NCERT Textbook: Cell: The Unit of Life | Biology Class 11 - NEET PDF Download

 Page 1


UNIT 3
Biology is the study of living organisms. The detailed description of
their form and appearance only brought out their diversity. It is the
cell theory that emphasised the unity underlying this diversity of forms,
i.e., the cellular organisation of all life forms. A description of cell
structure and cell growth by division is given in the chapters comprising
this unit. Cell theory also created a sense of mystery around living
phenomena, i.e., physiological and behavioural processes. This mystery
was the requirement of integrity of cellular organisation for living
phenomena to be demonstrated or observed. In studying and
understanding the physiological and behavioural processes, one can
take a physico-chemical approach and use cell-free systems to
investigate. This approach enables us to describe the various processes
in molecular terms. The approach is established by analysis of living
tissues for elements and compounds. It will tell us what types of organic
compounds are present in living organisms. In the next stage, one can
ask the question: What are these compounds doing inside a cell? And,
in what way they carry out gross physiological processes like digestion,
excretion, memory, defense, recognition, etc. In other words we answer
the question, what is the molecular basis of all physiological processes?
It can also explain the abnormal processes that occur during any
diseased condition. This physico-chemical approach to study and
understand living organisms is called ‘Reductionist Biology’. The
concepts and techniques of physics and chemistry are applied to
understand biology. In Chapter 9 of this unit, a brief description of
biomolecules is provided.
CELL: STRUCTURE AND FUNCTIONS
Chapter 8
Cell: The Unit of Life
Chapter 9
Biomolecules
Chapter 10
Cell Cycle and
Cell Division
2024-25
Page 2


UNIT 3
Biology is the study of living organisms. The detailed description of
their form and appearance only brought out their diversity. It is the
cell theory that emphasised the unity underlying this diversity of forms,
i.e., the cellular organisation of all life forms. A description of cell
structure and cell growth by division is given in the chapters comprising
this unit. Cell theory also created a sense of mystery around living
phenomena, i.e., physiological and behavioural processes. This mystery
was the requirement of integrity of cellular organisation for living
phenomena to be demonstrated or observed. In studying and
understanding the physiological and behavioural processes, one can
take a physico-chemical approach and use cell-free systems to
investigate. This approach enables us to describe the various processes
in molecular terms. The approach is established by analysis of living
tissues for elements and compounds. It will tell us what types of organic
compounds are present in living organisms. In the next stage, one can
ask the question: What are these compounds doing inside a cell? And,
in what way they carry out gross physiological processes like digestion,
excretion, memory, defense, recognition, etc. In other words we answer
the question, what is the molecular basis of all physiological processes?
It can also explain the abnormal processes that occur during any
diseased condition. This physico-chemical approach to study and
understand living organisms is called ‘Reductionist Biology’. The
concepts and techniques of physics and chemistry are applied to
understand biology. In Chapter 9 of this unit, a brief description of
biomolecules is provided.
CELL: STRUCTURE AND FUNCTIONS
Chapter 8
Cell: The Unit of Life
Chapter 9
Biomolecules
Chapter 10
Cell Cycle and
Cell Division
2024-25
G.N. RAMACHANDRAN, an outstanding figure in the field of protein
structure, was the founder of the ‘Madras school’ of
conformational analysis of biopolymers. His discovery of the triple
helical structure of collagen published in Nature in 1954 and his
analysis of the allowed conformations of proteins through the
use of the ‘Ramachandran plot’ rank among the most outstanding
contributions in structural biology. He was born on October 8,
1922, in a small town, not far from Cochin on the southwestern
coast of India. His father was a professor of mathematics at a
local college and thus had considerable influence in shaping
Ramachandran’s interest in mathematics. After completing his
school years, Ramachandran graduated in 1942 as the top-
ranking student in the B.Sc. (Honors) Physics course of the
University of Madras. He received a Ph.D. from Cambridge
University in 1949. While at Cambridge, Ramachandran met
Linus Pauling and was deeply influenced by his publications on
models of the a-helix and ß-sheet structures that directed his
attention to solving the structure of collagen. He passed away at
the age of 78, on April 7, 2001. G.N. Ramachandran
(1922 – 2001)
2024-25
Page 3


