From cells to organ systems
A school has students, classes, departments and the whole school. Each level has a job, and the school runs well because the jobs are shared. A multicellular body is organised in the same way.
A tissue is a group of similar cells with intercellular substances that perform a specific function. Tissues form an organ (stomach, lung, heart, kidney). Organs working together form an organ system. The result is division of labour.
In the figure : Cells, tissues, organs and organ systems form the animal body.
Rule 1: Four basic tissues — All complex animals have only four basic tissues: epithelial, connective, muscular and neural. They are organised in a specific pattern and proportion. The heart contains all four. Epithelia are sheet-like tissues lining the body surface and the cavities, ducts and tubes, and have one free surface facing a body fluid or the outside.
- Easy : The skin surface is covered by epithelium.
- Medium : Stomach is an organ made of epithelial, connective, muscular and neural tissue.
- Exam-type : Q: Name the four basic animal tissues. A: Epithelial, connective, muscular and neural.
Rule 2: Organ systems, morphology and anatomy — Two or more organs that work together by physical or chemical interaction form an organ system, such as the digestive system. Morphology is the study of external form and visible features, and anatomy is the study of the internal organs.
- Easy : Looking at the outside of a frog is morphology.
- Medium : Dissecting a frog to see the stomach is anatomy.
- Exam-type : Q: Differentiate morphology and anatomy. A: Morphology deals with external features, while anatomy deals with internal organs.
Cell → tissue → organ → organ system → organism.
Saying that the heart is made of muscular tissue only. It has all four tissue types.
Memory clue: ECMN — Epithelial, Connective, Muscular, Neural.
Spot the difference
| Level | Meaning | Example |
|---|---|---|
| Tissue | Similar cells doing one job | Epithelium |
| Organ | Tissues combined to do a job | Stomach |
| Organ system | Organs working together | Digestive system |
Practice questions
- 1Define a tissue.
- 2List the four basic animal tissues.
- 3What is an organ system? Give one example.
- 4What is meant by morphology in animals?
- 5What is anatomy?
- 6Why is the body of a multicellular animal organised into organs and systems?
- 7What are epithelia?
- 8Which tissues are present in the heart?
Show answers (try them yourself first)
- 1A group of similar cells along with intercellular substances that performs a specific function.
- 2Epithelial, connective, muscular and neural tissue.
- 3Organs working together for a common function; for example, the digestive system.
- 4The study of external form and visible features.
- 5The study of internal organs.
- 6It gives division of labour, so activities are more efficient and coordinated.
- 7Sheet-like tissues lining the body surface and cavities, ducts and tubes, with one free surface.
- 8All four: epithelial, connective, muscular and neural.
Frog: habit, habitat and adaptations
After the first monsoon showers, frogs start calling from puddles and fields. During hot summer and cold winter, they vanish, as they sleep in burrows.
The frog is an amphibian that lives on land and in freshwater. The common Indian species is Rana tigrina. It is poikilothermic (cold blooded): its body temperature changes with the environment.
In the figure : A frog escapes extreme heat by summer sleep and extreme cold by winter sleep.
Rule 1: Poikilotherm and camouflage — Frogs cannot keep a constant body temperature, so they are called cold-blooded or poikilotherms. They can change the colour of the skin to match the surroundings, a protective colouration to hide from enemies, which the book calls mimicry.
- Easy : A frog on green grass looks green.
- Medium : A reptile and a fish are also poikilotherms.
- Exam-type : Q: What is a poikilotherm? A: An animal whose body temperature varies with the environment.
Rule 2: Aestivation and hibernation — In peak summer a frog hides in deep burrows to avoid heat; this is summer sleep (aestivation). In peak winter, it hides to avoid cold; this is winter sleep (hibernation). During these periods gas exchange occurs through the skin.
- Easy : Frogs are not seen in the middle of a hot May.
- Medium : A frog buried in mud in winter breathes through the skin.
- Exam-type : Q: Name the winter sleep of frogs. A: Hibernation.
Aestivation = summer sleep; hibernation = winter sleep. Both help to survive extreme conditions.
Mixing aestivation and hibernation. A is for 'aestas' (summer) and H is for 'hiems' (winter).
Memory clue: A-S, H-W — Aestivation = Summer, Hibernation = Winter.
Spot the difference
| Point | Aestivation | Hibernation |
|---|---|---|
| Season | Summer | Winter |
| Avoids | Heat | Cold |
| Respiration | Through skin | Through skin |
Practice questions
- 1Name the common Indian frog.
- 2What does cold-blooded mean?
- 3Define aestivation.
- 4Why do frogs change colour?
- 5Where do frogs live?
- 6What is poikilothermy?
- 7How do frogs breathe during hibernation?
