Zoology प्राणीशास्त्र

Quick summary

Classification means arranging animals into groups based on their similarities and differences. It makes the study of millions of animals easy. This topic covers the basis of classification, binomial nomenclature, taxonomic hierarchy and the outline classification of the animal kingdom (non-chordates and chordates).

Key points

• Carolus Linnaeus is the father of taxonomy. He introduced binomial nomenclature in Systema Naturae (10th edition, 1758).
• In binomial nomenclature each animal has two names: genus (capital first letter) and species (small letter), printed in italics or underlined separately when handwritten. Example: Panthera tigris (tiger), Homo sapiens (human).
• R. H. Whittaker (1969) gave the five-kingdom classification: Monera, Protista, Fungi, Plantae, Animalia.
• Taxonomic hierarchy (वर्गीकरण श्रेणी): Kingdom → Phylum → Class → Order → Family → Genus → Species.
• Species is the basic unit of classification: a group of similar individuals that can interbreed and produce fertile offspring.
• Basis of classification: levels of organisation, symmetry, germ layers, body cavity (coelom), segmentation and notochord.
• Levels of organisation: cellular (Porifera), tissue (Cnidaria), organ (Platyhelminthes), organ-system (higher phyla).
• Symmetry: asymmetrical (most sponges), radial (Cnidaria, adult Echinodermata), bilateral (most other phyla).
• Germ layers: diploblastic (ectoderm and endoderm – Cnidaria) and triploblastic (with mesoderm – Platyhelminthes onwards).
• Coelom: acoelomate (Platyhelminthes), pseudocoelomate (Aschelminthes), coelomate/eucoelomate (Annelida onwards).
• Non-chordates lack a notochord; chordates have a notochord, a dorsal hollow nerve cord and pharyngeal gill slits at some stage.

Important terms

• Taxon – any unit of classification (e.g. a family or a genus).
• Coelom – fluid-filled body cavity lined by mesoderm on both sides.
• Pseudocoelom – body cavity not fully lined by mesoderm.
• Metamerism – division of body into similar segments (e.g. earthworm).
• Protostome – mouth forms from blastopore (Annelida, Arthropoda, Mollusca).
• Deuterostome – anus forms from blastopore (Echinodermata, Chordata).

Key facts

5 important questions

Q1. What is binomial nomenclature?

It is the system of naming every organism by two words: genus name and species name. It was introduced by Linnaeus. Example: Rana tigrina (Indian bullfrog; now also named Hoplobatrachus tigerinus).

Q2. Differentiate between diploblastic and triploblastic animals.

Diploblastic animals have two germ layers, ectoderm and endoderm, with a non-cellular mesoglea between them (e.g. Hydra). Triploblastic animals have three germ layers including mesoderm (e.g. earthworm, human).

Q3. Explain the taxonomic hierarchy with an example.

• Arrange the seven main ranks from kingdom to species.
• Each higher rank includes several lower ranks.
• Example (human): Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species sapiens.
• Lower ranks show more similarity among members.

Q4. Describe the basis of classification of animals.

• Level of organisation (cellular to organ-system).
• Symmetry (asymmetry, radial, bilateral).
• Germ layers (diploblastic, triploblastic).
• Coelom (acoelomate, pseudocoelomate, coelomate).
• Segmentation and notochord.
• Give one example for each.

Q5. Write an outline classification of non-chordates with one example each.

• List the nine phyla from Protozoa to Echinodermata.
• Give one key character and one Indian example for each (as in the table).
• Mention that Hemichordata (Balanoglossus) was once placed with chordates but is now often treated separately.

Exam tip

Learn the table of phyla with one character and one example. Write scientific names correctly: genus capitalised, species small, both in italics or underlined.

Quick summary

Protozoa are microscopic, unicellular, eukaryotic animal-like organisms. A single cell does all life functions: feeding, movement, respiration, excretion and reproduction. In the five-kingdom system they are placed in Kingdom Protista. This topic covers general characters, classification by locomotion, type study and diseases.

Key points

• Protozoa means "first animals" (प्रोटोझोआ / आदिजीव). The term was given by Goldfuss (1818).
• They are found in fresh water, sea water, moist soil and as parasites in other animals.
• Body is a single cell, but it shows division of labour among its organelles (protoplasmic level of organisation).
• Body may be naked (Amoeba) or covered by a pellicle (Paramecium, Euglena) or a shell/test (Arcella, foraminiferans).
• Locomotion is by pseudopodia, flagella or cilia; parasitic forms may have no locomotor organs.
• Nutrition: holozoic (Amoeba), holophytic (Euglena in light), saprozoic, or parasitic (Plasmodium).
• Contractile vacuoles remove excess water (osmoregulation) in fresh-water forms.
• Respiration and excretion occur by diffusion through the body surface.
• Asexual reproduction: binary fission, multiple fission, budding.
• Sexual reproduction: syngamy and conjugation (Paramecium).
• Many form cysts to survive bad conditions.

Important terms

• Pseudopodia – temporary finger-like projections of cytoplasm (false feet).
• Pellicle – thin, firm, flexible outer covering.
• Macronucleus – large nucleus controlling daily metabolism (Paramecium).
• Micronucleus – small nucleus involved in reproduction.
• Trichocysts – defensive organelles below the pellicle of Paramecium.
• Sporozoite – infective stage of Plasmodium injected by mosquito.
• Schizogony – asexual multiple fission of Plasmodium in human cells.

Key facts

• Plasmodium vivax causes benign tertian malaria; female Anopheles mosquito is the vector.
• Ronald Ross (1897) showed the malaria parasite's development in the mosquito; Nobel Prize 1902.
• Entamoeba histolytica causes amoebic dysentery; Trypanosoma gambiense causes sleeping sickness (tsetse fly vector); Leishmania donovani causes kala-azar (sand fly vector).

5 important questions

Q1. What are contractile vacuoles?

They are clear, fluid-filled vacuoles in fresh-water protozoa. They collect excess water from the cytoplasm and throw it out, maintaining water balance (osmoregulation).

Q2. Why is Euglena called a plant-animal?

Euglena has chloroplasts and makes food by photosynthesis like plants. It also has a flagellum, no cell wall and can feed saprozoically in darkness like animals.

Q3. Describe the life cycle of Plasmodium vivax.

• Two hosts: human (intermediate host, asexual) and female Anopheles (definitive host, sexual).
• Mosquito bite injects sporozoites → liver cells (pre-erythrocytic schizogony).
• Merozoites enter RBCs → trophozoite → signet ring → schizont → RBCs burst every 48 hours (fever with chills).
• Some form gametocytes, taken up by mosquito.
• In mosquito: gametes fuse → ookinete → oocyst on stomach wall → sporozoites reach salivary glands.

Q4. Describe conjugation in Paramecium.

• Two individuals of compatible mating types join at the oral groove.
• Macronucleus degenerates; micronucleus divides by meiosis into four; three degenerate.
• The remaining one divides into a stationary and a migratory pronucleus.
• Migratory pronuclei are exchanged and fuse to form a synkaryon.
• Conjugants separate; further divisions restore normal nuclei.
• It brings genetic variation and rejuvenation.

Q5. Give the general characters of Protozoa.

• Unicellular, microscopic, eukaryotic.
• Free-living or parasitic.
• Locomotion by pseudopodia, flagella or cilia.
• Nutrition holozoic, holophytic, saprozoic or parasitic.
• Contractile vacuoles; diffusion for respiration and excretion.
• Asexual and sexual reproduction; encystment.

Exam tip

The Plasmodium life cycle is the most-asked question. Write it in two clear parts: in man and in mosquito, with all stage names.

Quick summary

Porifera are sponges – simple, mostly marine, sedentary animals with many pores on the body. They are multicellular but have no true tissues or organs. Water flows through a special canal system for food, respiration and excretion. This topic covers characters, canal system, skeleton, reproduction and examples.

Key points

• The name Porifera means "pore-bearing". The phylum name was given by Robert Grant.
• Sponges (स्पंज) are mostly marine; a few are fresh-water (Spongilla).
• They are sessile (fixed) and mostly asymmetrical; some are radially symmetrical (Sycon).
• Cellular level of organisation; no true tissues.
• Body wall has an outer pinacoderm and an inner choanoderm, with jelly-like mesenchyme (mesohyl) between them.
• Choanocytes (collar cells) line the inner cavities; their flagella create water current and they trap food.
• Water enters through small pores called ostia, passes through the spongocoel or canals, and leaves through a large opening called the osculum.
• Skeleton is made of spicules (calcareous or siliceous) and/or spongin fibres.
• Digestion is intracellular.
• No nervous, respiratory, excretory or circulatory systems.
• Reproduction: asexual by budding and gemmules (internal buds in fresh-water sponges); sexual by gametes. Most are hermaphrodite. Development includes a free-swimming larva (amphiblastula or parenchymula).
• Very high regeneration power.

Important terms

• Ostia – small inhalant pores.
• Osculum – large exhalant opening.
• Spongocoel – central cavity of the sponge.
• Choanocyte – flagellated collar cell.
• Porocyte – tubular cell forming an ostium (in simple sponges).
• Amoebocytes / archaeocytes – wandering cells in mesenchyme; form other cells.
• Gemmule – asexual reproductive body of fresh-water sponges that survives bad conditions.

Key facts

• Classes: Calcarea (calcareous spicules – Sycon, Leucosolenia), Hexactinellida (six-rayed siliceous spicules – Euplectella, Venus's flower basket), Demospongiae (siliceous spicules and/or spongin – Spongilla, Euspongia bath sponge).

5 important questions

Q1. What are choanocytes?

Choanocytes are flagellated cells with a collar of microvilli around the flagellum. They line the spongocoel or flagellated chambers, create water current and trap food particles.

Q2. What is a gemmule?

