic Theory of evolution.

• Proposed by Dobzhansky, Fisher, Haldane, Wright, Mayr, Huxley and others.
• Gene mutations and chromosomal changes create variation.
• Recombination during meiosis reshuffles genes.
• Natural selection changes allele frequencies.
• Reproductive isolation separates populations into species.
• Genetic drift and gene flow also act.

Q5. Give an outline of human evolution.

• Dryopithecus and Ramapithecus (ape-like ancestors, about 15 million years ago).
• Australopithecus (East Africa; walked upright).
• Homo habilis (handy man; first stone tools).
• Homo erectus (used fire; Java man, Peking man).
• Homo neanderthalensis; Homo sapiens (arose in Africa, spread worldwide).

Exam tip

For theories, write the main points as a numbered list and add one criticism. The Hardy–Weinberg numerical is a quick, sure-mark question.

Quick summary

Population genetics studies how often genes and genotypes occur in a population and how these frequencies change over generations. The Hardy–Weinberg law states that, without outside forces, these frequencies stay constant. Any change shows that evolution is taking place. This topic covers gene pool, allele and genotype frequencies, the Hardy–Weinberg equation with worked problems, and the factors that disturb the equilibrium.

Key points

• A population is a group of interbreeding individuals of one species in an area. The total of all alleles in it is the gene pool.
• Allele frequency = number of copies of one allele ÷ total copies of all alleles of that gene.
• The Hardy–Weinberg law was stated independently by G. H. Hardy (English mathematician) and W. Weinberg (German physician) in 1908.
• Statement: in a large, randomly mating population, allele and genotype frequencies remain constant from generation to generation if there is no mutation, migration, selection or genetic drift.
• If p = frequency of dominant allele A and q = frequency of recessive allele a, then p + q = 1.
• Genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1.
• Conditions for equilibrium: very large population, random mating, no mutation, no gene flow (migration), no natural selection.
• Factors that change allele frequencies (evolutionary forces): mutation, gene flow (migration), genetic drift (chance changes, strong in small populations), natural selection and non-random mating (inbreeding).
• Genetic drift includes the founder effect (a few individuals start a new population) and the bottleneck effect (a population is drastically reduced by a disaster).
• The law is a "null model": real populations are compared with it to detect evolution.

Important terms

• Gene pool – all alleles of all genes in a population.
• Allele frequency – proportion of one allele among all alleles of a gene.
• Genetic equilibrium – state in which allele frequencies do not change.
• Gene flow – transfer of alleles between populations by migration.
• Genetic drift – random change of allele frequency by chance.

Formulas

• p + q = 1
• p² + 2pq + q² = 1
• q = √(frequency of recessive phenotype), when the recessive phenotype is aa only.
• Frequency of carriers (heterozygotes) = 2pq.

5 important questions

Q1. State the Hardy–Weinberg law.

In a large, randomly mating population with no mutation, migration, selection or drift, the allele frequencies and genotype frequencies remain constant generation after generation; genotype frequencies are given by p² + 2pq + q² = 1.

Q2. What is the founder effect?

When a small group of individuals leaves a population and starts a new one, the new population carries only the alleles of the founders. Its allele frequencies may be very different from the parent population, purely by chance.

Q3. In a population of 1,000 people, 160 show a recessive trait (aa). Find p, q and the number of each genotype.

1. q² = 160 ÷ 1000 = 0.16
2. q = √0.16 = 0.4
3. p = 1 − 0.4 = 0.6
4. AA = p² = 0.36 → 0.36 × 1000 = 360
5. Aa = 2pq = 2 × 0.6 × 0.4 = 0.48 → 480
6. aa = 160
7. Check: 360 + 480 + 160 = 1000 ✓

Q4. Albinism occurs in 1 in 10,000 people. What proportion are carriers?

1. q² = 1/10,000 = 0.0001
2. q = √0.0001 = 0.01
3. p = 1 − 0.01 = 0.99
4. Carriers = 2pq = 2 × 0.99 × 0.01 = 0.0198, about 1 in 50 people.
5. So carriers are far more common than affected persons; most recessive alleles hide in heterozygotes.

