CBSE Class 9 Science Notes Chapter 5 The Fundamental Unit of Life

 

Class 9 Science Notes 

Chapter 5     The Fundamental Unit of Life

Cells are the building blocks of all living beings. Complex organisms' primary structural and functional unit is the cell. The smallest functional unit of life is a cell, discovered by Robert Hooke in 1665. A cell can independently perform all necessary activities to sustain life.

1. Plasma/Cell membrane: This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell so the cell membrane is called a selectively permeable membrane.

Some substances like CO2 or O2  gases can move across the cell membrane by a process called diffusion. The movement of water molecules (liquid) through such a selectively permeable membrane is called osmosis. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.

If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. If the medium has a lower water concentration then the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.

The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. The flexibility of cell membrane also enables the cell to engulf in food and other material from its external environment. Such process is known as endocytosis. It is observed in Amoeba.

2. Cell wall (Protective wall): Plants cells, in addition to the plasma membrane have another rigid outer covering called cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. It is a complex substance and provides structural strength to plant cells. When a living plant loses water through osmosis there is shrinkage or contraction of contents of the cell away from cell wall. This phenomenon is known as plasmolysis.

3. Nucleus (Brain of a cell): The nucleus has a double-layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, i.e., to the cytoplasm.The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.

Chromosomes — contain information for inheritance of features from parents to next generation in form of DNA [Deoxyribo Nucleic Acid] molecules. Chromosomes are composed of DNA and protein. Functional segments of DNA are called genes. The nucleus plays a central role in cellular reproduction.

Prokaryotic Cells: In some organisms like bacteria, the nuclear material is not enclosed by nuclear membrane and membrane bound cell organelle are absent. Such nucleus is called nucleoid and such cells are known as prokaryotic cell. Such cells have single chromosome.

Eukaryotic Cells: Cells having well defined nucleus and having membrane bound cell organelle is termed as eukaryotic cell. Such cells have more than one chromosomes.

4. Cytoplasm: The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.

5. Cell Organelles: Every cell has a membrane around it to keep its content separate from the external environment. The different components of cell perform different function and these components are called cell organelles.

(i) Endoplasmic Reticulum (ER) (Channels, Network for transport):

The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags.
There are two types of ER-Rough endoplasmic reticulum [RER] and smooth endoplasmic reticulum [SER]. RER has particles called ribosomes attached to its surface. The ribosomes Endoplasmic Reticulum are the sites of protein manufacture.The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.

(ii) Golgi Apparatus (Packaging): The golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles arranged approximately, parallel to each other in stacks called cisterns.
The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. It’s function include the storage, modification and packages of products in vesicles. In some cases complex sugar may be made from simple sugar in the Golgi apparatus. It is also involved in the formation of lysosomes.

(iii) Lysosomes [Suichge bags] (Cleanliness of cell): Lysosomes are a kind of waste dispatch and disposal system of the cell. Lysosome help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cells such as bacteria or food, as well as old organelles, end up in the lysosome, which break them up into small pieces. They are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. Under abnormal condition, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore they are also known as “suicide bags”

iv) Mitochondria (Powerhouse, Energy provider): Mitochondria are known as powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP [Adenosine Triphosphate] molecules. ATP is known as energy currency of the cell. Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. They are able to make some of their own protein.

(v) Plastids: Plastids are present only in plant cells. There are two types of plastids chromoplasts and leucoplasts. Chromoplasts are the coloured plastids present in leaves, flowers and fruits. Plastids containing the pigment chlorophyll are known as chloroplasts. They are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are found primarily in organelles in which materials such as starch, oils and protein granules are stored.

The internal organisation of the plastids consists of numerous membrane layers embedded in a material called stroma. Plastids are similar to mitochondria in external structure. Plastids have their own DNA and ribosomes.

(vi) Vacuoles (Storage): Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small-sized in animal cells while plant cells have very large vacuoles [50% to 90% cell volume].

In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. In Amoeba, the food’vacuole contain the food items that is consumed it and contractile vacuoles expels excess water and some wastes from the cell.

