Zoology प्राणीशास्त्र

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.