Q5. Explain the factors that disturb Hardy–Weinberg equilibrium.

• Mutation: adds new alleles.
• Gene flow: migration in and out.
• Genetic drift: founder and bottleneck effects; stronger in small populations.
• Natural selection: favours some genotypes (e.g. sickle-cell trait in malaria areas).
• Non-random mating: inbreeding raises homozygotes.
• Conclusion: these forces cause evolution.

Exam tip

In numericals, always start from q² (the recessive phenotype), find q, then p. Show the check step; it earns full marks and catches mistakes.

Quick summary

Fossils are the preserved remains or traces of organisms that lived in the past. They are the direct evidence of evolution and help us arrange the history of life on a time chart called the geological time scale. This topic covers how fossils are formed, types of fossils, methods of dating them, and the eras and periods of the geological time scale with the main animal events.

Key points

• The study of fossils is palaeontology; Georges Cuvier is called its father. Fossil is जीवाश्म in Marathi.
• Fossils usually form in sedimentary rocks, when dead organisms are quickly buried by mud or sand so that they do not decay.
• Types of fossils:
• Petrified fossils – tissues replaced by minerals such as silica (petrified wood, bones).
• Moulds and casts – a hollow impression of the body (mould) later filled by minerals (cast).
• Impressions/compressions – thin films or prints of leaves, feathers or soft bodies.
• Whole preservation – insects in amber (tree resin); woolly mammoths frozen in ice in Siberia.
• Trace fossils – footprints, burrows, and coprolites (fossil dung).
• Dating of fossils: relative dating by position in rock layers (lower layers are older); absolute (radiometric) dating using decay of radioactive elements.
• Carbon-14 dating (half-life about 5,730 years) is useful for remains up to about 50,000 years. Potassium–argon and uranium–lead methods date very old rocks.
• Earth is about 4.6 billion years old. The time scale is divided into eons, eras, periods and epochs.
• Living fossils are present-day organisms that have changed very little over long periods: Limulus (king crab), Latimeria (coelacanth), Sphenodon (tuatara).
• Connecting links such as Archaeopteryx (between reptiles and birds) show transitions between groups.

Important terms

• Half-life – time in which half of a radioactive substance decays.
• Index fossil – fossil of a widespread, short-lived species used to date rock layers.
• Mass extinction – loss of a large number of species in a short geological time.
• Stratigraphy – study of rock layers (strata).

Key facts

• The Mesozoic era is called the "Age of Reptiles"; the Cenozoic is the "Age of Mammals".

5 important questions

Q1. What are living fossils? Give two examples.

Living fossils are present-day organisms that closely resemble their fossil ancestors and have changed very little for millions of years. Examples: Limulus (king crab) and Latimeria (coelacanth fish).

Q2. Write the principle of carbon dating.

Living organisms take in carbon-14 along with carbon-12. After death, C-14 decays to nitrogen-14 with a half-life of about 5,730 years while C-12 stays constant. The C-14 : C-12 ratio in a fossil tells how long ago the organism died.

Q3. Describe the process of fossilisation and types of fossils.

• Conditions: quick burial, hard parts, sedimentary rock, absence of decay.
• Types: petrification, moulds and casts, impressions, amber, freezing, trace fossils.
• Examples of each.

Q4. Describe the geological time scale.

• Age of Earth; eons, eras, periods.
• Table of eras and periods with main events.
• Mass extinctions at end-Permian and end-Cretaceous.

Q5. Explain how fossils provide evidence of evolution.

• Fossils show simple forms in older rocks and complex forms in newer rocks.
• Connecting links: Archaeopteryx, Seymouria, therapsids.
• Fossil series: evolution of horse.
• Dating confirms the sequence.

Exam tip

Learn the order of periods with a memory line such as "Camels Often Sit Down Carefully, Perhaps Their Joints Creak" (Cambrian → Cretaceous).

Quick summary

Gametogenesis is the formation of male and female gametes (sperms and ova) in the gonads. Fertilisation is the fusion of a sperm and an ovum to form a diploid zygote. This topic covers spermatogenesis, oogenesis, the structure of sperm and egg, types of eggs, and the steps and significance of fertilisation.

