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Structural Organisation in Animals

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces the concept of tissue-level organisation in multicellular animals, explaining how cells, tissues, organs, and organ systems work together to perform life functions efficiently. It then provides a detailed study of the morphology and anatomy of the frog (Rana tigrina) as a representative vertebrate. The one big idea a student must walk away with is that all complex animals are built from only four basic tissue types, and the frog's body systems illustrate how these tissues are organised into functional organs and organ systems.
Key Concepts (25)
Tissue
A group of similar cells along with intercellular substances that perform a specific function.
e.g. The epithelial tissue lining the stomach forms a protective layer and secretes digestive juices.
Organ
A structure made of one or more types of tissues organised in a specific proportion and pattern to perform a particular function.
e.g. The heart consists of all four types of tissues: epithelial, connective, muscular and neural.
Organ system
When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system.
e.g. The digestive system includes the mouth, oesophagus, stomach, intestine, liver and pancreas.
Morphology
The study of form or externally visible features of an organism or its parts.
e.g. In frogs, morphology includes the shape of the head, trunk, limbs, and the presence of webbed digits.
Anatomy
The study of the morphology of internal organs in animals.
e.g. The anatomy of a frog includes the structure of its heart, lungs, kidneys, and reproductive organs.
Poikilotherms (cold-blooded)
Animals that do not have a constant body temperature; their body temperature varies with the temperature of the environment.
e.g. Frogs are poikilotherms, so they become inactive in extreme heat or cold.
Camouflage (mimicry)
The ability of an animal to change its colour to hide from enemies, providing protective coloration.
e.g. Frogs change colour when they are in grasses versus on dry land to avoid predators.
Aestivation
Summer sleep; a period of dormancy during peak summer to protect from extreme heat.
e.g. Frogs take shelter in deep burrows during summer to avoid heat.
Hibernation
Winter sleep; a period of dormancy during winter to protect from extreme cold.
e.g. Frogs take shelter in deep burrows during winter to avoid cold.
Sexual dimorphism
Differences in external appearance between males and females of the same species.
e.g. Male frogs have sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs, which are absent in females.
Nictitating membrane
A transparent membrane that covers the eye of a frog, protecting it while in water.
e.g. Frogs have bulged eyes covered by a nictitating membrane.
Tympanum
A membranous structure on either side of the eyes in frogs that receives sound signals; it functions as an ear.
e.g. The tympanum is visible externally in frogs and is an organ of hearing and balance.
Cloaca
A small, median chamber in frogs that is used to pass faecal matter, urine, and sperms (or ova) to the exterior.
e.g. The rectum, ureters, and reproductive ducts all open into the cloaca in frogs.
Chyme
Partially digested food that is passed from the stomach to the duodenum.
e.g. After action of HCl and gastric juices, food becomes chyme in the frog's stomach.
Villi and microvilli
Numerous finger-like folds in the inner wall of the intestine that increase surface area for absorption of digested food.
e.g. In frogs, digested food is absorbed by villi and microvilli in the intestine.
Cutaneous respiration
Respiration through the skin, which acts as an aquatic respiratory organ in frogs.
e.g. In water, frogs exchange dissolved oxygen through their skin by diffusion.
Pulmonary respiration
Respiration through the lungs, which occurs on land in frogs.
e.g. Air enters through nostrils into the buccal cavity and then to the lungs for pulmonary respiration.
Hepatic portal system
A special venous connection between the liver and the intestine in frogs.
e.g. Blood from the intestine is carried to the liver via the hepatic portal system.
Renal portal system
A special venous connection between the kidney and the lower parts of the body in frogs.
e.g. Blood from the lower body is carried to the kidney via the renal portal system.
Nephron (uriniferous tubule)
The structural and functional unit of the kidney, responsible for filtering blood and forming urine.
e.g. Each frog kidney is composed of numerous nephrons.
Ureotelic animal
An animal that excretes urea as the primary nitrogenous waste.
e.g. Frogs are ureotelic animals because they excrete urea.
Mesorchium
A double fold of peritoneum that adheres the testes to the upper part of the kidneys in male frogs.
e.g. The testes in male frogs are found adhered to the kidneys by mesorchium.
Bidder's canal
A canal in the kidney of male frogs into which the vasa efferentia open, eventually communicating with the urinogenital duct.
e.g. Vasa efferentia enter the kidneys and open into Bidder's canal.
Vasa efferentia
10-12 fine tubules that arise from the testes in male frogs and carry sperm to the kidneys.
