NCERT Solutions for Class 11 Biology Chapter 7: Structural Organisation in Animals

Chapter at a glance — complete revision notes

Introduction

• In unicellular organisms all functions — digestion, respiration, reproduction — are performed by a single cell. In the complex body of multicellular animals the same basic functions are carried out by different groups of cells in a well-organised manner; Hydra has different cell types in thousands, while the human body has billions of cells.
• A tissue = a group of similar cells along with intercellular substances performing a specific function. All complex animals consist of only four basic types of tissues (epithelial, connective, muscular, neural), organised in specific proportion and pattern to form an organ (stomach, lung, heart, kidney). When two or more organs perform a common function by physical and/or chemical interaction they form an organ system (digestive, respiratory, …). Cells, tissues, organs and organ systems show division of labour and contribute to survival of the body as a whole.

7.1 Organ and Organ System

• Basic tissues organise into organs, which associate into organ systems — essential for more efficient and better coordinated activities of millions of cells. Each organ is made of one or more tissues; for example the heart consists of all four tissue types — epithelial, connective, muscular and neural.
• The complexity of organs and organ systems shows a discernible evolutionary trend (studied in Class XII). In this chapter the morphology and anatomy of the frog represent vertebrates. Morphology = study of form or externally visible features; anatomy (conventionally) = study of morphology of internal organs.

7.2 Frogs

• Frogs live on land and in fresh water and belong to class Amphibia of phylum Chordata. The most common species in India is Rana tigrina (Indian bullfrog).
• Frogs have no constant body temperature — they are cold-blooded (poikilotherms). They can change colour to hide from enemies (camouflage; this protective colouration is called mimicry). During peak summer and winter they shelter in deep burrows: summer sleep = aestivation, winter sleep = hibernation.

7.2.1 Morphology

• Skin is smooth and slippery due to mucus and always moist; the dorsal side is generally olive green with dark irregular spots; the ventral side is uniformly pale yellow. The frog never drinks water but absorbs it through the skin.
• The body is divisible into head and trunk (Fig. 7.1); neck and tail are absent. Above the mouth is a pair of nostrils. Eyes are bulged and covered by a nictitating membrane that protects them in water. On either side of the eyes is a membranous tympanum (ear) which receives sound signals.
• Forelimbs and hind limbs help in swimming, walking, leaping and burrowing. Hind limbs end in five digits, are larger and more muscular than forelimbs (four digits); feet have webbed digits that help in swimming.
• Sexual dimorphism: males have sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs, absent in females.

7.2.2 Anatomy

• The body cavity holds the digestive, circulatory, respiratory, nervous, excretory and reproductive systems (Fig. 7.2).

Digestive system

• Alimentary canal and digestive glands. The canal is short because frogs are carnivores (intestine length reduced). Path: mouth → buccal cavity → pharynx → oesophagus (short tube) → stomach → intestine → rectum → cloaca (opens outside by the cloacal aperture).
• Liver secretes bile stored in the gall bladder; pancreas produces pancreatic juice with digestive enzymes. Food is captured by the bilobed tongue. In the stomach digestion is by HCl and gastric juices; partially digested food, chyme, passes to the first part of the small intestine, the duodenum, which receives bile and pancreatic juice through a common bile duct. Bile emulsifies fat; pancreatic juice digests carbohydrates and proteins; final digestion takes place in the intestine. Digested food is absorbed by finger-like folds, villi and microvilli; undigested solid waste goes to the rectum and out through the cloaca.

Respiratory system

• In water the skin is the aquatic respiratory organ (cutaneous respiration) — dissolved oxygen is exchanged by diffusion. On land the buccal cavity, skin and lungs act as respiratory organs; respiration by lungs = pulmonary respiration. Lungs are a pair of elongated, pink, sac-like structures in the upper part of the trunk (thorax); air enters through nostrils into the buccal cavity and then to the lungs. During aestivation and hibernation gaseous exchange occurs through the skin.

