LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 11 : FORCE AND PRESSURE

FORCE

 

·         A push or pull on an object is called force.   

·         Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing.

 

Force is due to an interaction:

 

·         When two objects interact with each other, they can exert forces on each other. These forces can either attract or repel the objects, depending on the nature of the interaction.

 

Force has magnitude as well as direction:

 

·         The strength of a force is expressed by its magnitude. 

·         The magnitude of force is expressed in the the SI unit of force called Newton. Its symbol is N.

·         1 Newton is the force which can make an object of 1 kg mass to move an acceleration of 1 m/s².

·         When two forces act along the same direction, then the net force acting on the object is equal to the sum of the two forces and when they act in opposite direction, and then the net force acting on the object is equal to the difference between the two forces. 

·         If the two forces applied to an object are equal in magnitude and act in opposite directions, then the net force acting on the object is zero.

 

EFFECTS OF FORCE:

 

A force can produce the following effects.

·         A force can move a stationary object.

·         A force can stop a moving object.

·         A force can change the speed of a moving object. 

·         A force can change the direction of a moving object. 

·         A force can change the shape and size of an object.

 

TYPES OF FORCES

The most common types forces are:

·         Muscular force

·         Frictional force

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

CONTACT FORCES

A force which can be exerted bi an object on another object only through physical touching or contact is called a contact force. Examples of contact forces are

·         Muscular force and

·         Frictional force

 

MUSCULAR FORCE

The force exerted by the muscles of the body is called muscular force.

As the muscular force can be applied to an object only when our body or body of an animal is in contact with the object, therefore, muscular force is a contact force.

 

FRICTIONAL FORCE OR FRICTION

The force which always opposes the motion of one body over another body is called frictional force or friction. 

Since frictional force arises only when the surfaces of two objects are in touch with each other, frictional force is an example of of contact force. 

NON CONTACT FORCES

A force which can be exerted bi an object on another object even from a distance without touching each other is called a non contact force.

The examples of non contact forces are

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

MAGNETIC FORCE

The force exerted by a magnet is called magnetic force. 

Since a magnet can exert its magnetic force on iron objects from a distance even without touching them, therefore, magnetic force is a non contact force. The magnetic force between a magnet and an iron object is always that of attraction. A magnet can attract or repel another magnet. 

 

A magnet has two poles, North Pole and South Pole. 

 

There is a magnetic force of repulsion between the like poles of two magnets and there is a magnetic force of attraction between the unlike poles of two magnets. 

 

The magnetic force is widely used in our everyday life. 

 

ELECTROSTATIC FORCE

There are two types of electric charges. They are Positive electric charges and Negative electric charges. 

 

The force exerted by an electrically charged object is called electrostatic force. As an example, when a plastic comb is rubbed in dry hair, it gets electric charges and this electrically charged comb can attract tiny pieces of paper. 

 

Electrostatic forces may be attractive or repulsive. Electrostatic force between two like charges is repulsive and between two unlike charges is attractive. Therefore we can say, a positive charge repel another positive charge but attracts a negative charge. 

 

The electrostatic force can be exerted by a charged object on another object from a distance even when they are not in touch with each other; therefore, electrostatic force is an example of non contact forces. 

 

GRAVITATIONAL FORCE

The pull exerted by objects possessing mass on another mass is called gravitational forces. Gravitational force between two objects is a force of attraction. 

 

It is the gravitational force between the sun and the earth which holds the earth in its orbit around the sun. 

The force with which the earth is the objects towards it, is called the force of gravity or simply gravity. 

The force of gravity causes all the objects to fall towards the earth. 

The gravitational force of Earth or gravity acts on objects from a distance without there being a physical contact, therefore, gravitational force or gravity is an example of non contact forces. 

PRESSURE

 

Pressure is produced when a force acts on an object. The force acting on an object per unit surface area of the object is called pressure. 

The effect of a force depends on the area of the object on which it acts. 

Pressure = Force / Area

The SI unit of pressure is Newton per square metre ( N/m²), which is also called Pascal (Pa).

1 Pa = 1 N/m² 

1 kilo Pascal (1 kPa = 1000 Pa) is equal to 1000 Pascal. 

FACTORS ON WHICH PRESSURE DEPENDS

The pressure depends on two factors.