A gemmule is an internal bud formed in fresh-water sponges like Spongilla. It is a group of archaeocytes covered by a protective coat with spicules. It survives drought and cold and grows into a new sponge.

Q3. Describe the canal system in sponges.

• Function: water current for food, oxygen and removal of waste.
• Ascon type: simplest; spongocoel lined by choanocytes.
• Sycon type: body wall folded; radial canals lined by choanocytes.
• Leucon type: most complex; choanocytes in small flagellated chambers.
• Draw the water path for each type.

Q4. Give the general characters of Porifera.

• Aquatic, sessile, mostly marine.
• Asymmetrical or radial; cellular level.
• Ostia, osculum and canal system; choanocytes.
• Spicules or spongin skeleton.
• Intracellular digestion; no organ systems.
• Budding, gemmules, sexual reproduction; high regeneration.

Q5. Describe the structure of Sycon with a labelled diagram.

• Vase-like body fixed at the base; osculum at top surrounded by a fringe of spicules.
• Body wall folded into radial and incurrent canals.
• Pinacoderm outside, choanocytes in radial canals, mesenchyme with spicules and amoebocytes.
• Labels: osculum, spongocoel, ostia, incurrent canal, radial canal, prosopyle, apopyle, spicules, choanocytes.

Exam tip

The canal system is a favourite long question. Remember the order of water flow, and that water always leaves through the osculum.

Quick summary

Cnidaria (also called Coelenterata) are aquatic, mostly marine, radially symmetrical animals with stinging cells called cnidocytes. They are the first animals with a tissue level of organisation and a single body cavity (gastrovascular cavity) with one opening. This topic covers characters, body forms, polymorphism, type study of Obelia and corals.

Key points

• The name comes from cnidoblasts/cnidocytes (stinging cells). The older name Coelenterata was given by Leuckart (1847).
• Mostly marine; Hydra is a common fresh-water example.
• Radial symmetry; diploblastic – outer ectoderm (epidermis) and inner endoderm (gastrodermis) with non-cellular mesoglea between.
• Tissue level of organisation.
• Single cavity called the gastrovascular cavity (coelenteron) with one opening, the mouth, surrounded by tentacles.
• Digestion is both extracellular and intracellular.
• Cnidocytes with nematocysts are present on tentacles for defence and capturing prey.
• Nerve net is the simplest nervous system; no brain.
• Two body forms: polyp (cylindrical, fixed, asexual) and medusa (umbrella-like, free-swimming, sexual).
• Metagenesis (alternation of generations): polyp produces medusae asexually; medusae produce polyps through gametes and a planula larva (e.g. Obelia).
• Polymorphism: different forms of individuals in one colony (e.g. Physalia – Portuguese man-of-war).

Important terms

• Nematocyst – capsule inside a cnidocyte with a coiled thread that shoots out to sting or hold prey.
• Hypostome – conical raised region around the mouth.
• Planula – ciliated, free-swimming larva of cnidarians.
• Velum – shelf-like inner fold at the margin of a hydromedusa.
• Manubrium – tube hanging from the medusa carrying the mouth.
• Coral reef – large structure built from calcium carbonate skeletons of coral polyps.

Key facts

• ***Obelia*: colonial marine hydroid. Colony has a horizontal hydrorhiza and upright hydrocaulus. It bears two kinds of polyps: feeding gastrozooids (hydranths) and reproductive blastostyles (gonangia) that bud off medusae. Polyps covered by a chitinous perisarc**.
• Coral reefs in India: Gulf of Kutch, Gulf of Mannar, Lakshadweep, Andaman and Nicobar Islands.
• Types of reefs (Darwin's classification): fringing reef, barrier reef, atoll.

5 important questions

Q1. What is a nematocyst?

A nematocyst is a stinging capsule inside a cnidocyte. It contains a coiled hollow thread and poison. On touch, the thread shoots out to paralyse prey or for defence.

Q2. Define metagenesis.

Metagenesis is the regular alternation of an asexual polyp generation and a sexual medusa generation in the life cycle of some cnidarians, such as Obelia.

Q3. Describe the structure and life cycle of Obelia.

• Fixed colony: hydrorhiza, hydrocaulus, branches with hydranths and blastostyles.
• Perisarc forms hydrotheca and gonotheca.
• Blastostyle buds off free medusae.
• Medusae are dioecious; gametes fuse in water.
• Zygote → planula larva → settles → new polyp colony.
• Labels: hydranth, hydrotheca, tentacles, hypostome, blastostyle, gonotheca, medusa bud, perisarc, coenosarc.

Q4. Explain polymorphism in Cnidaria.

• Presence of more than one type of individual in a species for division of labour.
• Dimorphic: gastrozooids and blastostyles (Obelia).
• Polymorphic: Physalia has pneumatophore (float), dactylozooids (defence), gastrozooids (feeding) and gonozooids (reproduction).
• It increases efficiency of the colony.

Q5. Write a note on coral reefs.

• Formed by calcareous skeletons of anthozoan polyps over long time.
• Need warm, clear, shallow sea water.
• Types: fringing, barrier and atoll.
• Importance: rich biodiversity, protect coasts, tourism, source of lime.
• Threats: pollution, coral bleaching due to rising sea temperature, mining.
• Indian reef areas as listed above.

Exam tip

Draw Obelia colony and medusa together and mention metagenesis. Remember "diploblastic with mesoglea" as a key character.

Quick summary

Platyhelminthes are flatworms: dorsoventrally flattened, triploblastic, acoelomate animals, many of them parasites. Aschelminthes (Nemathelminthes or Nematoda) are roundworms: cylindrical, unsegmented and pseudocoelomate. This topic covers characters, examples, parasitic adaptations and the life cycles of liver fluke, tapeworm and Ascaris.

Key points

• Platyhelminthes (चपटे कृमी): bilaterally symmetrical, triploblastic, acoelomate; first animals with organ level of organisation.
• Body flat like a leaf or ribbon; digestive system incomplete (one opening) or absent (tapeworm).
• Excretion by flame cells (protonephridia).
• Mostly hermaphrodite (monoecious); fertilisation internal.
• Parasitic forms have hooks, suckers and a thick tegument.
• Classes: Turbellaria (Planaria, free-living, high regeneration), Trematoda (flukes – Fasciola hepatica), Cestoda (tapeworms – Taenia solium).
• Aschelminthes (गोल कृमी): body round, unsegmented, tapering at both ends; covered by a tough cuticle.
• Pseudocoelom present; digestive tract complete (mouth and anus).
• Sexes are separate (dioecious); male usually smaller with a curved tail. Sexual dimorphism is seen.
• No circular muscles; only longitudinal muscles, so movement is whip-like.
• Examples: Ascaris lumbricoides (roundworm), Wuchereria bancrofti (filarial worm), Enterobius (pinworm), Ancylostoma (hookworm).

Important terms

• Flame cell – excretory cell with a tuft of beating cilia like a flickering flame.
• Scolex – head of tapeworm with hooks and suckers.
• Proglottids – segments of tapeworm body; mature ones have both sex organs.
• Strobilisation – formation of proglottids behind the neck.
• Cysticercus – bladder-worm larval stage of Taenia in pig muscle.
• Intermediate host – host in which larval stages develop.
• Pseudocoelom – cavity between body wall and gut not lined by mesoderm on the gut side.

Key facts

• Fasciola larval stages (in order): miracidium → sporocyst → redia → cercaria → metacercaria.
• Ascaris has a single host; larvae migrate through liver, heart and lungs before settling in the intestine.

5 important questions

Q1. What are flame cells?

Flame cells are excretory cells of flatworms. Each cell has a cavity with a bunch of cilia whose beating looks like a flame. They push waste fluid into excretory canals.

Q2. Give two differences between Platyhelminthes and Aschelminthes.

Platyhelminthes are flat, acoelomate and mostly hermaphrodite with an incomplete gut. Aschelminthes are round, pseudocoelomate and dioecious with a complete gut.

Q3. Describe the life cycle of Taenia solium.

• Adult in human small intestine, fixed by scolex.
• Gravid proglottids with eggs pass out with faeces.
• Pig eats eggs → onchosphere (hexacanth embryo) hatches → reaches muscles.
• Forms cysticercus (bladder worm) – "measly pork".
• Man eats undercooked pork → cysticercus becomes adult.
• Labels for scolex diagram: rostellum, hooks, suckers, neck.

Q4. Explain parasitic adaptations of helminths.

• Organs of attachment: hooks, suckers.
• Thick tegument/cuticle resists host enzymes.
• Reduced or absent digestive system (tapeworm absorbs digested food).
• Anaerobic respiration.
• Hermaphroditism and huge egg production.
• Larval stages in intermediate hosts; resistant egg shells.

Q5. Describe the life cycle of Ascaris lumbricoides.

• Monogenetic; lives in human small intestine.
• Fertilised eggs pass out in faeces; develop in soil to infective stage.
• Swallowed with food/water; larvae hatch in intestine.
• Migrate via blood to liver, heart, lungs; moult; travel up trachea; swallowed again.
• Mature in intestine; cause ascariasis, malnutrition, intestinal blockage.

Exam tip

Learn larval stages of Fasciola in correct order and the hosts of each parasite. A table of parasites and diseases gives quick marks.

Quick summary

The physical environment of animals has three parts: the atmosphere (air), the hydrosphere (water) and the lithosphere (rocks and soil). Life exists where these three meet, in the biosphere. This topic covers the layers and composition of air, the distribution and properties of water, soil formation and erosion, and renewable and non-renewable energy sources.

