HSSC/ SSC Biology Class 1st By Aarif Sir

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Hello students in this class we will discuss about the cell and its components for ssc and hssc level

Compiled from NCERT Science Textbooks Class 6-12

I will discuss here only important topic and cell organelles like 

Cell - Cell Organelles: Plasma Membrane, Cell Wall, Cytoplasm, Nucleus, Mitochondria.

Prokaryotic Cells vs. Eukaryotic Cells. Plant Cell vs. Animal Cell.

What is Cell ?

Cell is the basic fundamental structure and functional unit of life.

Robert Hooke    Discovered and coined the term cell in 1665 

Robert Brown     Discovered Cell Nucleus in 1831

Schleiden and Schwann     Presented The cell theory, that all the plants and animals are

composed of cells and that the cell is the basic unit of life. Schleiden (1838) and Schwann (1839)  With the discovery of the electron microscope in 1940, it was possible to observe and

understand the complex structure of the cell and its various organelles.

Plasma Membrane or Cell Membrane

 Cell membrane is also called the plasma membrane. It can be observed only through an electron microscope  Plasma membrane is the outermost covering of the cell that separates the contents

of the cell from its external environment.

Endocytosis

 The plasma membrane is flexible and is made up of organic molecules called lipids

and proteins.  The flexibility of the cell membrane also enables the cell to engulf in food and other

material from its external environment. Such processes are known as endocytosis

(endo → internal; cyto → of a cell). Amoeba acquires its food through such processes.

Diffusion

 Plasma membrane is a selectively permeable membrane [The plasma membrane is

porous and allows the movement of substances or materials both inward and

outward].

 Some substances like carbon dioxide or oxygen can move across the cell membrane

by a process called diffusion [spontaneous movement of a substance from a region of

high concentration (hypertonic solution) to a region where its concentration is low

(hypotonic solution)].

  Thus, diffusion plays an important role in gaseous exchange between the cells as well

as the cell and its external environment.

Osmosis

  Water also obeys the law of diffusion. The movement of water molecules through  selectively permeable membrane is called osmosis.  Osmosis is the passage of water from a region of high water concentration through  semi-permeable membrane to a region of low water concentration. Thus, osmosis i a special case of diffusion through a selectively permeable membrane.  Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.  Thus, diffusion is important in exchange of gases and water in the life of a cell.  additions to this, the cell also obtains nutrition from its environment   Different molecules move in and out of the cell through a type of transport requiring

use of energy in the form of ATP.

Reverse Osmosis (RO)

  Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water. Osmosis  Water also obeys the law of diffusion. The movement of water molecules through  selectively permeable membrane is called osmosis.  Osmosis is the passage of water from a region of high water concentration through  semi-permeable membrane to a region of low water concentration. Thus, osmosis i a special case of diffusion through a selectively permeable membrane.  Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.  Thus, diffusion is important in exchange of gases and water in the life of a cell. In additions to this, the cell also obtains nutrition from its environment.  Different molecules move in and out of the cell through a type of transport require in use of energy in the form of ATP Reverse Osmosis (RO  Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water.

Cell Wall 

 Cell wall is absent in animals. Plant cells, in addition to the plasma membrane, have another rigid outer coverin called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substanc and provides structural strength to plants.

Plasmolysis

When a living plant cell loses water through osmosis there is shrinkage or

contraction of the contents of the cell away from the cell wall. This phenomenon is

known as plasmolysis (plasma → fluid; lysis → disintegration, decomposition).  Only living cells, and not dead cells, are able to absorb water by osmosis. Cell walls

permit the cells of plants, fungi and bacteria to withstand very dilute [hypotonic]

external media without shrinkage. In such media the cells tend to lose water by osmosis. The cell shrinks, building u pressure against the cell wall. The wall exerts an equal pressure against the

shrunken cell.  Cell wall also prevents the bursting of cells when the cells are surrounded by a

hypertonic medium (medium of high concentration)  In such media the cells tend to gain water by osmosis. The cell swells, building u pressure against the cell wall. The wall exerts an equal pressur against the swolle cell.  Because of their walls, plant cells can withstand much greater changes in the

surrounding medium than animal cells.