·         The force applied

·         Area over which force acts. 

The same force can produce different pressures depending on the area over which it acts. For example, when a force acts over a large area of an object, it produces small pressure. But if the same force acts over a small area of the object, it produces a large pressure. 

 

WHY SCHOOL BAGS HAVE WIDE STRAPS

A school bag or a shoulder bag has wide strap made of thick cloth so that the weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure it is more comfortable to carry the heavy school bag. 

 

WHY A SHARP KNIFE CUTS BETTER THAN A BLUNT KNIFE

A sharp knife has a very thin edge to its blade. A sharp knife cuts objects like vegetables better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hands falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty. 

 

WHY THE TIP OF A NEEDLE IS SHARP

The tip of a sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth producing a large pressure sufficient to Pierce the cloth being stitched. A knife Razer blade and an x are the cutting tools where is a sewing needle is a piercing tool. 

 

WHY THE DEPRESSION IS MUCH MORE WHEN A MAN STANDS ON THE CUSHION THEN WHEN HE LIES DOWN ON IT

When a man stands on a cushion then only his two feet having small area are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion then his whole body having larger area is in contact with the cushion producing much smaller pressure. This smaller pressure produces a very little depression in the cushion. 

 

PRESSURE EXERTED BY LIQUIDS

All the liquids exert pressure on the base and the walls of their containers. All the liquids have weight and this weight acts on the base of the vessel producing a pressure. 

The pressure exerted by a liquid increases with increasing depth inside the liquid. 

The pressure of a liquid increases with depth. The greater the depth of a point in a liquid the greater is the pressure. Liquids also exert pressure on the walls of the vessel in which they are stored. The sideways pressure exerted by liquids also increases with the depth of the liquid. 

The formation of mountains of water from the leaking pipes of water supply pipeline tells us that water exerts pressure on the walls of the container. 

A liquid exerts pressure in all directions even upwards. 

 

PRESSURE EXERTED BY GASES

 

All the gases exert pressure on the walls of their containers. Air pressure arises due to the constant solutions of the tiny molecules of the gases present in air with the walls of the container or vessel in which it is enclosed. 

High air pressure produces by the gas molecules on the walls of balloon causes it to extend and get inflated. 

 

Two examples that gases like air exert pressure are:

 

·         When air is filled into a balloon with our mouth the balloon gets inflated.

·         When air is filled into a bicycle tube with a pump the tube gets integrated and makes the tyre feel hard. 

 

ATMOSPHERIC PRESSURE

The layer of air above the earth is called atmosphere. Air has weight show the atmosphere consisting of tremendous amount of air has enormous weight the weight of atmosphere exerts a pressure on the surface of the earth and on all the objects on the Earth including ourselves. This pressure is known as atmospheric pressure. The atmospheric pressure is due to the weight of air present in the atmosphere above us. Atmospheric pressure also acts in all directions. 

 

MAGNITUDE OF ATMOSPHERIC PRESSURE

 

Atmospheric pressure is equal to the weight of air present in a BD tall column of air standing on one square metre area of the earth.  As we go up in the atmosphere from the surface of Earth the atmospheric pressure goes on decreasing. On the surface of Earth, the atmospheric pressure is maximum at the sea level. The atmospheric pressure on the top of a high mountain will be much less than at its base. 

Although pressure is is measured in Pascal, atmospheric pressure is measured in mm of Mercury. The atmospheric pressure on the surface of earth at the sea level is 760 mm of Mercury. 

 

OUR BODY AND ATMOSPHERIC PRESSURE

Change the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside we do not get crushed. But there is an effect of low atmospheric pressure in our body. At higher altitudes, atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes. 

 

APPLICATIONS OF ATMOSPHERIC PRESSURE IN EVERYDAY LIFE

 

1. DRINKING STRAW

 

The drinking straw works on the existence of atmospheric pressure. The lower end of drinking straw is dipped in the soft drinks. When we suck at the upper end of the straw with our mouth, the pressure of air inside the straw and in our mouth is reduced. The pressure acting on the surface of the soft drink is equal to atmospheric pressure. So, the greater atmospheric pressure acting on the surface of the soft drink pushes the soft drink up the straw into our mouth. 

