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Chapter 5 : The Fundamental Unit Of Life




Table Of Contents

Introduction

  • The body of all organisms is made up of tiny microscopic units called cell.
  • The cell is a fundamental, structural and functional unit of living organisms and basic unit of life.
  • All living organisms are made up of cells. Cells make tissues, tissues together make organ, organs make organ system and organ systems make body of organisms. So cell is called structural unit of life.
  • All the basic functions of the body like respiration, excretion etc. are carried out by cell through  its cell organelles, so cell is called functional unit of life.
  • Cell biology is the study of cells in all aspects of structure and functions.
 

Discovery of cell:

  • Cell was first discovered by Robert Hook in 1665. He observed the cell in a cork slice with the help of a primitive microscope.
  • Anton Von Leeuwenhoek (1674) was the first to observe free cells, like bacteria, protozoa, red blood cells and sperms in his home made microscope.

Cell theory:

  • The cell theory, that all the plants and animals are composed of cells and that the cell is basic unit of life, was presented by two biologists, M. Schleiden (1838) and T. Schwann (1839).
  • The cell theory was further expanded by a german physiologist, Rudolf Virchow (1855). He gave the phrase Omnis cellula-e-cellula, i.e., all cells arises from pre-existing cells.
  • Modified cell theory is termed as cell principle or modern cell theory which postulates that:
    • All living organisms are composed of cells or cell products.
    • All living cells arise from pre-existing cells.
    • All cells are basically alike in chemical composition and metabolic processes.
The functions of an organism as a whole is the result of the activities and interaction of the constituent cells.

Shape, size and number of cells:

Cell shape:


  • The shape of cells is related to the specific function they perform.
  • Some cells like Amoeba and WBCs have changing shapes.
  • In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape.

Cell size:


  • The size of the cell also varies considerably in different animals and plants.
  • The average cell size varies from 0.5 to 20µ (µ = micrometer).
  • In human body, the smallest cell is RBC and the longest one is the nerve cells.

Cell number:


  • The number of cells in living beings differs from the one in unicellular organisms to many in multicellular forms.
  • The number of cells in not definite in multicellular organisms, and may increase along with the growth and volume of organism.


Types of Cells:


Differences between Prokaryotic and Eukaryotic cells:


S. No.

Prokayotic cell

Eukaryotic cell

1
primitive and incomplete cells
Advance and complete
2
Prokaryotes are always unicellular organisms.
 
Unicellular  and multicellular
3
Nucleus is not well defined and known as nucleoid
Well defined nucleus is present
4
membrane bound organelles, such as Mitochondria, Golgi complex etc. are absent.
 
membrane bound organelles, such as Mitochondria, Golgi complex etc. are also present.
 
5
Ribosomes are smaller and scattered randomly in the cytoplasm.
 
Ribosomes are bigger. They are either attached to endoplasmic reticulum or are found free.
 
6
The prokaryotes include archaebacteria, bacteria and cyanobacteria (blue green algae).
 
Eukaryotes include all living organisms, except bacteria and blue-green algae

Structure of cell:

Structurally the cell is formed of three major parts:
  1. Plasma membrane or cell membrane
  2. Cytoplasm and its contents
  3. Nucleus

Plasma membrane (cell membrane):

  • Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.
  • Plasma membrane is living, thin, delicate, elastic, selectively permeable membrane.
  • The plasma membrane is flexible and made up of organic molecules called lipids and proteins.
Functions:
  • Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other material. Therefore, the plasma membrane is called a selectively permeable membrane.
  • Substances can pass across a membrane by two processes- diffusion and osmosis.
    Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.
    “Diffusion is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.”
    Example: movement of carbon dioxide and oxygen during respirationin organisms.
    Osmosis:
    The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called osmosis.
  1. Let us see what will happen if you put an animal cell or plant cells into solution of sugar or salt prepared in water?
  1. If the medium surrounding the cell has a higher water concentration than the cell, i.e., if solution is a very diluted solution, the cell will gain water by osmosis. Such a dilute solution is called hypotonic solution.
Water molecule will pass across the plasma membrane in both directions, but more water molecules will enter the cell than will leave. The cell will therefore, swell up and increase in volume. This process is called endosmosis.
  1. If the medium surrounding the cell is exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane resulting in no change in the size of the cell. Such a solution is called isotonic solution.
  2. If the medium has a lower concentration of water than the cell, i.e., if it is very concentrated solution, the cell will lose water by osmosis. Such a concentrated solution is called hypertonic solution.
In this case too, water crosses the plasma membrane in both the directions, but this time more water leaves the cell than enter it. The cell will therefore, shrink and reduce in volume. This process is known as exosmosis.