e.g. Vasa efferentia enter the kidneys and open into Bidder's canal.
Metamorphosis
The process of transformation from a larval stage (tadpole) to an adult frog.
e.g. Frog eggs hatch into tadpoles, which undergo metamorphosis to become adult frogs.
Formulas & Equations (1)
No chemical equations or mathematical formulas are present in this chapter.
Reads as: Not applicable
Variables: Not applicable
Use when: Not applicable
⚠ Not applicable
How It Works
Digestion in Frog
  1. Food is captured by the bilobed tongue.
  2. Food enters the mouth and passes into the buccal cavity, then through the pharynx into the oesophagus.
  3. The oesophagus opens into the stomach, where HCl and gastric juices partially digest food into chyme.
  4. Chyme passes from the stomach to the duodenum (first part of the small intestine).
  5. The duodenum receives bile from the gall bladder (via the common bile duct) and pancreatic juice from the pancreas.
  6. Bile emulsifies fats; pancreatic juices digest carbohydrates and proteins.
  7. Final digestion occurs in the intestine; digested food is absorbed by villi and microvilli.
  8. Undigested solid waste moves into the rectum and passes out through the cloaca.
💡 The alimentary canal is short because frogs are carnivores; bile is stored in the gall bladder but secreted by the liver.
Respiration in Frog
  1. In water: Cutaneous respiration occurs — dissolved oxygen is exchanged through the skin by diffusion.
  2. On land: Pulmonary respiration occurs — air enters through the nostrils into the buccal cavity and then to the lungs.
  3. During aestivation and hibernation: Gaseous exchange takes place through the skin alone.
💡 Frogs have three respiratory surfaces: skin (cutaneous), buccal cavity, and lungs (pulmonary).
Circulation in Frog (Closed Type)
  1. The heart pumps blood; it has three chambers: two atria and one ventricle, covered by the pericardium.
  2. Sinus venosus joins the right atrium and receives blood from major veins (vena cava).
  3. The ventricle opens into the conus arteriosus on the ventral side of the heart.
  4. Arteries carry blood from the heart to all parts of the body (arterial system).
  5. Veins collect blood from different parts of the body and return it to the heart (venous system).
  6. Special portal systems exist: hepatic portal system (liver-intestine) and renal portal system (kidney-lower body).
  7. Blood consists of plasma and cells: RBCs (nucleated, contain haemoglobin), WBCs, and platelets.
  8. Lymph (lacking few proteins and RBCs) circulates through the lymphatic system.
💡 Frogs have single circulation (blood passes through the heart once per circuit) and nucleated RBCs with haemoglobin.
Excretion in Frog
  1. A pair of kidneys (compact, dark red, bean-shaped) filter nitrogenous wastes from blood.
  2. Each kidney contains numerous nephrons (uriniferous tubules) as functional units.
  3. In males, two ureters emerge from kidneys and act as urinogenital ducts, opening into the cloaca.
  4. In females, ureters and oviducts open separately into the cloaca.
  5. A thin-walled urinary bladder (ventral to rectum) also opens into the cloaca.
  6. The frog excretes urea (ureotelic).
💡 Ureters in males also carry sperm (urinogenital duct), but in females ureters and oviducts open separately.
Nervous System of Frog
  1. Central nervous system: Brain (enclosed in cranium) and spinal cord (enclosed in vertebral column).
  2. Brain is divided into fore-brain (olfactory lobes, cerebral hemispheres, diencephalon), mid-brain (optic lobes), and hind-brain (cerebellum, medulla oblongata).
  3. Medulla oblongata passes through foramen magnum and continues as spinal cord.
  4. Peripheral nervous system: 10 pairs of cranial nerves and spinal nerves.
  5. Autonomic nervous system: Sympathetic and parasympathetic divisions.
  6. Sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), hearing and balance (tympanum with internal ears).
💡 Frogs have 10 pairs of cranial nerves (not 12 like humans); external ear is absent, only tympanum is visible.
Reproduction in Frog
  1. Male: Testes (paired, yellowish, ovoid) adhered to kidneys by mesorchium; vasa efferentia (10-12) carry sperm to kidneys, then to Bidder's canal, then to urinogenital duct, and finally to cloaca.
  2. Female: Ovaries (paired) near kidneys; oviducts open separately into cloaca; a mature female lays 2500-3000 ova at a time.
  3. Fertilisation is external and takes place in water.
  4. Development involves a larval stage called tadpole.
  5. Tadpole undergoes metamorphosis to form the adult frog.
💡 Fertilisation is external; the tadpole stage is a key feature of amphibian life cycle.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'ureter' with 'urinary bladder' function
✗ The ureter stores urine in frogs.
→ The ureter is a duct that carries urine from the kidney to the cloaca; it does not store urine. The urinary bladder stores urine.
✓ The ureter transports urine from the kidney to the cloaca; the urinary bladder stores urine temporarily.
⚠ Saying frogs have four-chambered heart
✗ Frogs have a four-chambered heart like mammals.
