LECTURE: 1 : CLASS XI: PHYSICS : UNITS & MEASUREMENTS

CLASS XI   |    PHYSICS    |    CHAPTER 2

      notes prepared by subhankar Karmakar

PHYSICAL QUANTITIES:

All those quantities which can be measured directly or indirectly and in terms of which the laws of Physics can be expressed are called physical quantities. For example, length, mass, time, speed, temperature, force, electric current, angle etc are physical quantities.

TYPES OF PHYSICAL QUANTITIES:

Physical quantities are of two types:

a. Fundamental quantities

b. Derived quantities

a. FUNDAMENTAL QUANTITIES:

The physical quantities which can be treated as independent of other physical quantities and are not usually defined in terms of other physical quantities are call fundamental quantities. There are seven fundamental quantities, and two supplementary fundamental quantities.

The fundamental or base quantities are:

1. Mass

2. Length

3. Time

4. Electric current

5. Thermodynamic temperature

6. Luminous intensity

7. Amount of substance

Two supplementary fundamental quantities are:

1. Angle

2. Solid angle

b. DERIVED QUANTITIES:

The physical quantities who's defining operations are based on other physical quantities are called derived quantities. All physical quantities other than fundamental quantities are derived quantities. For example, velocity, speed, acceleration, force, momentum etc are derived quantities.

THE MEASURING PROCESS:

Measurement: The measurement of a physical quantity is the process of comparing this quantity with a standard amount of the physical quantity of the same kind, called its unit. 

The measurement of a physical quantity has two components.

1. The unit in which the quantity is measured (u)

2. The numerical value or the magnitude of the quantity. (n)

If the measure of a physical quantity = Q

Numerical value of the physical quantity = n

Size of the unit = u, then

Q = nu

If n₁ and n₂ are the numerical values for a physical quantity Q corresponding to the units u₁ and u₂ , then

Q = n₁u₁ =  n₂ u₂

PHYSICAL UNIT:

The standard amount of a physical quantity chosen to measure the physical quantity of the same kind is called a physical unit.

Desirable characteristics of a physical unit:

1. It should be well defined.

2. It should be of convenient size. Neither too small, nor too large.

3. It should not change with time.

4. It should be easily reproducible.

5. It should be imperishable or indestructible.

6. It should not be affected by the change in physical conditions such as variation of pressure temperature etc.

7. It should be internationally acceptable.

8. It should be easily accessible.

FUNDAMENTAL & DERIVED UNITS:

Fundamental units: The physical units which can neither be derived from one another, nor they can be further resolved into more simpler units are called fundamental units. Units of fundamental quantities are fundamental units.

Derived units: All the other physical units which can be expressed in terms of the fundamental units are call derived units. Like unit of force is Newton, but it can be expressed in terms of fundamental units.

Force = Mass x acceleration

1 N = 1 kg x 1 m /s² = 1 kg m /s²

DIFFERENT SYSTEM OF UNITS:

a. cgs system = It was set up in France. It is based on centimetre, gram and second as the fundamental units of length, mass and time respectively.

b. mks system = It is also a French system based on metre, kilogram and second as the fundamental units of length, mass and time respectively.

c. SI system = It is the international system of units. What is the modernization extended form of the mks system. 

BASIC SI QUANTITIES AND UNITS:

Basic quantity - basic unit - symbol

1. Length - metre - m

2. Mass - kilogram - kg

3. Time - second - s

4. Temperature - kelvin - K

5. Electric Current - ampere - A

6. Luminous Intensity - candela - cd

7. Quantity of matter - mole - mol

Supplementary units

1. Angle - radian - rad

2. Solid angle - steradian - sr

Definition of Radian and Steradian:

1. Radian: it is defined as the plane angle subtended at the centre of a circle by an Arc equal in length to the radius of the circle.

θ (in radians) = arc/radius = l/r

2. Steradian: it is defined as the solid angle subtended at the centre of a sphere why the surface of the sphere equal in area to that of a square having each side equal to the radius of the sphere. 

Ω (in steradian) = surface area/ radius²

Some common practical units:

1. Fermi = it is also known as femtometre. It is a very small unit of distance. It is used to measure nuclear distance. The radius of a proton is 1.2 fermi.

1 fermi = 10⁻¹⁵ m

2. Angstrom (Å): 

It is also a small unit of distance. It is used express wavelength of light.

1 Å = 10⁻¹⁰ m = 10⁻⁸ cm

3. Nano-metre:

It is also used to express wavelength of light.

1 nano metre = 10⁻⁹ m

4. Micron (μm)

It is also known as micro metre. 

1 μm = 10⁻⁶ m

5. Astronomical Unit ( AU)

It is defined as the the mean distance of the earth from the sun. It is a practical unit used for measuring large distances. It is used in astronomy to measure distances of planets.

1 AU = 1.496 x 10¹¹ m

6. Light year (ly)

It is the distance travelled by light in vacuum in one year. Light year is used in astronomy to measure distances of nearby stars. Like alpha centauri, the nearest are outside the solar system is 4.3 light years away from the Earth.

1 ly = 9.467 x 10¹⁵ m

7. Parsec or Parallactic Second:

It is the largest practical unit of distance used in astronomy. It is defined as the distance, at which an Arc of length 1 astronomical unit subtends an angle of 1 second of arc. 

1 parsec = 3.26 ly = 3.08 x 10¹⁶ m

Indirect method for measuring large distances:

a. Triangulation method for the height of an inaccessible object.

b. Parallax method to measure the distance of a nearby star. 

* Sextant: sextant is an instrument by which we can measure the angle of a distant object with the horizontal.

a. Triangulation method for the height of an inaccessible object.

Let AB = h be the height of the mountain to be measured. By using a sextant, we first measure the angle of elevation of its peak from my point C on the ground. Let it be θ₁ or ∠ACB  =  θ₁  . Move the sextant to another position D such that CD = d. Again measure the angle of elevation, ∠ADB = θ₂ . 

in right triangle ∆ABC, 

cot  θ₁ = CB/AB = x /h

in right triangle ∆ABD, 

cot  θ₂ = DB/AB = (d + x )/h

cot  θ₂ -  cot  θ₁ = (d + x )/h - x/h = d/h

∴ h = d / ( cot  θ₂ -  cot  θ₁ )

Hence, if we know d , the height h can be determined.

b. Parallax method

Parallax: parallax is the apparent shift in the position of an object with respect to another when we shift our eye side wise.

suppose we hold a pen O at a distance S from the eyes. Look at the pen first by the left eye L closing the right eye, and then buy the right eye R closing the left eye. The position of the  pen appears to change with respect to the background. This is called parallax. The distance between the two points of observation is called basis. In this case, the distance LR = b between the two eyes is the basis. ∠LOR = θ is called parallax angle or parallactic angle.

Parallax method can be used to find the

1. Distance of moon or any other planet.

2. Distance of a nearby star. 

1. Distance of moon or any other planet.

To measure the distance S of the moon or a faraway planet P, we observe it simultaneously from two different positions (observatories) A and B on the earth, separated by a large distance AB = b. We select a distant star S' whose position and direction can be taken approximately same from A and B. 

      Now, ∠PAS' = Φ₁ and ∠PBS' = Φ₂ are measured from the two observatories at the same time. As b<<S, so we can take AB as an arc of length b. 