- 8Why are frogs not seen during peak summer and peak winter?
Show answers (try them yourself first)
- 1Rana tigrina.
- 2The body temperature changes with the surrounding temperature.
- 3Summer sleep: hiding in burrows to avoid extreme heat.
- 4To blend with the surroundings and hide from enemies.
- 5On land and in freshwater.
- 6The condition of having a body temperature that varies with the environment.
- 7Through their skin.
- 8They shelter in deep burrows, in summer and winter sleep, to escape extreme heat and cold.
Frog: external features (morphology)
Pick up a frog's picture and look: bulging eyes, no neck, smooth wet skin and long hind legs for leaping. Each feature suits life in water and on land.
The frog's skin is smooth and slippery with mucus and is kept moist; it absorbs water through the skin and does not drink. The body has a head and a trunk; neck and tail are absent.
In the figure : External parts of a frog.
Rule 1: Skin, head and senses — The dorsal side is generally olive green with dark irregular spots, while the ventral side is uniformly pale yellow. Nostrils lie above the mouth. The bulging eyes are covered by a nictitating membrane that protects them in water. A membranous tympanum on each side of the eyes receives sound.
- Easy : A frog's belly is pale yellow.
- Medium : The nictitating membrane acts like a clear shield when the frog is under water.
- Exam-type : Q: What receives sound in a frog? A: The tympanum.
Rule 2: Limbs and sexual dimorphism — Hind limbs end in five digits and are longer and more muscular than the fore limbs, which end in four digits. Digits of the feet are webbed to help swimming. Males have sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs, which females lack.
- Easy : A frog uses the hind limbs for leaping.
- Medium : The croak of a male frog is amplified by the vocal sacs.
- Exam-type : Q: How can you tell a male frog from a female? A: By the vocal sacs and the copulatory pad on the first digit of forelimbs.
Fore limbs: 4 digits. Hind limbs: 5 digits, webbed. Males: vocal sacs and copulatory pad.
Saying that frogs have a tail and neck. Adult frogs have neither.
Memory clue: 4-5 — Fore 4, Hind 5.
Spot the difference
| Point | Fore limb | Hind limb |
|---|---|---|
| Digits | 4 | 5 |
| Size | Smaller | Larger, muscular |
| Webbing | Not webbed | Webbed |
Practice questions
- 1Which parts is the frog's body divided into?
- 2How many digits does the hind limb of a frog have?
- 3What is the function of the nictitating membrane?
- 4What is the tympanum?
- 5Why is a frog's skin slippery?
- 6How do male frogs differ from female frogs?
- 7Why are the toes of a frog webbed?
- 8A frog never drinks water, yet it does not die of thirst. Explain.
Show answers (try them yourself first)
- 1Head and trunk.
- 2Five.
- 3It protects the eyes when the frog is in water.
- 4The membranous ear drum that receives sound.
- 5It is covered with mucus.
- 6Males have vocal sacs and a copulatory pad on the first digit of the forelimb.
- 7Webbing helps it to swim.
- 8It absorbs water through its moist skin.
Frog: digestive system
A frog snaps up an insect with a flick of its tongue and swallows it whole. The food then moves through a short gut, as the frog is a meat-eater.
The frog's alimentary canal is short, because frogs are carnivores. It runs: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca. The liver and pancreas are the digestive glands.
In the figure : Path of food in a frog from the mouth to the cloaca.
Rule 1: Parts and glands — Food is caught by the bilobed tongue. The liver makes bile, stored in the gall bladder. The pancreas makes pancreatic juice. The stomach wall secretes HCl and gastric juices. Partly digested food (chyme) enters the duodenum, which receives bile and pancreatic juice through the common bile duct.
- Easy : The tongue flicks out to catch an insect.
- Medium : Bile is stored in the gall bladder before it is released.
- Exam-type : Q: What is chyme? A: Partially digested food that moves from the stomach to the duodenum.
Rule 2: Digestion and absorption — Bile emulsifies fats; pancreatic juice digests carbohydrates and proteins; final digestion happens in the intestine. Digested food is absorbed through villi and microvilli of the intestinal wall. Undigested waste moves to the rectum and exits via the cloaca.
- Easy : Fat droplets get broken into tiny drops by bile.
- Medium : Finger-like villi increase the area for absorption.
- Exam-type : Q: Where is digested food absorbed? A: In the intestine, through villi and microvilli.
Carnivores have a short gut; herbivores have a long gut.
Thinking that bile contains enzymes. Bile emulsifies fats, but it is the pancreatic juice that digests carbohydrates and proteins.
Memory clue: MOSIRC — Mouth, Oesophagus, Stomach, Intestine, Rectum, Cloaca.