Key points

• Gametogenesis has three phases: multiplication (mitosis), growth, and maturation (meiosis).
• Spermatogenesis occurs in the seminiferous tubules of the testes. Spermatogonia (2n) → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n) → meiosis II → spermatids (n) → spermiogenesis → spermatozoa.
• One primary spermatocyte gives four sperms.
• Sertoli cells nourish developing sperms; Leydig (interstitial) cells secrete testosterone.
• Oogenesis occurs in the ovary. It begins before birth; oogonia form primary oocytes that stay arrested in prophase I until puberty.
• One primary oocyte gives one ovum and two or three polar bodies (unequal division of cytoplasm).
• In humans, the secondary oocyte is released at ovulation, arrested at metaphase II; meiosis II completes only when a sperm enters.
• Sperm structure: head (nucleus + acrosome with enzymes like hyaluronidase), neck (centrioles), middle piece (mitochondria – energy), tail (flagellum for swimming).
• Egg coverings: primary (vitelline membrane, zona pellucida in mammals), secondary (follicle cells – corona radiata), tertiary (jelly coat in frog, albumen and shell in birds).
• Fertilisation in humans occurs in the ampulla of the fallopian tube.

Important terms

• Spermiogenesis – change of round spermatid into a tailed sperm (no division).
• Spermiation – release of sperms from Sertoli cells into the lumen.
• Graafian follicle – mature ovarian follicle with a fluid-filled antrum.
• Acrosome reaction – release of acrosomal enzymes that dissolve egg coverings.
• Cortical reaction – release of cortical granules that change the zona pellucida and block other sperms.
• Polyspermy – entry of more than one sperm; normally prevented.
• Capacitation – final activation of sperm in the female tract.
• Amphimixis – fusion of male and female pronuclei.

Key facts

• Types of eggs by yolk amount: microlecithal (little yolk; e.g. Amphioxus, eutherian mammals – often called alecithal), mesolecithal (moderate; frog), macrolecithal/megalecithal (much yolk; birds, reptiles).
• By yolk distribution: isolecithal / homolecithal (uniform), telolecithal (concentrated at the vegetal pole – frog, bird), centrolecithal (in the centre – insects).

5 important questions

Q1. What is the role of the acrosome?

The acrosome is a cap on the head of the sperm. It contains enzymes (like hyaluronidase and acrosin) that dissolve the coverings of the egg, helping the sperm to penetrate it.

Q2. Why does oogenesis give only one functional ovum?

During both meiotic divisions the cytoplasm divides unequally. Almost all of it goes to one cell (the ovum), so it has enough food for the embryo. The tiny polar bodies degenerate.

Q3. Describe spermatogenesis with a flow chart.

• Multiplication: spermatogonia divide by mitosis.
• Growth: primary spermatocytes (2n).
• Maturation: meiosis I → secondary spermatocytes (n); meiosis II → spermatids (n).
• Spermiogenesis: nucleus condenses, acrosome forms from Golgi, tail from centriole, mitochondria in middle piece.
• Hormonal control: FSH and LH (via testosterone).

Q4. Describe the structure of a human sperm with a labelled diagram.

• Head: nucleus (haploid) and acrosome.
• Neck: proximal and distal centrioles.
• Middle piece: mitochondrial spiral (energy for movement).
• Tail: axial filament (9 + 2 microtubules) covered by sheath.
• Labels: acrosome, nucleus, neck, centrioles, mitochondria, axial filament, tail.

Q5. Explain the process and significance of fertilisation.

• Sperm meets egg; capacitation; acrosome reaction.
• Sperm passes corona radiata and zona pellucida; membranes fuse.
• Cortical reaction blocks polyspermy.
• Secondary oocyte completes meiosis II; pronuclei fuse (amphimixis) → zygote.
• Significance: restores diploid number, brings variation, activates the egg, decides sex (in humans).

Exam tip

A comparison table of spermatogenesis and oogenesis is almost always asked. Draw both flow charts side by side with chromosome numbers.

Quick summary

After fertilisation, the zygote divides and develops into an embryo through a fixed series of stages: cleavage, blastulation, gastrulation and organogenesis. These early stages are studied in frog and chick as type examples. This topic covers types of cleavage, blastula, gastrula, germ layers and their derivatives, fate maps and foetal membranes.