Key points

• Atmosphere is the envelope of gases around the Earth, held by gravity. Its layers from the ground up are troposphere, stratosphere, mesosphere, thermosphere and exosphere.
• Troposphere (up to about 8–18 km) has nearly all water vapour and weather; temperature falls with height.
• Stratosphere (up to about 50 km) contains the ozone layer, which absorbs harmful ultraviolet (UV) rays.
• Composition of dry air (by volume): nitrogen about 78%, oxygen about 21%, argon about 0.93%, carbon dioxide about 0.04%, plus traces of other gases.
• Hydrosphere covers about 71% of the Earth's surface. About 97% of all water is salty sea water; only about 3% is fresh, and most of that is locked in ice caps and glaciers.
• Physico-chemical characters of water: temperature, colour, turbidity, pH, dissolved oxygen (DO), free CO₂, hardness, alkalinity and nutrients. These decide which animals can live in it.
• Water has a high specific heat and is densest at 4 °C, so ponds freeze from the top and aquatic life survives below.
• Lithosphere is the solid outer crust. Soil forms from rock by weathering (physical, chemical and biological) plus the addition of humus. This process is called pedogenesis.
• Soil erosion is the removal of the top fertile soil by water and wind. Causes: deforestation, overgrazing, faulty farming, mining and floods.
• Renewable energy (solar, wind, hydro, tidal, geothermal, biomass) is replaced naturally. Non-renewable energy (coal, petroleum, natural gas, nuclear fuel) is limited and takes millions of years to form.

Important terms

• Biosphere – the part of Earth where life exists (जीवावरण).
• Ozone layer – layer of O₃ in the stratosphere that filters UV radiation.
• Dissolved oxygen (DO) – amount of oxygen dissolved in water, in mg/L.
• Hardness of water – caused mainly by calcium and magnesium salts.
• Humus – dark, decomposed organic matter in soil.
• Soil profile – vertical section of soil showing layers (horizons O, A, B, C).
• Fossil fuels – coal, oil and gas formed from ancient buried organisms.

Key facts

• Good-quality fresh water for fish usually has DO above 5 mg/L and pH about 6.5–8.5.
• Soil conservation methods: afforestation, contour ploughing, terrace farming, strip cropping, check dams, controlled grazing.

5 important questions

Q1. What is the importance of the ozone layer?

The ozone layer in the stratosphere absorbs most of the harmful ultraviolet rays of the Sun. Without it, UV rays would cause skin cancer, cataract, damage to crops and harm to aquatic life.

Q2. Distinguish between renewable and non-renewable energy sources.

Renewable sources (solar, wind, hydro, biomass) are replaced naturally and do not run out; they cause little pollution. Non-renewable sources (coal, petroleum, natural gas) are limited, take millions of years to form and cause pollution when burnt.

Q3. Describe the major zones of the atmosphere and the composition of air.

• Define atmosphere.
• Five zones with height and features (table above).
• Composition: N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ 0.04%, water vapour variable.
• Importance: oxygen for respiration, CO₂ for photosynthesis, protection from UV, keeps Earth warm, weather and rainfall.

Q4. Describe the hydrosphere and the physico-chemical characteristics of water.

• Global distribution: oceans 97%, ice 2%, groundwater, lakes and rivers less than 1%.
• Physical: temperature, colour, turbidity, light penetration, density.
• Chemical: pH, DO, free CO₂, hardness, alkalinity, chlorides, nitrates and phosphates.
• Show how each affects aquatic animals (e.g. low DO kills fish).

Q5. Explain the formation of soil and the causes of soil erosion.

• Weathering of parent rock: physical (heat, frost, wind), chemical (oxidation, carbonation), biological (lichens, roots, burrowing animals like earthworms).
• Addition of humus; development of horizons.
• Causes of erosion: deforestation, overgrazing, shifting cultivation, mining, heavy rain and wind.
• Effects and control measures.

Exam tip

Draw a neat labelled diagram of atmospheric layers with heights. In the water question, always mention DO and pH, as practicals also test them.

Quick summary

An ecosystem is a natural unit in which living organisms interact with each other and with their non-living surroundings. Energy flows and nutrients cycle through it. This topic covers the components of an ecosystem, the main types (forest, grassland, desert and pond), and food chains and food webs.

Key points

• The term ecosystem was coined by A. G. Tansley (1935). Ecosystem is परिसंस्था in Marathi.
• Abiotic components: light, temperature, water, air, soil, minerals (inorganic and organic substances, climatic factors).
• Biotic components: producers (green plants, phytoplankton), consumers (herbivores = primary, carnivores = secondary and tertiary) and decomposers (bacteria, fungi).
• Forest ecosystem: tall trees, shrubs and herbs as producers; deer, insects, birds as primary consumers; fox, snake, tiger as higher consumers. Example: Tadoba forest in Vidarbha.
• Grassland ecosystem: grasses and herbs as producers; grasshoppers, rabbits, cattle as herbivores; frogs, snakes, hawks as carnivores.
• Desert ecosystem: low rainfall, extreme temperature; cacti and thorny shrubs; animals like camel, desert rat, lizards and snakes show water-saving adaptations.
• Pond ecosystem: phytoplankton and rooted plants (Hydrilla, lotus) as producers; zooplankton as primary consumers; small fish, frogs, then big fish and birds as higher consumers; bacteria and fungi in the bottom mud as decomposers.
• A food chain is a straight series of organisms through which food energy passes, each eating the one before it.
• Types of food chains: grazing food chain (starts with green plants), detritus food chain (starts with dead organic matter) and parasitic food chain (from host to parasites).
• A food web is a network of many interconnected food chains. It gives an ecosystem stability: if one food source is lost, consumers can use another.

Important terms

• Producer (autotroph) – makes its own food by photosynthesis.
• Consumer (heterotroph) – depends on other organisms for food.
• Decomposer – breaks down dead matter into simple nutrients.
• Trophic level – the feeding position of an organism in a food chain.
• Detritus – dead plant and animal remains.
• Ecotone – transition zone between two ecosystems.

Key facts

• Example grazing chain: grass → grasshopper → frog → snake → hawk.
• Example detritus chain: dead leaves → earthworm → bird.

5 important questions

Q1. Define an ecosystem. Who coined the term?

An ecosystem is a self-sustaining unit of nature made of living organisms and their physical environment, interacting through energy flow and nutrient cycling. The term was given by A. G. Tansley in 1935.

Q2. Distinguish between food chain and food web.

A food chain is a single straight path of energy transfer from producer to top consumer. A food web is a network of many interconnected food chains. A food web is more stable and realistic, because most animals eat more than one kind of food.

Q3. Describe the structure of a pond ecosystem.

• Abiotic: water, dissolved gases, minerals, light, temperature, bottom mud.
• Producers: phytoplankton (algae, diatoms) and macrophytes.
• Consumers: zooplankton (primary), small fish and insects (secondary), large fish and birds (tertiary).
• Decomposers: bacteria and fungi in the mud.
• Zones: littoral, limnetic and profundal.
• Draw a labelled diagram.

Q4. Explain the types of food chains with examples.

• Grazing food chain: plants → herbivores → carnivores (example from grassland or pond).
• Detritus food chain: detritus → detritivores → predators (example from forest floor).
• Parasitic food chain: host → parasite → hyperparasite (e.g. tree → bird → lice → bacteria).
• Importance: energy transfer, balance of populations.

Q5. Compare forest and desert ecosystems.

• Climate and rainfall.
• Producers and their adaptations (broad leaves vs spines and fleshy stems).
• Consumers and their adaptations (camel stores fat; desert rat gets water from food; nocturnal habits).
• Productivity: high in forest, low in desert.
• Decomposers: abundant in forest, few in desert.

Exam tip

For any ecosystem question, write abiotic and biotic components under clear sub-headings and finish with a food chain from that ecosystem.

Quick summary

Ecological pyramids are graphs that show the number, biomass or energy of organisms at each trophic level of a food chain. Energy flows in one direction through an ecosystem and much of it is lost at each step. This topic covers the three types of pyramids, their shapes in pond, forest and parasitic food chains, and the single-channel, Y-shaped and universal models of energy flow.

Key points

• Ecological pyramids were first described by Charles Elton (1927), so they are also called Eltonian pyramids.
• The producer level forms the base and the top consumer forms the apex.
• Pyramid of numbers: number of individuals at each level. Upright in grassland and pond; spindle-shaped in a tree-based forest chain; inverted in a parasitic food chain.
• Pyramid of biomass: total dry weight of living matter at each level. Upright on land; inverted in a pond, because the small mass of phytoplankton at any time supports a larger mass of zooplankton and fish (phytoplankton multiply very fast).
• Pyramid of energy: energy at each level per unit area per unit time. It is always upright, because energy is lost at every transfer.
• Energy flow is unidirectional: Sun → producers → herbivores → carnivores. Energy is not recycled; nutrients are.
• Ten percent law (Lindeman, 1942): only about 10% of energy at one trophic level passes on to the next; the rest is lost mainly as heat in respiration.
• Because of this loss, food chains usually have only 4–5 trophic levels.
• Single-channel model: energy flows in one straight line through the trophic levels.
• Y-shaped model (Odum): energy from producers splits into two arms, the grazing food chain and the detritus food chain, which work together.
• Universal model (Odum): applies to any living component; the energy taken in is partly used in respiration, partly stored as growth (production) and partly lost as unassimilated food.

Important terms

• Trophic level – feeding level in a food chain (पोषण पातळी).
• Biomass – total weight of living organisms in an area.
• Gross primary productivity (GPP) – total energy fixed by producers.
• Net primary productivity (NPP) – GPP minus energy used by producers in respiration.
• Standing crop – amount of living matter present at a given time.

Key facts

• NPP = GPP − R (R = respiration loss).
• If producers fix 10,000 J, herbivores get about 1,000 J, primary carnivores about 100 J and secondary carnivores about 10 J.

5 important questions

Q1. Why is the pyramid of energy always upright?

At each transfer only about 10% of energy passes to the next level and the rest is lost as heat. So the energy present at every higher level is always less than the level below it.

Q2. Why is the pyramid of biomass inverted in a pond?

Phytoplankton are tiny and short-lived, so their standing biomass at any moment is small. They reproduce very fast and support a larger biomass of zooplankton and fish, so the pyramid becomes inverted.