 

2. SYRINGE

 

A glass tube or plastic tube with a nozzle and piston for sucking in and ejecting liquid in a thin stream is called a syringe. The series works on the existence of atmospheric pressure. When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe. 

3. DROPPER

 

The dropper is a small glass tube with a rubber ball at one end and a nozzle at the other. Its functionality relies on atmospheric pressure. To use the dropper, we immerse the nozzle in the liquid and squeeze the rubber ball, causing air inside the tube and bulb to escape as bubbles. This action lowers the air pressure within the dropper. Once we release the rubber bulb, the higher atmospheric pressure outside the dropper forces the liquid to be drawn up into the tube.

 

4. RUBBER SUCKER

 

A rubber sucker, also known as a suction cup, is a rubber device that adheres firmly to flat and smooth surfaces when pressed. Its appearance resembles a small concave rubber cup. When we apply the rubber sucker to a flat, smooth surface and press it, the concave rubber cup flattens significantly, expelling most of the air from underneath. This compression results in a significant reduction in air pressure inside the rubber sucker. As a consequence, the higher atmospheric pressure from the outside effectively secures the rubber sucker in place on the flat surface.

 

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 10 : REACHING THE AGE OF ADOLESCENCE

DIFFERENT STAGES OF LIFE AND ROLE OF HORMONES:

The human beings can reproduce only after a certain age because their reproductive systems start working only after a certain age. 

It is the hormones which bring about changes that make a child grow into an adult. 

1. A young human being below the age of full physical development is called a child.

2. A mature human being who is fully grown and developed is called an adult.

3. The transitional period of physical and mental development which occurs between childhood and adulthood is called adolescence.

4. The period of life of a person when the body undergoes a lot of changes leading to reproductive maturity is called adolescence. 

5. A person who is in the process of growing from a child to an adult is called an adolescent. Adolescence is usually begins around the age of 10 or 11 and lasts up to 18 or 19 years. They are also known as teenagers. 

6. Adolescence is the time between childhood and adulthood. 

7. Puberty is the time when adolescent become sexually mature. The period during which adolescent boys and girls reach sexual maturity and become capable of reproduction is called puberty. 

8. Puberty tends to start earlier in girls than in boys. Girls attained puberty at a lower age of 10 to 13 years, while boys reach puberty at a comparatively higher age of 12 to 14 years. 

9. During the puberty development of secondary sexual characteristics occur in both boys and girls. 

10. All the changes which occur in boys and girls at puberty are brought about by various hormones. 

11. The most conspicuous change during puberty is the sudden increase in the height of of boys and girls. 

12. The height of a person depends on the genes inherited from the parents. 

13. When puberty sets in, a time of rapid changes in body shape and appearance starts in boys and girls which make the boys and girls look different from one another. 

14. Some of the changes in body shape and appearance in boys and girls brought about by the onset of puberty are as follows:

a. Boys develop broader shoulders and wider chests than girls.

b. Girls develop broader hips than boys. Due to this, the region below the waist becomes wider in girls. 

c. Boys develop more muscle than girls. So, the body of boys looks more muscular than that of girls. 

d. Girls develop breasts. This also changes the body shape of grown up girls and makes them look different from boys. 

e. Boys develop Adam's Apple which makes them look different from girls. 

f. Boys develop facial hair like moustache and beard, girls do not have facial hair. Growth of facial hair makes boys look different from girls. 

15. Changes at puberty:

a. Increasing height: 

When puberty sets in there is a rapid increasing height of boys and girls. Initially girls grow faster than boys. The rate of growth in height varies in different persons. 

Full height of a person = (present height of a person/percentage of full height at present age) x 100

Problem1: A 10 year old boy 125 cm tall. Is the present height of the boy is 75% of his full height, calculate the full height which the boy will eventually reach at the end of growth period. 