Key points

• Cleavage – rapid mitotic divisions of the zygote without growth; the cells are called blastomeres. The embryo stays about the same size.
• Types of cleavage by plane/extent: holoblastic (whole egg divides; microlecithal and mesolecithal eggs) – equal (Amphioxus) or unequal (frog); meroblastic (only part divides; macrolecithal eggs) – discoidal (bird, reptile) or superficial (insects).
• Types by pattern: radial (echinoderms, amphibians), spiral (annelids, molluscs), bilateral, rotational (mammals).
• Determinate cleavage – fate of each blastomere fixed early; indeterminate – each early blastomere can form a whole embryo (basis of identical twins).
• Morula – solid ball of blastomeres (looks like a mulberry).
• Blastula – hollow ball with a cavity called the blastocoel. Frog blastula has small micromeres at the animal pole and large yolky macromeres at the vegetal pole. Chick has a blastoderm (area pellucida and area opaca). Mammals form a blastocyst with an inner cell mass and outer trophoblast.
• Gastrulation – movement of cells to form the three germ layers: ectoderm, mesoderm and endoderm. A new cavity, the archenteron (primitive gut), forms and opens by the blastopore.
• Morphogenetic movements: epiboly (spreading over), emboly – invagination, involution, convergence, divergence.
• In frog, gastrulation starts at the grey crescent (dorsal lip of blastopore). In chick, a primitive streak forms.
• Organogenesis – formation of organs from germ layers; neurulation forms the neural tube from ectoderm (neural plate → neural folds → neural tube).
• Spemann's organiser: Spemann and Mangold (1924) showed that the dorsal lip of the blastopore can induce a second embryo (primary organiser; embryonic induction).

Important terms

• Blastomere – a cell formed by cleavage.
• Blastocoel – cavity of the blastula.
• Archenteron – cavity of the gastrula; future gut.
• Yolk plug – yolky cells visible inside the circular blastopore of frog gastrula.
• Fate map – map showing what each region of the blastula will become (Vogt, using vital stains).
• Notochord – forms from chorda-mesoderm in the roof of the archenteron.
• Extra-embryonic (foetal) membranes – amnion, chorion, allantois, yolk sac (in reptiles, birds and mammals).

Key facts

• Foetal membranes: amnion (fluid cushion against shock and drying), chorion (gas exchange; forms placenta in mammals), allantois (stores waste in reptiles and birds; forms umbilical blood vessels in mammals), yolk sac (food in birds; early blood formation in mammals).

5 important questions

Q1. What is the grey crescent?

The grey crescent is a grey, crescent-shaped area that appears on the frog egg opposite the point of sperm entry, after cortical rotation. It marks the future dorsal side, and gastrulation begins here (dorsal lip of the blastopore).

Q2. Differentiate between holoblastic and meroblastic cleavage.

In holoblastic cleavage the whole egg divides completely, as in frog and Amphioxus. In meroblastic cleavage only the yolk-free cytoplasm divides, as a disc in bird eggs, because heavy yolk blocks division.

Q3. Describe cleavage and blastulation in frog.

• Mesolecithal, telolecithal egg; holoblastic, unequal, radial cleavage.
• First and second cleavage meridional; third latitudinal, above the equator.
• Micromeres at animal pole, macromeres at vegetal pole.
• Morula formed, then blastula with blastocoel in the animal half.
• Labels: animal pole, vegetal pole, micromeres, macromeres, blastocoel.

Q4. Explain gastrulation in frog.

• Begins at dorsal lip of blastopore (grey crescent).
• Epiboly: micromeres spread over macromeres.
• Invagination and involution of cells through the dorsal lip.
• Archenteron forms; blastocoel shrinks; yolk plug seen.
• Three germ layers: ectoderm, mesoderm (with chorda-mesoderm), endoderm.
• Labels: archenteron, blastopore, yolk plug, ectoderm, mesoderm, endoderm.

Q5. Describe the foetal membranes of chick and their functions.