Spot the difference
| Gland | Secretion | Role |
|---|---|---|
| Liver | Bile | Emulsifies fats |
| Pancreas | Pancreatic juice | Digests carbohydrates and proteins |
| Stomach wall | HCl and gastric juice | Digests food |
Practice questions
- 1Why is the alimentary canal of a frog short?
- 2Name the digestive glands of a frog.
- 3Where is bile stored?
- 4What is the first part of the small intestine called?
- 5What opens into the cloaca?
- 6What is the role of the tongue?
- 7What are villi?
- 8Describe the path of food from the mouth to the cloaca.
Show answers (try them yourself first)
- 1Frogs are carnivores, so their intestine is short.
- 2Liver and pancreas.
- 3In the gall bladder.
- 4Duodenum.
- 5The rectum, the ureters and the urinary bladder (and the genital ducts).
- 6Capturing prey.
- 7Finger-like folds of the inner intestine wall that absorb food.
- 8Mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum, cloaca.
Frog: respiration and circulation
A frog sitting still on the bank takes quick throat movements, while the same frog under water breathes without any lung movement. It uses different organs in different places.
Frogs show cutaneous (skin), buccal (mouth) and pulmonary (lung) respiration. The circulatory system is closed, with a three-chambered heart (two atria and one ventricle).
In the figure : Blood enters the frog heart through the sinus venosus and leaves through the conus arteriosus.
Rule 1: Respiration — In water, the skin acts as the respiratory organ and oxygen dissolved in water diffuses in. On land, the skin, buccal cavity and lungs are used. The lungs are a pair of pink, sac-like organs in the thorax. Air comes in through the nostrils.
- Easy : A frog under water breathes through the skin.
- Medium : A frog on land uses the lungs and the buccal cavity.
- Exam-type : Q: What is respiration by the lungs called? A: Pulmonary respiration.
Rule 2: Heart, vessels and blood — The heart is covered by a pericardium and has two atria and one ventricle. The triangular sinus venosus joins the right atrium and receives blood from the vena cava; the ventricle opens into the conus arteriosus. Special veins link the liver and intestine (hepatic portal system) and the kidney and lower body (renal portal system). RBCs are nucleated and contain haemoglobin. The frog also has a lymphatic system.
- Easy : A frog's heart has three chambers.
- Medium : Blood from the intestine goes to the liver first through the hepatic portal vein.
- Exam-type : Q: Are the RBCs of a frog nucleated? A: Yes.
Frog heart: 2 atria + 1 ventricle; RBCs are nucleated; portal systems: hepatic and renal.
Saying that a frog's heart has four chambers. It has three.
Memory clue: 2+1 — Two atria and one ventricle.
Spot the difference
| Place | Respiratory organ | Method |
|---|---|---|
| In water | Skin | Cutaneous |
| On land | Skin, buccal cavity, lungs | Cutaneous, buccal, pulmonary |
| Hibernation/aestivation | Skin | Cutaneous |
Practice questions
- 1How many chambers does a frog's heart have?
- 2Which organ does a frog use to breathe under water?
- 3What is the sinus venosus?
- 4Name the two portal systems in a frog.
- 5What does the pericardium do?
- 6What is special about the RBCs of a frog?
- 7How is the lymph different from blood?
- 8Name three respiratory organs of a frog on land.
Show answers (try them yourself first)
- 1Three: two atria and one ventricle.
- 2The skin.
- 3A triangular chamber that joins the right atrium and receives blood from the vena cava.
- 4The hepatic portal system and the renal portal system.
- 5It is the membrane that covers the heart.
- 6They are nucleated and contain haemoglobin.
- 7Lymph lacks some proteins and RBCs.
- 8Skin, buccal cavity and lungs.
Frog: excretion, control and coordination
A frog removes waste from the blood through its kidneys and reacts to a shadow within a flash. Both jobs need organs that sense, decide and clean.
The frog's excretory system has a pair of kidneys, ureters, cloaca and urinary bladder. It is ureotelic (excretes urea). The control system includes the neural system and endocrine glands.
In the figure : The frog's brain has three parts that continue into the spinal cord.
Rule 1: Excretion — The kidneys are compact, dark red, bean-like organs on either side of the vertebral column. Each has many nephrons (uriniferous tubules). In males the ureter acts as a urinogenital duct that opens into the cloaca; in females, the ureters and oviducts open separately. The bladder lies ventral to the rectum.
- Easy : A frog's urine is mostly urea.
- Medium : In a male frog the ureter also carries sperms.
- Exam-type : Q: What type of excretion is found in a frog? A: Ureotelic.