      Now  θ = Arc/Radius = b/s

                      ∴ S = b/θ

where θ = ∠APB = Φ₁ + Φ₂ , is the parallactic angle. 

2. Distance of a nearby star. 

Suppose N is a nearby star whose distance d from the earth is to be found. F is a far off star whose direction and position is fixed for all the position of the earth in its orbital motion. When the earth is at position A, the parallax angle between distance star F and nearby star N is determined. Let it be θ₁ . After 6 months, the earth is at diametrically opposite position B. The parallax angle ∠NBF = θ₂ is measured. 

Total parallax angle subtended by N on the earth's orbital diameter AB is 

                θ = θ₁ +  θ₂

As,           θ = Arc/Radius

                θ = AB/d

             ∴ d = AB/θ

This Parallax method is useful for measuring distances of stars which are less than 100 light years away from the Earth. 

LECTURE -2 : CLASS VIII : SCIENCE : CHAPTER 8 : CELL STRUCTURE & FUNCTIONS

CLASS VIII   |    SCIENCE    |    CHAPTER 8

      notes prepared by subhankar Karmakar

Comparison of Plant Cells and Animal Cells:

* A General diagram of a plant cell

* A general diagram of an animal cell

The main similarities between plant cells and animal cells are given below:

1. Plant cells and animal cells have a cell membrane or plasma membrane around them.

2. Plant cells and animal cells have cytoplasm.

3. Plant cells and animal cells have a nucleus.

4. Plant cells and animal cells have a nuclear membrane.

5. Plant cells and animal cells have mitochondria.

The main differences between plant cells and animal cells are given below:

1. A plant cell has a cell wall around it, but an animal cell does not have a cell wall around it.

2. A photosynthetic plant cell has chloroplasts in it. Other plant cells have different plastids in them, but in an animal cell there is no chloroplasts or other plastids.

3. A plant cell has a large vacuole in it, but an animal cell usually does not have any vacuole, some animal cells have small vacuoles.

Prokaryotic cells and Eukaryotic cells:

a. Prokaryotic cells and prokaryotes:

The primitive cells, where nucleus of the cell is not properly organised and they do not have any nuclear membrane around the nuclear material and they are called prokaryotic cells. In prokaryotic cells, the nuclear material is in direct contact with the cytoplasm.

The organisms made of prokaryotic cells are called prokaryotes. 

 All the prokaryotes are simple, unicellular organisms. Most of the bacteria and blue green algae are prokaryotes. 

 b. Eukaryotic cells and Eukaryotes:

The cells having nuclear material enclosed by a nuclear membrane are called Eukaryotic cells. Eukaryotic cells have a proper, well organised nucleus and nucleus are separated from the cytoplasm by nuclear membrane. Eukaryotic cells are more advanced then prokaryotic cells. 

The organisms which are made of Eukaryotic cells are called Eukaryotes. All the organisms other than bacteria and blue green algae are Eukaryotes. For example, Amoeba is an Eukaryote. Eukaryotes may be unicellular or multicellular. 

Different organisms having variety in cell number, cell shape and cell size:

A. Variety in the number of cells:

Different organisms have different number of cells in their bodies.

B. Variety in shape of cells:

The cells in multicellular organisms have many different shapes.

C. Variety in size of cells:

The cells in multicellular organisms can have many different sizes.

A. Variety in the number of cells:

Depending on the number of the cells, in the body of an organism, an organism is called unicellular or multicellular. 

(i) Unicellular Organisms:

The organisms which are made up of only one cell are called unicellular organism. It is also known as single celled organism. Some of the examples of unicellular organisms are:

Amoeba, Paramecium, Euglena, Chlamydomonas and bacteria.

The single cell of all the unicellular organisms behaves as a complete organisms. A unicellular organism can perform all the necessary life functions with the help of just one cell. For example, Amoeba is a tiny animal which consists of only one cell but can perform all the basic functions of life like taking food, digestion, respiration, movement, response to environmental changes, removal of waste and reproduction etc. 

(ii) Multicellular Organisms:

The organisms which are made up of many cells are called multicellular organisms. Most of the plants and animals around us, including us are multicellular organisms. Multicellular organisms millions and billions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions. A multicellular organism starts its life as a single cell called fertilized egg cell or Zygote. 

B. Variety in shape of cells:

There are many types of cells in the bodies of multicellular organisms. These cells differ in shapes. For example, the shape of nerve cell in animals is very different from the shape of a muscle cell. A nerve cell is long and branched whereas a muscle cell is pointed at both ends and has a spindle shape and an epithelial cell is rectangular shape. Cells are different in shapes and sizes so that they can perform different functions. Some of the examples of animal cells which have different sizes and shapes are: nerve cell or neuron, muscle cell, epithelial cell, red blood cell, white blood cells, bone cell and cartilage cell. 

Different shape of a cell helps in its functioning. Like nerve cells are long and have projections so that they can make contacts with many other Nerve cells and carry messages over long distances like between brain and other parts of body. Muscle cells bring about the moment at body parts by contracting and relaxing, hence, they are pointed at the both ends and spindle shaped. 

Some of the important plant cells are: epidermal cells, xylem cells, phloem cells, and photosynthetic cells. The epidermal cells form a layer around the plant organs and protect the cells below from injury, xylem cells are the tube like plant cells having thick and strong walls which carry water and mineral salts from the roots of the plant to the leaves. Phloem cells are also tubelight plant cells having thin walls which carry the food made by leaves to all other parts of the plant. The photosynthetic cells of the plant contain chlorophyll and prepare food by photosynthesis. The mesophyll cells of leaf are the photosynthetic plant cells. These cells in the leaf of a plant are specially adapted for making food by photosynthesis. 

Amoeba: 

All the plant and animal cells are not capable of independent existence, the single celled organism like amoeba can exist independently. 

The shape of amoeba cell is irregular. In fact the Amoeba cell has no fixed shape. The 

Amoeba cell keeps on changing its shape continuously. Shape of Amoeba cell changes because amoeba can make its cytoplasm in any direction it wants to. The Amoeba cell finger like projections of varying lengths protruding out of his body which is called pseudopodia. 

Amoeba derives two advantages by changing shape: 

The changing of shape due to the formation of pseudopodia helps amoeba in 

1. movement

2. In capturing food.

C. Variety in size of cells:

The cells are of many different sizes. Some are so small that they cannot be seen with the naked eyes, whereas large cells also exists. Generally most of the cells are extremely small in size, like bacteria cells have a length of 0.1 micrometer ~ 0.5 micrometer. The smallest cell is bacteria mycoplasma, which is only 0.1 micrometre long. Whereas, the birds eggs are very large cells, they can be seen easily with naked eye. Each egg of the bird is a single cell, like the hen's egg is a single cell. The biggest cell is the ostrich egg which is approximately 17 cm long. The size of cells has no relation with the size of the body of an animal. Rather it is related to its function. 

LECTURE 1 : CLASS XI: PHYSICS : MOTION IN ONE DIMENSION

CLASS XI   |    PHYSICS    |    CHAPTER 3

      notes prepared by subhankar Karmakar

Rest : An object is said to be at rest if it does not change its position with respect to its surroundings with time.