Q3. Describe the pyramids of number with examples.

• Define pyramid of numbers.
• Upright: grassland (grass → grasshopper → frog → snake).
• Spindle: forest tree supporting many insects and birds.
• Inverted: parasitic chain (one tree → many birds → many more lice).
• Draw all three shapes.

Q4. Explain energy flow in an ecosystem with the Y-shaped model.

• Sun's energy fixed by producers (GPP, NPP).
• Two channels: grazing food chain and detritus food chain.
• Both chains interact; detritus chain handles a large share of energy in forests.
• Lindeman's 10% law; heat loss at each level.
• Draw the Y-shaped diagram.

Q5. Describe the single-channel and universal models of energy flow.

• Single-channel: straight flow from producers to top consumer; respiration loss at each level.
• Universal model: intake (I) → assimilation (A) → production (P) + respiration (R); non-assimilated energy (NU) lost.
• Unidirectional nature of flow and the first and second laws of thermodynamics.

Exam tip

Always draw pyramid shapes and energy-flow diagrams; marks are given for labelled figures. Remember the one exception: the energy pyramid is never inverted.

Quick summary

Biodiversity is the variety of all living organisms on Earth, from genes to species to ecosystems. India is one of the world's megadiverse countries, but biodiversity is falling fast because of human activities. This topic covers levels and value of biodiversity, causes of its loss, in-situ and ex-situ conservation, the Wildlife Protection Act 1972, the Biological Diversity Act 2002, the Zoological Survey of India, hotspots, and Tadoba, Nagzira and Kanha.

Key points

• The term biodiversity was popularised by E. O. Wilson (1988). It is जैवविविधता in Marathi.
• Three levels: genetic diversity (variation within a species), species diversity (variety of species in an area) and ecosystem diversity (variety of habitats such as forests, wetlands, deserts).
• Values: food, medicines, timber, fibres, ecological services (pollination, soil fertility, clean water), aesthetic, cultural and ethical value.
• India has about 2.4% of the world's land but about 7–8% of recorded species.
• Causes of loss (HIPPO): Habitat destruction, Invasive species, Pollution, human Population pressure, Over-exploitation (poaching, overfishing). Climate change adds to these.
• In-situ conservation (in the natural habitat): national parks, wildlife sanctuaries, biosphere reserves, tiger reserves, sacred groves.
• Ex-situ conservation (outside the habitat): zoos, botanical gardens, seed banks, gene banks, cryopreservation, captive breeding.
• Biodiversity hotspot: a region with very high endemic species that has lost most of its original vegetation. The concept was given by Norman Myers (1988).
• India's hotspots: Himalaya, Indo-Burma, Western Ghats and Sri Lanka, Sundaland (Nicobar Islands).
• Zoological Survey of India (ZSI): set up in 1916, headquarters Kolkata. It surveys, identifies and documents Indian fauna, keeps national animal collections, and publishes the Red Data Book and fauna of India.

Important terms

• Endemic species – found only in one particular region.
• Endangered species – at high risk of extinction (e.g. tiger, great Indian bustard).
• National park – area strictly protected for wildlife; no human activities like grazing.
• Wildlife sanctuary – protected area where some regulated human activity is allowed.
• Red Data Book – list of threatened species kept by IUCN.

Key facts

• Wildlife (Protection) Act, 1972: bans hunting of listed animals; has Schedules giving levels of protection; provides for national parks and sanctuaries; controls trade in wildlife and its products; sets up wildlife advisory boards and the post of Chief Wildlife Warden.
• Biological Diversity Act, 2002: follows the Convention on Biological Diversity (1992); three-tier system – National Biodiversity Authority (Chennai), State Biodiversity Boards, local Biodiversity Management Committees; access and benefit-sharing; People's Biodiversity Registers.

5 important questions

Q1. What is a biodiversity hotspot? Name the hotspots in India.

A hotspot is a region very rich in endemic species that has already lost much of its natural habitat. India has parts of four: Himalaya, Indo-Burma, Western Ghats and Sri Lanka, and Sundaland.

Q2. Write the role of the Zoological Survey of India.

ZSI (1916, Kolkata) surveys and documents India's animals, identifies specimens, maintains national zoological collections, publishes fauna volumes and status reports, and advises the government on animal conservation.

Q3. Explain the causes of reduction of biodiversity.

• Habitat loss and fragmentation (deforestation, dams, roads, urbanisation).
• Over-exploitation: poaching, illegal wildlife trade, overfishing.
• Invasive alien species (e.g. water hyacinth, African catfish).
• Pollution of air, water and soil; pesticides.
• Climate change; forest fires.
• Give one Indian example for each.

Q4. Give the salient features of the Wildlife Protection Act, 1972 and the Biological Diversity Act, 2002.

• Write the points listed under Key facts.
• Mention penalties and the role of each authority.
• Add the importance of these laws for animal conservation.

Q5. Write an introductory account of Tadoba, Nagzira and Kanha.

• Location, area type (national park / sanctuary / tiger reserve).
• Vegetation: dry deciduous teak forest (Tadoba, Nagzira), sal forest and meadows (Kanha).
• Fauna: tiger, leopard, sloth bear, gaur, sambar, chital, wild dog; barasingha in Kanha.
• Conservation role and ecotourism.

Exam tip

Questions on local national parks are common in RTMNU papers. Learn at least five animals and the district for each park.

Quick summary

Pollution is the addition of harmful substances or energy to the environment that damages living beings. This topic covers air, water and noise pollution – their sources, effects and control – along with acid rain, the greenhouse effect, ozone depletion, global warming, mercury poisoning, and bioaccumulation and biomagnification.

Key points

• Pollution is प्रदूषण in Marathi. A pollutant is any substance or energy that causes pollution.
• Air pollution sources: vehicles, industries, thermal power plants, burning of crops and garbage, dust from construction. Main pollutants: CO, SO₂, NOₓ, particulate matter (PM2.5, PM10), hydrocarbons.
• Effects of air pollution: asthma, bronchitis, lung cancer, eye irritation; damage to plants and buildings.
• Control: catalytic converters, CNG and electric vehicles, electrostatic precipitators and scrubbers in factories, tall chimneys, planting trees.
• Acid rain: SO₂ and NOₓ dissolve in rain to form sulphuric and nitric acids (pH below 5.6). It harms fish, forests, soil and monuments (marble cancer of the Taj Mahal).
• Greenhouse effect: CO₂, methane, nitrous oxide, CFCs and water vapour trap heat radiated by the Earth. Too much of it causes global warming – melting glaciers, rising sea level, changed rainfall.
• Ozone depletion: CFCs release chlorine that breaks down ozone in the stratosphere; the ozone hole forms over Antarctica. Ozone is measured in Dobson Units. The Montreal Protocol (1987) controls ozone-depleting substances.
• Water pollution sources: sewage, industrial waste, fertilisers and pesticides from farms, oil spills, hot water from power plants.
• Effects: water-borne diseases (cholera, typhoid), fall in dissolved oxygen, eutrophication (algal bloom due to excess nitrates and phosphates), fish death.
• Mercury pollution: methyl mercury in fish caused Minamata disease in Japan (1950s) – nerve damage, numbness, paralysis and birth defects.
• Noise pollution: unwanted loud sound measured in decibels (dB). Sources: traffic, loudspeakers, firecrackers, factories, aircraft. Effects: hearing loss, stress, high blood pressure, sleeplessness.

Important terms

• Bioaccumulation – build-up of a persistent chemical inside one organism over its lifetime.
• Biomagnification – increase in concentration of a persistent chemical at each higher trophic level.
• BOD – biological oxygen demand; high BOD means heavily polluted water.
• Eutrophication – nutrient enrichment of water leading to algal bloom and oxygen loss.
• Smog – smoke plus fog.

Key facts

• DDT biomagnification example: water 0.000003 ppm → zooplankton 0.04 ppm → small fish 0.5 ppm → large fish 2 ppm → fish-eating birds 25 ppm. In birds it causes thin eggshells and population decline.
• Safe noise limit in residential areas in India: about 55 dB by day and 45 dB at night (CPCB norms). Above 80–90 dB for long periods damages hearing.
• Indian laws: Water Act 1974, Air Act 1981, Environment (Protection) Act 1986.

5 important questions

Q1. What is biomagnification?

Biomagnification is the increase in the concentration of a non-degradable pollutant such as DDT or mercury at each successive level of a food chain. Top consumers like fish-eating birds get the highest dose.

Q2. What is acid rain? Give its effects.

Rain with pH below 5.6, formed when SO₂ and NOₓ react with water to give sulphuric and nitric acid. It kills fish, damages leaves and soil, and corrodes buildings and monuments.

Q3. Describe the sources, effects and control of air pollution.

• Natural and man-made sources; main pollutants.
• Effects on humans, animals, plants and materials.
• Global effects: greenhouse effect, global warming, ozone depletion.
• Control: devices, cleaner fuels, laws, green belts.

Q4. Write a note on water pollution and the effects of mercury.

• Sources: domestic, industrial, agricultural, thermal.
• Effects: disease, eutrophication, low DO, fish kills.
• Mercury: methylation in water, biomagnification in fish, Minamata disease symptoms.
• Control: sewage treatment plants, effluent treatment, less fertiliser use.

Q5. Explain the greenhouse effect, global warming and ozone depletion.

• Greenhouse gases and how they trap heat; normal role in keeping Earth warm.
• Global warming: causes and effects.
• Ozone layer, CFCs, ozone hole, Dobson Unit, Montreal Protocol.
• Effects of UV on life.

Exam tip

Draw the DDT biomagnification chain with numbers; it earns easy marks and works for both bioaccumulation and pollution questions.