Rule 2: Nervous system and sense organs — The central nervous system has the brain and spinal cord. The peripheral nervous system has cranial and spinal nerves, and the autonomic system has sympathetic and parasympathetic parts. There are ten pairs of cranial nerves. Fore-brain includes olfactory lobes, cerebral hemispheres and diencephalon; the mid-brain has optic lobes; the hind-brain has cerebellum and medulla oblongata. Sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), eyes and ears. Endocrine glands include pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
- Easy : The optic lobes of the mid-brain process what the eye sees.
- Medium : The ear of a frog gives hearing and balance.
- Exam-type : Q: How many pairs of cranial nerves does a frog have? A: Ten.
Frog: ureotelic, 10 pairs of cranial nerves, simple eyes, tympanum but no external ear.
Saying that frogs have an external ear. Only the tympanum can be seen.
Memory clue: F-M-H — Fore-brain, Mid-brain, Hind-brain.
Spot the difference
| Brain part | Includes | Feature |
|---|---|---|
| Fore-brain | Olfactory lobes, cerebral hemispheres, diencephalon | Smell, higher control |
| Mid-brain | Optic lobes | Vision |
| Hind-brain | Cerebellum, medulla oblongata | Balance, links to spinal cord |
Practice questions
- 1Name the excretory organs of a frog.
- 2What is the structural unit of a kidney called?
- 3What does ureotelic mean?
- 4Name the parts of the frog's brain.
- 5Which part of the brain has optic lobes?
- 6Which sense organs of frog are well organised?
- 7How do the ureters differ in male and female frogs?
- 8Name any four endocrine glands in a frog.
Show answers (try them yourself first)
- 1A pair of kidneys.
- 2The nephron (uriniferous tubule).
- 3Excreting nitrogenous waste mainly as urea.
- 4Fore-brain, mid-brain and hind-brain.
- 5The mid-brain.
- 6The eyes and the internal ears.
- 7In males they act as urinogenital ducts that open into the cloaca; in females, they open separately from the oviducts.
- 8Pituitary, thyroid, adrenal and pancreatic islets (or parathyroid, thymus, pineal, gonads).
Frog: reproduction, development and importance
In the monsoon, you may see a jelly-like mass of eggs in a pond and, a few weeks later, tiny tadpoles with tails. The tadpoles then turn into frogs.
Frogs have separate sexes with well-organised reproductive systems. Fertilisation is external, in water. The egg becomes a tadpole that undergoes metamorphosis into the adult frog.
In the figure : Life cycle of a frog from egg to adult.
Rule 1: Reproductive systems — Males have a pair of yellowish ovoid testes attached to the kidneys by mesorchium. 10–12 vasa efferentia enter the kidney and open into Bidder's canal and then to the urinogenital duct and cloaca. Females have a pair of ovaries with no connection to the kidneys, and a pair of oviducts that open into the cloaca separately. A female lays 2500–3000 ova at a time.
- Easy : Male frogs have testes that lie near the kidneys.
- Medium : A female frog lays thousands of eggs, as only a few survive.
- Exam-type : Q: How many ova does a female frog lay? A: 2500 to 3000 at a time.
Rule 2: Development and importance — Fertilisation and development are external. Eggs hatch into tadpoles, which undergo metamorphosis to form frogs. Frogs eat insects and protect crops, are links in food chains and webs, and in some countries their legs are eaten.
- Easy : A tadpole breathes with gills and later grows lungs.
- Medium : Frogs keep insect numbers down in a paddy field.
- Exam-type : Q: Why are frogs useful to farmers? A: They eat insect pests.
External fertilisation + external development + larval stage (tadpole) + metamorphosis.
Saying that a tadpole is the adult stage. It is the larval stage.
Memory clue: E-T-M — Egg, Tadpole, Metamorphosis.
Spot the difference
| Point | Male | Female |
|---|---|---|
| Gonad | Testes | Ovaries |
| Connection with kidney | Yes, through vasa efferentia | None |
| Cloaca | Opens through the urinogenital duct | Ureters and oviducts open separately |
Practice questions
- 1Where does fertilisation take place in a frog?
- 2What is a tadpole?
- 3What is metamorphosis?
- 4How many ova can a female frog lay at a time?
- 5Name the male reproductive organs of a frog.
- 6What are vasa efferentia?
- 7Why does a female frog lay thousands of eggs?
- 8List two ways in which frogs are useful.
Show answers (try them yourself first)
- 1Outside the body, in water.
- 2The larval stage of a frog.
- 3The change from the larval stage to the adult form.
- 4About 2500 to 3000.
- 5A pair of testes.
- 6Fine tubules (10–12) that carry sperms from the testes to the kidney.
- 7Fertilisation and development are external, so many eggs are lost, and a large number ensures that some survive.
- 8They eat insects that damage crops and are links in food chains and webs.