This MCQ module is based on: Frog
TOPIC 29 OF 45
Frog
🎓 Class 11
Biology
CBSE
Theory
Ch 7 – Structural Organisation in Animals
⏱ ~14 min
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Frog
7.6 Frog — Rana tigrina
Frogs belong to class Amphibia of Phylum Chordata. Frogs are seen to live both on land and in freshwater (ponds, ditches). They are referred to as amphibians as they can live both in aquatic as well as on land. They are cold-blooded (poikilotherms), i.e., they do not have a body temperature regulation mechanism. Their body temperature varies with the temperature of the environment.
Frogs can change the colour to hide them from predators (camouflage). This is called mimicry. They are not seen during peak summer and winter. During this period they take shelter in deep burrows to protect them from extreme heat and cold. This is known as aestivation and hibernation, respectively.
7.6.1 Morphology
The skin is smooth, slippery due to the presence of mucus. Skin is always maintained in moist condition. The colour of the dorsal side of the body is generally olive green with dark irregular spots. On the ventral side the skin is uniformly pale yellow. The frog never drinks water but absorbs it through the skin. The body of the frog is divisible into head and trunk; no neck or tail. A pair of nostrils is present above the mouth. Eyes are bulged and covered by a nictitating membrane that protects them while in water. On either side of the eyes a membranous tympanum (ear drum) receives sound signals.
The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs end in five digits and forelimbs in four digits. Feet have webbed digits that help in swimming. Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of sound producing vocal sacs and also a copulatory pad on the first digit of the fore limbs which are absent in the female frogs.
7.6.2 Digestive System
The digestive system consists of alimentary canal and digestive glands. The alimentary canal is short because frogs are carnivores and hence the length of intestine is reduced. The mouth opens into the buccal cavity that leads to the oesophagus through pharynx. Oesophagus is a short tube that opens into the stomach which in turn continues as the intestine, rectum and finally opens outside by the cloacal aperture. Cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms or ova.
Liver secretes bile that is stored in the gall bladder. Pancreas, a digestive gland produces pancreatic juice containing digestive enzymes. Food is captured by the bilobed tongue. Partially digested food called chyme is passed from stomach to the first part of the intestine, the duodenum. The duodenum receives bile from gall bladder and pancreatic juices from the pancreas through a common bile duct. The bile emulsifies fat and pancreatic juices digest carbohydrates and proteins. Final digestion takes place in the intestine. Digested food is absorbed by the numerous finger-like folds in the inner wall of intestine called villi and microvilli. The undigested solid waste moves into the rectum and passes out through cloaca.
7.6.3 Respiratory System
Frogs respire on land and in the water by two different methods. In water, skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion. On land, the buccal cavity, skin and lungs act as the respiratory organs.
The lungs are a pair of elongated, pink-coloured sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to lungs. During aestivation and hibernation gaseous exchange takes place through the skin.
7.6.4 Circulatory System
Frog has a well-developed closed circulatory system with heart, blood vessels and blood. Since frogs also have lymph and lymph channels, the lymphatic system is also present. The heart is a muscular structure situated in the upper part of the body cavity. It has three chambers, two atria and one ventricle and is covered by a membrane called pericardium. A triangular structure called sinus venosus joins the right atrium. It receives blood through the major veins called vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart. The blood from the heart is carried to all parts of the body by the arteries (arterial system). The veins collect blood from different parts of body to the heart and form the venous system. Special venous connections between liver and intestine as well as the kidney and lower parts of the body are present in frogs. The former is called hepatic portal system and the latter is called renal portal system.
The blood is composed of plasma and cells. The cells are RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes and platelets. RBC's are nucleated and contain red coloured pigment namely haemoglobin. The lymph is different from blood. It lacks few proteins and RBCs. The blood carries nutrients, gases and water to the respective sites during the circulation.
7.6.5 Excretory System
The excretory system consists of a pair of kidneys, ureters, cloaca and urinary bladder. These are compact, dark red and bean-like structures situated a little posteriorly in the body cavity on both sides of vertebral column. Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons. Two ureters emerge from the kidneys in the male frogs. The ureters act as urinogenital duct which opens into the cloaca. In females the ureters and oviduct open separately in the cloaca. The urinary bladder is also present which opens in the cloaca. The frog excretes urea and thus is an ureotelic animal. The excretory wastes are carried by blood into the kidney where it is separated and excreted.