Important Questions for Class 12 Physics Chapter 4 Moving Charges and Magnetism

 

Important Questions for Physics Chapter 4 Moving Charges and Magnetism Class 12 

Question 1.
Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why? (Delhi 2008)
Answer:
At the edges of the solenoid, the field lines get diverged due to other fields and/or non-availability of dipole loops, while in toroids the dipoles (in loops) orient continuously.

Question 2
An electron does not suffer any deflection while passing through a region of uniform magnetic field. What is the direction of the magnetic field? (All India 2009)
Answer:

∴Magnetic field will be in the line of the velocity of electron.

Question 3    Beam of particles projected along +x-axis, experiences a force due to a magnetic field along the +y-axis. What is the direction of the magnetic field? (All India 2009)

Answer:

Direction of the magnetic field is towards negative direction of z-axis

Question 4
A beam of electrons projected along +x-axis, experiences a force due to a magnetic field along the +y/-axis. What is the direction of the magnetic field? (All India 2010)

Answer:
Direction of the magnetic field is F = q (v × B) towards positive direction of z-axis

Question 5.
Depict the direction of the magnetic field lines due to a circular current carrying loop. (Comptt. Delhi 2012)
Answer:
Direction of the magnetic field lines is given by right hand thumb rule.

Question 6.
Why do the electric field lines not form closed loops? (Comptt. All India 2012)
Answer:
Electric field lines do not form closed loops because the direction of an electric field is from positive to negative charge. So one can regard a line of force starting from a positive charge and ending on a negative charge. This indicates that electric field lines do not form closed loops.

Question 7
Two charges of magnitudes – 2Q and + Q are located at points (a, 0) and (4a,0) respectively. What is the electric flux due to these charges through a sphere of radius ‘3a’ with its centre at the origin? (All India 2013)
Answer:

Question 8
Write the expression for the work done on an electric dipole of dipole moment p in turning it from its position of stable equilibrium to a position of unstable equilibrium in a uniform electric
field E. (Comptt. Delhi 2013)
Answer:
Torque, acting on the dipole is, τ = pE sin θ

Question 9
Why do the electrostatic field lines not form closed loops? (All India 2014)
Answer:
Electric field lines do not form closed loops because the direction of an electric field is from positive to negative charge. So one can regard a line of force starting from a positive charge and ending on a negative charge. This indicates that electric field . lines do not form closed loops.

Question 10
Why do the electric field lines never cross each other? (All India)
Answer:
The electric lines of force give the direction of the electric field. In case, two lines of force intersect, there will be two directions of the electric field at the point of intersection, which is not possible.

Question 11.

distance ‘d’ apart as shown in the figure. The electric field intensity is zero at a point ’P’ on the line joining them as shown. Write two conclusions that you can draw from this. (Comptt. Delhi 2014)
Answer:

• Two point charges ‘ q1‘ and ‘ q2 should be of opposite nature.
• Magnitude of charge ql must be greater than that of charge q2.

 Question 12. What is the electric flux through a cube of side 1 cm which encloses an electric dipole? (Delhi 2015)

Answer:
Zero because the net charge of an electric dipole (+ q and – q) is zero.

Question 13.
Why are electric field lines perpendicular at a point on an equipotential surface of a conductor? (Comptt. All India 2015)
Answer:
If the electric field lines were not normal to the equipotential surface, it would have a non-zero component along the surface. To move a unit test charge against the direction of the component of the field, work would have to be done which means this surface cannot be equipotential surface.
Hence, electric field lines are perpendicular at a point on an equipotential surface of a conductor.

Question 14.

Is the potential difference VA – VB positive, negative or zero? (Delhi 2016)
Answer:
The potential difference is positive.

Question 15.
How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased? (Delhi 2016)
Answer:
The electric flux due to a point charge enclosed by a spherical gaussian surface remains ‘unaffected’ when its radius is increased.

Question 16.
Show on a plot the nature of variation of the

• Electric field (E) and
• potential (V), of a (small) electric dipole with the distance (r) of the field point from the centre of the dipole. (Comptt. Outside Delhi 2016)

Answer:

Question 17.
Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer. (Delhi 2017)
Answer:
No, it does not, because the charge resides only on the surface of the conductor.