→ Frogs have a three-chambered heart (two atria and one ventricle). They do not have a four-chambered heart.
✓ Frogs have a three-chambered heart: two atria and one ventricle.
⚠ Mixing up aestivation and hibernation
✗ Aestivation is winter sleep and hibernation is summer sleep.
→ Aestivation is summer sleep (to avoid heat), hibernation is winter sleep (to avoid cold).
✓ Aestivation is summer sleep; hibernation is winter sleep.
⚠ Claiming frogs drink water through mouth
✗ Frogs drink water through their mouth.
→ Frogs never drink water; they absorb it through their skin.
✓ Frogs absorb water through their skin; they do not drink water.
⚠ Confusing 'hepatic portal system' with 'renal portal system'
✗ The hepatic portal system connects the kidney to the lower body.
→ The hepatic portal system connects the liver to the intestine; the renal portal system connects the kidney to the lower parts of the body.
✓ The hepatic portal system is between the liver and intestine; the renal portal system is between the kidney and lower body.
⚠ Stating that frogs have external ears
✗ Frogs have external ears that are visible.
→ Frogs do not have external ears; only the tympanum (a membranous structure) is visible externally.
✓ Frogs lack external ears; the tympanum is the externally visible ear structure.
Compare & Contrast
Season and purpose
Aestivation
Hibernation
Aestivation is summer sleep to avoid extreme heat; hibernation is winter sleep to avoid extreme cold.
Organ and environment
Cutaneous respiration
Pulmonary respiration
Cutaneous respiration occurs through the skin in water; pulmonary respiration occurs through the lungs on land.
Organs connected
Hepatic portal system
Renal portal system
Hepatic portal system connects the liver and intestine; renal portal system connects the kidney and lower parts of the body.
Ducts and openings
Male frog reproductive system
Female frog reproductive system
In males, ureters act as urinogenital ducts and open into the cloaca; in females, ureters and oviducts open separately into the cloaca.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (2-3 marks), 1 Long Answer (5 marks) — typically on frog anatomy or organ systems.
Most asked: Morphology of frog (external features, sexual dimorphism) · Digestive system of frog (alimentary canal and digestive glands) · Respiratory system (cutaneous and pulmonary respiration) · Circulatory system (heart structure, hepatic and renal portal systems) · Excretory system (kidneys, ureters, ureotelic nature) · Reproductive system (male and female, external fertilisation, metamorphosis)
Don't skip: Draw a labelled diagram of the digestive system of frog (5 marks).; Explain the structure and function of the frog's heart and mention the portal systems (3-5 marks).; Describe the male and female reproductive systems of frog (3-5 marks).; Differentiate between cutaneous and pulmonary respiration (2-3 marks).
One-Line Revision (read aloud before the exam)
  1. A tissue is a group of similar cells with intercellular substances performing a specific function.
  2. All complex animals consist of only four basic types of tissues: epithelial, connective, muscular, and neural.
  3. Frogs belong to class Amphibia, phylum Chordata, and the common Indian species is Rana tigrina.
  4. Frogs are poikilotherms (cold-blooded) and exhibit camouflage (mimicry).
  5. Frogs undergo aestivation (summer sleep) and hibernation (winter sleep).
  6. Frog skin is smooth, slippery, moist, and never drinks water — it absorbs water through skin.
  7. Frog body is divisible into head and trunk; neck and tail are absent.
  8. Male frogs have vocal sacs and a copulatory pad on the first digit of forelimbs (sexual dimorphism).
  9. Frogs have a short alimentary canal because they are carnivores.
  10. The liver secretes bile stored in the gall bladder; the pancreas produces pancreatic juice.
  11. Digestion begins in the stomach with HCl and gastric juices; chyme passes to the duodenum.
  12. Bile emulsifies fats; pancreatic juices digest carbohydrates and proteins.
  13. Absorption occurs through villi and microvilli in the intestine.
  14. Cutaneous respiration occurs through skin in water; pulmonary respiration occurs through lungs on land.
  15. Frogs have a three-chambered heart (two atria, one ventricle) with single circulation.
  16. RBCs in frogs are nucleated and contain haemoglobin.
  17. The hepatic portal system connects liver and intestine; the renal portal system connects kidney and lower body.
  18. Frogs are ureotelic — they excrete urea.
  19. Each kidney is composed of nephrons (uriniferous tubules).
  20. In male frogs, ureters act as urinogenital ducts; in females, ureters and oviducts open separately into the cloaca.
  21. The frog brain has fore-brain (olfactory lobes, cerebral hemispheres, diencephalon), mid-brain (optic lobes), and hind-brain (cerebellum, medulla oblongata).
  22. Frogs have 10 pairs of cranial nerves.
  23. External ear is absent; tympanum is the visible ear structure.
  24. Fertilisation is external and occurs in water; development includes a tadpole larval stage that undergoes metamorphosis.
  25. A mature female frog lays 2500-3000 ova at a time.

