# Structural Organisation in Animals (NEET Biology Class 11): The Frog (Rana tigrina) Source: https://tracknation.in/story/structural-organisation-in-animals-neet-biology-class-11-the-frog-rana-tigrina (TrackNation β€” free UPSC notes with PYQs) > NEET Class 11 Structural Organisation in Animals, mined from NCERT: the frog Rana tigrina β€” its basics (Amphibia, poikilotherm, aestivation/hibernation), morphology, and digestive, respiratory (cutaneous + pulmonary), circulatory (3-chambered heart, sinus venosus, conus arteriosus), excretory, nervous (cerebellum, medulla, cranial nerves), endocrine (pituitary to gonads) and reproductive systems (mesorchium, vasa efferentia, Bidder's canal). With 11 MCQs, 6 NEET PYQs and an Active Recall prompt. ## The Four Animal Tissues β€” Epithelial, Connective, Muscular, Neural 🌱 **NEET priority: Foundational.** *1–2 questions a year. Short chapter β€” learn the frog "firsts" (Rana tigrina, 3-chambered heart, sinus venosus, dual respiration) and move on.* Cells of the same kind, doing the same job, are grouped into a **tissue**. All the complexity of an animal body is built from just **four** of them β€” **epithelial, connective, muscular and neural**. Learn these four before the frog: NEET asks them directly, and they explain every organ that follows. ### 1. Epithelial tissue β€” the covering and lining tissue - Epithelium has a **free surface** facing a body fluid or the outside, and rests on a **basement membrane**. There is very little intercellular material. Two broad types: **simple epithelium** (a single layer, for absorption and secretion) and **compound epithelium** (many layers, for protection). - **Simple epithelium comes in three shapes.****Squamous** β€” flat tiles forming a thin diffusion barrier, found in the alveoli of the lungs and the walls of blood vessels. **Cuboidal** β€” cube-shaped, found in the ducts of glands and the tubular parts of the nephron, where it aids secretion and absorption. **Columnar** β€” tall and pillar-like, lining the stomach and intestine, with nuclei at the base. - Columnar or cuboidal cells bearing **cilia** form **ciliated epithelium**, which moves mucus or eggs along β€” found in the bronchioles and the fallopian tubes. When columnar cells of differing heights give a false impression of layering, the tissue is called **pseudostratified**. Epithelial cells specialised for secretion form **glandular epithelium**: **exocrine** glands release their product through a duct (saliva, mucus, oil, earwax, milk, digestive enzymes), while **endocrine** glands have **no duct** and pour hormones straight into the blood. - **Cell junctions** hold epithelium together β€” three of them, each with a distinct job. **Tight junctions** seal neighbouring cells so nothing leaks between them; **adhering junctions** cement cells together mechanically; and **gap junctions** leave channels that let ions and small molecules pass directly from one cell's cytoplasm to the next. ### 2. Connective tissue β€” the most abundant and most widely distributed - Connective tissues link and support other tissues. Except for blood, they all secrete **collagen or elastin fibres** into a ground substance called the **matrix** β€” and it is the matrix, not the cells, that gives each type its character. - **Loose connective tissue** has cells and fibres loosely arranged: **areolar tissue** sits beneath the skin and acts as a support framework for epithelium, while **adipose tissue** is packed with fat-storing cells. **Dense connective tissue** has densely packed fibres β€” **dense regular** (orderly parallel bundles) forms **tendons**, which attach muscle to bone, and **ligaments**, which attach bone to bone; **dense irregular** tissue, with fibres running every way, is found in the skin. - **Specialised connective tissue** covers cartilage, bone and blood. **Cartilage** has a solid but pliable matrix that resists compression, with cells called **chondrocytes** enclosed in small cavities; it is found in the tip of the nose, the outer ear, and between adjacent bones of the vertebral column. **Bone** has a hard matrix rich in **calcium salts and collagen**, with cells called **osteocytes** in spaces called **lacunae**; the bone marrow of some bones is the site of blood-cell production. - **Blood is a connective tissue** β€” the point NEET most often tests here. It is a **fluid connective tissue** whose matrix is **plasma**, and unlike every other connective tissue it contains **no fibres** in the matrix. It carries the red blood cells, white blood cells and platelets. ### 3. Muscular tissue and 4. Neural tissue - **Muscle tissue** is made of long cells called **muscle fibres**, and comes in three kinds. **Skeletal (striated)** muscle is attached to bones, striped, **multinucleate** and under **voluntary** control. **Smooth (unstriated)** muscle has spindle-shaped cells with a single nucleus, no stripes, and is **involuntary** β€” found in the walls of the stomach, intestine and blood vessels. **Cardiac** muscle is found only in the heart: it is **striated like skeletal muscle but involuntary like smooth muscle**, and its cells are joined end to end by **intercalated discs** that let the whole heart contract as one unit. - **Neural tissue** exerts the greatest control over the body's responsiveness. **Neurons** are the excitable cells that carry impulses; **neuroglia** make up more than half the volume of neural tissue and protect and support the neurons. When a neuron is suitably stimulated an electrical disturbance travels swiftly along its plasma membrane, and on arriving at the tip it triggers events that stimulate the next cell. ### The four animal tissues β€” match-the-pair table | **Tissue** | **Sub-type** | **Where it is found** | **The exact fact NEET asks** | | Epithelial | Squamous | Alveoli, blood vessel walls | Thin flat cells for diffusion | | Epithelial | Cuboidal | Gland ducts, nephron tubules | Secretion and absorption | | Epithelial | Columnar | Stomach and intestine lining | Tall cells, nuclei at the base | | Epithelial | Ciliated | Bronchioles, fallopian tubes | Moves mucus and eggs along | | Connective | Areolar / Adipose | Beneath skin | Adipose stores fat | | Connective | Tendon / Ligament | Muscle-to-bone / bone-to-bone | Both are DENSE REGULAR tissue | | Connective | Cartilage / Bone | Nose tip, ear / skeleton | Chondrocytes vs osteocytes in lacunae | | Connective | Blood | Circulatory system | Fluid tissue; matrix = plasma; NO fibres | | Muscular | Skeletal | Attached to bones | Striated, multinucleate, voluntary | | Muscular | Smooth | Stomach, intestine, vessels | Unstriated, uninucleate, involuntary | | Muscular | Cardiac | Heart only | Striated BUT involuntary; intercalated discs | | Neural | Neuron / Neuroglia | Brain, cord, nerves | Neuroglia are over half the volume | **Exam traps.** **Blood is a connective tissue**, not an epithelial or a separate category β€” the single most-missed fact in this chapter. **Cardiac muscle is striated but involuntary**, which is why "striated equals voluntary" is a false rule. **Tendon joins muscle to bone; ligament joins bone to bone** β€” the two are swapped in options constantly. And **endocrine glands are ductless**, whereas exocrine glands release their product through a duct. ## The Frog β€” Basics (Rana tigrina) Having met the four tissues, we can now see how they are assembled into organs and organ systems in one representative vertebrate β€” the **frog**. ### The frog β€” basics - The common Indian frog is **Rana tigrina**, class **Amphibia**, phylum **Chordata**. - It is **cold-blooded (poikilotherm)**; it changes colour to hide (**camouflage / mimicry**), takes **summer sleep (aestivation)** and **winter sleep (hibernation)**. - Skin is moist and slippery; the frog **never drinks water** β€” it **absorbs water through the skin**. ## Morphology, Digestion & Respiration ### Morphology - Body = **head + trunk** (no neck, no tail). A pair of nostrils above the mouth; bulged eyes with a protective **nictitating membrane**; a **tympanum** (ear) on each side. - **Hind limbs** end in **5 webbed digits** (for swimming); **fore limbs** end in **4 digits**. - **Sexual dimorphism:** the male has **vocal sacs** and a **copulatory pad** on the first fore-limb digit; the female has neither. ### Digestive & respiratory systems - The