Solution: present height of the boy = 125 cm

                Percentage of full height = 75%

So, full height of the boy = (125/75) x 100 cm

                                           = 166.67 cm

b. Change in body shape and appearance

a. Boys develop broader shoulders and wider chests than girls.

b. Girls develop broader hips than boys. Due to this, the region below the waist becomes wider in girls. 

c. Boys develop more muscle than girls. So, the body of boys looks more muscular than that of girls. 

d. Girls develop breasts. This also changes the body shape of grown up girls and makes them look different from boys. 

e. Boys develop Adam's Apple which makes them look different from girls. 

f. Boys develop facial hair like moustache and beard, girls do not have facial hair. Growth of facial hair makes boys look different from girls. 

c. Change in voice

The growth of voice box in boys is much more than the growth of voice box in girls. The bigger boys box in boys gives deeper voice to the boys. The bulge or projection the front of throat or neck in grown up boys is called Adam's Apple. The smaller voice box in girls gives shrill voice or high pitched voice to the girls.

d. Development of pimples and acne on the face.

Pimples are small red spots on the face of a person. Acne is a skin condition marked by the eruption of numerous red pimples on the face. The pimples and acne are formed due to the increased activity of of sebaceous glands and sweat glands present in the skin. 

e. Development of sex organs

The testes begin to produce male sex cells called sperms at puberty. The ovaries start releasing mature eggs or ova at puberty. 

f. Reaching mental, intellectual, and emotional maturity. 

Adolescence is a period which brings maturity is a person's way of thinking. 

The changes which take place in body and mind during adolescence are a natural part of the growing up process. Every human being has to pass through this stage. 

Adolescence is a period which also brings intellectual maturity. Adolescence is the time in one's life when the brain has the greatest capacity for learning.

16. HORMONES:

Hormones are the chemical substances whitch co-ordinate the activities of living organisms and also their growth. 

The hormones involved in the development and control of the reproductive organs and secondary sexual characteristics are called sex hormones. 

The male sex hormone is known as testosterone and female sex hormone is known as oestrogen. 

17. Exocrine glands and endocrine glands

A gland is a structure which secretes a specific substance or substances in the body. A gland which secretes is product into a duct or tube is called and exocrine gland. For example, salivary glands secrets is product into a duct called salivary duct. Sweat glands and sebaceous glands are also exocrine glands. 

A gland which does not have a duct and secrets is product directly into the bloodstream is called an endocrine gland. An endocrine gland makes a chemical substance called hormone. Pituitary gland, thyroid gland, adrenal glands are example of endocrine gland. 

17: SEX HORMONES:

The onset of puberty is controlled by hormones. The testes make male sex hormone called testosterone and ovaries make female sex hormone called oestrogen. The production of sex hormone is controlled by pituitary gland. 

18. FUNCTIONS OF OF TESTOSTERONE AND OESTROGEN:

Testosterone: Testosterone hormone performs the following functions:

a. Testosterone hormone produces male secondary sexual characteristics in boys at puberty.

b. Testosterone hormone causes the growth and development of male sex organs at puberty.

c. Testosterone hormone causes growth spurt in boys at puberty.

Oestrogen: Oestrogen hormone perform the following functions. 

a. Oestrogen hormone produces female secondary sexual characteristics in girls at puberty.

b. Oestrogen hormone causes the growth and development of female sex organs at puberty. 

c. Oestrogen hormone brings about the monthly preparation of uterus for pregnancy.

19. Menstrual cycle:

In females the reproductive phase of life begins at puberty at the age of 10 to 12 years and generally lasts till the age of approximately 45 to 50 years. 

PREGNANCY:

With the onset of puberty, the eggs begin to mature in the ovaries of a woman. One mature egg is released by one of the over is of the women once in about 28 to 30 days. During this period, the inner lining of uterus grows and becomes thick and spongy, and prepares itself to receive the fertilized egg. So, in case the fertilization of egg cell occurs by a sperm, the fertilized Excel begins to divide to form an embryo. The embryo then gets embedded in the thick uterus lining. This results in pregnancy. 

MENSTRUATION:

If fertilization does not occur, then the egg released by the ovary dies within a few days and the thick lining breaks down. Since the thick uterus lining contains a lot of blood vessels therefore the breaking down of uterus lining produces blood along with other tissues. This blood and the other tissues come out of vagina of women in the form of a bleeding.

The bleeding from the uterus which occurs in a woman every month is called menstrual flow or menstruation. Menstruation occurs once in in about 28 to 30 days.

The first occurrence of menstruation activity is called Menarche. 

The permanent stoppage of menstruation in a woman is called menopause.

The process of ovulation and menstruation in women is called menstrual cycle and it is controlled by hormones. 

