How to Use This Section
This chapter is examined in a very particular way. Almost every question is "name the structure", "state its function", or "give the one difference" - there is very little reasoning to build up. That makes it one of the easiest chapters to score full marks in, and one of the easiest to lose marks in through a swapped name.
Work through the three tiers in order. Tier 1 fixes the names. Tier 2 puts you in front of a described structure and asks you to identify it. Tier 3 asks for the comparisons and the joined-up answers. The two chapter-end exercises are answered in full at the end.
The Facts These Questions Draw On
Levels of organisation. Cells - tissues - organs - organ systems. A tissue is a group of cells along with intercellular substances performing one or more functions. All complex animals consist of only four basic types of tissues - epithelial, connective, muscular and neural.
Epithelial tissue. Simple (one layer) - squamous (flat, in walls of blood vessels and air sacs, for diffusion), cuboidal (in ducts of glands and tubular parts of nephrons, for secretion and absorption), columnar (in stomach and intestine lining, for secretion and absorption). Ciliated - moves particles or mucus in a specific direction. Glandular - unicellular (goblet cells) or multicellular (salivary gland); exocrine secrete through ducts, endocrine have no ducts and release hormones into blood. Compound - many layers, mainly protective, on the dry surface of skin, the moist surface of the buccal cavity, pharynx and the ducts of most glands. Junctions - tight junctions stop leaking, adhering junctions cement cells together, gap junctions allow rapid transfer of ions and molecules.
Connective tissue. Loose - areolar (beneath the skin, a support framework), adipose (stores fat). Dense - regular (tendons join muscle to bone, ligaments join bone to bone), irregular. Specialised - cartilage with chondrocytes, bone with osteocytes in lacunae, blood as a fluid connective tissue.
Muscular tissue. Skeletal - attached to bones, striated, voluntary. Smooth - in the wall of hollow organs, not striated, involuntary. Cardiac - only in the heart, striated, involuntary, with intercalated discs.
Neural tissue. Neurons conduct impulses; neuroglia support them and make up more than one half of the volume of neural tissue.
The frog. Rana tigrina, class Amphibia. Poikilotherm, mimicry, aestivation (summer), hibernation (winter). Head and trunk, no neck, no tail; five hind digits, four fore digits; nictitating membrane, tympanum; male has vocal sacs and a copulatory pad. Digestive canal short because carnivore; cutaneous and pulmonary respiration; three-chambered heart with sinus venosus and conus arteriosus; hepatic and renal portal systems; nucleated RBCs; kidneys with nephrons, ureotelic; ten pairs of cranial nerves; mesorchium, vasa efferentia, Bidder's canal; external fertilisation, tadpole, metamorphosis.
Tier 1 - Concept Checks
Question 1
Q. Arrange the levels of organisation in an animal body in order.
Answer. Cells - tissues - organs - organ systems. Each level is built from the one before it, and together they show division of labour.
Question 2
Q. Define a tissue.
Answer. A group of cells along with intercellular substances performing one or more functions in the body.
Question 3
Q. Name the four basic types of tissues found in complex animals.
Answer. Epithelial, connective, muscular and neural.
Question 4
Q. What is the one feature all epithelia share?
Answer. Every epithelium has one free surface facing a body fluid or the outside environment, and its cells are structurally and functionally connected at junctions.
Question 5
Q. Where is squamous epithelium found and what does it do?
Answer. In the walls of blood vessels and air sacs of the lungs. Its job is to form a diffusion boundary, which suits its flat, thin cells.
Question 6
Q. Name the epithelium lining the tubular parts of the nephron.
Answer. Cuboidal epithelium. It does secretion and absorption, and is also found in the ducts of glands.
Question 7
Q. What is the difference between a unicellular and a multicellular gland? Give one example of each.
Answer. A unicellular gland is a single isolated glandular cell - the goblet cells of the alimentary canal. A multicellular gland is a cluster of cells - the salivary gland.
Question 8
Q. How do exocrine and endocrine glands differ?
Answer. Exocrine glands secrete their products through ducts - mucus, saliva, earwax, oil, milk, digestive enzymes. Endocrine glands do not have ducts; their products are hormones released into the blood.
Question 9
Q. Name the three types of cell junctions and what each does.
Answer. Tight junctions - stop substances from leaking across a tissue. Adhering junctions - cement neighbouring cells together. Gap junctions - allow rapid transfer of ions and small molecules between cells.
Question 10
Q. What is areolar tissue and where is it found?
Answer. A loose connective tissue present beneath the skin. It serves as a support framework for epithelium.
Question 11
Q. Distinguish between a tendon and a ligament.
Answer. A tendon attaches skeletal muscle to bone. A ligament attaches one bone to another bone. Both are dense regular connective tissue.