Motion: An object is said to be in motion if it changes its position with respect to its surroundings with time.

Distance / path length: (s or x)

It is the length of the actual path travelled by a body between its initial and final positions. It is a scalar quantity.

The SI unit of distance is metre

The CGS unit of distance is centimetre

Displacement: (s or x)

The displacement of an object is the change in the position of an object in a fix direction. It is a vector quantity.

The SI unit of displacement is  metre 

The CGS unit of displacement is centimetre

Speed: (v)

The distance travelled per unit time is called speed. 

Speed = distance travelled/ time taken

         v = s/t

It is a scalar quantity

The SI unit of speed is m/s or ms⁻¹

The CGS unit of speed is cm/s or cms⁻¹

The dimensional formula of speed is [M⁰L¹T⁻¹]

Velocity: (v) 

The rate of change of position of an object which time in  a given direction is called its velocity.(v)

Velocity= displacement/time

It is a vector quantity.

SI unit of velocity is m/s

CGS unit of velocity is cm/s

The dimensional formula of velocity is [M⁰L¹T⁻¹]

Acceleration: 

The rate of change of velocity of an object with time is called its acceleration. (a)

Acceleration= change in velocity/time taken

It is a vector quantity.

The SI unit of acceleration is m/s²

The CGS unit of acceleration is cm/s²

The dimensional formula of acceleration is [M⁰L¹T⁻²]

Different types of speed

There are four types of speed

1. Uniform speed

2. Variable speed

3. Average speed

4. Instantaneous speed

Uniform speed: if a body covers equal distances in equal intervals of time the body is said to be in uniform speed.

Variable speed: if a body covers unequal distances in equal intervals of time the body is in variable speed.

Average speed: the ratio of Total distance travelled and total time taken is called average speed

Instantaneous speed: the speed of an object at any particular instant of time is called instantaneous speed of that object.

Different types of velocity:

There are basically four types of velocity

1. Uniform velocity

2. Variable velocity

3. Average velocity

4. Instantaneous velocity

Uniform velocity: a body is said to be moving with uniform velocity if it covers equal displacements in equal intervals of time.

Variable velocity: a body is said to be moving with variable velocity if either its speed changes or direction of motion changes or or both change with time

Average velocity: the ratio of total displacement and total time is called average velocity

Instantaneous velocity: the velocity of an object at a particular instant of time or at a particular point of its path is called its instantaneous velocity.

How to calculate average speed in different situations:

a. A body covering different distances with different speeds:

Suppose a body covers distances s₁, s₂, s₃.... with speeds v₁, v₂, v₃... respectively, then its average speed will be

vₐ = (s₁+ s₂ + s₃... )/(t₁ + t₂ + t₃... )

⟹ vₐ =  (s₁+ s₂ + s₃... )/(s₁/v₁ + s₂/v₂ +  s₃/v₃ .....)

(i) if s₁ = s₂ = s

vₐ =  (s₁+ s₂)/(s₁/v₁ + s₂/v₂)

⟹ vₐ = (s + s)/(s/v₁ + s/v₂)

⟹ vₐ = (2s)/s(1/v₁ + 1/v₂)

⟹ vₐ = (2v₁v₂)/(v₁ + v₂)

As an example suppose a body travels from A to B at 40 m/s and from B to A at 60 m/s. Calculate the average speed.

Soln. Here the distances of same ie.  s₁ = s₂ = s

As AB distance = BA distance

∴  vₐ = (2v₁v₂)/(v₁ + v₂)

    ⟹ vₐ = (2x40x60)/(40 + 60)

    ⟹ vₐ = (2x40x60)/100

    ⟹ vₐ = 48 m/s

b. A body moving with different speeds in different time intervals:

Suppose a body travels with speeds v₁, v₂, v₃, ...

In time intervals   t₁,  t₂,  t₃ .......

Respectively, then

Total distance travelled = v₁t₁ +  v₂t₂ + v₃t₃ + .....

Total time taken = t₁ + t₂ + t₃ + ........

∴  vₐ = (v₁t₁ +  v₂t₂ + v₃t₃ + .....)/( t₁ + t₂ + t₃ + ..)

As an example, if a body travels along a straight line for the first 10 minutes with speed 30 km/h, and the next 20 minutes with speed 60 km/h. Find the average speed of the body.

Soln. Here,  v₁ = 30 km/h; v₂ = 60 km /h

t₁ = 10 min = 10/60 h = 1/6 h

t₂ = 20 min = 20/60 h = 2/6 h

∴  t₁ + t₂ = 1/6 + 2/6 = 3/6 = 1/2 h

We know,  vₐ = (v₁t₁ +  v₂t₂)/( t₁ + t₂)

∴  vₐ = {30x(1/6)+60x(2/6)}/(1/2)

⟹ vₐ = (5 + 20)/(1/2)

⟹ vₐ = 25x2 = 50 km/h

CLASS XI: PHYSICS : CHAPTER-1 : PHYSICAL WORLD

CLASS XI   |    PHYSICS    |    CHAPTER 1

      notes prepared by subhankar Karmakar

Q1. What is scientific method? Mention the various steps involved in it.

Ans. Scientific method: the step-by-step approach used by a scientist in studying natural phenomena and establishing laws which govern these phenomena is called scientific method. 

Generally, it involves the following steps:

a. Taking a large number of systematic observations by doing controlled experiments.

b. Studying these observations and making qualitative and quantitative reasoning.

c. Suggesting mathematical models to account for the observed behaviour.

d. Predicting new phenomena on the basis of suggested model.

e. Modifying the theory, if necessary, the light of fresh evidences.

Q2. Name the four basic forces in nature. Arrange them in the order of their increasing strengths.

Ans. Fundamental forces in nature: 

In the macroscopic world, we observe several kinds of forces. All the forces between microscopic objects arise from two fundamental forces:

a. Gravitational force(F_g) 

b. Electromagnetic force. (F_e)

In the microscopic world, there are two more basic forces:

c. Strong nuclear force(F_s)

d. Weak nuclear force (F_w)

(F_g) : (F_w) : (F_e) : (F_s) = 1: 10²⁵ : 10³⁶ : 10³⁸

Q3. Describe briefly the fundamental forces in nature.

Ans. There are four fundamental forces in nature. They are as follows:

a. Gravitational force(F_g) 

b. Electromagnetic force. (F_e)

c. Strong nuclear force(F_s)

d. Weak nuclear force (F_w)

Gravitational force(F_g) : it is a force of mutual attraction between two bodies by virtue of their masses. According to the Newton's law of gravitation, the gravitational attraction between two bodies of masses m₁ and m₂ separated by a distance r is given by

   F_g = Gm₁m₂/r²

Where G is the universal gravitational constant.

Electromagnetic force (F_e): 

The force acting between two electric charges at rest is called electrostatic force. Electrostatic force is a part of electromagnetic force. According to Coulomb's law, the magnitude of the electrostatic force (F_e) between two point charges q₁ and q₂ separated by a distance r in vacuum is given by

(F_e) = (1/4πεₒ).q₁q₂ / r²

Where εₒ is the permittivity of vacuum. 

The force acting between two magnetic poles is called magnetic force.

Together electrostatic force and magnetic force is known as electromagnetic force.