Quick summary

Annelida are segmented worms. They are bilaterally symmetrical, triploblastic animals with a true coelom and a body divided into ring-like segments. They live in moist soil, fresh water and the sea. This topic covers general characters, classification and the type study of the earthworm (Pheretima posthuma).

Key points

• Annelida means "little rings" (annulus = ring). The earthworm is called गांडूळ in Marathi.
• Body is long, cylindrical and metamerically segmented; segmentation is both external and internal.
• First animals with a true schizocoelous coelom.
• Organ-system level of organisation.
• Locomotion by setae (chaetae), parapodia (in marine forms) or suckers (leeches); body has circular and longitudinal muscles.
• Closed circulatory system; respiratory pigment haemoglobin is dissolved in plasma.
• Excretion by nephridia.
• Nervous system: a pair of cerebral ganglia, nerve ring and a ventral double nerve cord with ganglia.
• Sexes separate (Nereis) or hermaphrodite (earthworm, leech).
• Marine forms have a trochophore larva; development direct in earthworm.
• Classes: Polychaeta (Nereis, Aphrodite), Oligochaeta (Pheretima), Hirudinea (Hirudinaria – Indian cattle leech).

Important terms

• Metamerism – repetition of similar segments along the body.
• Clitellum – thick glandular band in segments 14–16 of Pheretima; forms the cocoon.
• Setae – S-shaped chitinous bristles used in locomotion.
• Typhlosole – internal fold in the intestine of earthworm (from about the 26th segment) that increases absorptive area.
• Nephridia – coiled excretory tubules.
• Hirudin – anticoagulant in leech saliva.

Key facts

• ***Pheretima posthuma* external features:** about 100–120 segments; prostomium overhangs the mouth; clitellum in segments 14–16; one female genital pore on segment 14; a pair of male genital pores on segment 18; four pairs of spermathecal pores between segments 5–9.
• Digestive system: mouth → buccal cavity → pharynx → oesophagus → muscular gizzard (segment 8–9, grinds soil) → stomach → intestine with typhlosole → anus.
• Nephridia types: septal, integumentary and pharyngeal.
• Hearts: four pairs – two pairs of lateral hearts (segments 7 and 9) and two pairs of lateral oesophageal hearts (segments 12 and 13).
• Reproduction: hermaphrodite but cross-fertilisation (protandrous); cocoon formed by clitellum; direct development (no larva).
• Vermicomposting: earthworms convert organic waste into rich manure; earthworms are called "friends of farmers". Charles Darwin wrote a book (1881) on their role in forming soil.

5 important questions

Q1. What is metamerism?

Metamerism is the division of the body into a series of similar segments called metameres. Each segment repeats organs like nephridia, setae and ganglia. Example: earthworm.

Q2. What is the function of the clitellum?

The clitellum is a glandular band in segments 14–16 of earthworm. It secretes the cocoon in which eggs are fertilised and develop.

Q3. Describe the digestive system of earthworm.

• Mouth in first segment, buccal cavity and muscular pharynx.
• Oesophagus, gizzard to grind soil particles.
• Stomach with calciferous glands that neutralise humic acids.
• Intestine with typhlosole and intestinal caeca; absorbs food.
• Anus at the last segment.
• Labels: pharynx, gizzard, stomach, intestine, typhlosole, caeca.

Q4. Give the general characters of Annelida.

• Segmented, bilaterally symmetrical, triploblastic, coelomate.
• Setae or parapodia for locomotion.
• Closed circulation; haemoglobin in plasma.
• Nephridia for excretion.
• Ventral double nerve cord.
• Trochophore larva in marine forms.

Q5. Write the economic importance of earthworms.

• Burrowing loosens and aerates soil; improves drainage.
• Castings enrich soil with nutrients.
• Vermicompost from farm and kitchen waste.
• Used as fish bait and in poultry feed.
• Used in laboratory study.

Exam tip

Learn the segment numbers in Pheretima (clitellum 14–16, female pore 14, male pores 18). These are standard short-answer questions.

Quick summary

Arthropoda is the largest phylum of the animal kingdom. Its members have jointed legs, a segmented body and a hard chitinous exoskeleton. Insects, crabs, prawns, spiders and centipedes all belong here. This topic covers general characters, classification, the type study of cockroach and prawn, and economic importance.

Key points

• Arthropoda means "jointed feet" (arthron = joint, podos = foot). The name was given by von Siebold.
• Bilaterally symmetrical, triploblastic, segmented animals with an organ-system level of organisation.
• Body divided into head, thorax and abdomen, or cephalothorax and abdomen.
• Exoskeleton of chitin, shed from time to time by moulting (ecdysis).
• Coelom is reduced; the body cavity is a blood-filled haemocoel.
• Open circulatory system; dorsal heart.
• Respiration by gills (aquatic), trachea (insects), book lungs (spiders, scorpions) or body surface.
• Excretion by Malpighian tubules (insects) or green glands (antennal glands, in crustaceans).
• Compound eyes and simple eyes (ocelli); antennae as sense organs.
• Mostly dioecious; fertilisation internal; development direct or through larval stages and metamorphosis.

Important terms

• Ecdysis – shedding of the old exoskeleton during growth.
• Haemocoel – body cavity filled with blood (haemolymph).
• Ommatidium – visual unit of the compound eye.
• Mosaic vision – image formed by many ommatidia, each giving part of the picture.
• Metamorphosis – change from larva to adult (complete: egg-larva-pupa-adult; incomplete: egg-nymph-adult).
• Ootheca – egg case of cockroach containing about 16 eggs.

Key facts

• Onychophora (Peripatus) is called a connecting link between Annelida and Arthropoda.
• Cockroach: head with antennae, compound eyes and biting-chewing mouthparts (labrum, mandibles, first maxillae, labium/second maxillae, hypopharynx); thorax of prothorax, mesothorax and metathorax; forewings called tegmina; 13-chambered heart; Malpighian tubules for excretion; spiracles (10 pairs) for breathing; incomplete metamorphosis (nymphs).
• Useful arthropods: honey bee (honey, wax, pollination), silkworm (silk), lac insect (Kerria lacca), prawns and crabs (food).
• Harmful arthropods: mosquito (malaria, dengue), housefly (carries germs), locust and termites (crop and wood damage).

5 important questions

Q1. What is moulting?

Moulting (ecdysis) is the periodic shedding of the hard chitinous exoskeleton. It allows the arthropod to grow, as the rigid exoskeleton cannot expand.

Q2. Name the excretory organs of cockroach and prawn.

Cockroach excretes through Malpighian tubules, which open at the junction of the midgut and hindgut. Prawn excretes through green glands (antennal glands) at the base of the antennae.

Q3. Describe the mouthparts of cockroach.

• Biting and chewing type.
• Labrum (upper lip) – single flap.
• Mandibles – a pair of hard toothed jaws for chewing.
• First maxillae – help hold and push food; bear palps.
• Labium (lower lip, fused second maxillae) with labial palps.
• Hypopharynx – tongue-like, in the middle.
• Labels for diagram: labrum, mandible, maxilla, labium, hypopharynx, palps.

Q4. Give the general characters of Arthropoda.

• Jointed appendages; segmented body.
• Chitinous exoskeleton; moulting.
• Haemocoel; open circulation.
• Gills, trachea or book lungs.
• Malpighian tubules or green glands.
• Compound eyes; usually dioecious; metamorphosis common.

Q5. Describe the economic importance of insects.

• Useful: honey and wax, silk, lac, pollination of crops, natural enemies of pests (ladybird beetle), scavengers.
• Harmful: crop pests (locust, aphids), stored grain pests, vectors of diseases (mosquito, housefly, sand fly), termites damage wood.
• Give one example for each point.

Exam tip

Classify arthropods with a table and one example each. Practise the labelled diagram of cockroach mouthparts – very frequently asked.

Quick summary

Mollusca is the second largest phylum of animals. Molluscs have a soft, unsegmented body usually protected by a calcareous shell. Snails, slugs, oysters, mussels, octopus and cuttlefish are molluscs. This topic covers general characters, classification, the type study of Pila and economic importance.

Key points

• Mollusca means "soft-bodied" (mollis = soft). The study of molluscs is malacology; the study of shells is conchology.
• Bilaterally symmetrical (some become asymmetrical by torsion, e.g. gastropods), triploblastic, coelomate (coelom reduced).
• Body divided into head, visceral mass and a muscular foot.
• A fold of skin called the mantle (pallium) covers the visceral mass and secretes the shell.
• Space between mantle and body is the mantle cavity, which holds gills.
• Respiration by gills called ctenidia, or by a pulmonary sac (lung) in land snails.
• Feeding organ in many is the radula, a rasping, tongue-like ribbon with teeth (absent in bivalves).
• Circulation mostly open; closed in cephalopods. Blood pigment is haemocyanin (copper-based).
• Excretion by kidneys (organs of Bojanus in bivalves).
• Larval stages: trochophore and veliger.
• Octopus and squid have well-developed eyes and brain; they are the most intelligent invertebrates.

Important terms

• Mantle – skin fold that secretes the shell.
• Radula – rasping organ with rows of chitinous teeth.
• Torsion – 180° twisting of the visceral mass in gastropods during development.
• Ctenidium – comb-like gill of molluscs.
• Osphradium – sense organ that tests water entering the mantle cavity.
• Operculum – lid closing the shell mouth in Pila.
• Pearl – shiny body formed when the mantle covers a foreign particle with nacre.

Key facts

• ***Pila globosa* (apple snail): fresh-water, amphibious; shell dextral (right-handed) and spirally coiled; respires by ctenidium in water and by a pulmonary sac** in air; has an operculum; sexes separate.
• Neopilina is a connecting link between Annelida and Mollusca.

5 important questions

Q1. What is the function of the mantle?

The mantle is a fold of body wall covering the visceral mass. It secretes the shell, forms the mantle cavity for gills and, in pearl oysters, produces pearls.

Q2. What is torsion?

