Structural Organisation in Animals – Class 11 Biology Chapter 7 Notes & NCERT PDF

Class 11BiologyChapter 7NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 7 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 7: Structural Organisation in Animals

Introduction

In unicellular organisms one cell does everything; in multicellular animals, groups of cells share the work. A group of similar cells with intercellular substances performing a specific function is a tissue. All complex animals have only four basic tissue types – epithelial, connective, muscular and neural – organised into organs (stomach, lung, heart, kidney). Two or more organs working together form an organ system (digestive, respiratory, etc.). This division of labour helps the whole body survive. The chapter then studies the morphology (external form) and anatomy (internal organs) of the frog, representing vertebrates.

Key Concepts

7.1 Organ and Organ System

Tissues → organs → organ systems; this organisation gives more efficient, better-coordinated activity of millions of cells.

Example: the heart contains all four tissue types.

Increasing complexity of organs shows an evolutionary trend (detailed in Class XII).

Morphology = form or externally visible features; in animals, the external appearance of organs/parts. Anatomy = study of internal organs.

7.2 Frogs

Live on land and in freshwater; class Amphibia, phylum Chordata. Common Indian species: *Rana tigrina*.

Cold blooded (poikilotherms) – body temperature varies with the environment.

They change colour to hide from enemies (camouflage); the book calls this protective coloration mimicry.

Aestivation = summer sleep; hibernation = winter sleep (shelter in deep burrows).

#### 7.2.1 Morphology

Skin smooth and slippery due to mucus, always moist. Dorsal side olive green with dark irregular spots; ventral side uniformly pale yellow.

Frog never drinks water – absorbs it through the skin.

Body = head and trunk; neck and tail absent.

A pair of nostrils above the mouth; bulged eyes covered by a nictitating membrane (protection in water); a membranous tympanum (ear) on either side receives sound.

Hind limbs: five digits, larger and muscular; fore limbs: four digits. Webbed digits help swimming.

Sexual dimorphism: males have vocal sacs and a copulatory pad on the first digit of fore limbs.

#### 7.2.2 Anatomy

Digestive system

Alimentary canal is short because frogs are carnivores.

Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca.

Bilobed tongue captures food. Liver secretes bile (stored in gall bladder); pancreas makes pancreatic juice.

Stomach walls secrete HCl and gastric juices; partly digested food = chyme → duodenum, which receives bile and pancreatic juice through a common bile duct. Bile emulsifies fat; pancreatic juice digests carbohydrates and proteins.

Absorption by finger-like villi and microvilli; waste exits through cloaca.

Respiration

In water: cutaneous respiration (through skin, by diffusion).

On land: buccal cavity, skin and lungs; lung breathing = pulmonary respiration. Lungs are a pair of elongated pink sacs in the thorax.

During aestivation and hibernation, gas exchange is through the skin.

Circulation

Closed vascular system; also a lymphatic system (lymph, lymph channels, lymph nodes).

Heart: three chambers – two atria and one ventricle – covered by pericardium. Sinus venosus joins the right atrium and receives blood from vena cava; the ventricle opens into conus arteriosus.

Special venous connections: hepatic portal system (liver–intestine) and renal portal system (kidney–lower body).

Blood = plasma + RBCs (nucleated, with haemoglobin), WBCs and platelets. Lymph lacks a few proteins and RBCs.

Excretion

Pair of kidneys, ureters, cloaca and urinary bladder. Kidneys are compact, dark red, bean-like; units = uriniferous tubules / nephrons.

In males, ureters act as urinogenital ducts; in females, ureters and oviducts open separately into the cloaca.

Frog excretes urea → ureotelic.

Control and coordination

Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, gonads.

Nervous system: CNS (brain, spinal cord), PNS (cranial and spinal nerves), autonomic (sympathetic and parasympathetic). Ten pairs of cranial nerves.

Brain in cranium: fore-brain (olfactory lobes, cerebral hemispheres, diencephalon), mid-brain (pair of optic lobes), hind-brain (cerebellum, medulla oblongata). Medulla passes through foramen magnum into the spinal cord.

Sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), hearing (tympanum with internal ears). Eyes are simple eyes; external ear absent; ear serves hearing and balance.