Weak nuclear force (F_w):

It is a force that appears only between elementary particles involved in a nuclear process such as the β-decay of a nucleus.  It is a short range force that operates only over the size of a nucleus (= 10⁻¹⁵ m)

Strong nuclear force(F_s):

The strong attractive force which binds together the protons and neutrons in a nucleus is called strong nuclear force. It is a short range force that operates only over the size of a nucleus (= 10⁻¹⁵ m)

Q4. What are the different conservation laws in Physics?

Ans. Conservation laws: in any physical process involving the different forces, some physical quantities, remain unchanged with time. Such quantities are called conserved quantities. The laws which govern the conservation of these quantities are called conservation laws.

There are four basic conservation laws:

a. Law of conservation of energy.

b. Law of conservation of linear momentum

c. Law of conservation of angular momentum

d. Law of conservation of charge.

Q5. Name the scientist who was first awarded two Nobel prizes.

Ans. Madame Marie Curie , for physics ine 1903 and for chemistry in 1911.

Q6. Who first discovered radioactivity?

Ans. A H Becquerrel in 1896.

Q7. Mention some important contributions of Albert Einstein to physics.

Ans. a. Mass energy equivalence

         b. Photoelectric effect

         c. Special theory of relativity

         d. General theory of relativity.

Q8. Name the Indian physicist who was first awarded the Nobel prize.

Ans. C V Raman.

Q9. What was the important discovery of CV Raman?

Ans. Inelastic scattering of light by molecules.

Q10. What was the major contribution of Indian physicist SN Bose?

Ans. Quantum statistics ( Bose Einstein statistics)

Q11. With which field work was the famous Indian physicist H J bhabha associated?

Ans. Cascade process in cosmic radiation.

Q12. Name the Indian physicist associated with the triple helical structure of proteins.

Ans. G N Ramachandran.

CLASS XI: PHYSICS : MOTION IN ONE DIMENSION : NUMERICALS

CLASS XI  |    PHYSICS    |    CHAPTER 3

      notes prepared by subhankar Karmakar

Q.1. A car is moving with a velocity 54 km per hour. It suddenly applied brake and comes to rest within 0.6 s. How much distance it will travel before it comes to rest?

Q2. A body travels a distance s₁ with velocity v₁ and distance s₂ with velocity v₂ in the same direction. Prove that the average velocity of the body

vₐ = {(s₁ + s₂)v₁v₂}/(s₁v₂ +  s₂v₁)

Q3. A car travels along a straight line for the first half time which speed 50 km/h and the second half time with speed 60 km/h. Find the average speed of the car.

Q4. On a foggy day, two driver spot each other when they are just 80m apart. They are travelling at 72 km/h and 54 km/h respectively. Both of them applied brakes retarding their cars at the rate of 5 m.s⁻². Determine whether they never collision or not.

Q5. A car accelerates from rest at a constant rate α for some time, after which it decelerates at a constant rate β to come to rest. If the total time elapsed is t second, then prove that

(i) the maximum velocity attained by the car

  Vₘₐₓ = (αβt)/(α + β)

(ii) the total distance travelled buy the car

  X  = (αβt²)/2(α + β)

Q6. A body covers 12 m in 2nd second and 20 m in 4th second. How much distance will it cover in 4 seconds after the 5th second?

Q7. Two buses A and B are at positions 50 m and 100 m from the origin at time t = 0. They start moving in the same direction simultaneously with uniform velocity of 10 m/s and 5 m/s. Determine the time and position at which A overtakes B. 

Q8.  A body covers a distance of 20 m in the 7th second and 24 m in the 9th second. How much shall it cover in 15th second?

Q9. An object is moving with uniform acceleration its velocity after 5 seconds is 25 m/s and after 8 seconds it is 34 m/s. Find the the distance travelled by the object in 12th second.

Q10. A ball thrown vertically upwards with a speed of 19.6 m/s from the top of a tower returns to the earth  in 6 second. Find the height of the tower. 

Q11. A ball is thrown vertically upwards with a velocity 20 m/s from the top of a multistoried building. The height of the point from where the ball is thrown is 25 m from the ground.

(a) How high will the ball rise? 

(b) How long will it be before the ball hits the ground?    (Take g = 10 m/s²)

Q12. Two balls are thrown simultaneously, A vertically upwards with a speed of 20 m/s from the ground, and B vertically downwards from a height of 40 m with the same speed and along the same line of motion. At what point do the two balls collide? 

LECTURE -1 : CLASS VIII : SCIENCE : CHAPTER 8 : CELL STRUCTURE & FUNCTIONS

CLASS VIII   |    SCIENCE    |    CHAPTER 8

      notes prepared by subhankar Karmakar

Cell : Cells are the basic units of life. All the living things both plants and animals, are made from cells. Cells are tiny basic unit of structure and function of all living organisms. Cells are the building blocks of plants and animals. Therefore, we can say a cell is the smallest unit of life which has a definite structure and performs specific function.

Microscope: The instrument used to study cells is called microscope.

Discovery of cell: an English scientist named Robert hooke discovered cell in 1665.

In 1838, a German scientist Schleiden first suggested that all plants are made of cells. 

In 1839, another German scientist Schwann suggested that all animals are made of cells too. 

Cell division: all cells arise from pre-existing cells by a method which is called cell division.

Classification of cell: All cells are primarily categorised into two main types. Animal cells and plant cells. 

Structures of a cell: Each cell has a number of smaller parts or internal structures in it. Some of the structures are present in both animal and plant cells, while some structures are only present in plant cells, they are not present in animal cells. They are also called parts of a cell.

The parts of a cell present in both animal and plant cells: 

Cell membrane or plasma membrane, Cytoplasm, Nucleus and Mitochondria. These basic parts are present in in both animal as well as plant cells. 

.                  Parts of a cell

The parts of a cells present in plant cells only:

Cell wall, Chloroplasts, Large vacuole are present in plant cell only. 

.               Parts of a Plant Cell

1. Cell membrane or plasma membrane:

Every cell is covered by a thin sheet of skin which is called cell membrane or plasma membrane. The cytoplasm and nucleus are enclosed within the cell membrane. Therefore we can say, cell membrane encloses the living part of the cell which is known as protoplasm. 

The cell membrane protects the cell and also gives shape to the cell. The cell membrane has tiny pores in it. Through this pores, the cell membrane controls the movement of substances "into the cell" and "out of the cell" like the dissolved glucose and oxygen can enter into the cell whereas the waste products like carbon dioxide can go out from the cell through this pores of the cell membrane. 

Cell membrane separates the cell from other cells as well as from the surrounding medium. The cell membrane or plasma membrane is a living part of the cell.

2. Cytoplasm: 

Cytoplasm is a transparent jelly like material which fills the cell between nucleus and the cell membrane. It acts like a chemical factory of the cell. In cytoplasm, new substances are built from materials taken into the cell and energy is released and stored. It is a place where most of the chemical reactions which keep the cell alive take place. Cytoplasm has many internal structures which are known as organelles. The most prominent internal structure in the cytoplasm is nucleus. It also contains mitochondria, Golgi bodies, ribosomes etc. Cytoplasm for plant cells also contain chloroplast. Cytoplasm and the nucleus taken together is called protoplasm.

