Structural Organisation in Animals: Frog for ISC Class 11 Biology

What Structural Organisation in Animals: Frog is about

Weightage: Structural Organisation in Animals: Frog belongs to the unit Structural Organisation in Animals and Plants, which CISCE weights at 11 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

External morphology and adaptations

Body form: streamlined, divided into head and trunk; no neck or tail. Skin: smooth, moist, slimy because of mucus glands; the dorsal side is olive green with dark irregular spots, the ventral side pale yellow. The frog can change skin shade to match its surroundings (camouflage), which protects it from predators. Eyes: bulging, protected by a transparent nictitating membrane when under water. Tympanum (eardrum): a circular membrane behind each eye that receives sound. Nostrils: a pair above the mouth. Limbs: forelimbs with four digits, hindlimbs larger and muscular with five webbed digits, used for leaping and swimming. Sexual dimorphism: males have vocal sacs (for croaking) and a copulatory (nuptial) pad on the first digit of each forelimb; both are absent in females. Body temperature: frogs are poikilothermic; they aestivate in hot, dry weather and hibernate in winter, staying inactive in burrows or mud.

Worked example

How would you tell a male frog from a female frog in the field?

Try it first, then show the solutionSolution

1. Look at the throat: males have vocal sacs that inflate when croaking
2. Look at the first finger of the forelimb: males have a thickened copulatory pad used to grip the female

Answer: Vocal sacs and copulatory pads are present only in males

Digestive system

The gut is short because frogs are carnivores (they feed on insects and small animals), and meat is easier to digest than plant matter. Path: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca → outside through the cloacal aperture. The tongue is bilobed at the tip and attached at the front of the mouth, so it can be flicked out to catch insects. Small teeth on the upper jaw (maxillary teeth) and on the roof of the mouth (vomerine teeth) hold prey; the lower jaw has none. Liver makes bile, stored in the gall bladder; the pancreas makes pancreatic juice. Food is digested in the stomach by HCl and gastric juices and becomes a semi-fluid chyme; it passes into the duodenum, where bile emulsifies fats and pancreatic juice digests carbohydrates and proteins. Final digestion and absorption take place in the intestine, whose inner surface has villi and microvilli to increase area. Undigested matter is stored in the rectum and passed out through the cloaca.

Worked example

Why is the frog's tongue attached at the front and not at the back of the mouth?

Try it first, then show the solutionSolution

1. Attachment at the front lets the free, bilobed tip be thrown forward out of the mouth
2. This allows the frog to catch a flying or crawling insect quickly and pull it in

Answer: Front attachment allows the tongue to be flicked out to catch prey

Respiratory system

Frogs breathe in three ways. (1) Cutaneous respiration: oxygen dissolved in water or in the moist film on the skin diffuses into blood capillaries just below the skin; this is the main method in water and during hibernation and aestivation. (2) Buccopharyngeal respiration: on land, air drawn in through the nostrils is exchanged across the moist, vascular lining of the buccal cavity and pharynx. (3) Pulmonary respiration: on land, air enters a pair of thin-walled, elastic, sac-like lungs in the upper trunk through the nostrils, buccal cavity and larynx; the lungs have folded inner walls rich in capillaries. Because the frog has no diaphragm or ribs, it pushes air into the lungs by raising the floor of the buccal cavity with the nostrils closed (a pumping action). The skin must stay moist for gas exchange.

Worked example

A frog spends the winter buried in mud at the bottom of a pond. How does it get oxygen?

Try it first, then show the solutionSolution

1. Its lungs cannot be ventilated while buried; its metabolic rate falls very low
2. Oxygen diffuses through the moist, thin, highly vascular skin into the blood

Answer: By cutaneous respiration through the skin

Circulatory system

The frog has a well-developed closed blood vascular system and also a lymphatic system. Heart: a muscular, three-chambered organ (two atria and one ventricle) in the upper body cavity, enclosed in a double membrane, the pericardium. A thin-walled triangular chamber, the sinus venosus, on the dorsal side collects blood from the venae cavae and opens into the right atrium. The left atrium receives oxygenated blood from the lungs (pulmonary veins). The ventricle pumps blood into a sac-like conus arteriosus on the ventral side, which leads to the arteries (the arterial system). Veins return blood to the heart (the venous system). Two portal systems carry blood from one organ to another before it returns to the heart: the hepatic portal system (intestine to liver) and the renal portal system (hind limbs to kidneys). Blood: plasma and cells: red blood cells (oval, nucleated, with haemoglobin), white blood cells and platelets. Lymph is like blood without red blood cells and with fewer proteins. Because one ventricle receives both oxygenated and deoxygenated blood, some mixing occurs; skin breathing partly makes up for this.