• Amnion: encloses embryo in amniotic fluid; protection.
• Chorion: outer membrane; with allantois forms chorio-allantoic membrane for respiration.
• Allantois: stores nitrogenous waste (uric acid); respiration.
• Yolk sac: digests and absorbs yolk; blood formation.
• Draw a labelled diagram of the chick embryo with membranes.

Exam tip

Learn the derivatives table of germ layers – it gives quick marks. Always label animal and vegetal poles in frog diagrams.

Quick summary

The endocrine system is a group of ductless glands that release chemical messengers called hormones directly into the blood. Hormones act on distant target organs and control growth, metabolism, reproduction and homeostasis. This topic covers the human endocrine glands, their hormones, functions and disorders, the mechanism of hormone action and feedback control.

Key points

• Endocrine glands are ductless (अंतःस्रावी ग्रंथी). Hormones were named by Bayliss and Starling (secretin, 1902). Thomas Addison is called the father of endocrinology.
• Hormones may be peptides/proteins (insulin, growth hormone), steroids (cortisol, testosterone, oestrogen) or amino acid derivatives (thyroxine, adrenaline).
• Hypothalamus produces releasing and inhibiting hormones that control the anterior pituitary; it also makes oxytocin and ADH, stored in the posterior pituitary.
• Pituitary (hypophysis) is called the master gland: anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).
• Thyroid makes thyroxine (T₄) and T₃ (need iodine) and calcitonin.
• Parathyroid makes parathormone (PTH) which raises blood calcium.
• Adrenal cortex makes glucocorticoids (cortisol), mineralocorticoids (aldosterone) and sex hormones. Adrenal medulla makes adrenaline and noradrenaline – the "fight or flight" hormones.
• Pancreas (islets of Langerhans, described by Paul Langerhans): α-cells – glucagon (raises blood sugar); β-cells – insulin (lowers blood sugar); δ-cells – somatostatin. Banting and Best (1921) isolated insulin.
• Gonads: testes – testosterone; ovaries – oestrogen and progesterone.
• Pineal gland – melatonin (body clock, day–night rhythm). Thymus – thymosins (maturation of T-lymphocytes).
• Mechanism: protein hormones bind receptors on the cell membrane and act through second messengers like cyclic AMP (Sutherland); steroid hormones enter the cell, bind intracellular receptors and act on genes.
• Negative feedback keeps hormone levels steady (e.g. high thyroxine reduces TRH and TSH).

Important terms

• Target organ – organ on which a hormone acts.
• Tropic hormone – hormone that controls another endocrine gland (TSH, ACTH, FSH, LH).
• Second messenger – molecule inside the cell (cAMP, Ca²⁺) that carries the hormone signal.
• Homeostasis – maintenance of a steady internal environment.
• Goitre – enlarged thyroid, often due to iodine deficiency.

Key facts

5 important questions

Q1. Why is the pituitary called the master gland?

Its anterior lobe secretes tropic hormones (TSH, ACTH, FSH, LH, prolactin, GH) that control other endocrine glands like the thyroid, adrenal cortex and gonads. But it is itself controlled by the hypothalamus.

Q2. Differentiate between diabetes mellitus and diabetes insipidus.

Diabetes mellitus is due to lack of insulin or its action; blood sugar is high and glucose appears in urine. Diabetes insipidus is due to lack of ADH; large amounts of dilute urine are passed without sugar.

Q3. Describe the hormones of the anterior pituitary and their functions.

• GH (somatotropin): growth of bones and muscles.
• TSH: stimulates thyroid.
• ACTH: stimulates adrenal cortex.
• FSH: growth of follicles; spermatogenesis.
• LH (ICSH in males): ovulation, corpus luteum; testosterone secretion.
• Prolactin: milk production.
• Mention disorders of GH.

Q4. Explain the mechanism of hormone action.

• Hormone reaches target cell; binds a specific receptor.
• Peptide hormones: membrane receptor → G-protein → adenylate cyclase → cAMP → protein kinase → response (e.g. adrenaline breaking glycogen).
• Steroid hormones: pass membrane → bind cytoplasmic/nuclear receptor → hormone–receptor complex binds DNA → gene expression → new proteins.
• Draw both pathways as flow charts.

