Structural Organisation in Animals Class 11 Biology: Organ Systems of the Frog

What Structural Organisation in Animals is about

Weightage: This chapter is part of Unit II, Structural Organization in Plants and Animals, which carries 10 of the 70 theory marks in the school annual exam together with Morphology of Flowering Plants and Anatomy of Flowering Plants.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit II Structural Organization in Plants and Animals

Key concepts

Organ and organ system; division of labour

NCERT section 7.1 Organ and Organ System

In multicellular animals, a tissue is a group of similar cells, together with the material between them, that performs a particular function. There are only four basic tissues: epithelial, connective, muscular and neural.

Tissues arranged in a particular pattern and proportion make an organ (stomach, lung, heart, kidney). An organ may contain several tissue types; the heart has all four.

When two or more organs perform a common function through physical and/or chemical interaction, they form an organ system (digestive system, respiratory system).

Cells, tissues, organs and organ systems share the body's work: this is division of labour, and it allows millions of cells to function efficiently and in coordination for the survival of the whole body. The complexity of organs and systems shows a recognisable evolutionary trend across animal groups.

Morphology in animals means the study of the external appearance of the body and its parts; anatomy is the study of internal organs.

Worked example

Explain the terms tissue, organ and organ system with one example each, and say what is meant by division of labour. (3 marks)

Try it first, then show the solutionSolution

1. Tissue: similar cells with intercellular material doing a specific job, e.g. muscular tissue. (½) Organ: tissues organised to do a function, e.g. heart (with all four tissues). (½)
2. Organ system: organs working together for a common function, e.g. digestive system (mouth, stomach, intestine, liver, pancreas). (1)
3. Division of labour: work is shared among cells, tissues, organs and systems so that each does a specific job efficiently for the survival of the whole body. (1)

Answer: Tissue (muscle), organ (heart), organ system (digestive); work shared at every level is division of labour.

Frog: habitat, adaptations and external morphology

NCERT section 7.2 Frogs (7.2.1 Morphology)

Frogs live both on land and in fresh water; they belong to class Amphibia, phylum Chordata. The commonest Indian species is *Rana tigrina*.

General features:

• Cold-blooded (poikilotherms): body temperature changes with the surroundings.
• Colour change: frogs alter their colour to match grass or dry ground, hiding from enemies (camouflage; NCERT calls this protective colouration mimicry).
• Aestivation and hibernation: during the hottest summer and coldest winter they shelter in deep burrows: summer sleep (aestivation) and winter sleep (hibernation).

Morphology:

• Skin: smooth and slippery because of mucus, always kept moist. Back (dorsal) side olive green with dark irregular spots; underside (ventral) uniformly pale yellow. A frog never drinks water; it absorbs water through its skin.
• Body: head and trunk only; no neck and no tail.
• Head: a pair of nostrils above the mouth; bulging eyes covered by a nictitating membrane that protects them under water; a membranous tympanum (ear) on each side of the eyes receives sound.
• Limbs: forelimbs and hind limbs for swimming, walking, leaping and burrowing. Hind limbs are larger and more muscular, ending in five digits; forelimbs end in four digits. Feet have webbed digits for swimming.
• Sexual dimorphism: males have vocal sacs (sound-producing) and a copulatory pad on the first digit of each forelimb; females lack both.

Worked example

How would you tell a male frog from a female frog by external features? Name two adaptations of the frog for life in water. (3 marks)

Try it first, then show the solutionSolution

1. Male: vocal sacs and a copulatory pad on the first digit of each forelimb; female: both absent. (1½)
2. Aquatic adaptations (any two): webbed hind feet for swimming; nictitating membrane protecting the eyes under water; moist skin for breathing in water. (1½)

Answer: Vocal sacs and copulatory pads in males; webbed feet, nictitating membrane, cutaneous respiration.

Digestive system of the frog

NCERT section 7.2.2 Anatomy (digestive system)

The digestive system consists of the alimentary canal and digestive glands.

Alimentary canal: frogs are carnivores, so the gut is short (a long intestine is not needed to digest meat). Path of food:
mouth → buccal cavity → pharynx → oesophagus (a short tube) → stomach → intestine (first part duodenum) → rectum → cloaca → outside.