Worked example

Name the chamber that receives blood from the body before the atria, and the chamber that receives blood from the ventricle.

Try it first, then show the solutionSolution

1. Venous blood from the venae cavae collects in the sinus venosus and passes to the right atrium
2. The ventricle pumps blood into the conus arteriosus, which leads to the arteries

Answer: Sinus venosus; conus arteriosus

Excretory system

The excretory system has a pair of kidneys, a pair of ureters, a urinary bladder and the cloaca. The kidneys are compact, dark red, bean-shaped organs lying towards the back of the body cavity on either side of the vertebral column. Each has many functional units, the uriniferous tubules (nephrons), that filter blood. In the male the ureters carry both urine and sperm, so each is a urinogenital duct opening into the cloaca. In the female the ureters and the oviducts open separately into the cloaca. A thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca. The adult frog is ureotelic: it excretes nitrogenous waste mainly as urea, which is less toxic than ammonia and can be held in the body for a while. (The aquatic tadpole excretes mainly ammonia.)

Worked example

Why can an adult frog excrete urea while its tadpole excretes ammonia?

Try it first, then show the solutionSolution

1. Ammonia is very toxic but very soluble; a tadpole living in water can wash it away continuously
2. The adult spends time on land where water is limited, so it converts ammonia into less toxic urea that needs less water to remove

Answer: Urea suits the adult's partly terrestrial life; ammonia suits the fully aquatic tadpole

Nervous system and sense organs

The nervous system has three parts. (1) Central nervous system: brain and spinal cord. The brain lies in a bony box, the cranium, and is divided into: forebrain, with olfactory lobes (smell), paired cerebral hemispheres and the unpaired diencephalon; midbrain, with a pair of optic lobes (vision); and hindbrain, with the cerebellum (balance and posture) and the medulla oblongata, which passes out through the foramen magnum and continues as the spinal cord in the vertebral column. (2) Peripheral nervous system: ten pairs of cranial nerves from the brain and spinal nerves from the spinal cord. (3) Autonomic nervous system: sympathetic and parasympathetic divisions that control internal organs. Sense organs: touch (sensory papillae in the skin), taste (taste buds), smell (nasal epithelium), sight (eyes) and hearing and balance (tympanum with internal ears). Of these, the eyes and internal ears are well-organised structures; the rest are clusters of cells around nerve endings. The frog's eyes are simple eyes (a single lens each).

Worked example

A frog with damaged optic lobes can still smell food. Explain using the parts of its brain.

Try it first, then show the solutionSolution

1. Optic lobes are in the midbrain and deal with vision
2. Smell is handled by the olfactory lobes in the forebrain, which are undamaged

Answer: Smell depends on forebrain olfactory lobes, not the midbrain optic lobes

Reproductive system and development

Sexes are separate and the reproductive organs are well organised. Male: a pair of yellowish, ovoid testes attached to the upper part of the kidneys by a fold of peritoneum, the mesorchium. About 10 to 12 vasa efferentia leave each testis, enter the kidney on that side and open into Bidder's canal; from there sperm pass into the urinogenital duct (ureter), which opens into the cloaca. The cloaca is a small median chamber through which faeces, urine and sperm leave. Female: a pair of ovaries near the kidneys, with no functional connection to them. A pair of oviducts starts near the ovaries and opens separately into the cloaca. A mature female may lay about 2500 to 3000 eggs at one time. Fertilisation is external, in water: the male grips the female (amplexus) and releases sperm over the eggs as they are laid. Development is indirect: the fertilised egg develops into an aquatic, gill-breathing larva, the tadpole, which changes into the adult by metamorphosis (it grows limbs, loses the tail and gills, and develops lungs).

Worked example

In the male frog the urinogenital duct carries both urine and sperm. Trace the route of sperm from testis to the outside.

Try it first, then show the solutionSolution

1. Testis → vasa efferentia → kidney (Bidder's canal)
2. → urinogenital duct (ureter) → cloaca → cloacal aperture

Answer: Testis, vasa efferentia, Bidder's canal, urinogenital duct, cloaca, outside

Structural Organisation in Animals: Frog 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, Nervous and Reproductive Systems
• Frog: Morphology, Digestion and Respiration

Structural Organisation in Animals: Frog: key facts

Path of food

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

Short gut: carnivore.

Blood through the heart

$\text{venae cavae}\to \text{sinus venosus}\to \text{right atrium};\text{pulmonary veins}\to \text{left atrium};\text{atria}\to \text{ventricle}\to \text{conus arteriosus}$

Three chambers: two atria, one ventricle.

Sperm route (male)

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

In females, oviducts and ureters open separately into the cloaca.