Q5. Describe the role of hormones in regulating blood sugar.

• Insulin (β-cells) lowers blood glucose: uptake by cells, glycogen formation in liver and muscle.
• Glucagon (α-cells) raises glucose: glycogen breakdown, gluconeogenesis.
• Adrenaline and cortisol also raise glucose during stress.
• Negative feedback keeps fasting glucose normal.
• Disorder: diabetes mellitus.

Exam tip

Learn a table of gland → hormone → function → disorder. It covers most short and long questions on this topic.

Quick summary

A nerve impulse is a wave of electrical change that travels along a nerve fibre. It is produced by the movement of sodium and potassium ions across the neuron membrane. At a synapse the impulse passes from one neuron to the next, usually through a chemical messenger. This topic covers the structure of a neuron, resting and action potentials, conduction in non-myelinated and myelinated fibres, and synaptic transmission.

Key points

• A neuron (चेतापेशी) has a cell body (cyton), short branched dendrites and a long axon. Many axons are covered by a myelin sheath formed by Schwann cells, with gaps called nodes of Ranvier.
• Resting membrane potential: the inside of a resting neuron is negative, about −70 mV, compared with the outside. The membrane is then polarised.
• It is maintained by the Na⁺–K⁺ pump (3 Na⁺ out, 2 K⁺ in), by the membrane being more permeable to K⁺ than Na⁺, and by large negative proteins inside.
• Action potential: a stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in and the inside becomes positive (about +30 to +40 mV) – depolarisation.
• Then Na⁺ channels close and K⁺ channels open; K⁺ flows out and the inside becomes negative again – repolarisation. A brief overshoot below −70 mV is hyperpolarisation.
• The stimulus must reach the threshold (about −55 mV). The impulse follows the all-or-none law: below threshold nothing happens, above it a full impulse is produced.
• During the refractory period the fibre cannot fire again; this keeps the impulse moving in one direction.
• In non-myelinated fibres, the impulse moves as a continuous wave through local currents – slow.
• In myelinated fibres, the impulse jumps from one node of Ranvier to the next – saltatory conduction – fast (up to about 100–120 m/s) and energy-saving.
• The ionic basis was worked out by Hodgkin and Huxley using the squid giant axon (Nobel Prize 1963).
• A synapse is the junction between two neurons. Most are chemical synapses: the impulse reaches the axon terminal → Ca²⁺ enters → synaptic vesicles release a neurotransmitter (e.g. acetylcholine) into the synaptic cleft → it binds receptors on the next neuron → ion channels open → new impulse.
• Acetylcholinesterase breaks down acetylcholine so the signal stops.
• Electrical synapses pass ions directly through gap junctions; they are faster but less common.

Important terms

• Polarisation – resting state with negative inside and positive outside.
• Depolarisation – inside becomes less negative or positive.
• Synaptic delay – short delay (about 0.5 ms) at a chemical synapse.
• EPSP / IPSP – excitatory or inhibitory postsynaptic potential.
• Neurotransmitter – chemical that carries the signal across a synapse (acetylcholine, noradrenaline, dopamine, serotonin, GABA).

Key facts

5 important questions

Q1. State the all-or-none law.

A nerve fibre either does not respond to a stimulus below threshold, or gives a full-sized impulse to any stimulus at or above threshold. A stronger stimulus does not increase the size of the impulse; it only increases the frequency of impulses.

Q2. What is saltatory conduction?

In myelinated fibres the impulse jumps from one node of Ranvier to the next, because ion exchange occurs only at the nodes. This makes conduction much faster and saves energy.

Q3. Explain the ionic basis of resting and action potential.

• Distribution of Na⁺, K⁺ and proteins.
• Na⁺–K⁺ pump and K⁺ leak channels.
• Depolarisation, repolarisation, hyperpolarisation with voltage-gated channels.
• Threshold and refractory periods.
• Draw the action potential graph.

Q4. Describe the conduction of nerve impulse in myelinated and non-myelinated fibres.

• Structure of each fibre.
• Local current flow in non-myelinated fibres.
• Saltatory conduction in myelinated fibres.
• Comparison of speed and energy use; diagrams.