7.6.6 Nervous and Endocrine System
The system for control and coordination is highly evolved in the frog. It includes both neural system and endocrine glands. The chemical coordination of various organs of the body is achieved by hormones which are secreted by the endocrine glands. The prominent endocrine glands found in frog are pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
The nervous system is organised into a central nervous system (CNS) — brain and spinal cord, a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic). There are ten pairs of cranial nerves arising from the brain. Brain is enclosed in a bony structure called brain box (cranium). The brain is divided into fore-brain, mid-brain and hind-brain. Forebrain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon. The midbrain is characterised by a pair of optic lobes. Hindbrain consists of cerebellum and medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
Frog has different types of sense organs, namely organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Out of these, eyes and internal ears are well-organised structures and the rest are cellular aggregations around nerve endings.
7.6.7 Reproductive System
The male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia are 10-12 in number that arise from testes. They enter the kidneys on their side and open into Bidder's canal. Finally it communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca. The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms.
The female reproductive organs include a pair of ovaries. The ovaries are situated near kidneys and there is no functional connection with kidneys. A pair of oviduct arising from the ovaries opens into the cloaca separately. A mature female can lay 2500 to 3000 ova at a time. Fertilisation is external and takes place in water. Development involves a larval stage called tadpole. The tadpole undergoes metamorphosis that forms the adult.
| System | Key feature in frog |
|---|---|
| Skin | Smooth, moist, mucus-coated; cutaneous respiration; pigment cells for camouflage |
| Digestive | Bilobed sticky tongue, short carnivore gut, cloaca |
| Respiratory | Buccal cavity + skin + lungs (3 modes; skin only during hibernation/aestivation) |
| Circulatory | 3-chambered heart (2 atria + 1 ventricle); sinus venosus, conus arteriosus; hepatic and renal portal systems |
| Excretory | Pair of bean-shaped kidneys; nephrons; ureotelic; cloaca |
| Nervous | 10 pairs of cranial nerves; brain = forebrain + midbrain + hindbrain |
| Reproductive | Sexual dimorphism (♂ vocal sacs, copulatory pads); external fertilisation; tadpole larva |
7.6.8 Economic Importance of Frogs
Frogs are beneficial:
- They eat insects and protect the crops; thus they help in maintaining ecological balance.
- In some countries, the muscular legs of frogs are used as food by man.
- Frogs are widely used as an experimental specimen in biological research and dissection laboratories.
Interactive: Compare the Three Model Animals
Choose a body system and see how it differs across earthworm, cockroach, and frog.
| Earthworm | Cockroach | Frog |
|---|---|---|
| — | — | — |
Activity 7.4 — Compare Frog Skin Wet vs Dry
Setup: Two filter-paper-lined Petri dishes; a frog model (or a preserved frog under teacher supervision). One paper is moistened with water; the other left dry.
Predict: Which of the two dishes will the frog 'prefer'? Why?
My prediction: …
- If the frog is alive, ethically observe — do NOT touch with bare hands (skin absorbs chemicals).
- Place the frog briefly on the dry paper, then on the moist paper. Note its behaviour.
- Look at the moist paper after 5 minutes — what wet patches/secretions are visible?
- Why does the frog seem more 'at home' on the moist paper?
Observation: The frog stays on the moist paper. Reason: cutaneous respiration requires the skin to stay wet so dissolved oxygen can diffuse into the underlying capillaries. On dry paper, the mucus dries and gas exchange is severely impaired. Also, the frog absorbs water through skin (it doesn't 'drink'), so it dehydrates fast on a dry surface. This is why frogs are tied to damp habitats — pond margins, rainy forests, garden pots after monsoon.
7.7 Worked Examples
Worked Example 1: Three respiratory modes
Why does a frog need three respiratory surfaces (skin, buccal cavity, lungs)?
Each surface is best at a different time/place:
- Skin (cutaneous): works underwater AND during hibernation — even when the lungs are inactive. The only mode when the frog is fully submerged in cold ponds for months.