Question 12
Q. Name the cells of cartilage and of bone, and say where the bone cells lie.
Answer. Cartilage cells are chondrocytes. Bone cells are osteocytes, and they are present in spaces called lacunae.
Question 13
Q. Which muscle tissue is striated but involuntary, and what is its special feature?
Answer. Cardiac muscle, found only in the heart. Its special feature is the intercalated disc, the junction between adjoining cells.
Question 14
Q. What are neuroglia and how much of the neural tissue do they make up?
Answer. They are the supporting cells of neural tissue - they protect and support neurons. They make up more than one half of the volume of neural tissue in the body.
Question 15
Q. Give the scientific name of the common Indian frog and its class.
Answer. Rana tigrina, class Amphibia.
Question 16
Q. What is aestivation and how does it differ from hibernation?
Answer. Aestivation is summer sleep and hibernation is winter sleep. In both, the frog takes shelter in deep burrows to protect itself from extreme heat or cold.
Question 17
Q. How many digits do the hind limbs and the forelimbs of a frog have?
Answer. Hind limbs have five digits; forelimbs have four. The hind limbs are larger and more muscular, with webbed digits that help in swimming.
Question 18
Q. How can you tell a male frog from a female one?
Answer. The male has sound-producing vocal sacs and a copulatory pad on the first digit of the forelimb. Both are absent in the female. This is sexual dimorphism.
Question 19
Q. How many chambers does the frog heart have, and what are they?
Answer. Three - two atria and one ventricle. The heart is covered by a membrane called the pericardium.
Question 20
Q. Name the excretory products of a frog and say what type of excretion this makes it.
Answer. The frog excretes urea, so it is a ureotelic animal.
Tier 2 - Application and Identification
Question 21
Q. A tissue is a single layer of tall, slender cells lining the stomach. Name it and give one function.
Answer. Columnar epithelium. Its functions are secretion and absorption. If the free surface also carried cilia it would be ciliated columnar epithelium, which moves mucus in a specific direction.
Question 22
Q. A layer of cells lines the inner surface of a bronchiole and carries hair-like structures on its free surface. What is it and what is it doing?
Answer. Ciliated epithelium. Its cilia move particles or mucus in a specific direction over the epithelium - in the bronchioles, out of the airway.
Question 23
Q. A tissue has many layers of cells and covers the dry surface of the skin. Name it and give its main function.
Answer. Compound epithelium. It is many-layered and its main function is to give protection against chemical and mechanical stresses, so it has a limited role in secretion and absorption.
Question 24
Q. Two epithelial cells are joined so that ions and small molecules pass rapidly from the cytoplasm of one straight into the other. Which junction is this?
Answer. A gap junction. It facilitates the cells to communicate with each other by connecting the cytoplasm of adjoining cells.
Question 25
Q. A connective tissue stores fat and lies beneath the skin. Identify it and say what the stored fat is used for.
Answer. Adipose tissue. The excess of fat is stored in its cells and acts as the body's fat store.
Question 26
Q. A fluid tissue carries cells but has no fibres in its matrix. What is it, and why is it called connective tissue?
Answer. It is blood, a fluid connective tissue. It is connective because it has cells suspended in a large amount of intercellular matrix - here plasma - and it links and transports between the parts of the body.
Question 27
Q. A muscle in the wall of the intestine is spindle-shaped, has no striations and is not under your control. Name it.
Answer. Smooth muscle, also called unstriated or involuntary muscle. It is found in the wall of hollow visceral organs such as the intestine.
Question 28
Q. A frog is found deep in a burrow in peak summer, and its lungs are not being used. How is it breathing?
Answer. Through its skin, by cutaneous respiration. During aestivation and hibernation, gaseous exchange takes place through the skin, which stays moist and highly vascularised for exactly this reason.
Question 29
Q. A frog's alimentary canal is noticeably shorter than that of a plant-eating animal of the same size. Explain why.
Answer. Because frogs are carnivores, and hence the length of the intestine is reduced. Animal food is easier to digest and needs less gut length than plant food.
Question 30
Q. Blood returning from the intestine of a frog does not go straight to the heart. Where does it go, and what is that route called?
Answer. It goes to the liver first, through the hepatic portal system - the special venous connection between the liver and the intestine.
Question 31
Q. A blood smear from an animal shows red cells that still carry nuclei. Could this be a frog? Explain.
Answer. Yes. The RBCs of a frog are nucleated and contain the red coloured pigment haemoglobin. Nucleated RBCs are exactly what separates the frog from a human being here.
Question 32
Q. In a dissected male frog, a single duct leaving the kidney carries both urine and sperms. Name it and explain why this happens.
Answer. It is the urinogenital duct. In male frogs the two ureters act as urinogenital ducts opening into the cloaca, because the vasa efferentia enter the kidney and open into Bidder's canal, which then joins the same duct.
