Structural Organisation in Animals | CBSE Class 11 Biology Notes

This note covers structural organisation in animals: how cells, tissues, organs and organ systems share the work of the body, the four basic tissues and epithelia, the meaning of morphology and anatomy, and the frog as a representative vertebrate. For the frog it covers habitat and seasonal habits, external features and sexual dimorphism, and the digestive, respiratory, circulatory, excretory, neural, endocrine, sensory and reproductive systems, development and importance to humans.

What does structural organisation mean in animals?

In unicellular organisms, all functions like digestion, respiration and 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.

The body of a simple organism like Hydra is made of different types of cells, and the number of cells in each type can be in thousands. The human body is composed of billions of cells that perform various functions.

How do these cells work together? In multicellular animals, a group of similar cells along with intercellular substances performs a specific function. Such an organisation is called a tissue.

Definition: A tissue is a group of similar cells along with intercellular substances that performs a specific function in a multicellular animal.

All complex animals consist of only four basic types of tissues. These tissues are organised in a specific proportion and pattern to form an organ, such as the stomach, lung, heart and kidney.

When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system, for example the digestive system and the respiratory system.

The levels of organisation

1. Cells: in multicellular animals, different groups of cells carry out the basic functions of the body in a well organised manner.
2. Tissues: a group of similar cells along with intercellular substances performs a specific function.
3. Organs: tissues are organised in a specific proportion and pattern to form organs such as the stomach, lung, heart and kidney.
4. Organ systems: two or more organs perform a common function by physical and/or chemical interaction, as in the digestive and respiratory systems.

Cells, tissues, organs and organ systems split up the work in a way that exhibits division of labour. Together they contribute to the survival of the body as a whole.

How do tissues build organs, and what are morphology and anatomy?

The basic tissues organise to form organs, which in turn associate to form organ systems in multicellular organisms. Such an organisation is essential for more efficient and better coordinated activities of the millions of cells that constitute an organism.

Each organ in our body is made of one or more types of tissues. For example, our heart consists of all the four types of tissues: epithelial, connective, muscular and neural.

Epithelia

Epithelia are sheet-like tissues lining the body's surface and its cavities, ducts and tubes. Epithelia have one free surface facing a body fluid or the outside environment.

The cells of an epithelium are structurally and functionally connected at junctions.

The evolutionary trend

After some careful study, the complexity in organs and organ systems is seen to display a certain discernible trend. This discernible trend is called the evolutionary trend.

Morphology and anatomy

The frog is used to represent the vertebrates in learning morphology and anatomy. Its morphology covers its external features, and its anatomy covers the internal organs and organ systems in its body cavity.

Where do frogs live, and how do they cope with their surroundings?

Frogs can live both on land and in freshwater. They belong to class Amphibia of phylum Chordata. The most common species of frog found in India is Rana tigrina, the Indian bullfrog.

Body temperature

Frogs do not have a constant body temperature. Their body temperature varies with the temperature of the environment. Such animals are called cold blooded animals or poikilotherms.

Change of colour

Frogs show changes in colour while they are in grasses and on dry land. They have the ability to change colour to hide themselves from their enemies. This is camouflage, and this protective coloration is called mimicry.

Summer sleep and winter sleep

Frogs are not seen during peak summer and winter. During these periods they take shelter in deep burrows to protect themselves from extreme heat and cold.

Note: Aestivation is summer sleep and hibernation is winter sleep. During both, gaseous exchange in the frog takes place through the skin.

What are the external features of a frog?

The skin

The skin of a frog is smooth and slippery due to the presence of mucus. Mucous glands are present in the skin. The skin is always maintained in a moist condition.

The skin is highly vascularised and helps in respiration in water and on land. The frog never drinks water but absorbs it through the skin.

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.

Head, trunk and sense organs on the head

The body of a frog is divisible into a head and a trunk. A neck and a tail are absent. Above the mouth, a pair of nostrils is present.

The eyes are bulged and covered by a nictitating membrane that protects them while the frog is in water. On either side of the eyes, a membranous tympanum (ear) receives sound signals.

