Describe the different phases of mitosis with the help of a diagram.

MITOSIS:

 Mitosis is the process by which a nucleus divides into two identical daughter cells. Mitosis takes place in all parts of plants and animals. The time taken for mitosis of one nucleus changes with the species of organisms and prevailing temperature. Drosophila, fruit fly, takes 7 minutes while man, 100 minutes. 

Phases of Mitosis:

 Mitosis is a continuous process, but four main phases are recognized, each of which merges into the other. 

(1) Prophase:

 (I) Chromosomes become visible as two lengthwise halves or chromatids. The chromatids are attached to each other at the centromere. The chromosomes are visible because the chromatin material has shortened and thickened by condensation. 

(ii) The centrosome divides to form two centrioles. The centrioles move towards the opposite pole of the cell and form spindle fibres. Centrioles are absent in higher plants. 

(iii) The nuclear membrane disappears. 

(iv) Nucleoli (singular nucleolus) disappear. 

(v) From each centriole, fibres originate forming a star shaped aster. Spindle fibres, centrioles and aster are collectively called mitotic apparatus. 

(2) Metaphase: 

Chromosomes arrange themselves on the cell equator position or midline of the spindle fibre. Each chromosome is attached to a separate spindle fibre by its centromere. 

(3) Anaphase: 

(i) The centromere divides and the two chromatids are drawn apart and become chromosomes. 

(ii) The chromosomes move towards the opposite poles drawn by the spindle fibres. 

(4) Telophase: 

(i) Chromosomes reform by a.n uncoiling process, become thinner, and finally disappear. 

(ii) Nucleoli reform. 

(iii) Nuclear membranes reappear While the nucleus is dividing by mitosis, the cell cytoplasm and plant cell wall are also dividing.

    Animal cells and amoebae divide by a 'furrowing' of the plasma membrane cytoplasm along the cell equator position. Plant cells develop a cell plate along the cell equator position which grows inwards to meet the opposite cell plate. Cellulose is then deposited on either side of the cell plate, the original cell now has formed two daughter cells. 

Give an illustrated account of the external and internal structures of the leaf of mustard (Brassica campestris).

 EXTERNAL STRUCTURE OF LEAF:

 Leaves grow from nodes on stems and branches. Each leaf consists of two parts. The lower stalk like part is called the petiole and the upper green expanded portion is called lamina. Young leaves are without petioles and their margins are entire or smooth but in mature leaves the margin is wavy. There is a swollen vein in the middle of the leaf which is known as midrib. The veins of different thickenings originate from the midrib and spread in the leaf forming a network. This type of arrangement of leaves is called reticulate venation. The veins consist of xylem and phloem. This network of veins supports the leaf and keeps its lamina in an expanded position. The angle formed within the stem and leaf is called the axil. In the axils of leaves buds are present from where new branches grow. Upper and lower surfaces of the leaf are different from each other such leaves are called bifacial leaves. As the function of leaves is to prepare food for the plant during photosynthesis, all of its tissues are arranged in such a way that photosynthesis can take place easily.

 Internal Structure of Leaf:

 A T.S (transverse section) of leaf shows the following parts under a Microscope. 

Epidermis:

 This layer covers the upper and lower surfaces of the leaf. The upper layer of the leaf is called the upper epidermis. The layer on the lower surface of the leaf is called the lower epidermis. The lower epidermis contains more stomata than the upper epidermis.

 The singular of stomata is stoma.

Each stoma has a pore with two guard cells which are kidney shaped. Gases and water vapour are exchanged through stomata. 

Functions:

 (i) It provides protection to the leaf. 

(ii) it carries out transpiration in which gases and water vapour are exchanged through stomata. 

Function:

 (2) Mesophyll:

 The tissue present between the upper and lower epidermis is called mesophyll. The mesophyll cells .below the upper epidermis are longer than broad arid are closely packed. They lie vertically and are double layered. They are called the palisade mesophyll. The cells below the palisade layer are irregular in shape. The lower part has more intercellular spaces and is sponge like. It is called spongy mesophyll. Both the palisade and spongy mesophyll have chloroplasts in which chlorophyll is present.

Function:

 The mesophyll prepares food for the plants. 

Vascular Bundles:

 Midribs and other veins of the leaf contain vascular bundles, the xylem and the phloem. Xylems are present towards the upper part while phloems are present towards the lower part. 

Function:

 (i) Xylem transport water and mineral salts from the soil to all parts of the plant upwards. 

(ii) Phloems transport food prepared in the leaves to all parts of the plant upwards and downwards. 

Newton's Law of Motion

 First law of motion:

 According to Newton's first law of motion :
 "A body tends to continue it's state of rest or of uniform motion unless a force is applied on it ".
 First law of motion is also known as law of inertia 

Inertia:

 "The inability of matter to change it's state of rest or of uniform motion ,is known as inertia".
 inertia of a body depends upon it's mass . Greater the mass of a body larger is it's inertia.