Tier 3 - Comparisons and Long Answers
Question 33
Q. Compare simple and compound epithelium under structure, function and location.
Answer. Simple epithelium is one cell layer thick; it lines body cavities, ducts and tubes, and works as a lining or covering surface, doing absorption, diffusion, secretion and filtration. Compound epithelium is many-layered; it covers the dry surface of the skin, the moist surface of the buccal cavity, pharynx and the inner lining of ducts of most glands, and its main function is protection against chemical and mechanical stresses, with a limited role in secretion and absorption.
Question 34
Q. Give the differences between the three types of muscle tissue.
Answer. Skeletal muscle - attached to skeletal bones, striated, voluntary, cells arranged in bundles. Smooth muscle - in the wall of hollow visceral organs, not striated, involuntary, spindle-shaped cells. Cardiac muscle - only in the heart, striated, involuntary, with intercalated discs where adjoining cells meet.
Question 35
Q. Describe the alimentary canal of a frog from mouth to exterior, and name its two digestive glands.
Answer. Mouth - buccal cavity - pharynx - oesophagus - stomach - intestine - rectum - cloaca, which opens outside. The glands are the liver, which secretes bile stored in the gall bladder, and the pancreas, which produces pancreatic juice containing digestive enzymes.
Question 36
Q. Describe the heart of a frog and the two vessels associated with it.
Answer. The heart is a muscular structure in the upper part of the body cavity, covered by the pericardium, with three chambers - two atria and one ventricle. 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.
Question 37
Q. Compare the way the ureters behave in male and female frogs.
Answer. In the male, the two ureters emerging from the kidneys act as urinogenital ducts, so the same duct carries urine and sperms into the cloaca. In the female, the ureters and the oviduct open separately into the cloaca, so the ureter carries urine only.
Question 38
Q. Trace the path of a sperm in a male frog from where it is made to where it leaves the body.
Answer. Testes - vasa efferentia (10 to 12 in number) - kidney - Bidder's canal - urinogenital duct, which comes out of the kidney - cloaca - exterior. The testes themselves are held to the upper part of the kidneys by a double fold of peritoneum, the mesorchium.
Question 39
Q. Compare the male and female reproductive systems of a frog with respect to the kidneys.
Answer. In the male, the testes are attached to the upper part of the kidneys by the mesorchium, and the sperms actually pass through the kidney. In the female, the ovaries are only situated near the kidneys and have no functional connection with them, and the oviducts open into the cloaca separately.
Question 40
Q. Describe reproduction and development in the frog.
Answer. The male has a pair of yellowish ovoid testes, the female a pair of ovaries. 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 the tadpole, and the tadpole undergoes metamorphosis to form the adult.
Question 41
Q. Why are frogs important, both to farmers and to the ecosystem?
Answer. Frogs eat insects and protect the crop, which helps farmers directly. They also maintain ecological balance because they serve as an important link of the food chain and food web in the ecosystem. In some countries the muscular legs of frogs are used as food by man.
The Chapter-End Exercises
Both exercises at the end of this chapter are answered here in full, so you can check your own attempt against them.
| Exercise | Answered as |
|---|---|
| 1. Draw a neat diagram of the digestive system of frog. | Question 42 below, and also as Question 13 of the Digestive and Respiratory Systems section |
| 2. Mention the function of the Ureters in frog. | Question 43 below, and also as Question 13 of the Circulatory and Excretory Systems section |
Question 42
Q. Draw a neat diagram of the digestive system of frog. This is one of the chapter-end exercises.
Answer. Draw the body outline of the frog and lay the canal down the middle, then label it in the order the food travels.
The labels your diagram must carry, in order: mouth, buccal cavity, pharynx, oesophagus (a short tube), stomach, intestine, rectum, cloaca and the cloacal aperture, through which the canal finally opens outside.
The glands to show at the side, joined to the canal by their ducts: the liver, which secretes bile; the gall bladder, in which the bile is stored; and the pancreas, which produces pancreatic juice containing digestive enzymes. Bile from the gall bladder and pancreatic juice reach the intestine through a common bile duct.
Also mark the bilobed tongue attached to the front of the floor of the buccal cavity, since it is the structure that catches the prey.
A note to add under the diagram: the canal is short because the frog is a carnivore, and hence the length of the intestine is reduced.
Question 43
Q. Mention the function of the Ureters in frog. This is one of the chapter-end exercises.
Answer. The ureters carry urine from the kidneys to the cloaca, from where it passes out of the body.
In male frogs the two ureters that emerge from the kidneys also act as urinogenital ducts - the same duct carries both urine and sperms and opens into the cloaca.
In female frogs the ureters and the oviduct open separately into the cloaca, so there the ureter carries urine only.