Limbs

The fore limbs and hind limbs help in swimming, walking, leaping and burrowing. The feet have webbed digits that help in swimming.

A green frog is drawn from the side in a sitting posture, with dark stripes on its back and pale spots on its limbs. Labels point to the head, the eye, the trunk, the fore limb and the hind limb. No tail is drawn.

See Fig. 7.1 in your NCERT textbook

Sexual dimorphism

Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of the fore limbs. Both are absent in female frogs.

How is the digestive system of the frog arranged?

The body cavity of the frog accommodates different organ systems, such as the digestive, circulatory, respiratory, nervous, excretory and reproductive systems, with well developed structures and functions.

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

The alimentary canal is short because frogs are carnivores, and hence the length of the intestine is reduced.

The path of the alimentary canal

1. The mouth opens into the buccal cavity.
2. The buccal cavity leads to the oesophagus through the pharynx.
3. The oesophagus is a short tube that opens into the stomach.
4. The stomach continues as the intestine.
5. The intestine leads into the rectum.
6. The rectum finally opens outside by the cloaca.

The digestive glands

The main digestive glands are the liver and the pancreas. The liver secretes bile, which is stored in the gall bladder. The pancreas, a digestive gland, produces pancreatic juice containing digestive enzymes.

The heart sits at the top centre over a large brown liver, with the green gall bladder at the lower edge of the liver. A pale lung lies on each side. The oesophagus is a narrow tube passing down behind the liver into a broad, curved stomach, which continues as a long, coiled intestine. The intestine leads into a wider rectum that joins the cloaca, which ends at the cloacal aperture. In the middle, yellow finger-like fat bodies sit above the two kidneys, and a ureter runs from each kidney to the cloaca. A bilobed urinary bladder is attached to the cloaca. Labels: heart, oesophagus, liver, gall bladder, lung, stomach, fat bodies, kidney, intestine, ureter, rectum, urinary bladder, cloaca and cloacal aperture.

See Fig. 7.2 in your NCERT textbook

Note: When drawing the digestive system of the frog, show the alimentary canal in order from the oesophagus to the cloacal aperture, and label the liver, which secretes bile, and the gall bladder, which stores it.

How does a frog capture, digest and absorb its food?

Food is captured by the bilobed tongue. The tongue is muscular and is bilobed at the tip, and it is used in capturing the prey.

The sequence of digestion

1. Digestion of food in the stomach takes place by the action of HCl and gastric juices secreted from the walls of the stomach.
2. The partially digested food, called chyme, is passed from the stomach to the first part of the small intestine, the duodenum.
3. The duodenum receives bile from the gall bladder and pancreatic juices from the pancreas through a common bile duct.
4. Bile emulsifies fat, and pancreatic juices digest carbohydrates and proteins.
5. Final digestion takes place in the intestine.
6. Digested food is absorbed by the numerous finger-like folds in the inner wall of the intestine, called villi and microvilli.
7. The undigested solid waste moves into the rectum and passes out through the cloaca.

Definition: Chyme is the partially digested food that is passed from the stomach to the duodenum, the first part of the small intestine.

How does a frog respire on land and in water?

Frogs respire on land and in water by two different methods.

Respiration in water

In water, the skin acts as the aquatic respiratory organ. This is cutaneous respiration. Dissolved oxygen in the water is exchanged through the skin by diffusion.

Respiration on land

On land, the buccal cavity, skin and lungs act as the respiratory organs. Respiration by the lungs is called pulmonary respiration.

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

Respiration during aestivation and hibernation

During aestivation and hibernation, gaseous exchange takes place through the skin.

Note: The skin is used in every situation: it is the respiratory organ in water, one of the three respiratory organs on land, and the site of gaseous exchange during aestivation and hibernation.

How is blood circulated in the frog?

The vascular system of the frog is a well-developed closed type, with single circulation. Frogs have a lymphatic system also.

• The blood vascular system involves the heart, blood vessels and blood.
• The lymphatic system consists of lymph, lymph channels and lymph nodes.

The heart

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 the pericardium.