Cytoplasm

 It is the jelly-like substance present between the cell membrane and the nucleus.

The cytoplasm is the fluid content inside the plasma membrane.

 It also contains many specialized cell organelles [mitochondria, golgi bodies,

ribosomes, etc].  Each of these organelles performs a specific function for the cell.  Cell organelles are enclosed by membranes.  The significance of membranes can be illustrated with the example of viruses.

 Viruses lack any membranes and hence do not show characteristics of life until

they enter a living body and use its cell machinery to multiply.

Nucleus  It is an important component of the living cell.  It is generally spherical and located in the center of the cell.  It can be stained and seen easily with the help of a microscope.  Nucleus is separated from the cytoplasm by a double layered membrane called the

nuclear membrane.  This membrane is also porous and allows the movement of materials between the

cytoplasm and the inside of the nucleus [diffusion].  With a microscope of higher magnification, we can see a smaller spherical body in thenucleus. It is called the nucleolus.  In addition, nucleus contains thread-like structures called chromosomes. Thesecarry genes and help in inheritance or transfer of characters from the parents to theoffspring. The chromosomes can be seen only when the cell divides.  Gene is a unit of inheritance in living organisms. It controls the transfer of a

hereditary characteristic from parents to offspring. This means that your parents

pass some of their characteristics on to you. Nucleus, in addition to its role in inheritance, acts as control center of the activitie of the cell.  The entire content of a living cell is known as protoplasm [cytoplasm + nucleus]. I includes the cytoplasm and the nucleus. Protoplasm is called the living substance o the cell. The nucleus of the bacterial cell is not well organized like the cells of multicellula organisms. There is no nuclear membrane.

Every cell has a membrane around it to keep its own contents separate from the

external environment.

 Large and complex cells, including cells from multicellular organisms, need a lot of

chemical activities to support their complicated structure and function.  To keep these activities of different kinds separate from each other, these cells use

membrane-bound little structures (or ‘organelles’) within themselves.

Chromosomes

  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 the form of DNA (deoxyribo nucleic acid) molecules.  Chromosomes are composed of DNA and Protein.  DNA molecules contain the information necessary for constructing and organizing

cells. Functional segments of dna are called genes.

  In a cell which is not dividing, this dna is present as part of chromatin material.

Chromatin material is visible as entangled mass of thread like structures. Wheneve the cell is about to divide, the chromatin material gets organised into

chromosomes.

  The nucleus plays a central role in cellular reproduction, the process by which a

single cell divides and forms two new cells.

  It also plays a crucial part, along with the environment, in determining the way the

cell will develop and what form it will exhibit at maturity, by directing the chemical

activities of the cell.

Nucleoid

 In some organisms like bacteria, the nuclear region of the cell may be poorly defined

due to the absence of a nuclear membrane. Such an undefined nuclear region

containing only nucleic acids is called a nucleoid.

Vacuoles 

 Empty structure in the cytoplasm is called vacuole. It could be single and big as in

an onion cell (plant cell). Cheek cells (animal cells) have smaller vacuoles.

 Large vacuoles are common in plant cells. Vacuoles in animal cells are much

smaller.

 Vacuoles are storage sacs for solid or liquid contents.

 The central vacuole of some plant cells may occupy 50-90% of the cell volume.

 In plant cells vacuoles are full of cell sap and provide turgidity [swollen and

distended or congested] and rigidity to the cell.

 Many substances of importance in the life of the plant cell are stored in vacuoles.

These include amino acids, sugars, various organic acids and some proteins.

 In single-celled organisms like amoeba, the food vacuole contains the food items that

the amoeba has consumed.