Divisions of the brain

$\text{forebrain (olfactory lobes, cerebrum, diencephalon)};\text{midbrain (optic lobes)};\text{hindbrain (cerebellum, medulla)}$

Ten pairs of cranial nerves.

Structural Organisation in Animals: Frog: 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: Frog

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

Tricks to remember Structural Organisation in Animals: Frog

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

• Four in front, five behind

  Forelimb 4 digits; hindlimb 5 webbed digits.

  Use it when: External features.

• Skin, Mouth, Lungs

  Three ways a frog breathes: cutaneous, buccopharyngeal, pulmonary. Skin works always; lungs only on land.

  Use it when: Respiration.

• Sinus in, conus out

  Blood enters the heart through the sinus venosus and leaves through the conus arteriosus.

  Use it when: Heart.

• BUCK: Bidder's, Urinogenital, Cloaca, Kidney

  Male sperm path: through the Kidney, Bidder's canal, Urinogenital duct, Cloaca.

  Use it when: Male reproductive system.

• Hot = Aestivate, Hibernate = Hiemal (winter)

  Aestas is Latin for summer; hiems is winter.

  Use it when: Dormancy.

Structural Organisation in Animals: Frog: 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 D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 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: Frog

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

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

  Confusion with birds and mammals.

  Wrong working

  The frog heart has two atria and two ventricles.

  Right working

  Two atria and one ventricle, with a sinus venosus and conus arteriosus.

  ✓Do this: Amphibians: three chambers.

  Usually costs: 1 mark.

• ✗Mistake: Saying the female frog has a urinogenital duct

  The male arrangement is applied to both sexes.

  Wrong working

  In the female, eggs pass through the ureter.

  Right working

  In the female, oviducts and ureters open separately into the cloaca.

  ✓Do this: Only the male has a combined duct.

  Usually costs: 1 mark.

• ✗Mistake: Calling the frog ammonotelic

  It lives in water, so students assume ammonia.

  Wrong working

  Frogs excrete ammonia.

  Right working

  Adult frogs are ureotelic; tadpoles excrete ammonia.

  ✓Do this: Adult = urea.

  Usually costs: 1 mark.

• ✗Mistake: Wrong digit count

  Careless sketches.

  Wrong working

  Five digits on the forelimb.

  Right working

  Four digits on the forelimb, five webbed digits on the hindlimb.

  ✓Do this: Count before you label.

  Usually costs: 1 mark in a diagram.

• ✗Mistake: Writing too much detail

  Students try to learn every vessel and nerve.

  Wrong working

  Two pages on the arterial arches.

  Right working

  The syllabus asks for a brief account: organs in order and how the system works.

  ✓Do this: Learn a crisp 5-point summary for each system.

  Usually costs: Time.

Practice questions on Structural Organisation in Animals: Frog

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

   The frog's tongue is:
   (a) attached at the back and free in front
   (b) bilobed and attached in front
   (c) absent
   (d) fused to the floor of the mouth

   Hint

   Flicked out.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   The number of digits on the hindlimb of a frog is:
   (a) 3
   (b) 4
   (c) 5
   (d) 6

   Hint

   Webbed.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the membrane that protects the frog's eyes under water.

   Hint

   Transparent.

   Answer

   Nictitating membrane

4. Q4 · 1 mark · Very short answerExam level

   What is the cloaca?

   Hint

   Common chamber.

   Answer

   A small median chamber at the end of the gut into which the rectum, ureters (urinogenital ducts) and oviducts open; faeces, urine and gametes leave through it.

5. Q5 · 2 marks · Short answerExam level

   Why does a frog not need to drink water?

   Hint

   Skin.

   Answer

   It absorbs water through its thin, moist, permeable skin, which is kept slimy by mucus glands.

6. Q6 · 2 marks · Short answerHarder exam level

   Name the parts of the frog's forebrain, midbrain and hindbrain.

   Hint

   Lobes.

   Answer

   Forebrain: olfactory lobes, cerebral hemispheres, diencephalon. Midbrain: optic lobes. Hindbrain: cerebellum, medulla oblongata.

7. Q7 · 2 marks · Short answerHarder exam level

   Give the composition of frog blood. How do frog RBCs differ from human RBCs?

   Hint

   Nucleus.

   Answer

   Plasma with red blood cells, white blood cells and platelets. Frog RBCs are oval and nucleated; human RBCs are biconcave and lack a nucleus.

8. Q8 · 3 marks · Short answerHarder exam level

   Describe the female reproductive system of the frog.

   Hint

   Ovaries and oviducts.

   Answer

   A pair of ovaries near the kidneys with no functional connection to them; a pair of coiled oviducts arising near the ovaries and opening separately into the cloaca. A mature female lays about 2500–3000 eggs at a time; fertilisation is external.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe the excretory system of the frog.