3. Nucleus:

Nucleus is a large spherical organelle present in all the cells. In animal cell, nucleus lies in the centre of the cell whereas in plant cells the nucleus maybe on the periphery of the cell.  Nucleus is separated from the cytoplasm by a membrane called nuclear membrane. Nucleus controls all the activities of cell. The transmission of characteristics from the parents to the offsprings is called inheritance and Nucleus plays a role in inheritance.  

Chromosome: Nucleus contains thread like structure called chromosome. The function of chromosomes is to transfer the characteristics from the parents to the offspring through the genes and it is called inheritance. 

Nucleolus: The nucleus also contains a tiny round structure called nucleolus. The nucleus containing chromosomes and nucleolus is bound by a membrane called nuclear membrane. Nuclear membrane has tiny pores for the exchange of materials with cytoplasm. Some cells like red blood cells do not have a nucleus.

Gene: Gene is a unit of inheritance in living organisms, which is transferred from parent to offspring during reproduction and determine some characteristic feature of the offspring. Thus genes control the transfer of hereditary characteristics from parents to offsprings. 

Protoplasm: All the living matter in a cell is called protoplasm. Protoplasm is a liquid substance which is present inside the cell membrane. Protoplasm includes cytoplasm, nucleus and other organelles. Most of the protoplasm is made up of compounds of only 4 elements Carbon, Hydrogen, Nitrogen and Oxygen. It also contains water, carbohydrates, proteins, nucleic acids, fats and mineral salts.

4. Mitochondria: 

Mitochondria are the tiny rod-shaped or spherical organelles which are found in all the cells. Mitochondria provide energy for all the activities of the cell. This energy is produced by the process of respiration in which food such as glucose is broken down by oxygen. Thus mitochondria use glucose and oxygen to produce energy. Mitochondria are found in large numbers in the cytoplasm in all the cells.

5. Cell wall: The plant cells have a thick cell wall around them outside the cell membrane. The cell wall is made of a tough material called cellulose. Cell wall gives shape and support to the plant cell. Cell walls also hold the plant cells together and give plants most of their strength. Plants need protection against high wind speed, variation in temperature and atmospheric moisture etc. Since plants are fixed at a place so they cannot move to protect themselves from the various changes in their surroundings. The tough cell wall present in plant cells provides protection to plants. Cell wall is not there in animal cells. The cell wall is a non living part of the plant cells.

6. Chloroplasts: Chloroplasts are the green coloured organelles present in the cytoplasm of plant cells. The process of food making by plants known as photosynthesis takes place in chloroplasts. The green colour of chloroplasts is due to the presence of a green pigment called chlorophyll in them. Chlorophyll can absorb sunlight energy. In the chloroplasts, carbon dioxide and water combine in the presence of sunlight energy to produce food such as glucose. And this process of food making is called photosynthesis. 

The organelles containing pigments or colour matter present in the cytoplasm of plant cells are called plastids. Plastids can be of many different colours. The plastids containing green pigment or chlorophyll are known as chloroplasts. Chloroplast are present in the cells of leaves. It makes the leaves green. The plastids are may be of different colour like red colours of tomatoes are due to red plastids. Different colours of flowers are also due to this plastids.

7. Large Vacoule: 

Vacoule is a space in the cytoplasm of a cell which is enclosed by a membrane and is usually contains substances dissolved in water. A vacoule appears as an empty space under the microscope. All the plant cells have a large vacoule. The vacuole is filled with a liquid called "cell sap" , which contains dissolve sugar and salts. The function of vacuole in a cell is to store various substances including waste products of the cell.

* In amoeba vacuoles contain food particles so they are called food vacuoles.

Lecture: 11 :CLASS VIII : SCIENCE : Chapter 2: QUESTIONS & ANSWERS

CHAPTER TWO: MICRO-ORGANISMS: FRIENDS AND FOE

Very Short Answer Type Questions

1. Question. Name the instrument (or device) which is needed to see the micro-organisms.

Answer. Microscope is needed to see the micro-organisms.

2. Question. What is the name of micro-organisms which reproduce only inside the living cells of other organisms ?

Answer. Viruses are the smallest micro-organisms which reproduce only inside the living cells of other organisms.

3. Question. What are the major groups of micro-organisms?

Answer. There are five major groups of micro-organisms. These are: Bacteria, viruses, Protozoa, some fungi and Algae.

4. Question. Name any two human diseases caused by bacteria.

Answer. Typhoid and Cholera are caused by bacteria in human beings.

5. Question. Name any two human diseases caused by viruses.

Answer. The human diseases such as common cold and influenza are caused by virus. 

6. Question. Name any two human diseases caused by protozoa.

Answer. The human diseases amoebic dysentery and malaria are caused by protozoans.

7. Question. Name any two human diseases caused by fungi.

Answer. Diseases athlete’s food and ring worm are caused by fungi.

8. Question. Which micro-organism is utilized in making curd from milk ?

Answer. Lactobacillus bacterium is utilized in making curd from milk.

9. Question. Name the -micro-organism which is used for the large scale production of alcohol.

Answer. Yeast is used for the large-scale production of alcohol.

10. Question. Name any two antibiotics.

Answer. Streptomycin and erythromycin are two commonly known antibiotics which are made from fungi and bacteria.

11. Question. Name an antibiotic extracted from fungus (mould). Name the fungus.

Answer. An antibiotic Penicillin was extracted from fungus (mould) by Alexander Fleming in 1929. This antibiotic was made from the fungus called penicillium.

12. Question. Name any four diseases which can be prevented by vaccination.

Answer. Several diseases including tuberculosis, smallpox, cholera and hepatitis can be prevented by vaccination.

13. Question. Name the scientist who discovered the-vaccine for 'smallpox.

Answer. Edward Jenner discovered the vaccine for smallpox in 1978.

14. Question. Name the scientist who discovered 'penicillin'.

Answer. Alexander Fleming discovered the antibiotic “Penicillin” in 1929.

15. Question. State an important function performed by blue-green algae.

Answer. Blue green algae fix the atmospheric nitrogen to enrich soil with nitrogen and hence, increase soil fertility.

16. Question. Name one 'biological nitrogen-fixer'.

Answer. Blue green algae are known as 'biological nitrogen-fixer' as they fix the atmospheric nitrogen.

17. Question. Name two common insects which act as carriers of disease-causing micro-organisms (or disease-causing microbes).

Answer. The two common insects which act as carriers of disease-causing micro-organisms (or disease-causing microbes) are housefly and mosquito.

18. Question. Name any two diseases spread by housefly.

Answer. Diseases cholera and tuberculosis spread by housefly.

19. Question. Name the insect which is the carrier of parasite of malaria.

Answer. The carrier of parasite of malaria is Female Anopheles mosquito.

20. Question. Name the insect which carries dengue virus.

Answer. Female Aedes mosquito carries dengue virus.

21. Question. Which of the two spreads dengue: mosquito or housefly ?

Answer. Female Aedes mosquito acts as a carrier of dengue virus.

22. Question. Name two diseases spread by mosquitoes.

Answer. Malaria and dengue spread by mosquitoes.

23. Question. Name the microbe which causes malaria disease.

Answer. Protozoa Plasmodium causes malaria disease.