Torsion is the 180° anticlockwise twisting of the visceral mass and mantle over the foot during the larval development of gastropods. As a result, the mantle cavity and anus come to lie in front, above the head.

Q3. Describe the external features of Pila.

• Spiral, dextral shell with apex, body whorl and mouth; operculum closes the mouth.
• Head with snout, two pairs of tentacles and eyes on stalks.
• Large flat foot for creeping.
• Right and left nuchal lobes (siphons) for water and air.
• Mantle cavity with ctenidium and pulmonary sac.
• Labels: shell, apex, aperture, operculum, tentacles, eye stalk, foot, nuchal lobes.

Q4. Give the general characters of Mollusca.

• Soft unsegmented body; head, foot, visceral mass.
• Mantle and calcareous shell.
• Ctenidia; radula (except bivalves).
• Open circulation (closed in cephalopods); haemocyanin.
• Trochophore or veliger larva.

Q5. Write the economic importance of molluscs.

• Food: oysters, mussels, squids, clams.
• Pearls from Pinctada; mother-of-pearl buttons and ornaments.
• Shells for lime and decoration.
• Harmful: shipworm (Teredo) damages wooden boats; slugs and snails eat crops; snails are hosts of liver fluke.

Exam tip

Remember "head–foot–visceral mass–mantle" as the basic body plan. Name one example for each class.

Quick summary

Echinodermata are spiny-skinned marine animals such as starfish, sea urchins and sea cucumbers. Adults have pentaradial (five-part) symmetry, while their larvae are bilaterally symmetrical. Their most special feature is the water vascular system used for movement, feeding and respiration. This topic covers characters, classes and the type study of the starfish.

Key points

• Echinodermata means "spiny skin" (echinos = spiny, derma = skin).
• All are marine; none in fresh water or on land.
• Adults show pentaradial symmetry; larvae are bilateral, which shows they came from bilateral ancestors.
• Triploblastic, coelomate, and deuterostomes (enterocoelous coelom) – like chordates.
• Endoskeleton of calcareous plates called ossicles, with spines on the surface.
• Water vascular system (ambulacral system) with tube feet for locomotion, attachment and food capture.
• No head, no brain; nervous system of nerve ring and radial nerves.
• No excretory organs; open/reduced circulation.
• Sexes usually separate; fertilisation external; development indirect with free-swimming larvae (bipinnaria, pluteus, auricularia).
• Very high regeneration power; starfish can regrow lost arms.

Important terms

• Madreporite – sieve-like plate on the aboral surface through which water enters.
• Tube feet (podia) – small hollow, sucker-ended projections moved by water pressure.
• Ampulla – muscular bulb above each tube foot.
• Pedicellariae – small jaw-like structures that keep the surface clean.
• Tiedemann's bodies – small bodies on the ring canal; thought to form amoebocytes.
• Aristotle's lantern – chewing apparatus of sea urchins.
• Ambulacral groove – groove on the oral side of each arm bearing tube feet.

Key facts

• Water vascular system path in starfish: madreporite → stone canal → ring canal (with Tiedemann's bodies and Polian vesicles in some) → radial canals (one in each arm) → lateral canals → ampullae and tube feet.
• Starfish feeds on bivalves; it pulls the shells apart with tube feet and pushes its stomach out (eversion) to digest the prey outside.
• Larva of starfish: bipinnaria, later brachiolaria.

5 important questions

Q1. What is the madreporite?

The madreporite is a perforated, sieve-like calcareous plate on the aboral surface of a starfish. Sea water enters the water vascular system through it.

Q2. What are pedicellariae?

Pedicellariae are tiny pincer-like structures on the body surface of starfish and sea urchins. They remove debris and small organisms and protect the skin gills.

Q3. Describe the water vascular system of starfish with a labelled diagram.

• Madreporite on aboral side.
• Stone canal with calcareous wall leads down to ring canal around mouth.
• Five radial canals run into the arms.
• Lateral canals connect to ampullae and tube feet.
• Contraction of ampulla pushes water into tube foot, which extends and grips.
• Functions: locomotion, attachment, food capture, respiration, sense.
• Labels: madreporite, stone canal, ring canal, Tiedemann's bodies, radial canal, ampulla, tube foot.

Q4. Give the general characters of Echinodermata.

• Marine; pentaradial adults, bilateral larvae.
• Spiny calcareous endoskeleton.
• Water vascular system and tube feet.
• Enterocoelous coelom; deuterostomes.
• No excretory organs; no brain.
• Indirect development; high regeneration.

Q5. Why are echinoderms considered close to chordates?

• Both are deuterostomes (anus from blastopore).
• Both have enterocoelous coelom.
• Both have a mesodermal endoskeleton.
• Bilateral larvae resemble the tornaria larva of Balanoglossus (Hemichordata).
• Radial cleavage in both.

Exam tip

Draw the water vascular system in one clear flow diagram. The bilateral larva versus radial adult point is a common short question.

Quick summary

Hemichordata are soft, worm-like marine animals that show some chordate features, such as pharyngeal gill slits, but lack a true notochord. Balanoglossus, the acorn worm, is the type example. This topic covers general characters, external features and the digestive system of Balanoglossus, its development and tornaria larva, and its affinities with Annelida, Echinodermata and Chordata.

Key points

• Hemichordata means "half chordates". The name was given by Bateson (1885), who placed them under Chordata. Today they are treated as a separate non-chordate phylum, close to Echinodermata.
• All are marine, mostly burrowing in sand or mud in shallow seas.
• Body is soft, bilaterally symmetrical, triploblastic and enterocoelous (deuterostome).
• Body has three parts: proboscis, collar and trunk, each with its own coelom (protocoel, mesocoel, metacoel).
• They have a short stomochord (buccal diverticulum) in the proboscis. Earlier it was thought to be a notochord; now it is regarded as a different structure.
• Pharyngeal gill slits are present – a true chordate-like character.
• Circulatory system is open with a contractile heart vesicle; excretion by the glomerulus in the proboscis.
• Nervous system is simple, mostly in the epidermis; the collar nerve cord is hollow in some forms.
• Sexes are separate; fertilisation is external; development is indirect through a tornaria larva (in Balanoglossus).
• Two classes: Enteropneusta (Balanoglossus, Saccoglossus) and Pterobranchia (Cephalodiscus, Rhabdopleura).

Important terms

• Proboscis – front, acorn-shaped part used for burrowing.
• Collar – short ring-like middle part; mouth lies on its ventral side.
• Stomochord – hollow forward outgrowth of the gut into the proboscis.
• Hepatic caeca – green-brown sac-like outgrowths of the trunk gut, digestive in function.
• Tornaria – free-swimming ciliated larva of Balanoglossus.

Key facts

• **Digestive system of Balanoglossus: mouth (ventral, in collar) → buccal cavity (with stomochord) → pharynx (dorsal branchial part with gill slits, ventral food part) → oesophagus → intestine with hepatic caeca** → anus at the end of the trunk.
• It is a ciliary feeder: mucus on the proboscis traps food particles, and cilia carry them to the mouth.
• Tornaria was first described by Johannes Müller, who thought it was an echinoderm larva because it looks like the bipinnaria of starfish.
• Development: radial holoblastic cleavage → coeloblastula → gastrula by invagination → tornaria → metamorphosis into young worm.

5 important questions

Q1. Why is Balanoglossus called the acorn worm?

Its proboscis and collar together look like an acorn (oak fruit) sitting in its cup, and the long soft trunk looks like a worm. So it is called the acorn worm or tongue worm.

Q2. What is tornaria larva and why is it important?

Tornaria is the transparent, ciliated, free-swimming larva of Balanoglossus. It closely resembles the bipinnaria larva of echinoderms, which shows that hemichordates and echinoderms are closely related.

Q3. Describe the external features and digestive system of Balanoglossus.

• Size and colour; three body regions.
• Proboscis with proboscis stalk; collar with mouth; trunk divided into branchio-genital, hepatic and abdominal regions.
• Gill pores, genital wings and hepatic caeca.
• Digestive tract from mouth to anus and ciliary feeding.
• Draw a labelled diagram.

Q4. Discuss the affinities of Balanoglossus.

• With Chordata: pharyngeal gill slits; hollow dorsal collar nerve cord; stomochord once compared with notochord.
• With Echinodermata: tornaria resembles bipinnaria; enterocoelous coelom; similar mesoderm formation; both deuterostomes.
• With Annelida: worm-like burrowing body; ciliated larva resembles trochophore; similar blood vessel pattern.
• Conclusion: closest to Echinodermata; separate phylum.

Q5. Describe the development and tornaria larva of Balanoglossus.

• External fertilisation; holoblastic radial cleavage.
• Blastula, gastrula, formation of coelomic sacs.
• Structure of tornaria: ciliary bands, apical plate with eye spots, complete gut.
• Metamorphosis into adult.

Exam tip

Affinities is the favourite long question. Write it in three headed parts and end with a clear conclusion on the systematic position.

Quick summary

The cell is the basic structural and functional unit of life. Animal cells are eukaryotic: they have a true nucleus and membrane-bound organelles. This topic covers the cell theory, the plasma membrane (fluid mosaic model) and the structure and functions of the main cell organelles.