alimentary canal is **short** (frog is carnivorous): buccal cavity β†’ oesophagus β†’ stomach β†’ intestine β†’ rectum β†’ **cloaca**. Food is caught by a **bilobed tongue**; liver (bile, stored in gall bladder) and pancreas aid digestion; villi absorb food. - **Respiration:** in water through the **skin (cutaneous)**; on land through skin, buccal cavity and **lungs (pulmonary)**. During aestivation/hibernation, only through the skin. ## Circulation, Excretion, Nerves & Reproduction ### Circulatory system - A **closed** blood vascular system plus a **lymphatic system**. - The heart is **3-chambered β€” two atria and one ventricle** β€” covered by a **pericardium**. The **sinus venosus** joins the right atrium (receives blood via the vena cava); the ventricle opens into the **conus arteriosus**. - Frogs have a **hepatic portal** system (liver–intestine) and a **renal portal** system (kidney–lower body). ### Excretory, nervous & reproductive systems - **Excretory:** a pair of **mesonephric kidneys**, ureters, urinary bladder and cloaca; the frog is **ureotelic**. In males the ureter doubles as a **urinogenital duct**. - **Nervous:** CNS (brain + spinal cord), PNS (cranial + spinal nerves) and ANS (sympathetic + parasympathetic). The brain, enclosed in a bony **cranium**, has fore-, mid- and hind-brain; the hind-brain includes the **cerebellum** and **medulla oblongata**. There are **10 pairs of cranial nerves**. - **Endocrine:** the prominent glands are the **pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals** and the **gonads** β€” coordination of organs via hormones, alongside the nervous system. - **Reproductive:** sexes are separate. The male has a pair of yellowish, ovoid **testes** attached to the kidneys by a double fold of peritoneum, the **mesorchium**; **10–12 vasa efferentia** carry sperm from the testes into the kidneys, opening into **Bidder's canal**. **Fertilisation is external**, in water; development is indirect via a **tadpole** larva that undergoes **metamorphosis** into the adult. ### The frog's organ systems β€” one-glance summary table | **System** | **Key structures** | **The exact fact NEET asks** | | Digestive | Buccal cavity β†’ oesophagus β†’ stomach β†’ intestine β†’ rectum β†’ CLOACA; bilobed tongue; liver (bile, stored in gall bladder), pancreas | The alimentary canal is SHORT because the frog is carnivorous | | Respiratory | Skin (cutaneous), buccal cavity, lungs (pulmonary) | In WATER: skin only. On LAND: skin + buccal cavity + lungs. During aestivation/hibernation: skin ONLY | | Circulatory | 3-chambered heart (2 atria + 1 ventricle), pericardium, sinus venosus, conus arteriosus | CLOSED blood vascular system + lymphatic system. Has BOTH hepatic portal and renal portal systems | | Excretory | A pair of MESONEPHRIC kidneys, ureters, urinary bladder, cloaca | The frog is UREOTELIC. In MALES the ureter doubles as a urinogenital duct | | Nervous | CNS (brain + spinal cord), PNS (cranial + spinal nerves), ANS (sympathetic + parasympathetic) | 10 PAIRS of cranial nerves. Hind-brain = cerebellum + medulla oblongata. Brain enclosed in a bony CRANIUM | | Endocrine | Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, gonads | Coordination happens via hormones ALONGSIDE the nervous system | | Reproductive | Testes attached by MESORCHIUM; ~11 vasa efferentia β†’ kidneys β†’ Bidder's canal | Fertilisation is EXTERNAL, in water. Development is INDIRECT via a tadpole that undergoes metamorphosis | ### Why this matters for NEET - High-value one-liners: **Rana tigrina**; **3-chambered heart** (2 atria + 1 ventricle); **sinus venosus** & **conus arteriosus**; dual (cutaneous + pulmonary) respiration; nictitating membrane; **10 pairs of cranial nerves**. - Trap: the frog **absorbs water through the skin** (never drinks); fertilisation is **external**, not internal (unlike reptiles/birds/mammals); the **fat bodies** attached near the gonads are nutrient stores, not part of the reproductive tract itself. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** The common Indian frog is: - (a) Hyla arborea - (b) Rana tigrina - (c) Rana temporaria - (d) Bufo melanostictus **Answer: (b)** β€” Rana tigrina is the common Indian frog β€” class Amphibia, phylum Chordata. **Q2.