Reproduction

Male: pair of yellowish ovoid testes attached to kidneys by mesorchium; 10–12 vasa efferentia enter the kidneys and open into Bidder's canal, then the urinogenital duct → cloaca.

Female: pair of ovaries (no functional connection with kidneys); oviducts open separately into cloaca. A female lays 2500–3000 ova at a time.

External fertilisation in water; larval stage tadpole undergoes metamorphosis.

Frogs eat insects (protect crops), are a link in food chains, and their legs are eaten in some countries.

Diagrams to Know

External features of frog: head, trunk, eye, nostril, tympanum, fore limb (4 digits), hind limb (5 webbed digits).

Internal organs / digestive system: oesophagus, stomach, intestine, rectum, cloaca, liver, gall bladder, lungs, heart, kidneys, fat bodies, urinary bladder.

Male reproductive system: testes, vasa efferentia, kidney, urinogenital duct, cloaca.

Female reproductive system: ovaries with ova, oviducts, ureter, cloaca.

Key-Facts Sheet

Rana tigrina, class Amphibia; cold blooded; aestivation and hibernation.

No neck, no tail; nictitating membrane; tympanum; no external ear.

Heart three-chambered (2 atria, 1 ventricle); single circulation (summary); nucleated RBCs.

Hepatic portal and renal portal systems present.

Ureotelic; 10 pairs of cranial nerves; optic lobes in mid-brain.

Male: vocal sacs, copulatory pad; 10–12 vasa efferentia; Bidder's canal.

2500–3000 ova; external fertilisation; tadpole → metamorphosis.

Exam Focus (from exercises)

Draw a neat diagram of the frog's digestive system.

Function of ureters in male frogs (urinogenital ducts carrying urine and sperms to cloaca).

Differences between male and female frogs; frog's modes of respiration.

Key Terms

Tissue (ऊतक), Organ system (अंग तंत्र), Morphology (आकारिकी), Anatomy (शारीरिकी), Poikilotherm (असमतापी), Aestivation (ग्रीष्म निष्क्रियता), Hibernation (शीत निष्क्रियता), Nictitating membrane (निमेषक पटल), Tympanum (कर्णपटह), Cloaca (अवस्कर), Sexual dimorphism (लैंगिक द्विरूपता), Cutaneous respiration (त्वचीय श्वसन), Ureotelic (यूरिया उत्सर्जी), Metamorphosis (कायांतरण), Tadpole (टैडपोल)

Common Mistakes

Saying frogs drink water – they absorb it through the skin.

Giving the frog heart four chambers – it has three.

Writing that fore limbs have five digits – four on fore limbs, five on hind limbs.

Forgetting that frog RBCs are nucleated.

Confusing aestivation (summer) with hibernation (winter).

💡 Key Learning Points

✓Cells form tissues, tissues form organs, and organs form organ systems, showing division of labour.
✓All complex animals have only four basic tissues: epithelial, connective, muscular and neural.
✓The frog (Rana tigrina) is a cold-blooded amphibian showing camouflage, aestivation and hibernation.
✓Frog anatomy includes a short gut ending in a cloaca, cutaneous and pulmonary respiration, a three-chambered heart and ureotelic excretion.
✓Frogs show sexual dimorphism, external fertilisation and development through a tadpole stage with metamorphosis.

👨‍🏫 Teaching Tips

→Use a labelled model or chart of the frog instead of dissection, as many schools no longer dissect.
→Build a flow chart of food movement from mouth to cloaca and have students trace it aloud.
→Compare frog and human systems in a two-column list (heart chambers, RBCs, excretion product).
→Show videos of frog vocal sacs and tadpole metamorphosis to connect morphology with behaviour.
→Ask students to link each structure to function (e.g., nictitating membrane → eye protection in water).
→Run a quick 'male or female frog?' identification round using the book's features.

📋 Assessment Questions

Can the student explain the levels of organisation from cell to organ system?
Can the student describe the frog's modes of respiration?
Can the student draw and label the frog's digestive system?
Can the student list differences between male and female frogs?
Can the student describe the frog's heart and blood?

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