Cell - The Unit of Life Class 11 Notes Biology Chapter 8

 Page 1


Points to Remember
Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and 
was modified by Rudolf Virchow. Cell theory States.
(A) All living organisms are composed of cells and products of cells.
(B) All cells arise from pre-existing cells.
Cell : Cell is the structural and functional unit of life.
Prokaryotic Cell    Eukaryotic Cells
? Generally small sized (1–10 µm) ? Generally large sized (5–10 µm)
? Well defined nucleus absent ? Well defined nucleus  present
? Membrane bounded cell organelles ? Membrane bounded cell organelles
absent  present
? DNA without histone protein ? DNA with histone protein
 e.g., Bacteria, Mycoplasma, Blue e.g., Amoeba, Euglena and other
 green Algae  higher organism
 Gram Positive Bacteria Gram Negative Bacteria
? Bacteria that take up gram ? Bacteria do not take up gram stain
Stain. e.g., Bacillus e.g., Escherichia Coli
PROKARYOTIC CELL :
Modification of cell envelope
? Slime layer : Glycocalyx in form of loose sheath.
? Capsule : Glycocalyx in form of thick and tough sheath.
? Mesosomes : Extension of plasma membrane. These can be in the form of
vesicles, tubules and lamellae.
Page 2


Points to Remember
Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and 
was modified by Rudolf Virchow. Cell theory States.
(A) All living organisms are composed of cells and products of cells.
(B) All cells arise from pre-existing cells.
Cell : Cell is the structural and functional unit of life.
Prokaryotic Cell    Eukaryotic Cells
? Generally small sized (1–10 µm) ? Generally large sized (5–10 µm)
? Well defined nucleus absent ? Well defined nucleus  present
? Membrane bounded cell organelles ? Membrane bounded cell organelles
absent  present
? DNA without histone protein ? DNA with histone protein
 e.g., Bacteria, Mycoplasma, Blue e.g., Amoeba, Euglena and other
 green Algae  higher organism
 Gram Positive Bacteria Gram Negative Bacteria
? Bacteria that take up gram ? Bacteria do not take up gram stain
Stain. e.g., Bacillus e.g., Escherichia Coli
PROKARYOTIC CELL :
Modification of cell envelope
? Slime layer : Glycocalyx in form of loose sheath.
? Capsule : Glycocalyx in form of thick and tough sheath.
? Mesosomes : Extension of plasma membrane. These can be in the form of
vesicles, tubules and lamellae. Fluid Mosaic Model of Plasma Membrane ?
 Functions : Cell wall formation, DNA replication and distribution to daughter 
cells, respiration, secretion processes, to increase surface area of plasma 
membrane and enzyme content.
? Flagella : Extension of cell wall. It is composed of three structure–filament,
hook and basal body. It help in motility of bacteria.
? Pili and fimbriae : Surface structure of some bacteria which attaches them
to rocks in streams and to host tissues.
Genetic Material : It is not covered by nuclear envelope. In addition to the
genomic DNA (the single chromosome/circular DNA), many bacteria have small 
circular self replicating, double straned DNA which is called as plasmid, plasmid 
contain genes like antibiotic resistance.
Eukaryotic cells 
Possess an oragnized nucleus with nuclear envelope and have a variety of 
complex locomotory and cytoskeletal structures.
Cell Membrane — Singer and Nicolson (1972) gave ‘fluid mosaic model’. 
According to this the quasi-fluid nature of lipid enables lateral movement of 
proteins within the overall bilayer; two types of proteins (Peripheral and integral 
proteins) with cholesterol, glycolipids and glycoporteins. Erythrocyte membrane 
has 52% protein and 40% lipids.
Function—It is semi permeable and helps in transport of molecule across it.
Passive transport    Active transport
? Transport of molecules from ? Transport of molecules from lower to
higher to lower concentration. higher concentration
? It  do not utilise energy (ATP). ? It utilises energy (ATP)
 e.g., diffusion e.g., Na
+
/k
+
 ATPase Pump.
Page 3


Points to Remember
Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and 
was modified by Rudolf Virchow. Cell theory States.
(A) All living organisms are composed of cells and products of cells.
(B) All cells arise from pre-existing cells.
Cell : Cell is the structural and functional unit of life.