20. The two chromosomes that determine the sex of an offspring are called sex chromosomes. The two sex chromosomes are named X chromosome and Y chromosome. The baby develops from XX combination of sex chromosomes in zygote is called a girl. The baby develops from XY combination of sex chromosome in zygote is a boy. It is the sperm of man which determines the sex of the child. The egg cells of woman cannot decide the sex of the child because all the egg cells contain the same sex chromosome, X chromosome. 

Lecture-1 : CLASS-X: SCIENCE : Chapter: COLOURFUL WORLD

CLASS X  |    SCIENCE    |   Colourful World

      notes prepared by subhankar Karmakar

                                                                         

GLASS PRISM:

A prism is a wedge shaped portion of a transparent refracting medium bounded by two plane faces inclined to each other at a certain angle.

The two plane faces (ABED and ACFD) inclined to each other are called refracting faces of the prism.

The line (AD) along which the two refracting faces meet is called the refracting edge of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

The angle A included between the two refracting faces is called angle of the prism.

Any section of the prism cut by a plane perpendicular to the refracting edge is called principal section of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

ANGLE OF DEVIATION FOR A PRISM:

The angle of deviation D is the angle between the incident ray and the emergent ray. 

Difference between when light travels through a glass slab and light travels through a prism:

 

In refraction through a glass slab , the emergent ray is parallel to the incident ray, but in refraction throigh a glass prism, the emergent ray is not parallel to the incident ray.

DISPERSION OF LIGHT:

The phenomenon of splitting of white light into its component colours on passing through a refracting medium is called "dispersion of light." The pattern of the coloured bands obtained on the screen is called "spectrum."

CAUSES OF DISPERSION:

The dispersion of white light occurs because different colours of white light travel at different speeds through the glass prism. 

"More is the speed, less is the deviation"

(i) Red colour travels with maximum speed in glass prism, hence, red colour deviates least or minimum.

(ii) Violet colour travels with minimum speed in glass prism, therefore, violet colour deviates maximum.

RECOMBINATION OF LIGHT:

Opposite of dispersion is called recombination of light. 

Where as splitting of white light into seven colours is called dispersion, but when seven colours combined together to give white light, it is called "recombination" of light.

RAINBOW FORMATION:

The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun's light by raindrops in the atmosphere. Therefore, we can say, a rainbow is produced by the dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum of seven colours.

(i) Why planets don't twinkle

In comparison to the stars planets  are very near to us. Therefore size of the planets seem to bigger to us. The continuously changing atmosphere is unable to cause variations in the light coming from a big sized planet because of which the planet does not twinkle at all.

(ii) Atmospheric Refraction:

The refraction of light caused by the earth's atmosphere give two different layers of  varrying optical densities is called atmospheric refraction

The different phenomena occur due to optical refraction of light

1) twinkling of stars

2) the stars seem higher then they actually are

3) advance sunrise and delayed sunset

(iii) Why does twinkling of a star  occur?

Twinkling of a star occurs due to atmospheric refraction of star's light. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. Therefore the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and hence the star appears to twinkle at night.

In the fig the actual position of a star is given, but due to atmospheric refraction the light from the star gradual curved as shown in the figure. When it ultimately reach our eyes, we see the stars at a height which is more than its actual height. The light bends due to the difference of optical densities of different layers of atmosphere. Hence ,"The Stars Seem Higher Than They Actually Are."

(iv) Advance Sunrise and Delayed Sunset :

We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.

So we observe an advance sunrise and a delayed sunset.

When the sun is below the horizon actually the rays of sunlight get bend due to atmospheric refraction and reaches our eyes and we observe the sun rise although the sun is still below the horizon. 

Similarly when the Sun already goes below the horizon after actual sunset, but still rays from the sun get bend due to atmospheric refraction and we observe that sunset still not happen. Therefore, we see a delayed sunset.

SCATTERING OF LIGHT

Q1. What is scattering of light? 

Ans. Scattering of light means to through light in various random directions. When light rays fall on various types of suspended particles in its path, it is scattered in random directions. Depending on the size of particles, different colour of light are scattered. The most prominent examples of scattering of light are (i) blue colour of the sky, (ii) red colour of the sun at sunrise as well as at sunset. 