24. Question. Name one disease which spreads by breathing in air containing microorganisms.

Answer. Tuberculosis disease spreads by breathing in air containing micro-organisms.

25. Question. Name one disease which spreads through insect bites.

Answer. Malaria disease spreads through insect bites.

26. Question. Name one disease which spreads through infected food or water.

Answer. Cholera disease spreads through infected food or water.

27. Question. Name the causative micro-organisms of the following animal diseases:

(a) Foot and mouth disease 

(b) Anthrax

Answer. (a) Foot and mouth disease is caused by a virus.

(b) Anthrax disease is caused by a bacterium.

28. Question. Name two food materials which are preserved by sun-drying method in our homes.

Answer. Vegetables like methi leaves and spinach are preserved by sun-drying method in our homes.

29. Question. Name two food materials which are preserved by using common salt.

Answer. Meat and fish are preserved by using common salt.

30. Question. Name two food materials which can be preserved by using sugar.

Answer. Jams and jellies are preserved by using sugar.

31. Question. Name two food materials which are usually preserved by deep freezing.

Answer. Preservation by deep freezing means preservation of food material in the extreme cold. Low temperature inhibits the growth of micro-organisms completely. Food materials such meat and fish are usually preserved by deep freezing.

32. Question. Name some of the preservatives which are used in the preservation of fruits as jams arid jellies.

Answer. Sodium benzoate and sodium meta bisulphite are common preservatives which are used in the preservation of fruits as jams and jellies.

33. Question. Name some of the preservatives which are used in the preservation of fruits and vegetables as pickles.

Answer. Salt, oil and vinegar are some of the preservatives which are used in the preservation of fruits and vegetables as pickles.

34. Question. Name two food materials which can be preserved by using oil or vinegar.

Answer. Fish and meat are often preserved by using oil or vinegar.

35. Question. Name any two special chemicals which are used as food preservatives.

Answer. Sodium benzoate and sodium meta-bi-sulphite are common preservatives.

36. Question. Name the micro-organisms which can fix atmospheric nitrogen in the soil.

Answer. Certain bacteria and blue green algae present in the soil fix atmospheric nitrogen and convert it into compounds of nitrogen.

37. Question. What type of plants can ϐix nitrogen gas of the air into compounds of nitrogen?

Answer. Leguminous plants can fix atmospheric nitrogen into nitrogen compounds.

38. Question. Name the micro-organisms present in the soil and in the root nodules of leguminous plants which can fix atmospheric nitrogen. 

Answer. Some bacteria and blue green algae present in the soil and rhizobium present in the root nodules of leguminous plants can fix atmospheric nitrogen.

39. Question. Name two leguminous plants which can fix nitrogen.

Answer. Leguminous plants such as beans and peas can fix nitrogen.

40. Question. Fill in the following blanks with suitable words:

(a) Alcohol is produced with the help of ............

(b)Blue-green algae fix................directly from air to enhance fertility of soil.

(c) Micro-organisms can be seen with help of a..........

(d) Cholera is caused by..............

(e) Common salt has been used to preserve............. and ............ for ages.

(f) The food material which is preserved by pasteurization is .................

(g) As a result of nitrogen cycle, the percentage of nitrogen in the atmosphere remains more or less…….

Answer. (a) yeast (b) nitrogen (c) microscope (d) bacteria (e) meat; fish (f) milk (g) constant

Short Answer Type Questions

41. Question. How do viruses differ from other micro-organisms such as bacteria?

Answer. Viruses differ from other micro-organisms because they are too much smaller in comparison to other micro-organisms. They do not show most of the characteristics of the living organisms like respiration. They just reproduce. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Hence, virues differ from other organisms.

42. Question. What are micro-organisms? Give any two examples of micro-organisms.

Answer. The organisms which are too small to be seen by the naked eyes and can be seen only though a microscope are called micro-organisms or microbes. Bacteria and viruses are examples of micro-organisms.

43. Question. Can micro-organisms be seen with the naked eye? If not, how can they be seen?

Answer. Micro-organisms are too much smaller that they cannot be seen with naked eye. They can be seen with a magnifying glass called microscope.

44 A. Question. How do houseflies carry disease-causing microbes-(or pathogens)?

Answer. The houseflies feed on garbage, animal excreta and dead organic matter. When the houseϐly sits on garbage and animal excreta for feeding, then the millions of disease-causing micro-organisms present in them sticking to the body of housefly. And when this housefly sits on uncovered food, they may transfer the pathogens and food gets contaminated. When this contaminated food is consumed by a person, pathogens enter into his body and causes various diseases.

44 B. Question. State any two ways of preventing diseases spread by houseflies.

Answer. The spreading of diseases by houseflies can be prevented by following ways:

(i) Food should always be kept covered. 

(ii) Proper sanitation should be maintained in the kitchen.

45A. Question. How do mosquitoes carry disease-causing micro-organisms and spread diseases?

Answer. Mosquito acts a carrier of disease causing micro-organism and spreads diseases from one person to another. They carry disease-causing micro-organisms (microbes) inside its body. Female Anopheles mosquito is a carrier of malarial parasite. When it bits a person suffering from malaria, it sucks the blood of that person which contains the malarial parasite microbes. When this infected mosquito bites a healthy person, it transfers the microbes into his blood stream, hence the healthy person also gets malarial disease.

45 B. Question. Mention any three ways of preventing diseases spread by mosquitoes.

Answer. The spreading of diseases by mosquitoes can be prevented by following ways: 

(i) We should not let water collect anywhere in the neighborhood.

(ii) We should keep our surrounding clean and dry.

(iii) Mosquito repellant creams should be applied on the exposed parts of the body.

46 A. Question. What is meant by fermentation? Name the scientist who discovered fermentation.

Answer. The process of conversation of sugar into alcohol is known as fermentation. Louis Pasteur discovered fermentation in 1857.

46 B. Question. Which micro-organism converts sugar into alcohol during fermentation?

Answer. Yeast converts sugar into alcohol during fermentation. Yeast is used for commercial production of alcohol and wine.

47 A. Question. How do micro-organisms help in increasing soil fertility?

Answer. Some bacteria like rhizobium and blue green algae like Nostoc and Anabaena present in the soil are able to fix nitrogen gas from the atmosphere to enrich soil with nitrogen and increase its fertility. These micro-organisms are commonly called biological nitrogen fixers.

47 B. Question. How do micro-organisms help in cleaning the environment?

Answer. Some micro-organisms decompose the organic matter of dead plants and animals and convert them into simple substances which mix up with the soil. These substances are again used by other plants and animals. Thus, micro-organisms decompose the harmful and smelly dead remains of plants and animals into harmless materials and thereby clean up the environment.

48. Question. What are antibiotics? What precautions must be taken while taking antibiotics?

Answer. Medicines which kill or stop the growth of the disease-causing microorganisms are called antibiotics. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria.

Some of the precautions should be taken while taking antibiotics which are as follows:

● Antibiotics should be taken only on the advice of a qualified doctor.

● A person must finish the full course of antibiotics prescribed by the doctor.

● Antibiotics should not be taken unnecessarily.