How blood reaches and leaves the heart

1. The veins collect blood from different parts of the body to the heart; they form the venous system.
2. The major veins, called vena cava, bring blood into the sinus venosus.
3. The sinus venosus joins the right atrium.
4. The ventricle opens into the sac-like conus arteriosus on the ventral side of the heart.
5. The arteries carry blood from the heart to all parts of the body; they form the arterial system.

Portal systems

Frogs have special venous connections between the liver and the intestine, and between the kidney and the lower parts of the body.

Blood and lymph

The blood is composed of plasma and cells. The blood cells are RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes, and platelets.

RBCs are nucleated and contain a red coloured pigment, haemoglobin. The lymph is different from blood: it lacks a few proteins and RBCs.

The blood carries nutrients, gases and water to the respective sites during circulation. The circulation of blood is achieved by the pumping action of the muscular heart.

How does a frog eliminate nitrogenous wastes?

The elimination of nitrogenous wastes is carried out by a well developed excretory system. It consists of a pair of kidneys, ureters, the cloaca and the urinary bladder.

The kidneys

The kidneys are compact, dark red and bean-like structures. They are situated a little posteriorly in the body cavity, on both sides of the vertebral column.

Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons.

The ureters, urinary bladder and cloaca

In male frogs, two ureters emerge from the kidneys. The ureters act as a urinogenital duct, which opens into the cloaca. In females, the ureters and the oviduct open separately into the cloaca.

The thin-walled urinary bladder is present ventral to the rectum, and it also opens into the cloaca.

The frog is ureotelic

The frog excretes urea and thus is a ureotelic animal. Excretory wastes are carried by the blood into the kidney, where they are separated and excreted.

1. Excretory wastes are carried by the blood into the kidneys.
2. The wastes are separated in the kidney, which is made of uriniferous tubules or nephrons.
3. The ureters, which emerge from the kidneys, open into the cloaca.
4. The cloaca passes the urine to the exterior through the cloacal aperture.

How does the frog control and coordinate its body?

The system for control and coordination is highly evolved in the frog. It includes both the neural system and the endocrine glands.

Chemical coordination

The chemical coordination of the various organs of the body is achieved by hormones, which are secreted by the endocrine glands.

The prominent endocrine glands found in the frog are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

The nervous system

There are ten pairs of cranial nerves arising from the brain. The brain is enclosed in a bony structure called the brain box (cranium).

The brain and spinal cord

The brain is divided into the fore-brain, mid-brain and hind-brain. In order from the fore-brain to the spinal cord, the parts are as follows.

1. Fore-brain: includes the olfactory lobes, the paired cerebral hemispheres and the unpaired diencephalon.
2. Mid-brain: characterised by a pair of optic lobes.
3. Hind-brain: consists of the cerebellum and the medulla oblongata.
4. The medulla oblongata passes out through the foramen magnum and continues into the spinal cord, which is enclosed in the vertebral column.

What sense organs does a frog have?

The frog has different types of sense organs: organs of touch, taste, smell, vision and hearing.

Of these, the eyes and the internal ears are well-organised structures. The rest are cellular aggregations around nerve endings.

The eyes

The eyes in a frog are a pair of spherical structures situated in the orbit in the skull. These are simple eyes, possessing only one unit. The bulged eyes are covered by a nictitating membrane that protects them in water.

The ears

The external ear is absent in frogs, and only the tympanum can be seen externally. The ear is an organ of hearing as well as balancing (equilibrium).

Note: The tympanum is the membranous ear seen on either side of the eyes; it receives sound signals. Hearing needs the tympanum together with the internal ears.

How are the reproductive systems of the frog organised?

Frogs have well organised male and female reproductive systems.

The male reproductive system

The male reproductive organs consist of a pair of yellowish, ovoid testes. The testes are found adhered to the upper part of the kidneys by a double fold of peritoneum called the mesorchium.

1. The vasa efferentia, 10 to 12 in number, arise from the testes.
2. They enter the kidneys on their side and open into Bidder's canal.
3. Bidder's canal finally communicates with the urinogenital duct that comes out of the kidneys.
4. The urinogenital duct opens into the cloaca.
5. The cloaca, a small, median chamber, passes faecal matter, urine and sperms to the exterior.