Glands: the liver makes bile, stored in the gall bladder; the pancreas makes pancreatic juice with digestive enzymes.

Digestion:
1. Prey is caught by the bilobed tongue, which can be flicked out.
2. In the stomach, HCl and gastric juices from the stomach wall digest food partly; the partly digested food is chyme.
3. Chyme passes into the duodenum, which receives bile and pancreatic juice through a common bile duct. Bile emulsifies fats; pancreatic juice digests carbohydrates and proteins.
4. Final digestion occurs in the intestine.
5. Digested food is absorbed by numerous finger-like folds of the intestinal wall, the villi (with microvilli).
6. Undigested solid waste goes to the rectum and out through the cloaca.

Worked example

Why is the alimentary canal of a frog short? Trace the path of food from the duodenum to the outside, naming the secretions received by the duodenum and their roles. (3 marks)

Try it first, then show the solutionSolution

1. Frogs are carnivores; meat is digested more easily than plant matter, so a short intestine is enough. (1)
2. Duodenum → intestine (villi absorb food) → rectum → cloaca → outside. (1)
3. The duodenum receives bile (emulsifies fat) from the gall bladder and pancreatic juice (digests carbohydrates and proteins) through the common bile duct. (1)

Answer: Carnivorous diet; duodenum → intestine → rectum → cloaca; bile and pancreatic juice.

Respiratory system of the frog

NCERT section 7.2.2 Anatomy (respiratory system)

Frogs breathe by two different methods depending on where they are:

• In water: the skin is the respiratory organ (cutaneous respiration). Oxygen dissolved in water diffuses in through the moist skin, and CO₂ diffuses out.
• On land: the buccal cavity, skin and lungs all act as respiratory organs. Breathing through the lungs is pulmonary respiration.

The lungs are a pair of elongated, pink, sac-like structures in the upper trunk (thorax). Air enters through the nostrils into the buccal cavity and then into the lungs.

During aestivation and hibernation, when the frog is inactive in its burrow, gas exchange takes place only through the skin.

The skin must stay moist for gas exchange; this is why mucus glands keep it slimy and why frogs cannot survive long in very dry places.

Worked example

Distinguish between cutaneous and pulmonary respiration in the frog. Which method is used during hibernation, and why is it enough? (3 marks)

Try it first, then show the solutionSolution

1. Cutaneous: gas exchange through the moist skin, used in water. Pulmonary: breathing through the lungs, used on land (with buccal and skin respiration). (2)
2. During hibernation only the skin is used; the frog is inactive with a very low rate of metabolism, so diffusion through the skin supplies enough oxygen. (1)

Answer: Skin (in water) vs lungs (on land); skin alone during hibernation because metabolic demand is very low.

Circulatory and lymphatic system of the frog

NCERT section 7.2.2 Anatomy (circulatory system)

The frog's vascular system is well developed and closed. It has two parts:

• Blood vascular system: heart, blood vessels and blood.
• Lymphatic system: lymph, lymph channels and lymph nodes.

Heart: a muscular organ in the upper body cavity with three chambers: two atria (auricles) and one ventricle, enclosed in a membrane, the pericardium.

• A triangular chamber, the sinus venosus, joins the right atrium; it receives blood from the major veins, the venae cavae.
• From the ventricle, blood passes into the conus arteriosus, a sac on the front (ventral) face of the heart.

Vessels: arteries carry blood from the heart to all parts (arterial system); veins bring blood back (venous system).

Portal systems: special venous connections in which blood from one organ passes through another before returning to the heart:

• Hepatic portal system: between the intestine and the liver.
• Renal portal system: between the lower parts of the body and the kidneys.

Blood: plasma and cells: RBCs (erythrocytes), which are nucleated and contain red haemoglobin; WBCs (leucocytes); and platelets. Blood carries nutrients, gases and water. Lymph differs from blood: it lacks some proteins and RBCs.

The pumping of the muscular heart drives circulation.

Worked example

Describe the structure of the frog's heart, including the chambers attached to it. How does frog blood differ from human blood in one respect? (3 marks)

Try it first, then show the solutionSolution

1. Three chambers: two atria and one ventricle, covered by the pericardium. (1)
2. The sinus venosus (receiving the venae cavae) opens into the right atrium; the ventricle opens into the conus arteriosus on the ventral side. (1)
3. Frog RBCs are nucleated, while mature human RBCs lack a nucleus. (1)

Answer: Two atria, one ventricle, sinus venosus, conus arteriosus; nucleated RBCs.