49. Question. Why are antibiotics not effective against 'common cold' and 'flu'?

Answer. Antibiotics are not effective against 'common cold' and 'flu' as these are caused by viruses. And the viruses do not have cells but live in the cells of living beings. And the working mechanism of the antibiotics is to break the cell wall and attack on the bacteria.

50. Question. What is the full form of HIV? Name the disease caused by HIV. 

Answer. The full form of HIV is Human Immunodeficiency Virus. HIV causes AIDS disease.

51. Question. Describe how, curd is made from milk. Name the bacterium which converts milk into curd:

Answer. Milk is turned into curd by bacteria. Milk contains many microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. When a small amount of pre-made curd is added into warm milk, then lactobacillus bacterium present in curd multiplies in milk and converts it into curd. During this process, lactobacillus bacterium acts on lactose sugar present in milk and converts it into lactic acid. This lactic acid then converts milk into curd.

52. Question. Name the micro-organism used in bread-making which makes the bread-dough rise. How does it make the dough rise?

Answer. Yeast is used in baking industry for making bread. It increase the volume of dough. When yeast is mixed in dough for making bread, the yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume. This makes the dough rise. This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.

53. Question. What is food poisoning? How is food poisoning caused?

Answer. Microorganisms such bacteria and fungi that grow on our food items sometimes produce toxic substances. These make the food unfit for consumption. Consuming such food can cause a serious illness called food poisoning. Food hygiene is essential for preventing food poisoning.

54 A. Question. What is meant by food preservation? Name any five methods of preserving food.

Answer. Food preservation: Processing of food to prevent their spoilage and to retain their nutritive value for period is called food preservation. 

Food can be preserved using following methods:

(1) Preservation by Salt

(2) Preservation by Sugar

(3) Preservation by oil and vinegar

(4) Heat and cold treatment

(5) Storage and packing

54 B. Question. How do you preserve cooked food at home?

Answer. We preserve cooked food in the refrigerator at home. Low temperature inhibits the growth of micro-organisms. When cooked food is kept in a cold place, then the food does not get spoiled easily.

55 A. Question. Why should we not let water collect anywhere in the neighbourhood?

Answer. All mosquitoes breed in water . Hence, one should not let water collect anywhere, in coolers, tyres, flower pot etc.

55 B. Question.  Name one animal disease each caused: (i) by virus (ii) by bacteria (iii) by fungus.

Answer.  (i) Yellow vein mosaic of bhindi (Okra) (ii) Citrus canker (iii) Rust of wheat

56. Question. Where do Rhizobium bacteria live? What is their function?

Answer. Rhizobium bacterium is found in the soil. It lives in the root nodules of leguminous plants such as beans and peas, with which it has a symbiotic relationship. It is involved in the fixation of nitrogen in leguminous plants (pulses).

57. Question. Name any two

(a) bacteria

(b) viruses

(c) protozoa

(d) algae, and

(e) fungi.

Answer. (a) Bacteria – (I) Lactobacillus bacteria (II) Rhizobium bacteria

(b) Viruses – (I) HIV (II) Tobacco Mosaic virus (TMV)

(c) Protozoa – (I) Amoeba (II) Paramecium

(d) Algae – (I) Spirogyra (II) Chlamydomonas

(e) Fungi – (I) Penicillium (II) Aspergillus

58. Question. State the beneϐicial effects (or usefulness) of micro-organisms in our lives.

Answer. Microorganisms are used for various purposes such as:

● They are used in the preparation of curd, bread and cake.

● They are used in the preparation of alcohol, wine and acetic acid (vinegar).

● They are also used in cleaning up of the environment.

● In agriculture, they are used to increase soil fertility by fixing nitrogen.

● The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Penicillin is an antibiotic made from Penicillium.

● Certain microbes are also used in the biological treatment of sewage and industrial effluents.

● Yeast is used in making idlis, bhaturas, bread, pastries and cakes.

59. Question. Describe the method of pasteurization for the preservation of milk.

Answer.  The process of pasteurization for the preservation of milk is discovered by

Louis Pasteur. In this process, milk is heated to about 70°C for 15 to 30 seconds to kill bacteria. Next it is chilled very fast to prevent bacteria from growing. It is finally stored at low temperature. This Pasteurized milk can be

consumed without boiling as it is free from harmful microbes.

60. Question.  Name one plant disease each caused:

(a) by fungi

(b) by virus

(c) by bacteria

Answer. (a) The plant disease, Rust of wheat is caused by fungi.

(b) Yellow vein Virus Insect mosaic of bhindi (Okra), a plant disease, is caused

by virus.

(c) Citrus canker is caused by bacteria.

61. Question. Which disease is spread by:

(a) female Anopheles mosquito?

(b female Aedes mosquito?

Answer. (a) female Anopheles mosquito carries the parasite of malaria.

(b) Female Aedes mosquito acts as carrier of dengue virus.

62. Question. Name two fruits which are preserved:

(a) in the form of pickles

(b) in the form of jams.

Answer. (a) The fruits which are preserved in the form of pickles by using oil and vinegar as preservative are raw mango and lemon.

(b) The fruits which are preserved in the form of jams by using sugar as preservative are mango and orange.

63. Question. What is the mode of transmission of the following diseases?

(a) Rust of wheat

(b) Citrus canker

(c) Yellow vein mosaic of bhindi (Okra)

Answer

64. Question.  Name any two animal diseases and two plant diseases caused by micro-organisms.

Answer. The following diseases are caused by the micro-organisms:

65. Question. State the causative micro-organisms and modes of transmission of the

following human diseases :

(i) Tuberculosis

(ii) Measles

(iii) Chickenpox

(iv) Polio

(v) Cholera

(vi) Typhoid

(vii) Hepatitis B

(viii) Malaria

(ix) Dengue.

Answer:

Long Answer Type Questions:

66 A. Question. What is meant by communicable diseases? Name any two communicable diseases.

Answer. Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases. Cholera and tuberculosis are examples of communicable diseases.

66 B. Question.  What are the various ways in which communicable diseases can occur and

spread?  

Answer. The communicable diseases can occur and spread in the following ways:

● Through air we breathe

● Through water we drink

● Through food we eat, and

● Through carriers such as mosquito.

67 A. Question. Name any five human diseases caused by micro-organisms. Also name the

causative micro-organisms and mode of transmission for each of these diseases.

Answer. 

67 B. Question. State the various ways of preventing the occurrence and spreading of

communicable diseases.

Answer. There are various ways of preventing the occurrence and spreading of communicable diseases. These are as follows:

● Maintain personal hygiene and good sanitary habits.

● Drink boiled drinking water.

● Spray insecticides and control breeding of mosquitoes by not allowing

water to collect in the surroundings.

● Keep the infected person separated from others & advice him/ her to keep

a handkerchief on the nose and mouth while sneezing.

● Timely vaccination against diseases should also be taken.

● Food must be kept covered.

68 A. Question. What is a vaccine? How does a vaccine work?

Answer. A vaccine is a special kind of preparation which provides immunity against a

particular disease. Vaccine consists dead and weakened microbes which are harmless and do not cause any disease. Vaccine works as follows:

● When the vaccine containing dead or weakened microbes is introduced

into the body of a healthy person, the body fights and kills the microbes by producing some substance called antibodies.

● Some of the antibodies remain in the body for a long time and protect it from any future attack of disease germs. Thus, a vaccine develops the immunity from a disease.