Two kidneys are drawn side by side, each with white, finger-like fat bodies at its upper end. A pale yellow, oval testis lies at the upper part of one kidney, with fine vasa efferentia running from it into the kidney. A urinogenital duct runs from the lower end of each kidney to the cloaca. The rectum, a long tube, opens into the cloaca, and a small, pale yellow, speckled urinary bladder lies below it. Labels: vasa efferentia, testis, adrenal gland, fat bodies, kidney, urinogenital duct, rectum, urinary bladder, cloaca and cloacal aperture.

See Fig. 7.3 in your NCERT textbook

The female reproductive system

The female reproductive organs include a pair of ovaries. The ovaries are situated near the kidneys, and there is no functional connection with the kidneys.

A pair of oviducts arising from the ovaries opens into the cloaca separately. A mature female can lay 2,500 to 3,000 ova at a time.

Two kidneys lie in the middle, with a dark, granular ovary containing ova spread across and beside each one. A long, coiled oviduct runs down each side of the body from an open upper end to the cloaca. A ureter runs from each kidney to the cloaca. A lobed urinary bladder is attached to the cloaca, which ends at the cloacal aperture. Labels: oviduct, ovary, ova, ureter, cloaca, cloacal aperture and urinary bladder.

See Fig. 7.4 in your NCERT textbook

How does a frog develop, and why are frogs useful?

Fertilisation in the frog is external and takes place in water. Development is also external and involves a larval stage called the tadpole.

The life cycle

1. A mature female lays 2,500 to 3,000 ova at a time.
2. Fertilisation is external and takes place in water.
3. The eggs hatch into tadpoles, the larval stage.
4. The tadpole undergoes metamorphosis.
5. Metamorphosis forms the adult frog.

Definition: Metamorphosis is the change that the tadpole, the larval stage of the frog, undergoes to form the adult.

Importance of frogs

Frogs are beneficial for mankind in several ways, and they play a part in keeping the ecosystem in balance.

Glossary

• Tissue — A group of similar cells along with intercellular substances that performs a specific function in a multicellular animal.
• Organ system — Two or more organs that perform a common function by their physical and/or chemical interaction, such as the digestive system.
• Morphology — The study of form or externally visible features; in animals it refers to the external appearance of the organs or parts of the body.
• Anatomy — The term conventionally used in animals for the study of the morphology of internal organs.
• Poikilotherm — A cold blooded animal, such as the frog, whose body temperature is not constant but varies with the temperature of the environment.
• Aestivation — Summer sleep, in which frogs shelter in deep burrows during peak summer to protect themselves from extreme heat.
• Nictitating membrane — The membrane that covers the bulged eyes of the frog and protects them while the frog is in water.
• Chyme — The partially digested food passed from the stomach to the duodenum, the first part of the small intestine.
• Cutaneous respiration — Respiration through the skin, in which dissolved oxygen in water is exchanged by diffusion.
• Sinus venosus — A triangular structure that joins the right atrium of the frog's heart and receives blood through the major veins called vena cava.
• Hepatic portal system — The special venous connection between the liver and the intestine in frogs.
• Ureotelic — Describes an animal that excretes urea; the frog excretes urea and is therefore a ureotelic animal.
• Mesorchium — The double fold of peritoneum by which the testes of the frog are adhered to the upper part of the kidneys.
• Cloaca — A small, median chamber in the frog used to pass faecal matter, urine and sperms to the exterior.
• Metamorphosis — The change that the tadpole, the larval stage of the frog, undergoes to form the adult.