Cell wall:

  • In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane.
  • Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a kind of carbohydrate (polysaccharide) and it provides structural strength to the plant.
Functions:
  1. Cell wall protects cell membrane and the internal structures of the cell.
  2. It provides rigidity and determines the shape of the plant cell.
  3. It prevents drying of the cell and helps it to bear unfavourable conditions.
  4. It provides mechanical strength to the plant cells.
Protoplasm:
  • All cells contain living substance called protoplasm. It is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
  • Protoplasm can be distinguished in two forms:
    1. Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
    2. Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.
                                           Protoplasm = cytoplasm + nucleoplasm

Cytoplasm

The part of the cell which occurs between the plasma membrane and nucleus envelop is called the cytoplasm
  • Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage product (starch, lipid, etc.).
  • Chemically cytoplasm contains about 90% water,, 7% proteins, 2% carbohydrates and lipids and 1% inorganic minerals, minerals vitamins, etc.

Cell organelles:

  • A cell has to perform different functions with the help of its various membrane- bound cell organelles.
  • Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form living part of the cell and each of them has a definite shape, structure and function.

NAME OF THE ORGANELLE

STRUCTURE

FUNCTONS

ENDOPLASMIC  RETICULUM

It is absent in prokaryotic cells and matured RBCs of mammals.
ER occurs in three forms:
Cisternae (i.e., closed, fluid-filled sacs), vesicles and tubules.
ER is of two types:
1.Rough endoplasmic reticulum (RER) with ribosomes attached on its surface.
2.Smooth endoplasmic reticulum (SER) which is without ribosomes.

  1. It forms intracellular transporting system.
  2. SER takes part in the synthesis of lipids.
  3. RER is concerned with the transport of proteins which are synthesized by ribosomes on their surface.
 

GOLGI COMPLEX (GOLGI APPARATUS OR GOLGI BODY)

Golgi complex was discovered by Caomillo Golgi (1898).
It occurs in almost all eukaryotic cells. It is not found in prokaryotic cells.
Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicls, vacuoles and flattened cisternae.
1. It is known as packaging and dispatching unit of the cell.
2.It is involved in the synthesis of cell wall, plasma membrane and lysosomes.
3.It is involved in the formation of cell plate during cell dividion.
 

RIBOSOMES

They are present both in prokaryotic and eukaryotic cells.
Ribosomes are dense, spherical and granular particles which occur freely in cytoplasm or remain attached to the endoplasmic reticulum (RER).
Chemically ribosomes are made up of ribonucleic acid (RNA) and proteins.
Ribosomes are of two types: 70S and 80S
 
Ribosome is the site of protein synthesis so it is known as protein factory of the cell.

MITOCHONDRIA

They are present in eukaryotic cell but absent in prokaryotic cells.
Mitochondria exist in variable shapes.
It is a double membranous organelle. Outer membrane is smooth and inner membrane forms folds like structure called cristae.
The cristae consist of F1 particles on its surface.
The matrix inside it contains ribosomes, repiratory enzymes and a circular DNA.
1. Mitochondria generate energy in the form of ATP (Adenosine Tri Phosphate), that is why it is called powerhouse of the cell.
2. Mitochondria have their own ribosomes and DNA, they can synthesize their own proteins, and they are self-duplicating units. So, they are regarded as semiautonomous organelles.
 

LYSOSOMES

Lysosomes are dark, spherical, single membrane bound sacs containing several digestive enzymes. These enzymes are capable of digesting or breaking down all organic materials.
 
Lysosomes are formed by Golgi complex.They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.
 
  1. Lysosomes are involved in intracellular digestion.
  2. During starvation or aging, the cell digests its own organelles through lysosomal enzymes, a process called autophagy.
  3. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
  4. When a cell is destined to die, the lysosomal enzymes digest the whole cell, a process called autolysis. Therefore lysosomes are also known as ‘suicide bags’ of the cell.
 

PLASTIDS (The term ‘plastid’ was given by Haeckel in 1866. Plastids occur in most plant cells and are absent in animal cells.