- Buccal cavity: A thin mucous-lined cavity with capillaries beneath; effective for short bursts of breathing on land when the frog is resting.
- Lungs: Best during active phases on land — leaping, calling, escaping a predator — when oxygen demand is high.
Multiple modes give the frog great metabolic flexibility — a key reason amphibians can move between water and land.
- Skin (cutaneous): works underwater AND during hibernation — even when the lungs are inactive. The only mode when the frog is fully submerged in cold ponds for months.
- Buccal cavity: A thin mucous-lined cavity with capillaries beneath; effective for short bursts of breathing on land when the frog is resting.
- Lungs: Best during active phases on land — leaping, calling, escaping a predator — when oxygen demand is high.
Multiple modes give the frog great metabolic flexibility — a key reason amphibians can move between water and land.
Worked Example 2: 3-chambered heart efficiency
Birds and mammals have a 4-chambered heart that completely separates oxygenated and deoxygenated blood. Frog has only 3 chambers and some mixing occurs in the ventricle. Why doesn't this hurt the frog?
Frogs are poikilotherms (cold-blooded) with low metabolic rates. They don't need to deliver oxygen at the rapid, high-purity rate that warm-blooded animals do. So a small amount of mixing in the ventricle is not a fatal cost.
Also, the ventricle has internal ridges that partially separate blood streams — the right-side (deoxygenated) blood preferentially flows to the pulmocutaneous artery (to lungs/skin) and the left-side (oxygenated) blood preferentially flows to the systemic arteries (to body). The conus arteriosus also has a spiral valve that helps direct the streams. So in practice, frogs achieve much better separation than the simple 3-chambered design suggests.
Also, the ventricle has internal ridges that partially separate blood streams — the right-side (deoxygenated) blood preferentially flows to the pulmocutaneous artery (to lungs/skin) and the left-side (oxygenated) blood preferentially flows to the systemic arteries (to body). The conus arteriosus also has a spiral valve that helps direct the streams. So in practice, frogs achieve much better separation than the simple 3-chambered design suggests.
Worked Example 3: Why are frogs declining globally?
Frog populations are declining worldwide. Suggest three reasons grounded in frog biology.
1. Permeable skin — frogs absorb water and oxygen through skin, so they also absorb pollutants (pesticides, heavy metals) very efficiently. They are 'bio-magnifiers' of contamination.
2. Aquatic-larva requirement — tadpoles need clean ponds with no fish predators. Habitat drying, pollution, urbanisation destroy breeding sites.
3. Climate sensitivity — poikilotherms depend on environmental temperature. A warming climate disrupts the timing of hibernation, breeding, and metamorphosis.
4. (Bonus) Chytrid fungus (Batrachochytrium) kills frogs by infecting the skin and disrupting electrolyte balance — has wiped out hundreds of species.
This is why amphibian conservation is a priority globally.
2. Aquatic-larva requirement — tadpoles need clean ponds with no fish predators. Habitat drying, pollution, urbanisation destroy breeding sites.
3. Climate sensitivity — poikilotherms depend on environmental temperature. A warming climate disrupts the timing of hibernation, breeding, and metamorphosis.
4. (Bonus) Chytrid fungus (Batrachochytrium) kills frogs by infecting the skin and disrupting electrolyte balance — has wiped out hundreds of species.
This is why amphibian conservation is a priority globally.
Competency-Based Questions
Q1. The heart of frog has how many chambers? L1 Remember
Answer: (b) Three. Two atria and one ventricle. The sinus venosus and conus arteriosus are accessory chambers but not counted as 'heart chambers'.
Q2. Which structure helps the male frog produce mating calls? L2 Understand
Vocal sacs. These are membranous pouches on either side of the lower jaw that fill with air and resonate when the frog calls. Females lack vocal sacs; this is an example of sexual dimorphism.
Q3. Apply: A frog kept in a sealed jar with limited air survives longer than a small bird in the same jar. Explain biologically. L3 Apply
The frog is poikilothermic with a low metabolic rate; it needs much less oxygen per unit time than a bird. Birds are warm-blooded with very high metabolism (constantly maintaining body temperature, flying muscles). Additionally, the frog can rely partly on cutaneous respiration if the air gets low, which the bird cannot. So the same jar of air lasts the frog much longer.