** The heart of a frog has: - (a) Four chambers - (b) One chamber - (c) Two chambers - (d) Three chambers (two atria, one ventricle) **Answer: (d)** β€” The frog heart is 3-chambered β€” two atria and one ventricle, enclosed in a pericardium. **Q3.** In water, a frog respires mainly through its: - (a) Skin (cutaneous respiration) - (b) Gills - (c) Buccal cavity - (d) Lungs **Answer: (a)** β€” In water the moist skin acts as the respiratory organ (cutaneous); on land lungs (pulmonary) also work. **Q4.** The structure that joins the right atrium of the frog's heart is the: - (a) Pericardium - (b) Conus arteriosus - (c) Sinus venosus - (d) Vena cava **Answer: (c)** β€” The sinus venosus receives venous blood (via the vena cava) and joins the right atrium. **Q5.** A frog is a poikilotherm, which means it: - (a) Maintains a constant body temperature - (b) Cannot change colour - (c) Has a body temperature that varies with the environment - (d) Is warm-blooded **Answer: (c)** β€” Poikilotherms (cold-blooded) have a body temperature that changes with the surroundings. **Q6.** The hind limbs of a frog end in how many digits? - (a) Four - (b) Six - (c) Five - (d) Three **Answer: (c)** β€” Hind limbs have five webbed digits (for swimming); fore limbs have four digits. **Q7.** How many pairs of cranial nerves are present in a frog? - (a) 12 - (b) 31 - (c) 10 - (d) 8 **Answer: (c)** β€” The frog has 10 pairs of cranial nerves arising from the brain. **Q8.** Fertilisation in frogs is: - (a) Internal - (b) Through the skin - (c) Absent - (d) External, in water **Answer: (d)** β€” Sexes are separate and fertilisation is external in water; development is indirect via a tadpole. **Q9.** The hind-brain of a frog includes the: - (a) Olfactory lobes - (b) Optic lobes - (c) Cerebral hemispheres - (d) Cerebellum and medulla oblongata **Answer: (d)** β€” The hind-brain of a frog consists of the cerebellum and medulla oblongata. **Q10.** The double fold of peritoneum attaching the frog's testes to the kidneys is the: - (a) Mesorchium - (b) Vasa efferentia - (c) Cloaca - (d) Bidder's canal **Answer: (a)** β€” The mesorchium anchors the yellowish, ovoid testes to the upper part of the kidneys. **Q11.** Which of these is NOT a prominent endocrine gland in the frog? - (a) Pituitary - (b) Nictitating membrane - (c) Pancreatic islets - (d) Thyroid **Answer: (b)** β€” The nictitating membrane is an eye structure, not an endocrine gland; pituitary, thyroid and pancreatic islets are endocrine glands. **Q12.** Blood is classified as which type of tissue? - (a) Neural tissue - (b) Connective tissue - (c) Epithelial tissue - (d) Muscular tissue **Answer: (b)** β€” Blood is a FLUID connective tissue whose matrix is plasma. It is the one connective tissue with no fibres in its matrix β€” and this is the single most-missed fact in the chapter. **Q13.** Cardiac muscle is unique because it is: - (a) Striated and voluntary - (b) Unstriated and involuntary - (c) Striated but involuntary - (d) Unstriated and voluntary **Answer: (c)** β€” Cardiac muscle is striped like skeletal muscle yet works involuntarily like smooth muscle. Its cells are joined by intercalated discs so the whole heart contracts as one unit. 'Striated equals voluntary' is therefore a false rule. **Q14.** Tendon and ligament respectively connect: - (a) Muscle to muscle, and bone to cartilage - (b) Bone to cartilage, and muscle to muscle - (c) Bone to bone, and muscle to bone - (d) Muscle to bone, and bone to bone **Answer: (d)** β€” Tendon attaches muscle to bone; ligament attaches bone to bone. Both are dense REGULAR connective tissue, and NEET swaps them in options constantly. **Q15.** Which epithelium lines the alveoli of the lungs and the walls of blood vessels? - (a) Compound stratified - (b) Simple squamous - (c) Simple cuboidal - (d) Simple columnar **Answer: (b)** β€” Simple squamous epithelium is made of thin flat cells forming a delicate diffusion barrier β€” exactly what gas exchange in the alveoli requires. **Q16.