Key points

• Robert Hooke (1665) first saw and named "cells" in cork. Leeuwenhoek first saw living cells.
• Cell theory: proposed by M. Schleiden (1838, plants) and T. Schwann (1839, animals). Rudolf Virchow (1855) added "Omnis cellula e cellula" – all cells arise from pre-existing cells.
• Cell is पेशी in Marathi.
• Prokaryotic cells (bacteria) have no nuclear membrane and no membrane-bound organelles; eukaryotic cells have both.
• Animal cells have no cell wall, no plastids and usually small vacuoles; they have centrioles.
• Plasma membrane is described by the fluid mosaic model of Singer and Nicolson (1972): a lipid bilayer with proteins floating in it like a mosaic.
• Membrane is selectively permeable; transport by diffusion, osmosis, active transport, endocytosis and exocytosis.
• Mitochondria are the "powerhouse of the cell" – site of aerobic respiration (Krebs cycle in matrix, electron transport on cristae) producing ATP.
• Endoplasmic reticulum: rough ER (with ribosomes, protein synthesis) and smooth ER (lipid and steroid synthesis, detoxification).
• Golgi complex (discovered by Camillo Golgi, 1898) packs, modifies and secretes proteins; forms lysosomes.
• Lysosomes (discovered by Christian de Duve) contain hydrolytic enzymes; called "suicide bags".
• Ribosomes: 80S in eukaryotic cytoplasm (60S + 40S), 70S in prokaryotes, mitochondria and plastids; site of protein synthesis.
• Nucleus has a double-layered nuclear envelope with pores, chromatin and nucleolus (makes rRNA).

Important terms

• Cristae – inner folds of the mitochondrial membrane.
• Cisternae – flattened sacs of Golgi body and ER.
• Peroxisome – small vesicle with enzymes like catalase that break down hydrogen peroxide.
• Centrosome – region with two centrioles; forms spindle fibres in animal cell division.
• Cytoskeleton – network of microtubules, microfilaments and intermediate filaments giving shape and movement.
• Semi-autonomous organelle – organelle with its own DNA and ribosomes (mitochondria, chloroplast).

Key facts

• Centriole shows a 9 + 0 arrangement of microtubule triplets; cilia and flagella show 9 + 2 arrangement of doublets.
• Mitochondria contain circular DNA and 70S ribosomes – support for the endosymbiotic theory.

5 important questions

Q1. State the cell theory.

All living organisms are made of cells and cell products. The cell is the basic unit of structure and function. New cells arise only from pre-existing cells (Virchow).

Q2. Why are lysosomes called suicide bags?

Lysosomes contain strong hydrolytic enzymes. If the membrane bursts, these enzymes digest the cell's own contents (autolysis), so they are called suicide bags.

Q3. Explain the fluid mosaic model of the plasma membrane.

• Given by Singer and Nicolson in 1972.
• Phospholipid bilayer: hydrophilic heads face outside, hydrophobic tails face inside.
• Proteins: peripheral (on surface) and integral (through the bilayer).
• Lipids and proteins move sideways – membrane is fluid.
• Carbohydrates attached as glycolipids and glycoproteins act in cell recognition.
• Labels: phospholipid bilayer, integral protein, peripheral protein, glycoprotein, cholesterol.

Q4. Describe the structure and functions of mitochondria.

• Rod or oval shaped; double membrane.
• Outer membrane smooth; inner membrane folded into cristae with F₁ particles (oxysomes, ATP synthase).
• Matrix contains enzymes of Krebs cycle, circular DNA, 70S ribosomes.
• Functions: ATP production, Krebs cycle, electron transport, own protein synthesis.
• Labels: outer membrane, inner membrane, cristae, matrix, F₁ particles, DNA, ribosomes.

Q5. Describe the Golgi complex and its functions.

• Stack of flattened cisternae with tubules and vesicles; cis (forming) face and trans (maturing) face.
• Receives proteins from ER, modifies them (glycosylation) and packs them in vesicles.
• Secretion of hormones and enzymes.
• Formation of lysosomes and acrosome of sperm.
• Cell plate formation in plant cells.

Exam tip

Draw a labelled animal cell as seen under the electron microscope. Always give the year and names for the fluid mosaic model (Singer and Nicolson, 1972).

Quick summary

The plasma membrane is the thin, living boundary of every cell. It controls what enters and leaves the cell and helps cells stick to and communicate with each other. This topic covers the chemical composition and ultrastructure of the membrane, its models, passive and active transport, endocytosis and exocytosis, protein sorting in the endomembrane system, and cell junctions and adhesion molecules.

Key points

• The plasma membrane (पेशीपटल) is about 7.5–10 nm thick and is selectively permeable.
• Chemical composition: lipids (phospholipids, cholesterol, glycolipids), proteins and a small amount of carbohydrate (as glycoproteins and glycolipids). In many animal cells, protein and lipid are roughly equal by weight.
• Phospholipids are amphipathic: a water-loving (hydrophilic) head and two water-fearing (hydrophobic) fatty-acid tails. In water they form a bilayer.
• Membrane models: Danielli and Davson sandwich model (1935); Robertson's unit membrane model (1959); fluid mosaic model by Singer and Nicolson (1972) – the accepted model.
• In the fluid mosaic model, proteins float like icebergs in a fluid lipid sea. Integral (intrinsic) proteins pass into or through the bilayer; peripheral (extrinsic) proteins sit on the surface.
• Carbohydrate chains on the outer surface form the glycocalyx, used in cell recognition.
• Passive transport (no energy, along the concentration gradient): simple diffusion, facilitated diffusion (through carrier or channel proteins) and osmosis (water through the membrane).
• Active transport uses ATP and moves substances against the gradient. Example: the Na⁺–K⁺ pump, which sends 3 Na⁺ out and brings 2 K⁺ in for each ATP used.
• Endocytosis: the membrane folds inward to take in material – phagocytosis (cell eating), pinocytosis (cell drinking) and receptor-mediated endocytosis (through clathrin-coated pits).
• Exocytosis: vesicles fuse with the membrane and release their contents outside (e.g. hormones, enzymes, neurotransmitters).
• Endomembrane system: ER, Golgi complex, lysosomes and vesicles work together. Proteins made on rough ER are sorted in the Golgi and sent to lysosomes, the membrane or out of the cell, guided by signal sequences.

Important terms

• Selective permeability – lets some substances pass and stops others.
• Uniport, symport, antiport – carrier moves one substance; two in the same direction; two in opposite directions.
• Tight junction – seals neighbouring cells to stop leakage (intestinal lining).
• Desmosome – spot-weld junction giving mechanical strength (skin).
• Gap junction – channel joining the cytoplasm of two cells (heart muscle).
• Cell adhesion molecules (CAMs) – surface proteins that bind cells: cadherins, integrins, selectins and the immunoglobulin superfamily.

Key facts

5 important questions

Q1. What is the glycocalyx?

It is the sugar coat on the outer surface of the plasma membrane, made of carbohydrate chains of glycoproteins and glycolipids. It helps in cell recognition, adhesion and protection.

Q2. Differentiate between phagocytosis and pinocytosis.

Phagocytosis is the intake of large solid particles (bacteria, food) in large vesicles, as in Amoeba and white blood cells. Pinocytosis is the intake of fluid droplets with dissolved substances in tiny vesicles.

Q3. Describe the fluid mosaic model of the plasma membrane.

• Proposed by Singer and Nicolson (1972).
• Phospholipid bilayer; hydrophilic heads outside, tails inside.
• Integral and peripheral proteins; cholesterol for stability.
• Glycocalyx on the outer surface.
• Fluidity: lateral movement of lipids and proteins; asymmetry of the two layers.
• Draw a labelled diagram and mention earlier models briefly.

Q4. Explain the mechanisms of transport across the plasma membrane.

• Passive: diffusion, facilitated diffusion, osmosis.
• Active: primary (Na⁺–K⁺ pump) and secondary (glucose–Na⁺ symport).
• Bulk transport: endocytosis and exocytosis.
• Give examples and a table of differences.

Q5. Write an account of cell junctions and cell adhesion molecules.

• Need for cell–cell interaction in tissues.
• Tight junction, adherens junction, desmosome, hemidesmosome, gap junction: structure and function.
• CAMs: cadherins (calcium-dependent), integrins (bind extracellular matrix), selectins (white blood cells), Ig-CAMs (nerve cells).

Exam tip

Never skip the diagram of the fluid mosaic model; label integral protein, peripheral protein, cholesterol and glycocalyx.

Quick summary

The nucleus is the control centre of the cell. It holds the genetic material, DNA, packed with proteins as chromatin. This topic covers the structure and functions of the nucleus and nuclear envelope, the chemical composition of DNA and RNA, the double helix model, and how DNA is packed into chromatin and chromosomes.

Key points

• The nucleus (केंद्रक) was discovered by Robert Brown (1831).
• Nuclear envelope: a double membrane with a perinuclear space between the two layers. The outer membrane is continuous with the rough ER.
• Nuclear pores (with a protein nuclear pore complex) allow mRNA and ribosome subunits to go out and proteins to come in.
• Nucleoplasm contains chromatin, enzymes and one or more nucleoli. The nucleolus makes ribosomal RNA and assembles ribosome subunits.
• Functions of nucleus: stores hereditary information, controls cell activities through protein synthesis, takes part in cell division.
• Nucleic acids are polymers of nucleotides. Each nucleotide = pentose sugar + nitrogenous base + phosphate.
• DNA: deoxyribose sugar; bases adenine (A), guanine (G), cytosine (C) and thymine (T).
• RNA: ribose sugar; uracil (U) in place of thymine; usually single-stranded. Types: mRNA, tRNA, rRNA.
• Chargaff's rule: in double-stranded DNA, A = T and G = C, so purines = pyrimidines.
• Watson and Crick (1953) gave the double helix model, using X-ray data of Rosalind Franklin and Maurice Wilkins.
• Chromatin packing: DNA wraps around a core of eight histone proteins to form a nucleosome; histone H1 links them. This "beads on a string" fibre (11 nm) coils into a 30 nm solenoid fibre, then loops and further coils to form a chromosome.
• Euchromatin is loosely packed and active; heterochromatin is tightly packed and mostly inactive.

Important terms

• Nucleoside – sugar + base (no phosphate).
• Purines – A and G (two rings); pyrimidines – C, T and U (one ring).
• Histones – basic proteins (H1, H2A, H2B, H3, H4) around which DNA winds.
• Supercoiling – extra twisting of the DNA double helix, which helps compact it.
• Phosphodiester bond – links nucleotides in a strand.