Q5. Describe the structure of a synapse and the mechanism of synaptic transmission.

• Presynaptic knob, vesicles, cleft, postsynaptic membrane, receptors.
• Steps of chemical transmission (Ca²⁺, exocytosis, binding, EPSP).
• Removal by acetylcholinesterase.
• Electrical synapses; properties (one-way, delay, fatigue).

Exam tip

Draw the action potential curve with labelled phases and voltages; it is asked almost every year in physiology papers.

Quick summary

Muscles move the body and its organs by contracting. Skeletal muscle is made of long fibres packed with protein filaments, actin and myosin, which slide over each other when the muscle contracts. This topic covers types of muscle, the ultrastructure of skeletal muscle, the sliding filament theory, the role of calcium and ATP, and properties such as twitch, tetanus, fatigue and rigor mortis.

Key points

• Types of muscle: skeletal (striated, voluntary, attached to bones), smooth (unstriated, involuntary, in gut and blood vessels) and cardiac (striated, involuntary, only in the heart, with intercalated discs).
• A skeletal muscle is made of bundles (fascicles) of muscle fibres. Each fibre is a long, multinucleated cell with a membrane called sarcolemma and cytoplasm called sarcoplasm.
• Each fibre has many myofibrils showing alternate dark A bands (anisotropic) and light I bands (isotropic).
• The unit of contraction is the sarcomere, the part between two Z lines.
• The A band contains thick myosin filaments; the I band contains thin actin filaments; the paler middle part of the A band is the H zone, with the M line at its centre.
• Thin filaments also carry tropomyosin and troponin, which block myosin-binding sites on actin at rest.
• Sliding filament theory was proposed in 1954 by H. E. Huxley and J. Hanson, and independently by A. F. Huxley and R. Niedergerke.
• Steps of contraction: nerve impulse reaches the neuromuscular junction → acetylcholine released → muscle action potential spreads along sarcolemma and T-tubules → sarcoplasmic reticulum releases Ca²⁺ → Ca²⁺ binds troponin → tropomyosin moves aside → myosin heads bind actin forming cross-bridges → ATP is split and myosin heads pull actin towards the M line (power stroke) → new ATP detaches the head and the cycle repeats.
• Relaxation: Ca²⁺ is pumped back into the sarcoplasmic reticulum, the binding sites are covered again and the filaments slide back.
• During contraction the I band and H zone shorten, the sarcomere shortens, but the A band length stays the same.
• Energy sources: ATP, creatine phosphate (quick refill), aerobic respiration, and anaerobic glycolysis giving lactic acid.

Important terms

• Motor unit – one motor neuron and all the muscle fibres it supplies.
• Muscle twitch – a single quick contraction to one stimulus.
• Tetanus – smooth, sustained contraction due to rapid repeated stimuli.
• Muscle fatigue – fall in contraction power after long activity, partly due to lactic acid.
• Oxygen debt – extra oxygen needed after exercise to remove lactic acid.
• Rigor mortis – stiffening of muscles after death because no ATP is left to detach myosin from actin.

Key facts

5 important questions

Q1. What is the role of calcium in muscle contraction?

Calcium released from the sarcoplasmic reticulum binds troponin. This shifts tropomyosin and exposes the myosin-binding sites on actin, so cross-bridges can form and contraction starts.

Q2. What is rigor mortis?

It is the stiffness of muscles a few hours after death. ATP is used up, so myosin heads stay attached to actin and muscles cannot relax.

Q3. Describe the ultrastructure of skeletal muscle.

• Muscle → fascicle → fibre → myofibril → sarcomere.
• Bands and lines: A, I, H, Z, M.
• Thick and thin filaments with their proteins.
• Sarcoplasmic reticulum and T-tubules (triad).
• Labelled diagram of a sarcomere.

Q4. Explain the sliding filament theory of muscle contraction.

• Proposers and year.
• Excitation at the neuromuscular junction.
• Excitation–contraction coupling and Ca²⁺.
• Cross-bridge cycle and role of ATP.
• Changes in bands; relaxation. Diagrams of relaxed and contracted sarcomere.

Q5. Compare skeletal, smooth and cardiac muscles.