Excretory, nervous and sensory systems of the frog

NCERT section 7.2.2 Anatomy (excretory, nervous and sensory systems)

Excretory system: a pair of kidneys, ureters, the urinary bladder and the cloaca.

• Kidneys are compact, dark red, bean-shaped organs a little towards the back of the body cavity, on either side of the vertebral column.
• Each kidney is made of many structural and functional units, the uriniferous tubules (nephrons).
• In males, the ureters also carry sperms and act as urinogenital ducts, opening into the cloaca. In females, the ureters and oviducts open separately into the cloaca.
• The thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca.
• Frogs excrete urea: they are ureotelic. Blood brings wastes to the kidney, where they are separated and excreted.

Control and coordination: both neural and endocrine systems.

• Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads; their hormones coordinate organs chemically.
• Nervous system: central (brain and spinal cord), peripheral (cranial and spinal nerves) and autonomic (sympathetic and parasympathetic). There are ten pairs of cranial nerves.
• Brain, inside a bony cranium (brain box): fore-brain (olfactory lobes, paired cerebral hemispheres, unpaired diencephalon), mid-brain (a pair of optic lobes) and hind-brain (cerebellum and medulla oblongata). The medulla passes through the foramen magnum and continues as the spinal cord inside the vertebral column.

Sense organs: sensory papillae for touch, taste buds for taste, nasal epithelium for smell, eyes for sight, and the tympanum with internal ears for hearing. Eyes and internal ears are well organised; the rest are cell groups around nerve endings. The eyes are a pair of spherical simple eyes in the orbits. There is no external ear; only the tympanum is visible. The ear serves both hearing and balance.

Worked example

Why is the frog called ureotelic? Name the parts of its fore-brain, mid-brain and hind-brain. (3 marks)

Try it first, then show the solutionSolution

1. It excretes its nitrogenous waste mainly as urea. (1)
2. Fore-brain: olfactory lobes, cerebral hemispheres, diencephalon; mid-brain: optic lobes. (1)
3. Hind-brain: cerebellum and medulla oblongata. (1)

Answer: Urea excretion; fore-brain (olfactory lobes, cerebral hemispheres, diencephalon), mid-brain (optic lobes), hind-brain (cerebellum, medulla).

Reproductive system and importance of frogs

NCERT section 7.2.2 Anatomy (reproductive system)

Frogs have well-organised male and female reproductive systems.

Male:

• A pair of yellowish, oval testes, attached to the upper part of the kidneys by a double fold of peritoneum, the mesorchium.
• Vasa efferentia (10-12 in number) arise from each testis, enter the kidney on its side, and open into Bidder's canal.
• Bidder's canal connects with the urinogenital duct, which comes out of the kidney and opens into the cloaca.
• The cloaca is a small median chamber through which faeces, urine and sperms pass out.

Female:

• A pair of ovaries near the kidneys, with no functional connection to them.
• A pair of oviducts from the ovaries opens separately into the cloaca.
• A mature female can lay 2500-3000 ova at a time.

Fertilisation is external, in water. Development includes a larval stage, the tadpole, which undergoes metamorphosis into the adult.

Importance to humans: frogs eat insects and so protect crops; they are an important link in food chains and food webs, helping maintain ecological balance; in some countries frog legs are eaten.

Worked example

Trace the path of sperms from the testis to the outside in a male frog. Why does a female frog lay so many eggs? (3 marks)

Try it first, then show the solutionSolution

1. Testis → vasa efferentia (10-12) → kidney → Bidder's canal → urinogenital duct → cloaca → outside. (2)
2. Fertilisation is external in water, and many eggs and tadpoles are eaten or fail to develop, so laying 2500-3000 ova improves the chance that some survive. (1)

Answer: Testis → vasa efferentia → Bidder's canal → urinogenital duct → cloaca; many eggs offset the losses of external fertilisation.

Structural Organisation in Animals in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Frog: Circulation, Excretion and Reproduction
• Frog: Digestive and Respiratory Systems

Structural Organisation in Animals: key facts

Levels of organisation

$\text{cell}\to \text{tissue}\to \text{organ}\to \text{organ system}\to \text{organism}$

Four basic tissues: epithelial, connective, muscular, neural.