Q2. What is Tyndall effect?

Ans. The scattering of light by particles in its path is called tyndall effect. When a beam of sunlight enters a dusty room through his window the tiny dust particles present in the air of room scatter the beam of light all around the room, and thus its path becomes visible to us. 

Q3) On what factor the colour of scattered light depends upon. Explain.

Ans. The colour of scattered light depends upon the size of the scattering particles in the atmosphere. Like, the larger particles of dust and water droplets present in the atmosphere scatter the light of all the colours due to which the scattered light appears to be white. As larger particles scatter all the colours present in the white light. But air molecules in the upper atmosphere where the amount of dust particles are low, they are very small compared to air particles and hence they scatter only light with low wavelength like blue. Due to this reason, the atmosphere looks blue to us.

Q4. Why the colour of the sky is blue?

The scattering of blue component of the white sunlight by air molecules of small size are responsible for blue colour of the sky. As we know scattering of light depends upon the size of the scattering particles. Where as dust particles are larger and scattered all the colours of visible light but air molecules like nitrogen and oxygen are much smaller in size hence they only scatter colours with low wavelengths like blue. It is the real reason behind the fact that our sky looks blue in colour.

Q5. Why the sun appears red at sunrise and sunset.

Ans. The sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.

Q6. Perform an experiment to study the scattering of light.

Ans.

 To show the scattering of light we can perform an experiment. For this we shall take a source of light S placed at the focus of a convex lens L1 so as to produce a parallel beam of light rays. A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc d having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to focus the light rays to form an image on the screen R. If we dissolve about 200 grams of sodium thiosulphate in water of the glass tank. Then add 1 or 2 ml of concentrated sulphuric acid to the water, we will see the fine microscopic particles of sulphur begin to form in water and colloidal solution is obtained. As the sulphur particles begin to form in water we will see the blue light coming from the sides of the glass tank. This is due to the scattering of short wavelength blue light by the minute colloidal sulphur particles. This is how the sky looks blue.

©Subhankar Karmakar

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 9 : REPRODUCTION IN ANIMALS

CLASS VIII   |    SCIENCE    |    CHAPTER 9

      notes prepared by subhankar Karmakar

                                                                         

Reproduction in Animals:

REPRODUCTION: 

The production of new organisms from the existing organisms of the same species is known as reproduction. It is essential for the survival of species on this earth. The process of reproduction ensures continuity of life on earth. 

METHODS OF REPRODUCTION: 

There are two main methods of reproduction in living organisms.

a. Asexual reproduction

b. Sexual reproduction

Special reproductive cells are called sex cells or gametes.  

ASEXUAL REPRODUCTION: 

The production of a new organism from a single parent without the involvement of sex cells or gametes is called asexual reproduction. 

In asexual reproduction, a part of the parent organism separates of and grows into a new organism. Thus, in asexual reproduction, only one parent is needed to produce a new organism. 

Some of the examples of asexual reproduction are binary fission in amoeba and budding in hydra.

SEXUAL REPRODUCTION:

The production of a new organism from two parents by making use of their sex cells or gemetes is called sexual reproduction. 

In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called zygote.

This zygote then grows and develops to form a new organism. 

Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female. 

Male and female parents have special organs in which produce male sex cells and female sex cells respectively.

The humans, fish, frog, cats and dogs all produced by the method of sexual reproduction.

In asexual reproduction, the young one produced is an exact copy of the parent whereas in sexual reproduction, the young one produced is not an exact copy of the parents.

MALE AND FEMALE:

An animal having male sex cells called sperms in its body is called male and having female sex cells called eggs or ova in its body is called female. 

GAMETES:

The cells involved in sexual reproduction are called gametes. There are two types of gametes, (a) male gametes and (b) female gametes. The male gametes in animals is called sperm and female gamete in animals is called egg or ovum.

SPERMS: 

The sperms are extremely small cells and can be seen only through microscope. It is is about 0.05 millimetre long. It has a head, middle piece and a tail. Sperm is a single cell with all the  cell components like nucleus, cytoplasm and cell membrane. 

The purpose of tail of sperm is to move it by waving its tail from side to side. 