Question 18.
Draw a plot showing variation of electric field with distance from the centre of a solid conducting sphere of radius R, having a charge of +Q on its surface. (Comptt. Delhi 2017)
Answer:
Plot between E and r

Question 19.
A point charge +Q is placed in the vicinity of a conducting surface. Draw the electric field lines between the surface and the charge.
(Comptt. Outside Delhi 2017)
Answer:

Question 20.
Derive an expression for the torque experienced by an electric dipole kept in a uniform electric field. (Delhi 2017)
Answer:
Consider an electric dipole consisting of charges + q and – q and of length 2a placed in a uniform electric field E→ making an angle θ with it. It has a dipole moment of magnitude,

Question 21.
Define electric flux. Write its S.I. unit.
A charge q is enclosed by a spherical surface of radius R. If the radius is reduced to half, how would the electric flux through the surface change? (All India 2009)
Answer:
Electric flux over an area in an electric field is the total number of lines of force passing through the area. It is represented by ϕ . It is a scalar quantity. Its S.I unit is Nm2 C-1 or Vm.

Electric flux ϕ by qenclosed
Hence the electric flux through the surface of sphere remains same.

Question 22.
A spherical conducting shell of inner radius rx and outer radius r2 has a charge ‘Q’. A charge ‘q’ is placed at the centre of the shell.
(a) What is the surface charge density on the
(i) inner surface,
(ii) outer surface of the shell?
(b) Write the expression for the electric field at a point x > r2 from the centre of the shell. (All India 2010)
Answer:
(a) Surface charge density on the :

(b) Electric field at a point x > r2 from the centre of the shell will be E = 14πε0(q+Qx2)

Question 23.
Show that the electric field at the surface of a charged conductor is given by E→=σε0n^, where σ is the surface charge density and h is a unit vector normal to the surface in the outward direction. (All India 2010)
Answer:
Electric field at a point on the surface of charged conductor, E = 14πε0QR2
For simplicity we consider charged conductor as a sphere of radius ‘R’. If ‘σ’ is in surface charge density, then

…where [ n^ is a unit vector normal to the surface in the outward direction]

Question 24.
A thin straight infinitely long conducting wire having charge density X is enclosed by a cylindrical surface of radius r and length l, its axis coinciding with the length of the wire. Find the expression for the electric flux through the surface of the cylinder. (All India 2011)
Answer:
Since the field is everywhere radial, flux through the two ends of the cylindrical Gaussian surface is zero. At the cylindrical part of the surface, E is normal to the surface at every point, and its magnitude is constant, since it depends only on r. The surface area of the curved part is 2πrl, where l is the length of the cylinder.

Hence the net translating force on a dipole in a uniform electric field is zero. But the two equal and opposite forces act at different points of the dipole. They form a couple which exerts a torque.
Torque = Either force × Perpendicular distance between the two forces
x = qE × 2a sin θ
X = pE sin θ [ ∵ p = q × 2a; p is dipole moment]
As the direction of torque τ⃗  is perpendicular to p⃗  and E⃗ , so we can write τ⃗ =p⃗ ×E→

Flux through the Gaussian surface = Flux through the curved cylindrical part of the surface is zero. At the cylindrical part of the surface, E is normal to the surface at every point, and its magnitude is constant, since at every point, and its magnitude is constant, since it depends only on r. The surface area of the cylinder.

Flux through the Gaussian surface = Flux through the curved cylindrical part of the surface
= E × 2πrl

(a) Electric field due to an infinitely long thin straight wire is radial.
(b) The Gaussian surface for a long thin wire of uniform linear charge density
The surface includes charge equal to λl.
Gauss’s law then gives

Question 25.
Plot a graph showing the variation of coulomb force (F) versus (1r2), where r is the distance between the two charges of each pair of charges : (1µC, 2µC) and (2µC, – 3µC). Interpret the graphs obtained. (All India 2010)
Answer:

Here positive slope depicts that force is repulsive in nature and negative slope depicts that the force is attractive in nature.