Path of food in frog

$\text{mouth}\to \text{buccal cavity}\to \text{pharynx}\to \text{oesophagus}\to \text{stomach}\to \text{duodenum}\to \text{intestine}\to \text{rectum}\to \text{cloaca}$

Short gut: carnivore.

Frog heart

$\text{sinus venosus}\to \text{right atrium};\text{2 atria}+\text{1 ventricle}\to \text{conus arteriosus}$

Three-chambered, closed circulation.

Path of sperm

$\text{testis}\to \text{vasa efferentia}\to \text{Bidder’s canal}\to \text{urinogenital duct}\to \text{cloaca}$

Testes attached to kidneys by mesorchium.

Frog brain

$\text{fore-brain (olfactory lobes, cerebrum, diencephalon)};\text{mid-brain (optic lobes)};\text{hind-brain (cerebellum, medulla)}$

Ten pairs of cranial nerves.

Structural Organisation in Animals: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Structural Organisation in Animals

The question patterns to expect, and the method that works for each.

Tricks to remember Structural Organisation in Animals

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Five back, four front

  Hind limbs have five digits and webbing; forelimbs have four digits.

  Use it when: Morphology questions.

• S-A-V-C heart

  Sinus venosus → Atria → Ventricle → Conus arteriosus.

  Use it when: Frog heart structure.

• Hepatic = liver, Renal = kidney

  Hepatic portal links gut to liver; renal portal links lower body to kidneys.

  Use it when: Portal system questions.

• Test-Vas-Bid-Uro-Clo

  Testis, vasa efferentia, Bidder's canal, urinogenital duct, cloaca.

  Use it when: Path of sperm.

• Summer A, Winter H

  Aestivation in summer, hibernation in winter (A before H as summer before winter in many minds).

  Use it when: Distinguishing the two.

Structural Organisation in Animals: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Structural Organisation in Animals

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying frogs drink water

  Common sense from other animals.

  ✓Do this: Frogs absorb water through the skin; they do not drink.

  Usually costs: ½-1 mark.

• ✗Mistake: Giving the frog a four-chambered heart

  Human heart is assumed.

  ✓Do this: Frog heart: two atria, one ventricle, plus sinus venosus and conus arteriosus.

  Usually costs: 1 mark.

• ✗Mistake: Writing that female ureters carry eggs

  Male urinogenital ducts are generalised.

  ✓Do this: In females, oviducts and ureters open separately into the cloaca; only male ureters act as urinogenital ducts.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up aestivation and hibernation

  Both are inactivity periods.

  ✓Do this: Aestivation: summer sleep; hibernation: winter sleep.

  Usually costs: ½ mark.

• ✗Mistake: Labelling the frog as having an external ear

  The tympanum is mistaken for an ear flap.

  ✓Do this: No external ear; only the tympanum is visible.

  Usually costs: ½ mark.

NCERT exercise guide: Structural Organisation in Animals

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Structural Organisation in Animals

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   In frogs, the vasa efferentia open into:
   (a) cloaca (b) Bidder's canal (c) urinary bladder (d) oviduct

   Hint

   Inside the kidney.

   Answer

   (b) Bidder's canal

2. Q2 · 1 mark · MCQExam level

   The frog is described as ureotelic because it excretes mainly:
   (a) ammonia (b) uric acid (c) urea (d) creatinine

   Hint

   -telic ending.

   Answer

   (c) urea

3. Q3 · 2 marks · Very short answerExam level

   Name the protective membrane of the frog's eye and the structure that receives sound. (2 marks)

   Hint

   One for eye, one for ear.

   Answer

   Nictitating membrane (1); tympanum (1).

4. Q4 · 2 marks · Very short answerHarder exam level

   What is the function of the bilobed tongue? Where does final digestion and absorption occur? (2 marks)

   Hint

   Prey capture; villi.

   Answer

   It catches prey (1). Final digestion is in the intestine, and absorption is by the villi and microvilli of its inner wall (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   List the endocrine glands of the frog and state how they coordinate the body. (2 marks)

   Hint

   Eight glands.