MCQs — 10 Questions with Answers

Q1. Which of the following is NOT a basic tissue type found in multicellular animals?

  • A. Epithelial tissue
  • B. Connective tissue
  • C. Vascular tissue ✓
  • D. Nervous tissue

Answer: C — Vascular tissue is found only in plants; the four basic animal tissues are epithelial, connective, muscular, and neural.

Q2. The nictitating membrane in a frog protects the eye primarily by:

  • A. Regulating light entry
  • B. Preventing water entry while submerged ✓
  • C. Secreting lubricating mucus
  • D. Changing eye color for camouflage

Answer: B — The nictitating membrane is a transparent protective layer that shields the eye when the frog is underwater.

Q3. Why is the intestine of a frog relatively short compared to that of a herbivorous mammal?

  • A. Frogs have a slower metabolic rate
  • B. Protein-rich food (carnivorous diet) requires less digestive time than plant material ✓
  • C. Frogs absorb nutrients faster through their skin
  • D. The frog stomach performs most nutrient absorption

Answer: B — Meat is easier to digest than plants, so carnivorous frogs do not need an elongated intestine like herbivores do.

Q4. During winter, frogs survive in deep burrows through hibernation. The primary respiratory mechanism during this period is:

  • A. Pulmonary respiration via lungs
  • B. Buccal cavity respiration
  • C. Cutaneous respiration through moist skin ✓
  • D. Gills present in the throat

Answer: C — During hibernation (and aestivation), gaseous exchange occurs exclusively through the frog's moist skin by diffusion.

Q5. Which statement correctly pairs a frog's external feature with its function? (Assertion-based)

  • A. Assertion: Webbed digits help in swimming. Reason: They reduce water resistance during movement. ✓
  • B. Assertion: Vocal sacs produce sound in both males and females. Reason: They aid in mating calls.
  • C. Assertion: The copulatory pad is absent in males. Reason: It is used for egg fertilization.
  • D. Assertion: Nostrils open into the lungs directly. Reason: This speeds up oxygen absorption.