 In some unicellular organisms, specialized vacuoles also play important roles in

expelling excess water and some wastes from the cell.

Endoplasmic Reticulum (ER)

 The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and

sheets. It looks like long tubules or round or long bags (vesicles).

  The ER membrane is similar in structure to the plasma membrane.

  There are two types of ER –– rough endoplasmic reticulum (RER) and smooth

endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum RER – Ribosomes

RER looks rough under a microscope because it has particles called ribosomes

attached to its surface.

 The ribosomes, which are present in all active cells, are the sites of protein

manufacture.

 The manufactured proteins are then sent to various places in the cell depending on

need, using the ER.

Smooth Endoplasmic Reticulum SER

 The SER helps in the manufacture of fat molecules, or lipids, important for cell

function.

Functions of Endoplasmic Reticulum (ER)

 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.

 Although the ER varies greatly in appearance in different cells, it always forms a

network system.

 Thus, one function of the ER is to serve as channels for the transport of materials

(especially proteins) between various regions of the cytoplasm or between the

cytoplasm and the nucleus.

 The ER also functions as a cytoplasmic framework providing a surface for some of

the biochemical activities of the cell.

 In the liver cells of the group of animals called vertebrates, SER plays a crucial

role in detoxifying many poisons and drugs.

Lysosomes

 Lysosomes are a kind of waste disposal system of the cell.

 Lysosomes help to keep the cell clean by digesting any foreign material as well as

worn-out cell organelles.

 Foreign materials entering the cell, such as bacteria or food, as well as old organelles

end up in the lysosomes, which break them up into small pieces. Lysosomes are able

to do this because they contain powerful digestive enzymes capable of breaking

down all organic material.

 During the disturbance in cellular metabolism, for example, when the cell gets

damaged, lysosomes may burst and the enzymes digest their own cell. Therefore,

lysosomes are also known as the ‘suicide bags’ of a cell.

 Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes.

These enzymes are made by RER.

Golgi Apparatus or Golgi Complex

 The golgi apparatus consists of a system of membrane-bound vesicles arranged

approximately parallel to each other in stacks called cisterns.

 These membranes often have connections with the membranes of ER and therefore

constitute another portion of a complex cellular membrane system.

 The material synthesized near the ER is packaged and dispatched to various targets

inside and outside the cell through the golgi apparatus.

 Its functions include the storage, modification and packaging of products in

vesicles. In some cases, complex sugars may be made from simple sugars in the golgi

apparatus.

 The golgi apparatus is also involved in the formation of lysosomes.

Mitochondria

 Mitochondria are known as the powerhouse 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.

[If Mitochondria is the Power Plant. ATP is the Electricity].

 ATP is known as the energy currency of the cell.

 The body uses energy stored in ATP for making new chemical compounds and for

mechanical work.

 Mitochondria have two membrane coverings instead of just one.

 The outer membrane is very porous while the inner membrane is deeply folded.

These folds create a large surface area for ATP-generating chemical reactions.

 Mitochondria are strange organelles in the sense that they have their own DNA and

ribosomes. Therefore, mitochondria are able to make some of their own proteins

[ribosomes prepare proteins].

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Plastids

 You might have noticed several small colored bodies in the cytoplasm of the cells of

Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are

called plastids.

 They are of different colours. Some of them contain green pigment called

chlorophyll. Green coloured plastids are called chloroplasts. They provide green

colour to the leaves.

 Chloroplasts are important for photosynthesis in plants.

 Chloroplasts also contain various yellow or orange pigments in addition to

chlorophyll.

 Plastids are present only in plant cells. There are two types of plastids –

chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).

 Leucoplasts are primarily organelles in which materials such as starch, oils and

protein granules are stored.

 The internal organization of the plastids consists of numerous membrane layers

embedded in a material called the stroma.

 Plastids are similar to mitochondria in external structure. Like the mitochondria,

plastids also have their own dna and ribosomes.