Key facts

• B-DNA: right-handed helix, diameter 2 nm, one turn = 3.4 nm with about 10 base pairs; adjacent base pairs 0.34 nm apart.
• A–T pair has 2 hydrogen bonds; G–C pair has 3 hydrogen bonds.
• Nucleosome core: about 146–147 base pairs of DNA around a histone octamer (2 each of H2A, H2B, H3, H4).

5 important questions

Q1. State Chargaff's rule.

In double-stranded DNA the amount of adenine equals thymine and guanine equals cytosine. So the total purines equal total pyrimidines (A + G = C + T).

Q2. What is a nucleosome?

A nucleosome is the basic unit of chromatin. It is a core of eight histone molecules around which about 146 base pairs of DNA are wound nearly twice, joined to the next by linker DNA with histone H1.

Q3. Describe the structure and functions of the nucleus.

• Shape, size and number of nuclei.
• Nuclear envelope, perinuclear space and nuclear pores.
• Nucleoplasm, chromatin and nucleolus.
• Functions: heredity, control of metabolism, ribosome formation, cell division.
• Draw a labelled diagram.

Q4. Describe the Watson–Crick model of DNA.

• Two antiparallel polynucleotide strands (5'→3' and 3'→5').
• Sugar-phosphate backbone outside; bases inside.
• Complementary base pairing with hydrogen bonds.
• Right-handed helix; measurements (2 nm, 3.4 nm, 10 bp).
• Significance: replication and genetic code.

Q5. Explain chromatin organisation.

• Euchromatin and heterochromatin.
• Levels: double helix → nucleosome (11 nm) → solenoid (30 nm) → looped domains → chromatid.
• Role of histones and non-histone proteins; supercoiling.
• Significance of packing about 2 m of DNA in a small nucleus.

Exam tip

Learn the DNA measurements and the H-bond numbers; they are often asked as one-mark objective questions.

Quick summary

The cell cycle is the ordered series of events by which a cell grows, copies its DNA and divides into two. Mitosis produces two identical daughter cells for growth and repair, while meiosis produces four haploid cells for sexual reproduction. This topic covers the phases of the cell cycle and its regulation, and the stages and significance of mitosis and meiosis.

Key points

• The cell cycle (पेशीचक्र) has two main parts: interphase (long, preparation) and M phase (division).
• Interphase: G1 (growth, protein and RNA synthesis), S (DNA replication; DNA amount doubles) and G2 (preparation for division, synthesis of tubulin).
• Cells that stop dividing (nerve cells, mature muscle cells) leave G1 and enter a resting G0 phase.
• In a typical human cell in culture, one cycle takes about 24 hours; mitosis itself takes only about one hour.
• Regulation: proteins called cyclins combine with cyclin-dependent kinases (CDKs) to push the cell from one phase to the next. Cyclin B–CDK1 is called MPF (maturation/M-phase promoting factor).
• Checkpoints: G1/S, G2/M and the spindle (metaphase) checkpoint check DNA damage and correct chromosome attachment. The protein p53 stops the cycle if DNA is damaged. Failure of control can lead to cancer.
• Mitosis (equational division) occurs in somatic cells. Stages: prophase, metaphase, anaphase, telophase, followed by cytokinesis (furrowing in animal cells).
• Meiosis (reduction division) occurs in germ cells to form gametes. It has two divisions: meiosis I (homologous chromosomes separate; chromosome number halves) and meiosis II (sister chromatids separate, like mitosis).
• Prophase I is long, with five sub-stages: leptotene, zygotene, pachytene, diplotene, diakinesis.
• Crossing over in pachytene exchanges segments between non-sister chromatids and brings genetic variation.

Important terms

• Synapsis – pairing of homologous chromosomes in zygotene.
• Bivalent (tetrad) – a pair of synapsed homologues with four chromatids.
• Synaptonemal complex – protein structure that holds homologues together.
• Chiasma – X-shaped point where crossing over has occurred (seen in diplotene).
• Kinetochore – protein disc on the centromere where spindle fibres attach.
• Karyokinesis / cytokinesis – division of nucleus / division of cytoplasm.

Key facts

• Metaphase is the best stage to count and study chromosomes.
• If a cell has 2n = 46, after meiosis each gamete has n = 23.

5 important questions

Q1. What is G0 phase?

G0 is a resting, non-dividing stage that cells enter after leaving G1. The cells stay metabolically active but do not divide, as in nerve cells and heart muscle cells. Some can re-enter the cycle when needed.

Q2. What is crossing over? Give its significance.

Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during pachytene of meiosis I. It produces new gene combinations, causes variation and helps in making gene maps.

Q3. Describe the cell cycle and its regulation.

• Interphase: G1, S, G2 with events of each.
• M phase: karyokinesis and cytokinesis.
• G0 phase.
• Cyclins, CDKs and MPF.
• Checkpoints and p53; link with cancer.
• Draw the cell-cycle wheel.

Q4. Describe mitosis with diagrams.

• Prophase: chromatin condenses; nucleolus and nuclear envelope disappear; spindle forms.
• Metaphase: chromosomes on the equatorial plate; kinetochores attached.
• Anaphase: centromeres split; chromatids move to opposite poles.
• Telophase: nuclear envelope reforms; chromosomes decondense.
• Cytokinesis by cleavage furrow; significance.

Q5. Describe the stages of meiosis I.

• Prophase I: leptotene, zygotene (synapsis), pachytene (crossing over), diplotene (chiasmata), diakinesis (terminalisation).
• Metaphase I: bivalents on the plate.
• Anaphase I: homologues separate (not chromatids).
• Telophase I; brief interkinesis; outline of meiosis II.
• Significance of meiosis.

Exam tip

Draw small, neat diagrams of each stage; examiners give marks even for a simple sketch with correct labels. Keep the mitosis–meiosis table ready for short questions.

Quick summary

Cells constantly receive signals from their surroundings and respond to them. This is called cell signalling or signal transduction. Cells also age and finally die, either by planned self-destruction (apoptosis) or by injury (necrosis). This topic covers types of signalling, cell surface receptors, G-protein signalling and second messengers, cellular ageing and the differences between apoptosis and necrosis.

Key points

• Signal transduction is the process by which a signal outside the cell is converted into a response inside it.
• Steps: reception (signal molecule binds a receptor) → transduction (relay inside the cell) → response (change in enzyme activity, gene expression, secretion, etc.).
• Types of signalling: endocrine (hormones through blood, long distance), paracrine (to nearby cells), autocrine (cell signals itself), juxtacrine (direct contact) and synaptic (neurotransmitters).
• Intracellular receptors are in the cytoplasm or nucleus; they bind lipid-soluble signals like steroid hormones and thyroxine and act directly on genes.
• Cell surface receptors bind water-soluble signals. Three main kinds: ion-channel-linked receptors, G-protein-coupled receptors (GPCRs) and enzyme-linked receptors (e.g. receptor tyrosine kinase for insulin).
• GPCR pathway: hormone binds receptor → G-protein exchanges GDP for GTP and becomes active → it activates adenylate cyclase → ATP is converted to cyclic AMP (cAMP) → cAMP activates protein kinase A → target proteins are phosphorylated → response (e.g. glycogen breakdown by adrenaline).
• Second messengers: cAMP, cGMP, Ca²⁺, IP₃ and DAG. Earl Sutherland discovered cAMP as a second messenger (Nobel Prize 1971).
• Cellular ageing (senescence): a gradual fall in cell function. Causes: shortening of telomeres with each division (Hayflick limit – about 50 divisions for normal human cells in culture), damage by free radicals, accumulated DNA errors and wear of proteins.
• Apoptosis is programmed, orderly cell death. The cell shrinks, chromatin condenses, DNA breaks into fragments, and the cell splits into apoptotic bodies that are eaten by phagocytes, with no inflammation. It is carried out by enzymes called caspases.
• Necrosis is accidental death due to injury, toxins or lack of blood; the cell swells and bursts, causing inflammation.

Important terms

• Ligand – signal molecule that binds a receptor.
• First messenger – the hormone or signal outside the cell.
• Second messenger – small molecule inside the cell that relays the signal.
• Telomere – repeated DNA at chromosome ends that shortens with age.
• Telomerase – enzyme that rebuilds telomeres (active in stem and cancer cells).
• Caspases – protease enzymes that carry out apoptosis.

Key facts

• Importance of apoptosis: shaping organs in the embryo, removing damaged or infected cells, killing cancer-prone cells, controlling cell numbers in the immune system.

5 important questions

Q1. What is a second messenger? Give examples.

A second messenger is a small molecule inside the cell that carries the signal from a surface receptor to target proteins. Examples: cAMP, cGMP, calcium ions, IP₃ and DAG.

Q2. What is the Hayflick limit?

It is the limited number of times (about 50) a normal human cell can divide in culture before it stops. It is due to shortening of telomeres with each division and helps explain cellular ageing.

Q3. Describe signal transduction through G-protein-coupled receptors.

• Structure of GPCR (seven transmembrane parts).
• Binding of hormone; activation of G-protein (GDP → GTP).
• Adenylate cyclase, cAMP, protein kinase A.
• Amplification of the signal; switching off by GTP hydrolysis and phosphodiesterase.
• Example: adrenaline and glycogen breakdown in liver. Draw a flow diagram.

Q4. Explain types of cell signalling and receptors.

• Endocrine, paracrine, autocrine, juxtacrine, synaptic with examples.
• Intracellular receptors (steroids).
• Surface receptors: ion-channel, GPCR, enzyme-linked.

Q5. Write an account of cellular ageing and cell death.

• Theories of ageing: telomere, free radical, DNA damage, wear and tear.
• Apoptosis: steps, caspases, significance.
• Necrosis and the table of differences.

Exam tip

Write the GPCR pathway as a flow chart with arrows; it is quicker and scores better than a long paragraph.