Answer: A — Webbed digits in hind limbs increase surface area and reduce drag, enabling efficient swimming; assertions B and C contain factual errors, and D incorrectly describes nasal anatomy.

Q6. A frog's heart receives deoxygenated blood from the body through which structure?

  • A. Pulmonary artery
  • B. Sinus venosus via vena cava ✓
  • C. Conus arteriosus
  • D. Left atrium

Answer: B — The sinus venosus collects deoxygenated blood from the major veins (vena cava) and delivers it to the right atrium.

Q7. Organ systems in multicellular organisms exhibit division of labour. How does this benefit the organism as a whole?

  • A. It allows each system to operate independently without coordination
  • B. It increases efficiency by specialising cells for specific functions while maintaining coordinated survival ✓
  • C. It reduces the number of cells needed to perform all functions
  • D. It eliminates the need for nervous and circulatory systems

Answer: B — Division of labour through specialised tissues and organ systems allows efficient, coordinated function while maintaining overall body homeostasis.

Q8. If a frog's dorsal skin appears green with dark spots, what biological principle does this illustrate?

  • A. Thermoregulation
  • B. Protective coloration (mimicry) for camouflage in grass environments ✓
  • C. Increased melanin production for UV protection
  • D. Adaptation for absorbing more dissolved oxygen

Answer: B — The olive-green dorsal colouration with dark spots matches grass and vegetation, helping the frog avoid predators through camouflage.

Q9. A student observes that during dissection a frog's bile is stored separately from the pancreas. Which organ stores bile, and what is its role in digestion? (HOTS)

  • A. Stomach stores bile and churns food mechanically
  • B. Gallbladder stores bile and releases it into the duodenum to emulsify fats ✓
  • C. Pancreas stores bile and secretes digestive enzymes
  • D. Liver stores bile and directly absorbs digested nutrients

Answer: B — The gallbladder is the storage organ for bile produced by the liver; it releases bile into the duodenum where it breaks down fats into smaller droplets (emulsification) for enzyme action.

Q10. Which term best describes an organism whose body temperature fluctuates with environmental temperature?

  • A. Homeotherm
  • B. Poikilotherm (cold-blooded) ✓
  • C. Ectoderm
  • D. Endotherm

Answer: B — Poikilotherms like frogs cannot regulate internal body temperature and depend on the environment; homeotherms and endotherms maintain constant body temperature.

Flashcards

What is a tissue?

A group of similar cells along with intercellular substances that perform a specific function.

Name the four basic tissue types found in all animals.

Epithelial, connective, muscular, and neural tissues.

What is the difference between morphology and anatomy?

Morphology is the study of external features or form, while anatomy is the study of internal organ structure.

Define poikilotherm with an example.

An organism whose body temperature varies with the environment; frogs are poikilotherms (cold-blooded).

What is the function of the nictitating membrane in a frog's eye?

It protects the eye while the frog is underwater.

Why is the frog's intestine short compared to herbivores?

Because frogs are carnivores and meat is easier to digest, requiring less intestinal length.

What are the two respiratory methods used by frogs?

Cutaneous respiration (through skin in water) and pulmonary respiration (through lungs on land).

How many chambers does a frog's heart have and what are they?

Three chambers: two atria and one ventricle.

What is aestivation?

A period of inactivity during peak summer when frogs hide in burrows to avoid extreme heat.

What male frog feature distinguishes it from females?

Sound-producing vocal sacs and a copulatory pad on the first digit of the fore limb.

Important Board Questions

Define 'tissue' and name the four basic types of tissues found in animals. Give one example location for each tissue type in the frog body. [2 marks]

Tissue = similar cells + intercellular substance performing one function. Four types: epithelial (skin), connective (around organs), muscular (limbs), neural (brain/spinal cord).

Describe the path of food through the frog's digestive system from ingestion to elimination. Explain the role of bile and pancreatic juice in the duodenum. [5 marks]

Path: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca. In duodenum: bile emulsifies fats, pancreatic juice digests carbs and proteins; villi absorb final products.