   Hint

   Kidneys to cloaca.

   Answer

   A pair of compact, dark red kidneys beside the vertebral column with many uriniferous tubules; ureters (urinogenital ducts in males) leading to the cloaca; a thin-walled urinary bladder ventral to the rectum opening into the cloaca. Waste is excreted mainly as urea (ureotelic).

10. Q10 · 3 marks · Short answerHarder exam level

    List four adaptations of the frog for life in water and on land.

    Hint

    Feet, skin, eyes, lungs.

    Answer

    Webbed hind feet for swimming; nictitating membrane protecting eyes in water; moist skin for gas exchange and water absorption; lungs for breathing air on land; strong hindlimbs for leaping; camouflage colouring.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Describe the circulatory system of the frog. [3]
    (b) What is a portal system? Name two in the frog. [2]

    Hint

    Closed, three chambers.

    Answer

    (a) Closed system with a lymphatic system; three-chambered heart (two atria, one ventricle) in pericardium; sinus venosus receives venae cavae and opens into right atrium; left atrium receives pulmonary veins; ventricle sends blood through conus arteriosus to arteries; veins return blood. Blood: plasma, nucleated RBCs with haemoglobin, WBCs, platelets. (b) A vein that carries blood from one organ to another organ (through capillaries) before it returns to the heart: hepatic portal (intestine to liver) and renal portal (lower body to kidneys).

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Draw a neat, labelled diagram of the digestive system of a frog, showing the associated glands. [3]
    (b) Name the two digestive juices that act on food in the duodenum, give the source of each and state one function of each. [2]

    Hint

    Liver, gall bladder and pancreas all connect to the duodenum.

    Answer

    (a) Diagram (ventral view): oesophagus, curved stomach, duodenum, coiled intestine, wider rectum and cloaca; a lobed liver with the gall bladder beneath it and the bile duct opening into the duodenum; the pancreas lying in the loop between the stomach and duodenum. (b) Bile, made by the liver and stored in the gall bladder, emulsifies fats; pancreatic juice, from the pancreas, digests carbohydrates and proteins.

13. Q13 · 3 marks · Short answerHarder exam level

    Compare the frog's three-chambered heart with a four-chambered heart in terms of blood mixing. How does the frog manage despite mixing?

    Hint

    One ventricle.

    Answer

    With one ventricle, oxygenated blood from the left atrium and deoxygenated blood from the right atrium can mix partly, so blood leaving the heart is not fully oxygenated. A four-chambered heart keeps them fully separate. The frog manages because it is cold-blooded with a low energy need, and its skin adds oxygen directly to blood.

14. Q14 · 3 marks · Short answerHOTS / competency

    A frog kept in a dry glass jar becomes sluggish and dies sooner than one kept on wet moss, even though both can breathe air. Explain.

    Hint

    Moist skin.

    Answer

    In dry air the frog's thin skin loses water fast; it cannot replace it by drinking and absorbs water only through skin. A dry skin also cannot carry out cutaneous respiration, which supplies a large share of its oxygen. On wet moss the skin stays moist, so water balance and skin breathing continue.

15. Q15 · 3 marks · Short answerHOTS / competency

    Why is the frog chosen as a representative to study vertebrate organ systems, and what does its tadpole stage tell us about its ancestry?

    Hint

    Amphibian link.

    Answer

    The frog shows all major vertebrate organ systems in a simple, easily studied form. Its tadpole is aquatic, has gills and a tail, and excretes ammonia, resembling fishes; the adult has lungs and limbs. This change during metamorphosis reflects the transition of vertebrates from water to land.

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.Body regions of a frog?Show answer

   Head and trunk; no neck or tail.

2. 2.Summer sleep?Show answer

   Aestivation.

3. 3.Teeth on the roof of the mouth?Show answer

   Vomerine teeth.

4. 4.Chamber collecting blood from the venae cavae?Show answer

   Sinus venosus.

5. 5.Nitrogenous waste of adult frog?Show answer

   Urea.

6. 6.Number of cranial nerves?Show answer

   Ten pairs.

7. 7.Canal in the kidney receiving vasa efferentia?Show answer

   Bidder's canal.

8. 8.Larva of frog?Show answer

   Tadpole.

Quick revision checklist

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

• ☐Can you name five external features with their uses?
• ☐Can you give two differences between male and female frogs?
• ☐Can you trace food from mouth to cloaca?
• ☐Can you name the three modes of respiration?
• ☐Can you describe the three-chambered heart?
• ☐Can you explain why frogs are ureotelic?
• ☐Can you trace sperm from testis to cloaca?
• ☐Can you name the parts of the frog's brain?

Structural Organisation in Animals: Frog: frequently asked questions