CHLOROPLAST

They are spherical or discoidal in shape and are enclosed in double membrane. Inside a plastid, two definite regions are clearly visible- Grana and Stroma.
Grana are stacks of membrane-bounded, flatterened discoid sacs containing the molecules of chlorophyll.
Stroma is the homogeneous matrix in which grana are embedded.
Plastids contain their own DNA and ribosomes i.e., they have their own protein synthesizing machinery. They are also self-replicating organelles.
On the basis of colour, plastids are of following three types:
  1. Chloroplasts: Green-coloured plastids containing chlorophyll.
  2. Leucoplasts: Colourless plastids.
  3. Chromoplasts: Coloured plastids (except green colour).
 
 
  1. Chloroplasts are photosynthetic organelles. The chlorophyll present in them trap solar energy for the purpose of synthesizing food for the plant. So chloroplasts are the ‘Kitchens of the cells’.
  2. Leucoplasts help in storing food products like starch, protein and lipids.
  3. Chromoplasts provide colour to flower which in turn attracts insects for pollination.
 
 
 

 

CENTROSOMES

Centrosome is found only in animal cells.
 
It is not bounded by any membrane but consists of two granule-like centrioles. Centrioles are hollow cylindrical structures which are made up of microtubules.
 
1.      Centrosome helps in cell division in animal cells. During cell division centrioles migrate to the poles of animal cells and are involved in the formation of the spindle.
2.      They produce basal bodies from which cilia and flagella arise.
 

VACUOLES

Vacuoles are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
The vacuole is bounded by a membrane, called tonoplast. The vacuole is filled with cell sap which is watery solution rich in sugar, amino acids, proteins, mineralsand metabolic wastes.
 
 
  1. Vacuoles are meant for the storage of food, water and other substances.
  2. Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
  3. Vacuoles provide turgidity and rigidity to the plant cells.
 

PEROXISOMES

Peroxisomes are small and spherical organelles containing powerful oxidative enzymes.
They are bounded by a single membrane.
 
Peroxisomes are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cell.
 
 

Nucleus:

  • The nucleus is a major, centrally located spherical cellular component. It is the centre from where all cellular activities are controlled. It is the carrier of hereditary material in the cell.
  • It is bounded by two membranes, both forming a nuclear envelope. The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the nucleoplasm.
  • The nucleopores allow transfer of materials between the nucleoplasm and the cytoplasm. Within nucleoplasm are embedded two types of nuclear structures- nucleolus and chromatin material. 
Nucleolus:
  • The nucleolus may be one or more in number ad is not bounded by any membrane.
  • Nucleolus consists of DNA, RNA and protein.
  • It is the site of ribosome formation. Nucleolus is known as factory of ribosomes.
Chromatin material:
  • The chromatin is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins (i.e., histones).
  •  DNA stores all the information necessary for the cell to function (metabolism), to grow and to reproduce further cells of the next generation.
  • The chromatin is condensed into two or more thick ribbon-like chromosomes during the division of cell.
  • Functional segments of DNA are called genes.
  • Genes carry information for protein synthesis, which in turn control the development of characters. Since genes are present on chromosomes and chromosomes pass from one generation to the next through, genes are called hereditary units.


 Functions of nucleus:
  1. The nucleus controls all metabolic activities of the cell. If the nucleus is removed from the cell, the protoplasm ultimately dries up and dies.
  2. It regulates the cell cycle.
  3. It is the storehouse of genes which are concerned with the development and manifestation of all the body characters.
  4. Nucleus helps in the formation of ribosomes and RNA.
  5. It helps in the transmission of characters from one generation to the next.