EGGS or OVA:

The eggs or ova are also very small and we need a microscope to see the eggs are ova. The egg or ovum is also a single cell having a nucleus, cytoplasm and a cell membrane. Outside the cell membrane, an egg or ovum has a thin layer of jelly called jelly coat which allows only one sperm to enter into it during fertilisation. 

The nuclei of sperm and egg contain chromosomes which carry genes and transmit the characteristics of parents to the offspring. The fusion of a male gamete called sperm with the female gamete called egg gives rise to a new cell called zygote. Thus, the new cell which is formed by the fusion of a male gamete and female gamete is called zygote.

Zygote is a single cell which contains one nucleus. The nucleus of zygote is formed by the combination of nuclei of sperm and egg.

The process of fusion of gametes is called fertilisation. 

FERTILISATION:

Sexual reproduction will occur when a male gamete combines with a female gamete. the fusion of male gamete called sperm with female gamete called ovum to form a zygote  during sexual reproduction is called fertilisation. Therefore we can say the zygote is actually a fertilised egg or ovum. 

All the multicellular animals start their life from a single cell called zygote through sexual reproduction.

There are two types of fertilisation process known as internal fertilisation and external fertilisation.

INTERNAL FERTILISATION:

The fertilisation takes place inside the female body is called internal fertilisation. The process of fertilization in humans the fusion of male gamete of sperm and female gamete or a occurs inside the body of a female animal is called internal fertilisation. 

EXTERNAL FERTILISATION:

The fertilization which takes place outside the female body is called external fertilisation. In external fertilisation, the male and female animals released their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilised them outside the body of female frog or fish. 

External fertilisation is very common in aquatic animals such as frog fish and starfish etc. 

Although the animals like frog and fish which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs or fishes. 

This is because of the following reasons.

(i) frog of fish lay eggs or release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall and hence all the eggs do not get fertilised. 

(ii) many of the eggs of frogs are eaten by other animals which live in water. 

(iii) many of the larvae of frog and fish which are hatched from the fertilized eggs are eaten up by other animals in water and fail to develop into adult frogs or fishes.

SEXUAL REPRODUCTION IN ANIMALS

The whole process of sexual reproduction in animals involves the formation of sperms and eggs, joining together of sperm and egg to form a fertilized egg called zygote and growth and development of zygote to form a baby animal. 

The sexual reproduction in animals takes place in the following steps:

(a) the male parent produces male gametes called sperms.

(b) the female parent produces female gametes called eggs or ova.

(c) The sperm enters into egg. The nucleus of sperm fuses with the nucleus of egg cell to form a new cell called zygote. 

(d) The zygote divides repeatedly to form a hollow ball of hundreds of cells which is called embryo. 

(e) Embryo grows and becomes a foetus (in which all the main body features of the baby animal have formed).

(f) Foetus grows and develop to form a new baby animal. 

SEXUAL REPRODUCTION IN HUMANS

In human beings, there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation and make a zygote; and for the growth and development of a zygote into a baby. 

THE MALE REPRODUCTIVE SYSTEM

The human male reproductive system consists of the following organs: Testes, Scrotal sacs, Epididymis, Sperm ducts, Seminal vesicles and Penis. 

Testes are the real reproductive organs in men. Testes make male gametes called sperms. Millions of sperms are produced in the testes. The testes are enclosed in two small bags of skin called scrotal sacs. 

The sperms formed in testes come out and go into a coiled tube called Epididymis. From epididymis, the sperms are carried by a long tube called sperm duct into organs called seminal vesicles. The seminal vesicles join to another tube called urethra coming from the bladder. Urethra carries the sperms to an organ called penis which opens outside the body. These sperms are carried in a liquid called semen. When the penis introduces sperms into the woman, one of the sperms be able to fuse with an egg cell and bring about fertilisation. 

THE FEMALE REPRODUCTIVE SYSTEM

The human female reproductive system consists of the following organs: Ovaries, Oviducts, Uterus and Vagina. 

Ovaries are the real reproductive organs in a woman. Ovaries make the female gametes called eggs or ova. Just above the ovaries, there are tubes called oviducts or fallopian tubes. In human beings, one mature egg is released into oviduct every month by one of the ovaries. The fertilisation of egg by a sperm takes place in the oviduct. 

The process of reproduction in humans takes place in two steps:

1. Fertilisation, and

2. Development of embryo.