Explain how a frog demonstrates structural and functional adaptation to a dual (aquatic-terrestrial) lifestyle. Support your answer with specific examples of respiratory, locomotor, and sensory adaptations. [6 marks]

Respiratory: cutaneous (water) + pulmonary (land). Locomotor: webbed digits (swimming), strong hind limbs (jumping/burrowing). Sensory: nictitating membrane (water), tympanum (sound). Behavioural: mimicry, aestivation, hibernation. Show how each supports survival in both environments.

Assertion-Reason Questions for Structural Organisation in Animals

Q1. Assertion (A):

Frogs are classified as poikilothermic (cold-blooded) animals because their body temperature remains constant throughout the year.

Reason (R):

Poikilothermic animals maintain a fixed body temperature independent of environmental conditions through internal metabolic regulation.

Answer: D — A is false but R is true — Both the assertion and reason are false. Poikilothermic animals have body temperatures that vary with the environment, not remain constant. Frogs are poikilothermic, but the defining characteristic is variable temperature, not constant temperature. The reason incorrectly defines poikilothermic animals.

Q2. Assertion (A):

The alimentary canal of frogs is shorter than that of herbivorous animals because frogs are carnivores.

Reason (R):

Carnivorous diets contain more easily digestible proteins and require less intestinal length compared to plant-based diets that require extensive breakdown and nutrient extraction.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why carnivores have shorter digestive tracts. Animal proteins are simpler to digest than plant cellulose, requiring less intestinal processing time. The reason provides the correct biological explanation for the assertion.

Q3. Assertion (A):

Frogs can perform respiration both in water and on land using the same set of respiratory organs.

Reason (R):

Frogs use lungs for respiration in both aquatic and terrestrial environments because lungs are efficient at extracting both dissolved oxygen from water and gaseous oxygen from air.

Answer: C — A is true but R is false — The assertion is true — frogs do respire in both environments. However, the reason is false. Lungs cannot extract dissolved oxygen from water; instead, frogs use cutaneous (skin) respiration in water and pulmonary and buccal respiration on land. The statement misunderstands the respiratory mechanisms.

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Case-Based Questions (CBQ) for Structural Organisation in Animals

Case Study:

A wildlife biologist studying frogs in the Western Ghats of India observed that during the summer monsoon season, the frog population remained highly active, feeding on insects and reproducing actively. However, in March, as temperatures soared above 40°C and water bodies dried up, the frogs disappeared from the visible areas. Upon careful excavation of burrows, the scientist found frogs in a dormant state, with minimal movement, reduced body temperature matching soil temperature, and gaseous exchange occurring through the skin. The heart rate had dropped significantly, and the frog's metabolic rate was nearly 90% lower than during active seasons.

Q1. The disappearance of frogs during the summer months is primarily due to:

  • A. Migration to cooler regions in the mountains
  • B. Aestivation — a dormant state to survive extreme heat and drought ✓
  • C. Death due to inability to survive in high temperatures
  • D. Hibernation triggered by photoperiod changes

Answer: B

Q2. The observation that gaseous exchange occurred through the skin during dormancy indicates that:

  • A. Lungs become non-functional during aestivation
  • B. Cutaneous respiration is sufficient to meet the minimal oxygen demand of a metabolically suppressed frog ✓
  • C. Frogs do not require oxygen during aestivation
  • D. The skin produces oxygen through photosynthesis

Answer: B

Q3. The 90% reduction in metabolic rate during dormancy explains why:

  • A. Frogs can survive without food and water for extended periods
  • B. The heart rate and respiration rate decrease significantly
  • C. Oxygen demand becomes minimal, making skin respiration adequate
  • D. All of the above ✓

Answer: D

Q4. If the temperature of the soil remained at 45°C continuously without dropping even at night, the frog's survival strategy would likely fail because:

  • A. Aestivation can only occur in cooler burrows with temperatures below 30°C
  • B. Frogs cannot burrow deeper than a certain depth
  • C. The frog's skin would dry out despite mucus secretion
  • D. Sustained high temperature would prevent the frog's body temperature from dropping below lethal limits ✓

Answer: D

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