• Shape, nuclei, striations, control, location.
• Special features: intercalated discs (cardiac), spindle shape (smooth).
• Speed of contraction and fatigue.
• Present as a table with diagrams.

Exam tip

Always state which bands shorten and which stay constant during contraction; it is a frequent objective question.

Quick summary

Sense organs contain receptors that change light, sound, gravity and movement into nerve impulses. The eye is the organ of sight, and the ear is the organ of both hearing and balance. This topic covers the structure of the human eye, the working of rods and cones, accommodation and eye defects, and the structure of the ear with the mechanisms of hearing and equilibrium.

Key points

• The eye (डोळा) is a nearly spherical organ in the bony orbit. Its wall has three layers.
• Outer fibrous layer: white, tough sclera; its transparent front part is the cornea.
• Middle vascular layer: choroid (blood vessels and pigment), ciliary body (holds the lens by suspensory ligaments) and iris (coloured, controls the size of the pupil).
• Inner layer: retina, with photoreceptors – rods and cones – and nerve cells.
• Rods (about 120 million) contain rhodopsin (visual purple) and work in dim light; they give black-and-white vision.
• Cones (about 6 million) contain iodopsin and work in bright light; they give colour vision. There are three kinds for red, green and blue.
• Fovea centralis (in the yellow spot) has only cones and gives the sharpest vision. The blind spot, where the optic nerve leaves, has no receptors.
• The lens divides the eye into the front chamber with watery aqueous humour and the back chamber with jelly-like vitreous humour.
• Accommodation: ciliary muscles change the curvature of the lens to focus near and far objects.
• Photochemistry: rhodopsin = opsin + 11-cis retinal (from vitamin A). Light changes it to all-trans retinal, rhodopsin splits and a nerve impulse starts. Vitamin A deficiency causes night blindness.
• The ear has three parts: external ear (pinna, auditory canal, ending at the tympanic membrane), middle ear (three ossicles – malleus, incus, stapes – and the Eustachian tube to the throat) and internal ear (membranous labyrinth filled with endolymph and surrounded by perilymph).
• Hearing: sound waves vibrate the tympanum → ossicles amplify → stapes pushes the oval window → fluid waves in the cochlea → the basilar membrane vibrates → hair cells of the organ of Corti bend against the tectorial membrane → impulses go by the auditory nerve to the brain. Humans hear about 20–20,000 Hz.
• Balance: the vestibular apparatus – three semicircular canals with cristae (detect rotation; dynamic balance) and the utricle and saccule with maculae carrying otoliths (detect head position and straight-line movement; static balance).

Important terms

• Photoreceptor – cell sensitive to light.
• Organ of Corti – hearing organ on the basilar membrane of the cochlea.
• Otoliths – small calcium carbonate crystals in the maculae.
• Eustachian tube – connects the middle ear with the pharynx and equalises air pressure.

Key facts

5 important questions

Q1. Distinguish between rods and cones.

Rods are rod-shaped, numerous, contain rhodopsin, work in dim light and do not detect colour. Cones are cone-shaped, fewer, contain iodopsin, work in bright light and give colour vision; they are packed in the fovea.

Q2. What is the function of the Eustachian tube?

It connects the middle ear cavity with the pharynx and keeps the air pressure equal on both sides of the eardrum, so that the eardrum vibrates freely and does not rupture.

Q3. Describe the structure of the human eye.

• Three coats and their parts.
• Lens, chambers and humours.
• Structure of retina; fovea and blind spot.
• Labelled diagram of vertical section of the eye.

Q4. Explain the mechanism of vision.

• Path of light and focusing; accommodation.
• Rhodopsin cycle with vitamin A.
• Generation of impulse and optic pathway to the brain.
• Colour vision by three types of cones.

Q5. Describe the structure of the ear and the mechanism of hearing and balance.

• External, middle and internal ear with diagram.
• Cochlea and organ of Corti.
• Steps of hearing.
• Static and dynamic equilibrium by maculae and cristae.

Exam tip

Practise the labelled diagrams of the eye (vertical section) and the cochlea (T.S.); a good diagram can secure half the marks of the long answer.