Test your Knowledge part 1

Fill in the blanks 1) ______________are “small organs” of the cell and are found embedded in the cytosol.
2) ______ discovered Golgi complex in _____
3) ______________ apparatus consists of a set of membrane-bounded, fluid-filled vesicls, vacuoles and flattened cisternae.
4)_____________ are present both in prokaryotic and eukaryotic cells.
5) Chemically ribosomes consists of _______ and ________
6)_______________ is the site of protein synthesis so it is known as protein factory of the cell.
7) ____ is a double membranous organelle
8)) Mitochondria have their own ribosomes and __________________, they can synthesize their own proteins, and they are self-duplicating units.
9) ______________ are dark, spherical, single membrane bound sacs containing several digestive enzymes.
10) _____________ helps in the formation of ribosomes and RNA.
11) When a cell is destined to die, the lysosomal enzymes digest the whole cell.This process is known as __________
12) PLASTIDS term is given by __________________ in _____
13) ____________are spherical or discoidal in  shape   and are enclosed in double membrane.
14) ______________ is the homogeneous matrix in which grana are embedded.
15) Chloroplasts are Green-coloured plastids containing __________________.
16) The chlorophyll present in Chloroplasts trap solar energy for the purpose of synthesizing food for the __________________.
17) _____________ carry information for protein synthesis, which in turn control the development of characters.
18) _________ are small and spherical organelles containing powerful oxidative enzymes and are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cells
19) The _______________ is a major, centrally located spherical cellular component.
20) The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the __________________
21) Nucleolus made of __________________, RNA and protein.
22) Nucleolus is called as __________________ of ribosomes.
23) The __________________ is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins
24) Functional segments of __________________ are called genes.
25)______________are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
26) Lysosomes are formed by __________________ complex.They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.

Test your Knowledge part 2

Fill in the blanks
1) The cell is a fundamental, structural and functional __________________ of living organisms and basic __________________ of life.
2) All __________________  are made up of cells.
3) Cells make tissues, tissues together make organ, organs make organ __________________ and organ __________________s make body of organisms.
4) Cell is called structural unit of __________________.
5) All the basic functions of the body like respiration, excretion are carried out by __________________ through  its __________________ organelles, so __________________ is called functional unit of life.
6) Cell was first discovered by _____ in _____.
7) Two biologists ______ and _______presented the cell theory which says that all the plants and animals are composed of cells and that the cell is basic unit of life
8) All cells are basically alike in chemical __________________ and metabolic processes.
9) Some cells like __________________ and WBCs have changing shapes.
10) The smallest cell in human body is __________________ and the longest one is the ______
11) The prokaryotes include archaebacteria, bacteria and cyanobacteria (blue green __________________).
12) __________________ membrane is living, thin, delicate, elastic, selectively permeable membrane and is flexible and made up of organic molecules called lipids and proteins.
13) Substances can pass across a membrane by two processes- __________________ and _____
14) __________________ is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.
15) The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called __________________.
16) If the medium surrounding the cell has a higher water concentration than the cell, then the cell will gain water by osmosis. Such a dilute solution is called ____________
17) The swelling up and increase of volume of cell as a result of water movement inside the cell is known as ____________
18) if medium surrounding the cell is exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane resulting in no change in the size of the cell. Such a solution is called ______
19) If the medium surrounding the cell has a lower concentration of water than the cell, then cell will lose water by osmosis. Such a concentrated solution is called _________________
20) The shrinking and reduce in volume of cell as a result of water movement outside is known as__________
21)__________________ is a kind of carbohydrate (polysaccharide) and provides structural strength to the plant.
22) _____________protects cell membrane and the internal structures of the cell
23)All cells contain living substance called __________________ which is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
24)________ is part of protoplasm which surrounds the nucleus.
25) ______ is part of protoplasm which is located inside the nucleus.
26) Chemically cytoplasm contains about 90% _____, 7% _____, 2% _________ and lipids and 1% inorganic minerals, minerals vitamins


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Class 9 Maths Class 9 Science
Reference Books for class 9 science

Given below are the links of some of the reference books for class 9 science.

These books by S.Chand Publications are detailed in their content and are must have books for class 9 students.
  1. Science for Ninth Class Part1 Physics
  2. Science for Ninth Class Part2 Chemistry
  3. Science for Ninth Class Part3 Biology
Some other books that might be worth considering are
  1. Oswaal CBSE Question Bank Class 9 Science Chapterwise & Topicwise We all know importance of practicing what we have studied. This book is perfect for practicing what you have learned and studied in the subject.
  2. Pearson Foundation Series (IIT-JEE/NEET) Physics, Chemistry, Maths & Biology for Class 9 (Main Books) | PCMB Combo These Foundation books would be helpful for students who want to prepare for JEE/NEET exams. Only buy them if you are up for some challenge and have time to study extra topics. These might be tough for you and you might need extra help in studying these books.
  3. Foundation Science Physics for Class - 9 by H.C. Verma This is one of my favorite Physics book for class 9. Most of the book is within the limits of CBSE syllabus. It might overwhelm you with its language but I feel if you can understand the content is authentic with plenty of problems to solve.

You can use above books for extra knowledge and practicing different questions.







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