Circulatory system

• Well-developed closed blood vascular system plus a lymphatic system (lymph, lymph channels, lymph nodes). The heart is a muscular, three-chambered organ (two atria and one ventricle) covered by the pericardium, in the upper body cavity. A triangular sinus venosus joins the right atrium and receives blood through the vena cava; the ventricle opens into a sac-like conus arteriosus on the ventral side.
• Arteries carry blood to all parts (arterial system); veins collect blood to the heart (venous system). Special venous connections: hepatic portal system (liver–intestine) and renal portal system (kidney–lower parts of the body). Blood = plasma + cells: RBCs (erythrocytes) – nucleated, with haemoglobin; WBCs (leucocytes); platelets. Lymph differs from blood — it lacks some proteins and RBCs. Blood carries nutrients, gases and water; circulation is by the pumping of the heart.

Excretory system

• Excretion of nitrogenous wastes by a pair of kidneys, ureters, cloaca and urinary bladder. Kidneys are compact, dark red, bean-like, lying slightly posteriorly in the body cavity on both sides of the vertebral column; each is made of structural and functional units, the uriniferous tubules (nephrons).
• In male frogs two ureters emerge from the kidneys 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 opens into the cloaca. The frog excretes urea — a ureotelic animal. Wastes are carried by blood to the kidneys where they are separated and excreted.

Control and coordination

• Highly evolved, including both neural system and endocrine glands. Hormones are secreted by the endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
• Nervous system: central (brain and spinal cord), peripheral (cranial and spinal nerves) and autonomic (sympathetic and parasympathetic). Ten pairs of cranial nerves. Brain enclosed in the bony cranium (brain box); divided into fore-brain (olfactory lobes, paired cerebral hemispheres, unpaired diencephalon), mid-brain (pair of optic lobes) and hind-brain (cerebellum and medulla oblongata). The medulla oblongata passes through the foramen magnum and continues as the spinal cord within the vertebral column.

Sense organs

• Organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), hearing (tympanum with internal ears). Eyes and internal ears are well-organised; the others are cellular aggregations around nerve endings. Eyes are a pair of spherical simple eyes (single unit) in the orbits of the skull. External ear is absent; only the tympanum is visible; the ear is an organ of hearing and balancing (equilibrium).

Reproductive system

• Male: a pair of yellowish ovoid testes (Fig. 7.3) adhered to the upper part of the kidneys by a double fold of peritoneum, the mesorchium; 10–12 vasa efferentia arise from the testes, enter the kidneys and open into Bidder’s canal, which communicates with the urinogenital duct leaving the kidney and opening into the cloaca. The cloaca is a small median chamber used to pass faecal matter, urine and sperms to the exterior.
• Female: a pair of ovaries near the kidneys (no functional connection with the kidneys) (Fig. 7.4); a pair of oviducts arising from the ovaries open separately into the cloaca. A mature female lays 2500–3000 ova at a time. Fertilisation is external (in water); development involves a larval stage, the tadpole, which undergoes metamorphosis to the adult.
• Importance: frogs eat insects and protect crops; they are an important link in food chains and food webs (ecological balance); in some countries the muscular legs are eaten.

Summary (as in NCERT)

• A tissue is a group of cells with intercellular substances performing one or more functions. Epithelia are sheet-like tissues lining the body’s surface and its cavities, ducts and tubes; they have one free surface facing a body fluid or the outside environment and cells structurally and functionally connected at junctions.
• Rana tigrina: skin with mucous glands, highly vascularised, helps in respiration; head and trunk; muscular, bilobed tongue to capture prey; alimentary canal of oesophagus, stomach, intestine, rectum opening into the cloaca; liver and pancreas; skin respiration in water and lungs on land; closed circulation with single circulation; nucleated RBCs; central, peripheral and autonomic nervous system; urinogenital organs open into the cloaca; testes (male), ovaries (female); 2500–3000 ova; external fertilisation and development; tadpole → metamorphosis → frog.