Common errors and misconceptions

• Misconception: Frogs drink water through the mouth. Correct: The frog never drinks water; it absorbs water through its skin.
• Misconception: The frog's heart has four chambers. Correct: It has three chambers, two atria and one ventricle, and is covered by the pericardium.
• Misconception: Frogs have a long intestine. Correct: The alimentary canal is short because frogs are carnivores, and hence the length of the intestine is reduced.
• Misconception: Frogs breathe only through their lungs. Correct: In water the skin is the respiratory organ; on land the buccal cavity, skin and lungs are used; during aestivation and hibernation, gaseous exchange takes place through the skin.
• Misconception: The RBCs of the frog have no nucleus. Correct: The RBCs of the frog are nucleated and contain the red pigment haemoglobin.
• Misconception: Frogs have an external ear. Correct: The external ear is absent; only the tympanum can be seen externally, and the ear serves hearing as well as balancing.
• Misconception: In female frogs the ureters act as a urinogenital duct. Correct: The ureters act as a urinogenital duct in males; in females the ureters and oviducts open separately into the cloaca.
• Misconception: Fertilisation in frogs is internal. Correct: Fertilisation is external and takes place in water; development involves a tadpole larva that undergoes metamorphosis.

Exam-style questions with model answers

Q1. To which phylum and class do frogs belong? Name the most common species of frog found in India. [1 mark]

1. Frogs belong to phylum Chordata and class Amphibia.
2. The most common species of frog found in India is Rana tigrina, the Indian bullfrog.

Q2. Distinguish between aestivation and hibernation in frogs. [2 marks]

1. Aestivation is summer sleep: during peak summer, frogs take shelter in deep burrows to protect themselves from extreme heat.
2. Hibernation is winter sleep: during winter, frogs take shelter in deep burrows to protect themselves from extreme cold.
3. During both, gaseous exchange takes place through the skin.

Q3. Mention the function of the ureters in frog. [2 marks]

1. The ureters emerge from the kidneys and open into the cloaca, carrying the urine formed in the kidneys to the cloaca, from where it passes to the exterior.
2. In male frogs the ureters act as a urinogenital duct: the vasa efferentia open into Bidder's canal, which communicates with this duct, so it also carries sperms to the cloaca.
3. In female frogs the ureters and oviducts open separately into the cloaca.

Q4. How does a frog respire on land and in water? [3 marks]

1. In water, the skin acts as the aquatic respiratory organ (cutaneous respiration); dissolved oxygen in the water is exchanged through the skin by diffusion.
2. On land, the buccal cavity, skin and lungs act as the respiratory organs. Respiration by the lungs is called pulmonary respiration. Air enters through the nostrils into the buccal cavity and then to the lungs, a pair of elongated, pink, sac-like structures in the thorax.
3. During aestivation and hibernation, gaseous exchange takes place through the skin.

Q5. Describe the structure of the heart of the frog. [3 marks]

1. The heart is a muscular structure situated in the upper part of the body cavity, covered by a membrane called the pericardium.
2. It has three chambers: two atria and one ventricle.
3. A triangular sinus venosus joins the right atrium and receives blood through the major veins called vena cava.
4. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart, and arteries carry blood from the heart to all parts of the body.

Q6. Describe the digestion and absorption of food in the frog. [4 marks]

1. Food is captured by the bilobed tongue.
2. In the stomach, food is digested by the action of HCl and gastric juices secreted from the stomach walls. The partially digested food, chyme, passes to the duodenum.
3. The duodenum receives bile from the gall bladder and pancreatic juices from the pancreas through a common bile duct. Bile emulsifies fat, and pancreatic juices digest carbohydrates and proteins.
4. Final digestion takes place in the intestine. Digested food is absorbed by the finger-like folds of the inner intestinal wall, the villi and microvilli.
5. Undigested solid waste moves into the rectum and passes out through the cloaca.

Q7. Describe the nervous system of the frog. [4 marks]

1. The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic).
2. Ten pairs of cranial nerves arise from the brain, which is enclosed in a bony brain box (cranium).
3. The brain is divided into the fore-brain (olfactory lobes, paired cerebral hemispheres and unpaired diencephalon), the mid-brain (a pair of optic lobes) and the hind-brain (cerebellum and medulla oblongata).
4. The medulla oblongata passes out through the foramen magnum and continues into the spinal cord, enclosed in the vertebral column.