Prokaryotic Cell    Eukaryotic Cells
? Generally small sized (1–10 µm) ? Generally large sized (5–10 µm)
? Well defined nucleus absent ? Well defined nucleus  present
? Membrane bounded cell organelles ? Membrane bounded cell organelles
absent  present
? DNA without histone protein ? DNA with histone protein
 e.g., Bacteria, Mycoplasma, Blue e.g., Amoeba, Euglena and other
 green Algae  higher organism
 Gram Positive Bacteria Gram Negative Bacteria
? Bacteria that take up gram ? Bacteria do not take up gram stain
Stain. e.g., Bacillus e.g., Escherichia Coli
PROKARYOTIC CELL :
Modification of cell envelope
? Slime layer : Glycocalyx in form of loose sheath.
? Capsule : Glycocalyx in form of thick and tough sheath.
? Mesosomes : Extension of plasma membrane. These can be in the form of
vesicles, tubules and lamellae. Fluid Mosaic Model of Plasma Membrane ?
 Functions : Cell wall formation, DNA replication and distribution to daughter 
cells, respiration, secretion processes, to increase surface area of plasma 
membrane and enzyme content.
? Flagella : Extension of cell wall. It is composed of three structure–filament,
hook and basal body. It help in motility of bacteria.
? Pili and fimbriae : Surface structure of some bacteria which attaches them
to rocks in streams and to host tissues.
Genetic Material : It is not covered by nuclear envelope. In addition to the
genomic DNA (the single chromosome/circular DNA), many bacteria have small 
circular self replicating, double straned DNA which is called as plasmid, plasmid 
contain genes like antibiotic resistance.
Eukaryotic cells 
Possess an oragnized nucleus with nuclear envelope and have a variety of 
complex locomotory and cytoskeletal structures.
Cell Membrane — Singer and Nicolson (1972) gave ‘fluid mosaic model’. 
According to this the quasi-fluid nature of lipid enables lateral movement of 
proteins within the overall bilayer; two types of proteins (Peripheral and integral 
proteins) with cholesterol, glycolipids and glycoporteins. Erythrocyte membrane 
has 52% protein and 40% lipids.
Function—It is semi permeable and helps in transport of molecule across it.
Passive transport    Active transport
? Transport of molecules from ? Transport of molecules from lower to
higher to lower concentration. higher concentration
? It  do not utilise energy (ATP). ? It utilises energy (ATP)
 e.g., diffusion e.g., Na
+
/k
+
 ATPase Pump.
Cell Wall is non-living rigid structure which gives shape to the cell and 
protects cell from mechanical damage and infection, helps in cell-to-cell interaction 
and provides barrier to undesirable macromolecules.
Cell wall of algae is made of cellulose, galactans, mannans and minerals like 
calcium carbonate. Plant cell wall consists of cellulose, hemicellulose, pectins 
and proteins.
Middle lamella is made of calcium pectate which holds neighbouring cells  
together.
Plasmodesmata connect the cytoplasm of neighbouring cells.
Endoplasmic Reticulum (ER)
Consists of network of tiny tubular structure. ER divides the intracellular 
space into  two distinct compartments–luminal (inside ER) and extra luminal 
(cytoplasm).
(i) Rough Endoplasmic Reticulum (RER) : 
? Ribosomes attached to outer surface.
Function : ? Involved in protein synthesis and secretion.
(ii) Smooth Endoplasmic Reticulum (SER) : ? Lack ribosomes.
Function  ? Site for synthesis of lipid.
Golgi apparatus : First observed by Camillo Golgi (in 1898)
Consist of cisternae stacked parallel to each other. Two faces of the organelle 
are convex/cis or forming face and concave/trans or maturing face.
Functions : Performs packaging of materials, to be delivered either to the 
intra-cellar targets or secreted outside the cell. Important site of formation of 
glycoproteins and glycolipids.
Lysosomes :
Membrane bound vesicular structures formed by the process of packaging 
in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases, 
carbohydroses) which are active in acidic pH. Also called ‘Suicidal Bag’.
Function : Intracellular digestion.
V acuoles : Member bound space found in the cytoplasm. Contain water, sap, 
excretory product, etc. In plant cell, vacoule occupies 90% of space.
Function : In plants tonoplast (single membrane of vacuole) faciliates  
transport of ions and other substances.
Contractile vacuole for excretion in Amoeba and food vacuoles formed in 
protists for digestion of food.
Page 4


Points to Remember
Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and 
was modified by Rudolf Virchow. Cell theory States.
(A) All living organisms are composed of cells and products of cells.
(B) All cells arise from pre-existing cells.
Cell : Cell is the structural and functional unit of life.