 Second law of motion:

 According to Newton's second law of motion :
               "When an unbalanced force acts on a- body it produces acceleration in the body in it's own direction such that the magnitude of acceleration is directly proportional to the magnitude of force ".

Where "F " represents the unbalanced force and "a" is the acceleration produced in the body of mass "m ". The direction of "a" is same as the direction of "F ".

 Unit of force:

 The unit of force can be defined with the help of Newton's second law of motion . Since,

F = m a

 Therefore the M.K.S unit of force will be kg - m/s^2 which is commonly known as Newton .
 "Force is said to be one Newton (1 N) if it produces an acceleration of 1 m/s^2 in a body of mass 1 kg".
 C.G.S unit of force is "dyne", Whereas the British unit of force is "Pound".
                                       1 Newton = 10^5 dyne.

 Third law of motion:

 According to Newton's third law of motion :
                 "To every action there is always an equal and opposite reaction".
 Action and reaction are two forces which are exchanged between two bodies ,when they interact with each other .

Definition of Physics Ch # 3

Motion under gravity: 

 If A body is thrown vertically upward with a certain velocity then due to gravity it's velocity decreases at the rate of 9.8 m/s 2,
      It means that the velocity of the body will decrease by 9.8 m/s in every second. As a result of which it's velocity becomes zero after it has-attained a certain height. At this point the body can not remain suspended in the air, therefore it starts falling down under the action of gravity with the same acceleration of 9.8 m/s 2-. This time its velocity .Increases by 9.8 m/s in every second. It hits the ground with same velocity with which it was thrown.
    During the entire motion under gravity, acceleration of the body is in the downward direction (towards the center of the earth) therefore it is taken as "-g" for upward motion, whereas for the downward motion it is taken as "+g ". The body takes a certain time "t " to reach the highest point. In the absence of air resistance it takes exactly the same time to return to the ground. Hence the total time for which the body remained in the air is "2 t "

Important note: 

For complete up and down motion g is to be taken as negative . The reason for doing so in such cases is that the direction of initial velocity vi of the body is taken as reference. It is taken as positive and any vector quantity including g , whose direction is opposite to the direction of vi will be negative. Especially in case of projectile motion g is taken as negative throughout the motion. Because at the point of projection the initial velocity in the vertical direction is upward whereas the direction of g throughout the projectile motion is downward. Hence g is taken negative at all the points, whether the body moves upward or downward. All the equations of projectile motion are derived on this assumption. 

Definition of Physics CH # 3

Velocity: 

"Rate of displacement of a body is called velocity". 
                                                OR  
"Speed of a body in a particular direction is called velocity"- 

:Velocity is a vector quantity,it's direction is same as the direction of displacement . It's unit in M.K.S system is m/s 

Definition of Physics CH # 3

Uniform velocity :

 "If a body covers equal distances in equal intervals of time in a particular direction then it's velocity is said to be uniform. 
                                                                       OR 
"if speed of a body is constant in a particular direction then it's velocity will be uniform".
 If a body moves with uniform velocity then it's instantaneous and average velocities will be equal . 
v= uniform , when v av = v inst.   

Definition of Physics CH# 3

Average velocity: 

"Total displacement divided by total time taken. is called average velocity". 

Instantaneous velocity: 

"It is the velocity of a body at a particular instant".
 Mathematically the instantaneous velocity of a body is given by : 

Definition of Physics CH # 3

Velocity: 

"Rate of displacement of a body is called velocity".
                                                  OR
"Speed of a body in a particular direction is called velocity"- 

:Velocity is a vector quantity , it's direction is same as the direction of displacement . It's unit in M.K.S system is m/s 

Where Ar is the displacement of the body in time At 

Physics Definitions (Ch-2)

Motion

When a body continuously changes it's position with respect to it's surroundings it is said to be in motion .

Rest:

 If a body does not change it's position with respect to it's surroundings it is said to be at rest . State of rest or of motion is relative . It means that a body at rest with respect to a certain frame of reference may be in motion with respect to some other frame of reference.

Displacement: 

Displacement of a body may be defined as : 

       "The change in position of a body in a particular direction".

It may also be defined as : 

       "The shortest straight line distance between initial and final positions of a body ". 

Displacement is a vector quantity. 

It's magnitude is equal to the distance between the two positions of the body , it is directed towards the final position . 

Speed:

 "Distance covered per unit time is called speed of the body". Speed is a scalar quantity. It's unit in M.K.S system is meter per second (m/s) .

Average speed :

 "Total distance covered divided by total time taken is called average speed ".

Instantaneous speed: 

                                        "It is the a body at a particular instant".
 Instantaneous speed of a body can be calculated by finding the distance covered by it in a very short interval of time and then dividing it by the time interval . Mathematically instantaneous speed is given by: 

It means that the interval of time is very taken is very short and is close to zero