** The cell junction that permits ions and small molecules to pass directly between adjacent cells is the: - (a) Gap junction - (b) Tight junction - (c) Adhering junction - (d) Desmosome **Answer: (a)** β€” Gap junctions leave channels that connect the cytoplasm of neighbouring cells. Tight junctions seal cells so nothing leaks between them, and adhering junctions cement cells together mechanically. **Q17.** Which of the following is a characteristic of endocrine glands? - (a) They are made of muscular tissue - (b) They are ductless and secrete hormones directly into the blood - (c) They release their secretion through a duct - (d) They secrete only digestive enzymes **Answer: (b)** β€” Endocrine glands have no duct and pour hormones straight into the blood. Exocrine glands β€” salivary, mucus, oil, earwax, milk and digestive-enzyme glands β€” release their product through a duct. **Q18.** Chondrocytes and osteocytes are the cells of: - (a) Cartilage and bone respectively - (b) Blood and cartilage respectively - (c) Muscle and bone respectively - (d) Bone and cartilage respectively **Answer: (a)** β€” Chondrocytes sit in cartilage, whose matrix is solid but pliable and resists compression. Osteocytes sit in the lacunae of bone, whose matrix is hardened by calcium salts and collagen. **Q19.** Which muscle type is multinucleate? - (a) Smooth muscle - (b) Skeletal muscle - (c) Cardiac muscle - (d) All three **Answer: (b)** β€” Skeletal muscle fibres are multinucleate. Smooth muscle cells are spindle-shaped with a single nucleus, and cardiac muscle cells are typically uninucleate. **Q20.** Ciliated epithelium is found in all of the following EXCEPT: - (a) The fallopian tubes - (b) The alveoli of the lungs - (c) The bronchioles - (d) The trachea **Answer: (b)** β€” The alveoli have simple SQUAMOUS epithelium for gas exchange, not ciliated epithelium. Cilia occur where something must be swept along β€” mucus in the bronchioles and trachea, the egg in the fallopian tube. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q21.** The frog absorbs water mainly through its: (NEET PYQ) - (a) Mouth - (b) Skin - (c) Cloaca - (d) Nostrils **Answer: (b)** β€” Frogs never drink water β€” they absorb it through the moist skin. **Q22.** Sexual dimorphism in the male frog is shown by the presence of: (NEET PYQ) - (a) Webbed feet - (b) Vocal sacs and a copulatory pad - (c) Nictitating membrane - (d) A tail **Answer: (b)** β€” Male frogs have vocal sacs and a copulatory pad on the first fore-limb digit; females lack both. **Q23.** The excretory organs (kidneys) of the frog are of which type? (NEET PYQ) - (a) Metanephric - (b) Pronephric - (c) Protonephridial - (d) Mesonephric **Answer: (d)** β€” Frog kidneys are mesonephric; the frog is ureotelic, excreting urea. **Q24.** During hibernation and aestivation, a frog respires through its: (NEET PYQ) - (a) Gills - (b) Buccal cavity - (c) Skin - (d) Lungs **Answer: (c)** β€” When dormant in burrows, gaseous exchange occurs only through the skin (cutaneous respiration). **Q25.** Which membrane protects the frog's eye in water? (NEET PYQ) - (a) Pericardium - (b) Nictitating membrane - (c) Tympanum - (d) Conjunctiva **Answer: (b)** β€” The transparent nictitating membrane covers and protects the eye while the frog is in water. **Q26.** Vasa efferentia in the male frog carry sperm from the testes to the: (NEET PYQ) - (a) Fat bodies - (b) Cloaca directly - (c) Kidneys, opening into Bidder's canal - (d) Urinary bladder **Answer: (c)** β€” 10–12 vasa efferentia enter the kidneys and open into Bidder's canal, which connects onward to the urinogenital duct. ### Active Recall Prompt Write everything you can recall about the **frog**, naming each system first: its basics (Rana tigrina, Amphibia, poikilotherm, aestivation/hibernation); **morphology** (limbs, nictitating membrane, sexual dimorphism); **digestive & respiratory** systems (cloaca, cutaneous + pulmonary); the **circulatory** system (3-chambered heart, sinus venosus, conus arteriosus); the **excretory** system; the **nervous** system (cerebellum, medulla, cranial nerves) and **endocrine glands**; and the **reproductive** system (mesorchium, vasa efferentia, Bidder's canal). Begin each fact with its topic and end it with a full stop.