Prokaryotic Cell    Eukaryotic Cells
? Generally small sized (1–10 µm) ? Generally large sized (5–10 µm)
? Well defined nucleus absent ? Well defined nucleus  present
? Membrane bounded cell organelles ? Membrane bounded cell organelles
absent  present
? DNA without histone protein ? DNA with histone protein
 e.g., Bacteria, Mycoplasma, Blue e.g., Amoeba, Euglena and other
 green Algae  higher organism
 Gram Positive Bacteria Gram Negative Bacteria
? Bacteria that take up gram ? Bacteria do not take up gram stain
Stain. e.g., Bacillus e.g., Escherichia Coli
PROKARYOTIC CELL :
Modification of cell envelope
? Slime layer : Glycocalyx in form of loose sheath.
? Capsule : Glycocalyx in form of thick and tough sheath.
? Mesosomes : Extension of plasma membrane. These can be in the form of
vesicles, tubules and lamellae. Fluid Mosaic Model of Plasma Membrane ?
 Functions : Cell wall formation, DNA replication and distribution to daughter 
cells, respiration, secretion processes, to increase surface area of plasma 
membrane and enzyme content.
? Flagella : Extension of cell wall. It is composed of three structure–filament,
hook and basal body. It help in motility of bacteria.
? Pili and fimbriae : Surface structure of some bacteria which attaches them
to rocks in streams and to host tissues.
Genetic Material : It is not covered by nuclear envelope. In addition to the
genomic DNA (the single chromosome/circular DNA), many bacteria have small 
circular self replicating, double straned DNA which is called as plasmid, plasmid 
contain genes like antibiotic resistance.
Eukaryotic cells 
Possess an oragnized nucleus with nuclear envelope and have a variety of 
complex locomotory and cytoskeletal structures.
Cell Membrane — Singer and Nicolson (1972) gave ‘fluid mosaic model’. 
According to this the quasi-fluid nature of lipid enables lateral movement of 
proteins within the overall bilayer; two types of proteins (Peripheral and integral 
proteins) with cholesterol, glycolipids and glycoporteins. Erythrocyte membrane 
has 52% protein and 40% lipids.
Function—It is semi permeable and helps in transport of molecule across it.
Passive transport    Active transport
? Transport of molecules from ? Transport of molecules from lower to
higher to lower concentration. higher concentration
? It  do not utilise energy (ATP). ? It utilises energy (ATP)
 e.g., diffusion e.g., Na
+
/k
+
 ATPase Pump.
Cell Wall is non-living rigid structure which gives shape to the cell and 
protects cell from mechanical damage and infection, helps in cell-to-cell interaction 
and provides barrier to undesirable macromolecules.
Cell wall of algae is made of cellulose, galactans, mannans and minerals like 
calcium carbonate. Plant cell wall consists of cellulose, hemicellulose, pectins 
and proteins.
Middle lamella is made of calcium pectate which holds neighbouring cells  
together.
Plasmodesmata connect the cytoplasm of neighbouring cells.
Endoplasmic Reticulum (ER)
Consists of network of tiny tubular structure. ER divides the intracellular 
space into  two distinct compartments–luminal (inside ER) and extra luminal 
(cytoplasm).
(i) Rough Endoplasmic Reticulum (RER) : 
? Ribosomes attached to outer surface.
Function : ? Involved in protein synthesis and secretion.
(ii) Smooth Endoplasmic Reticulum (SER) : ? Lack ribosomes.
Function  ? Site for synthesis of lipid.
Golgi apparatus : First observed by Camillo Golgi (in 1898)
Consist of cisternae stacked parallel to each other. Two faces of the organelle 
are convex/cis or forming face and concave/trans or maturing face.
Functions : Performs packaging of materials, to be delivered either to the 
intra-cellar targets or secreted outside the cell. Important site of formation of 
glycoproteins and glycolipids.
Lysosomes :
Membrane bound vesicular structures formed by the process of packaging 
in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases, 
carbohydroses) which are active in acidic pH. Also called ‘Suicidal Bag’.
Function : Intracellular digestion.
V acuoles : Member bound space found in the cytoplasm. Contain water, sap, 
excretory product, etc. In plant cell, vacoule occupies 90% of space.
Function : In plants tonoplast (single membrane of vacuole) faciliates  
transport of ions and other substances.
Contractile vacuole for excretion in Amoeba and food vacuoles formed in 
protists for digestion of food.
Fluid Mosaic Model of Plasma Membrane ?
Mitochondria : Double membraned structure. Outer membrane smooth and 
inner membrane forms a number of infoldings called cristae The inner compartment 
is called matrix. The cristae increase the surface area.