Q4. Analyse: List three differences between the frog circulatory system and the cockroach circulatory system. L4 Analyse
1. Type: Frog has a closed system (blood stays in vessels); cockroach has an open system (haemolymph in haemocoel).
2. Heart structure: Frog has a 3-chambered muscular heart; cockroach has a long tubular heart with 13 chambers, each with a pair of ostia.
3. Oxygen transport: Frog blood carries oxygen via haemoglobin (in nucleated RBCs); cockroach haemolymph does NOT carry oxygen — tracheae handle it directly.
4. Portal systems: Frog has hepatic and renal portal systems for routing nutrient/waste-rich blood; cockroach has none.
2. Heart structure: Frog has a 3-chambered muscular heart; cockroach has a long tubular heart with 13 chambers, each with a pair of ostia.
3. Oxygen transport: Frog blood carries oxygen via haemoglobin (in nucleated RBCs); cockroach haemolymph does NOT carry oxygen — tracheae handle it directly.
4. Portal systems: Frog has hepatic and renal portal systems for routing nutrient/waste-rich blood; cockroach has none.
Q5. Create: A new species of amphibian is discovered in a desert with very little water. Suggest three anatomical adaptations it might show compared to a pond frog. L6 Create
1. Less permeable skin — with a waxy or keratinised layer to reduce water loss; cutaneous respiration would be sacrificed for water retention.
2. Larger lungs and stronger reliance on pulmonary respiration — to compensate for reduced skin respiration.
3. Long aestivation in mud burrows — with a mucus-cocoon that traps moisture for months/years.
4. Direct development (no tadpole stage) — to skip the water-dependent larval stage, e.g., by laying eggs in moist soil that hatch into tiny froglets.
5. Concentrated urine — perhaps switching from ureotelic to uricotelic to save water.
Such adaptations are actually found in the spadefoot toad (Scaphiopus) of American deserts!
2. Larger lungs and stronger reliance on pulmonary respiration — to compensate for reduced skin respiration.
3. Long aestivation in mud burrows — with a mucus-cocoon that traps moisture for months/years.
4. Direct development (no tadpole stage) — to skip the water-dependent larval stage, e.g., by laying eggs in moist soil that hatch into tiny froglets.
5. Concentrated urine — perhaps switching from ureotelic to uricotelic to save water.
Such adaptations are actually found in the spadefoot toad (Scaphiopus) of American deserts!
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Frog respires through its skin during hibernation.
R: Skin of frog is moist and well supplied with blood capillaries.
Answer: (A). Both true; R explains A. Moisture allows gases to dissolve; capillaries beneath the skin allow the dissolved gases to enter the blood.
A: Frog is regarded as a cold-blooded animal.
R: The body temperature of frog is always less than the surrounding temperature.
Answer: (C). Assertion is true. But Reason is false — being cold-blooded means body temperature matches the surroundings (poikilothermic), not that it is always lower. A frog basking in the sun can be warmer than the air!
A: The blood from the right atrium of frog enters the same ventricle as the blood from the left atrium.
R: Frogs have only one ventricle.
Answer: (A). Both true; R explains A. With a single ventricle, both atria deliver their blood into the same chamber, where some mixing occurs.
Frequently Asked Questions - Frog
What is the main concept covered in Frog?
In NCERT Class 11 Biology Chapter 7 (Structural Organisation in Animals), "Frog" covers the core biological structures, functions, and classifications students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and ecological/physiological significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Frog useful in real-life or applied biology?
Real-life applications of "Frog" from NCERT Class 11 Biology Chapter 7 include medical diagnostics, agriculture, food preservation, biotechnology, ecological monitoring, and public health. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems, not just textbook content.
What are the key terms students should memorize for Frog?
Key terms in "Frog" (NCERT Class 11 Biology Chapter 7 Structural Organisation in Animals) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of Chapter 7?
NCERT Class 11 Biology Chapter 7 (Structural Organisation in Animals) is structured so each part builds biological understanding sequentially. "Frog" connects to neighbouring parts via shared classifications, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Frog?
CBSE board questions from "Frog" typically include: (1) 1-mark MCQs on definitions and classification, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining structure + function + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Frog" lesson allows students to explore biological structures, classifications, or processes using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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