   Answer

   Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads (1); they secrete hormones that coordinate organs chemically (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   Suppose a student sees a frog with large webbed hind limbs, a copulatory pad on the forelimb and swollen throat sacs. (a) Identify the sex. (b) Give the function of the copulatory pad. (2 marks)

   Hint

   Grip during mating.

   Answer

   (a) Male (1). (b) It helps the male grip the female during mating so that sperms can be released over the eggs (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Describe the sense organs of the frog. Which are well organised? (3 marks)

   Hint

   Five senses.

   Answer

   Touch: sensory papillae; taste: taste buds; smell: nasal epithelium (1). Vision: a pair of simple eyes; hearing and balance: tympanum with internal ears (1). Eyes and internal ears are well organised; the others are cell groups around nerve endings (1).

8. Q8 · 3 marks · Short answerHarder exam level

   Distinguish between the nervous system divisions of the frog. (3 marks)

   Hint

   CNS, PNS, ANS.

   Answer

   Central: brain and spinal cord (1). Peripheral: cranial nerves (10 pairs) and spinal nerves (1). Autonomic: sympathetic and parasympathetic (1).

9. Q9 · 2 marks · Very short answerHarder exam level

   Explain how the frog is useful to humans and to the ecosystem. (2 marks)

   Hint

   Insects and food webs.

   Answer

   It eats insects and protects crops (1); it is a link in food chains and webs, maintaining ecological balance (and frog legs are eaten in some countries) (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a pond is polluted with chemicals that damage frog skin. Predict two effects on adult frogs and one effect on their population, giving reasons. (3 marks)

    Hint

    Skin breathes and absorbs.

    Answer

    Adults cannot exchange gases well through damaged skin, especially under water and in hibernation (1). They absorb water and possibly toxins through the skin, so they may become dehydrated or poisoned (1). With external fertilisation in water, eggs and tadpoles are also exposed, so fewer survive and the population falls (1).

11. Q11 · 5 marks · Long answerHOTS / competency

    (a) Describe the circulatory system of the frog. (b) Explain the importance of the hepatic portal system. (c) Why is the frog called cold-blooded, and how does this relate to its three-chambered heart? (5 marks)

    Hint

    Closed system; liver processes absorbed food.

    Answer

    (a) Closed system with a three-chambered heart (two atria, one ventricle) in a pericardium, sinus venosus into the right atrium, conus arteriosus from the ventricle, arteries and veins, plus a lymphatic system; nucleated RBCs (2). (b) Blood from the intestine passes through the liver first, which processes absorbed nutrients before they reach the rest of the body (1). (c) Its body temperature changes with the surroundings; its lower oxygen demand is met even though blood partly mixes in the single ventricle (2).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): The alimentary canal of the frog is short.
    Reason (R): Frogs are carnivores.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Diet and gut length.

    Answer

    (a)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Name the common Indian frog.Show answer

   Rana tigrina (Indian bullfrog).

2. 2.What is the nictitating membrane?Show answer

   A transparent membrane covering the eye that protects it under water.

3. 3.Which digits of the male frog bear copulatory pads?Show answer

   The first digit of each forelimb.

4. 4.What is chyme?Show answer

   Partly digested food passed from the stomach to the duodenum.

5. 5.What is cutaneous respiration?Show answer

   Gas exchange through the skin.

6. 6.What is the sinus venosus?Show answer

   A triangular chamber joining the right atrium that receives blood from the venae cavae.

7. 7.Are frog RBCs nucleated?Show answer

   Yes.

8. 8.How many pairs of cranial nerves does a frog have?Show answer

   Ten pairs.

9. 9.What is the mesorchium?Show answer

   A double fold of peritoneum attaching the testes to the kidneys.

10. 10.What is the larval stage of a frog called?Show answer

    Tadpole.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you name the four basic tissues and give an organ that has all four?
• ☐Can you list five external features of a frog?
• ☐Can you distinguish male and female frogs?
• ☐Can you trace food through the frog's gut and name the glands?
• ☐Can you describe breathing in water and on land?
• ☐Can you describe the frog heart and the two portal systems?
• ☐Can you name the three parts of the frog brain with their components?
• ☐Can you trace sperms and eggs to the cloaca?

Structural Organisation in Animals: frequently asked questions