DNA (Circular)
OUTER MEMBRANE
RIBOSOME (70S)
CRISTA
MATRIX
INNER MEMBRANE
OUTER MEMBRANE
Mitochondria
Function : Sites of aerobic respiration. Called ’power houses’ of cell as 
produce cellular energy in the form of ATP. Matrix possesses single circular DNA 
molecule, a few RNA molecules, ribsomes (70S). It divides by binary fission.
Plastids : Found in plant cells and in euglenoides. Chloroplasts, chromoplasts 
and leucoplasts are 3 types of plastids depending on pigments contained.
Types of Plastids
Chloroplast                                  Chromoplast                        Leucoplast
(green coloured plastids)   carotenoid (fat souble Amyloplast (Starch)
contains chlorophyll, double carotene (give colour to plant parts) Elaioplasts (oil + fat)
stranded DNA and 70S Xanthophylls. Aleuroplast 
ribosomes Others (Yellow, red, orange)  (store proteins)
Function : Site of photosynthesis, and imparts colours to fruits and flowers.
Chloroplast
Page 5


Points to Remember
Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and 
was modified by Rudolf Virchow. Cell theory States.
(A) All living organisms are composed of cells and products of cells.
(B) All cells arise from pre-existing cells.
Cell : Cell is the structural and functional unit of life.
Prokaryotic Cell    Eukaryotic Cells
? Generally small sized (1–10 µm) ? Generally large sized (5–10 µm)
? Well defined nucleus absent ? Well defined nucleus  present
? Membrane bounded cell organelles ? Membrane bounded cell organelles
absent  present
? DNA without histone protein ? DNA with histone protein
 e.g., Bacteria, Mycoplasma, Blue e.g., Amoeba, Euglena and other
 green Algae  higher organism
 Gram Positive Bacteria Gram Negative Bacteria
? Bacteria that take up gram ? Bacteria do not take up gram stain
Stain. e.g., Bacillus e.g., Escherichia Coli
PROKARYOTIC CELL :
Modification of cell envelope
? Slime layer : Glycocalyx in form of loose sheath.
? Capsule : Glycocalyx in form of thick and tough sheath.
? Mesosomes : Extension of plasma membrane. These can be in the form of
vesicles, tubules and lamellae. Fluid Mosaic Model of Plasma Membrane ?
 Functions : Cell wall formation, DNA replication and distribution to daughter 
cells, respiration, secretion processes, to increase surface area of plasma 
membrane and enzyme content.
? Flagella : Extension of cell wall. It is composed of three structure–filament,
hook and basal body. It help in motility of bacteria.
? Pili and fimbriae : Surface structure of some bacteria which attaches them
to rocks in streams and to host tissues.
Genetic Material : It is not covered by nuclear envelope. In addition to the
genomic DNA (the single chromosome/circular DNA), many bacteria have small 
circular self replicating, double straned DNA which is called as plasmid, plasmid 
contain genes like antibiotic resistance.
Eukaryotic cells 
Possess an oragnized nucleus with nuclear envelope and have a variety of 
complex locomotory and cytoskeletal structures.
Cell Membrane — Singer and Nicolson (1972) gave ‘fluid mosaic model’. 
According to this the quasi-fluid nature of lipid enables lateral movement of 
proteins within the overall bilayer; two types of proteins (Peripheral and integral 
proteins) with cholesterol, glycolipids and glycoporteins. Erythrocyte membrane 
has 52% protein and 40% lipids.
Function—It is semi permeable and helps in transport of molecule across it.
Passive transport    Active transport
? Transport of molecules from ? Transport of molecules from lower to
higher to lower concentration. higher concentration
? It  do not utilise energy (ATP). ? It utilises energy (ATP)
 e.g., diffusion e.g., Na
+
/k
+
 ATPase Pump.
Cell Wall is non-living rigid structure which gives shape to the cell and 
protects cell from mechanical damage and infection, helps in cell-to-cell interaction 
and provides barrier to undesirable macromolecules.
Cell wall of algae is made of cellulose, galactans, mannans and minerals like 
calcium carbonate. Plant cell wall consists of cellulose, hemicellulose, pectins 
and proteins.
Middle lamella is made of calcium pectate which holds neighbouring cells  
together.
Plasmodesmata connect the cytoplasm of neighbouring cells.
Endoplasmic Reticulum (ER)
Consists of network of tiny tubular structure. ER divides the intracellular 
space into  two distinct compartments–luminal (inside ER) and extra luminal 
(cytoplasm).