68 B. Question.  Why are children given vaccination ?

Answer. Vaccination is the process of giving a vaccine orally or by injection which gives protection against a particular disease. Vaccination protect the children from number of diseases. Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination. Since the immune system of children is not strong and they are more likely to catch any disease very easily, so all the children should be vaccinated at the proper ages to provide them immunity from certain diseases.

69. Question.  What is meant by 'nitrogen fixation' ? State two ways in which nitrogen gas of the atmosphere can be 'fixed' in nature to get nitrogen compounds in the soil.

Answer.  The process of converting atmospheric nitrogen into compounds of nitrogen is called nitrogen fixation. The two ways in which in which nitrogen gas of the atmosphere can be 'fixed' in nature to get nitrogen compounds in the soil are:

(i) Nitrogen fixation by lightning: Lightning also helps in fixing atmospheric nitrogen into the soil. The temperature and pressure during

lightning convert atmospheric nitrogen into oxides of nitrogen. These nitrogen compounds combine with rainwater to form nitric and nitrous acid which mix with the soil.

(ii) Nitrogen fixation by certain bacteria and blue-green algae: Certain bacteria and blue-green algae present in the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen.

70. Question.  Draw a neat, labelled diagram of nitrogen cycle in nature. Which natural

phenomenon occurring in the sky is responsible for nitrogen fixation?

Answer. A neat, labelled diagram of the nitrogen cycle in nature is shown below:

Lightning is a natural phenomenon which occurs in the sky and responsible for nitrogen fixation. The temperature and pressure during lightning convert atmospheric nitrogen into oxides of nitrogen. These nitrogen compounds

combine with rain water to form nitric and nitrous acid which mix with the soil. These acids are utilized by various organisms.

Multiple Choice Questions (MCQs)

71. Question. The bread dough rises because of:

A. heat

B. grinding

C. growth of yeast cells

D. Kneading

Answer. C

Yeast is used in baking industry for making bread. It increases the volume of dough.

When yeast is mixed in dough for making bread, the yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume.

72. Question. Yeast is used in the production of:

A. sugar

B. alcohol

C. hydrochloric acid

D. Oxygen

Answer. B

 Yeast is used for commercial production of alcohol and wine.

73. Question. The process of conversion of sugar into alcohol is called:

A. nitrogen fixation

B. moulding

C. fermentation

D. infection

Answer. C

The process of conversion of sugar into alcohol is known as fermentation. Louis Pasteur Discovered fermentation in 1857.

74. Question. Which of the following is an antibiotic?

A. sodium bicarbonate

B. streptomycein

C. alcohol

D. yeast

Answer. B

Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria.

75. Question. The most common carrier of communicable diseases is:

A. ant

B. housefly

C. dragonfly

D. spider

Answer. B

Housefly is most common carrier of communicable diseases. The houseflies

feed on garbage, animal excreta and dead organic matter. Diseases cholera and tuberculosis spread by housefly.

76. Question.  The carrier of malaria causing protozoan is:

A. female anopheles

B. cockroach

C. housefly

D. female aedes mosquito

Answer. A

The carrier of malaria causing protozoan called plasmodium is female anopheles mosquito.

77. Question. The vaccine for smallpox was discovered by:

A. Alexander Fleming

B. Edward jenner

C. Louis Pasteur

D. Rober coch

Answer.  B

Edward Jenner discovered the vaccine for smallpox in 1978.

78. Question.  Alcohol can be converted into vinegar by the action of micro-organisms

called:

A. viruses

B. yeast

C. protozoa

D. bacteria

Answer. D

The dilute solution of acetic acid is called vinegar. Alcohol can be converted

into vinegar by the action of micro-organisms called bacteria. In this process, first alcohol is produce by using yeast. The acetobacter bacteria is then added to alcohol. Acetobacter bacteria convert alcohol into acetic acid (or vinegar).

79. Question. 

The first antibiotic called penicillin was extracted from:

A. a bacterium

B. a protozoan

C. a fungus

D. an alga

Answer. C

An antibiotic Penicillin was extracted from fungus (mould) by Alexander

Fleming in 1929. This antibiotic was made from the fungus called penicillium.

80. Question. Which of the following is not a communicable disease?

A. cholera

B. cancer

C. chickenpox

D. malaria

Answer. B

Cholera, chickenpox and malaria are communicable disease while cancer is not a communicable disease.

81. Question. Which of the following increases the fertility of soil

(A) Lactobacillus bacteria

(B) Rhizobium bacteria

(C) Spirogyra algae

(D) Blue-green algae

A. A and B

B. B and C

C. A and D

D. B and D

Answer. B and D 

As these two organisms are able to fix nitrogen gas from atmosphere to enrich soil with nitrogen compounds hence increasing fertility of soil. 

82. Question. Which of the following cannot be used as a food preservative?

A. sodium metabisulphate

B. sodium hydroxide

C. sodium benzoate

D. citric acid

Answer B

sodium metabisulphate, sodium benzoate and citric acid are used as a food preservative whereas sodium hydroxide is not used as food preservative.

83. Question. Which of the following disease is not caused by bacteria?

A. cholera

B. typhoid

C. tuberculosis

D. measles

Answer. D

Cholera, typhoid and tuberculosis are caused by bacteria whereas measles is caused by a virus.

84. Question. The micro-organisms which can reproduce and multiply only cells of other

organisms are:

A. protozoa

B. fungi

C. bacteria

D. viruses

Answer. D

Viruses are too much smaller in comparison to other micro-organisms. They do not show most of the characteristics of the living organisms like respiration. They just reproduce. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.

85. Question.  The dengue disease spread by Aedes mosquito is caused by:

A. bacteria

B. virus

C. protozoan

D. fungus

Answer. B

Dengue disease is caused by a virus. Female Aedes mosquito acts as carrier of dengue virus.

86. Question.  Which of the following disease is not caused by viruses?

A. measles

B. smallpox

C. cholera

D. polio

Answer C

Measles, smallpox and polio are caused by viruses while cholera is caused by a bacterium.

87. Question. The micro-organism which is capable of converting sugar into alcohol and

carbon dioxide is:

A. bacterium

B. fungus

C. alga

D. protozoan

Answer. B

Yeast, a fungus is capable of converting sugar into alcohol and carbon dioxide.

88. Question. Which of the following is not a use of micro-organisms?

A. preparation of medicines (or drugs)

B. preparation of food by photosynthesis

C. recycling of materials in nature

D. increasing the fertility of soil

Answer B

Some of the micro-organisms are used to preparation of medicines. Some of micro-organisms are used recycling of materials in nature and increasing the fertility of soil. The synthesis of food by photosynthesis takes place in leaves with the help of inorganic materi like carbon-dioxide and water in the presence of sunlight and chlorophyll.

89. Question. The malaria disease is caused by a:

A. virus

B. protozoan

C. bacterium

D. fungus

Answer. B

The malaria disease is caused by a protozoan called plasmodium.

90. Question

The parasite called Plasmodium causes a disease known as:

A. measles

B. polio

C. malaria

D. dengue

Answer . C

The parasite called Plasmodium causes a disease known as malaria. Female Anopheles mosquito carries the parasite of malaria.