Q8. Draw a neat labelled diagram of the digestive system of the frog and describe its alimentary canal and digestive glands. [5 marks]

1. Diagram: draw the heart at the top over the liver, with the gall bladder at the lower edge of the liver. Draw the oesophagus leading into a broad stomach, a coiled intestine, the rectum and the cloaca ending at the cloacal aperture. Label the oesophagus, liver, gall bladder, stomach, intestine, rectum, cloaca and cloacal aperture.
2. The digestive system consists of the alimentary canal and the digestive glands.
3. The alimentary canal is short because frogs are carnivores, so the intestine is reduced in length.
4. The mouth opens into the buccal cavity, which leads to the oesophagus through the pharynx. The short oesophagus opens into the stomach, which continues as the intestine and rectum, finally opening outside by the cloaca.
5. The liver secretes bile, which is stored in the gall bladder. The pancreas produces pancreatic juice containing digestive enzymes. Both reach the duodenum through a common bile duct.

Q9. Describe the male and female reproductive systems of the frog. [5 marks]

1. Male: a pair of yellowish, ovoid testes is adhered to the upper part of the kidneys by a double fold of peritoneum called the mesorchium.
2. 10 to 12 vasa efferentia arise from the testes, enter the kidneys on their side and open into Bidder's canal.
3. Bidder's canal communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca, a small, median chamber that passes faecal matter, urine and sperms to the exterior.
4. Female: a pair of ovaries is situated near the kidneys, with no functional connection with them.
5. A pair of oviducts arising from the ovaries opens into the cloaca separately. A mature female can lay 2,500 to 3,000 ova at a time.
6. Fertilisation is external, in water, and development involves a tadpole larva that undergoes metamorphosis to form the adult.

Key takeaways

• Cells, tissues, organs and organ systems split up the work of the body, showing division of labour and contributing to the survival of the body as a whole.
• All complex animals consist of only four basic types of tissues; the heart contains all four: epithelial, connective, muscular and neural.
• Frogs belong to class Amphibia of phylum Chordata, and Rana tigrina, the Indian bullfrog, is the most common species in India.
• Frogs are poikilotherms, change colour for camouflage, and shelter in deep burrows during aestivation in summer and hibernation in winter.
• The frog's body has a head and trunk with no neck or tail; males have vocal sacs and a copulatory pad on the first digit of the fore limbs.
• The alimentary canal is short because frogs are carnivores; bile and pancreatic juice reach the duodenum through a common bile duct.
• Frogs respire through the skin in water and through the buccal cavity, skin and lungs on land.
• The closed circulatory system has a three-chambered heart, a sinus venosus, a conus arteriosus, hepatic and renal portal systems, and nucleated RBCs.
• The frog is ureotelic; in males the ureters act as a urinogenital duct, while in females the ureters and oviducts open separately into the cloaca.
• A female lays 2,500 to 3,000 ova at a time; fertilisation is external in water, and the tadpole undergoes metamorphosis into the adult.

Test yourself

What is a tissue?

A group of similar cells along with intercellular substances that performs a specific function in a multicellular animal.

Name the four types of tissues present in the heart.

Epithelial, connective, muscular and neural tissues. The heart consists of all four basic types of tissues.

Why are frogs called poikilotherms?

They do not have a constant body temperature; their body temperature varies with the temperature of the environment, so they are cold blooded.

How many digits do the fore limbs and hind limbs of a frog have?

The fore limbs end in four digits and the hind limbs end in five digits. The hind limbs are larger and more muscular.

What protects the frog's eyes while it is in water?

The nictitating membrane, which covers the bulged eyes of the frog.

What are the hepatic portal and renal portal systems?

Special venous connections in frogs: the hepatic portal system between the liver and the intestine, and the renal portal system between the kidney and the lower parts of the body.

Which parts make up the frog's fore-brain?

The olfactory lobes, the paired cerebral hemispheres and the unpaired diencephalon.

How are frogs useful to humans and to the ecosystem?

They eat insects and protect the crop, and they maintain ecological balance as an important link in food chains and food webs. In some countries their muscular legs are eaten.