(i) Rough Endoplasmic Reticulum (RER) : 
? Ribosomes attached to outer surface.
Function : ? Involved in protein synthesis and secretion.
(ii) Smooth Endoplasmic Reticulum (SER) : ? Lack ribosomes.
Function  ? Site for synthesis of lipid.
Golgi apparatus : First observed by Camillo Golgi (in 1898)
Consist of cisternae stacked parallel to each other. Two faces of the organelle 
are convex/cis or forming face and concave/trans or maturing face.
Functions : Performs packaging of materials, to be delivered either to the 
intra-cellar targets or secreted outside the cell. Important site of formation of 
glycoproteins and glycolipids.
Lysosomes :
Membrane bound vesicular structures formed by the process of packaging 
in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases, 
carbohydroses) which are active in acidic pH. Also called ‘Suicidal Bag’.
Function : Intracellular digestion.
V acuoles : Member bound space found in the cytoplasm. Contain water, sap, 
excretory product, etc. In plant cell, vacoule occupies 90% of space.
Function : In plants tonoplast (single membrane of vacuole) faciliates  
transport of ions and other substances.
Contractile vacuole for excretion in Amoeba and food vacuoles formed in 
protists for digestion of food.
Fluid Mosaic Model of Plasma Membrane ?
Mitochondria : Double membraned structure. Outer membrane smooth and 
inner membrane forms a number of infoldings called cristae The inner compartment 
is called matrix. The cristae increase the surface area.
DNA (Circular)
OUTER MEMBRANE
RIBOSOME (70S)
CRISTA
MATRIX
INNER MEMBRANE
OUTER MEMBRANE
Mitochondria
Function : Sites of aerobic respiration. Called ’power houses’ of cell as 
produce cellular energy in the form of ATP. Matrix possesses single circular DNA 
molecule, a few RNA molecules, ribsomes (70S). It divides by binary fission.
Plastids : Found in plant cells and in euglenoides. Chloroplasts, chromoplasts 
and leucoplasts are 3 types of plastids depending on pigments contained.
Types of Plastids
Chloroplast                                  Chromoplast                        Leucoplast
(green coloured plastids)   carotenoid (fat souble Amyloplast (Starch)
contains chlorophyll, double carotene (give colour to plant parts) Elaioplasts (oil + fat)
stranded DNA and 70S Xanthophylls. Aleuroplast 
ribosomes Others (Yellow, red, orange)  (store proteins)
Function : Site of photosynthesis, and imparts colours to fruits and flowers.
Chloroplast
Ribosomes
Composed of RNA and proteins; without membrane. Eucaryotic ribosomes 
are 80S. S = Svedberg’s unit)
Function : Site of protein synthesis.
Cytoskeleton : Network of filaments.
Proteinaceous structure in cytoplasm made up of microtubules and micro 
filaments.
Function : Mechanical support, motility, maintenance of the shape of the cell.
Cilia and Flagella
Cilia are small structures which work like oars which help in movement.
Flagella are longer and responsible for cell movement. They are covered 
with a plasma membrane. Core is called axoneme which has 9 + 2 arrangement 
of axonemal microtubules.
Centrosome and Centrioles
Centrosome contains two cylindrical structures called centrioles. Surrounded 
by amorphous pericentriolar material. Has 9 + 2 arrangement. Centrioles form the 
basal body of cilia or flagella and spindle fibres for cell division in animal cells. 
They produces spindle apparatus during cell divison.
Nucleus : With double membrane with perinuclear space and nuclear pores; 
has Chromatin, nuclear matrix and nucleoli (site for rRNA synthesis). (Named 
by Robert Brown – 1831)
Chromatin DNA + nonhistone proteins. (Named by Flemming)
Nucleoplasm – Nucleolus + Chromatin
Nulear membrane—It is with perinuclear space and nucleopores.
Chromosomes—DNA/RNA + Histone protein/Nonhistone protein.
Centromere : Primary constriction–disc is known as kinetochores.
No nucleus in Erythrocyctes (RBC) of mammals and sieve tubes in vascular 
plants.
Chromosomes (on basis of position of centromere) :
Metacentric : Middle centromere.
Sub-metacentric : Centromere nearer to one end of chromosomes.
Acrocentric : Centromere situated close to its end.
Telocentric :Has terminal centromere.
Satellite : Some chromosomes have non-staining secondary constructions at 
a constant location, which gives the appearance of small fragment called satellite.