UNIT 3
Biology is the study of living organisms. The detailed description of
their form and appearance only brought out their diversity. It is the
cell theory that emphasised the unity underlying this diversity of forms,
i.e., the cellular organisation of all life forms. A description of cell
structure and cell growth by division is given in the chapters comprising
this unit. Cell theory also created a sense of mystery around living
phenomena, i.e., physiological and behavioural processes. This mystery
was the requirement of integrity of cellular organisation for living
phenomena to be demonstrated or observed. In studying and
understanding the physiological and behavioural processes, one can
take a physico-chemical approach and use cell-free systems to
investigate. This approach enables us to describe the various processes
in molecular terms. The approach is established by analysis of living
tissues for elements and compounds. It will tell us what types of organic
compounds are present in living organisms. In the next stage, one can
ask the question: What are these compounds doing inside a cell? And,
in what way they carry out gross physiological processes like digestion,
excretion, memory, defense, recognition, etc. In other words we answer
the question, what is the molecular basis of all physiological processes?
It can also explain the abnormal processes that occur during any
diseased condition. This physico-chemical approach to study and
understand living organisms is called ‘Reductionist Biology’. The
concepts and techniques of physics and chemistry are applied to
understand biology. In Chapter 9 of this unit, a brief description of
biomolecules is provided.
CELL: STRUCTURE AND FUNCTIONS
Chapter 8
Cell: The Unit of Life
Chapter 9
Biomolecules
Chapter 10
Cell Cycle and
Cell Division
2024-25
G.N. RAMACHANDRAN, an outstanding figure in the field of protein
structure, was the founder of the ‘Madras school’ of
conformational analysis of biopolymers. His discovery of the triple
helical structure of collagen published in Nature in 1954 and his
analysis of the allowed conformations of proteins through the
use of the ‘Ramachandran plot’ rank among the most outstanding
contributions in structural biology. He was born on October 8,
1922, in a small town, not far from Cochin on the southwestern
coast of India. His father was a professor of mathematics at a
local college and thus had considerable influence in shaping
Ramachandran’s interest in mathematics. After completing his
school years, Ramachandran graduated in 1942 as the top-
ranking student in the B.Sc. (Honors) Physics course of the
University of Madras. He received a Ph.D. from Cambridge
University in 1949. While at Cambridge, Ramachandran met
Linus Pauling and was deeply influenced by his publications on
models of the a-helix and ß-sheet structures that directed his
attention to solving the structure of collagen. He passed away at
the age of 78, on April 7, 2001. G.N. Ramachandran
(1922 – 2001)
2024-25
When you look around, you see both living and non-living things. You
must have wondered and asked yourself – ‘what is it that makes an
organism living, or what is it that an inanimate thing does not have which
a living thing has’ ? The answer to this is the presence of the basic unit of
life – the cell in all living organisms.
All organisms are composed of cells. Some are composed of a single
cell and are called unicellular organisms while others, like us, composed
of many cells, are called multicellular organisms.
8.1 WHAT IS A CELL?
Unicellular organisms are capable of (i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete
structure of a cell does not ensure independent living. Hence, cell is the
fundamental structural and functional unit of all living organisms.
Anton Von Leeuwenhoek first saw and described a live cell. Robert
Brown later discovered the nucleus. The invention of the microscope and
its improvement leading to the electron microscope revealed all the
structural details of the cell.
8.2 CELL THEORY
In 1838, Matthias Schleiden, a German botanist, examined a large number
of plants and observed that all plants are composed of different kinds of
cells which form the tissues of the plant. At about the same time, Theodore
CELL: THE UNIT OF LIFE
CHAPTER  8
8.1 What is a Cell?
8.2 Cell Theory
8.3 An Overview of
Cell
8.4 Prokaryotic Cells
8.5 Eukaryotic Cells
2024-25
Page 4


UNIT 3
Biology is the study of living organisms. The detailed description of
their form and appearance only brought out their diversity. It is the
cell theory that emphasised the unity underlying this diversity of forms,
i.e., the cellular organisation of all life forms. A description of cell
structure and cell growth by division is given in the chapters comprising
this unit. Cell theory also created a sense of mystery around living
phenomena, i.e., physiological and behavioural processes. This mystery
was the requirement of integrity of cellular organisation for living
phenomena to be demonstrated or observed. In studying and
understanding the physiological and behavioural processes, one can
take a physico-chemical approach and use cell-free systems to
investigate. This approach enables us to describe the various processes
in molecular terms. The approach is established by analysis of living
tissues for elements and compounds. It will tell us what types of organic
compounds are present in living organisms. In the next stage, one can
ask the question: What are these compounds doing inside a cell? And,
in what way they carry out gross physiological processes like digestion,
excretion, memory, defense, recognition, etc. In other words we answer
the question, what is the molecular basis of all physiological processes?
It can also explain the abnormal processes that occur during any
diseased condition. This physico-chemical approach to study and
understand living organisms is called ‘Reductionist Biology’. The
concepts and techniques of physics and chemistry are applied to
understand biology. In Chapter 9 of this unit, a brief description of
biomolecules is provided.
CELL: STRUCTURE AND FUNCTIONS
Chapter 8
Cell: The Unit of Life
Chapter 9
Biomolecules
Chapter 10
Cell Cycle and
Cell Division
2024-25
G.N. RAMACHANDRAN, an outstanding figure in the field of protein
structure, was the founder of the ‘Madras school’ of
conformational analysis of biopolymers. His discovery of the triple
helical structure of collagen published in Nature in 1954 and his
analysis of the allowed conformations of proteins through the
use of the ‘Ramachandran plot’ rank among the most outstanding
contributions in structural biology. He was born on October 8,
1922, in a small town, not far from Cochin on the southwestern
coast of India. His father was a professor of mathematics at a
local college and thus had considerable influence in shaping
Ramachandran’s interest in mathematics. After completing his
school years, Ramachandran graduated in 1942 as the top-
ranking student in the B.Sc. (Honors) Physics course of the
University of Madras. He received a Ph.D. from Cambridge
University in 1949. While at Cambridge, Ramachandran met
Linus Pauling and was deeply influenced by his publications on
models of the a-helix and ß-sheet structures that directed his
attention to solving the structure of collagen. He passed away at
the age of 78, on April 7, 2001. G.N. Ramachandran
(1922 – 2001)
2024-25
When you look around, you see both living and non-living things. You
must have wondered and asked yourself – ‘what is it that makes an
organism living, or what is it that an inanimate thing does not have which
a living thing has’ ? The answer to this is the presence of the basic unit of
life – the cell in all living organisms.
All organisms are composed of cells. Some are composed of a single
cell and are called unicellular organisms while others, like us, composed
of many cells, are called multicellular organisms.
8.1 WHAT IS A CELL?
Unicellular organisms are capable of (i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete
structure of a cell does not ensure independent living. Hence, cell is the
fundamental structural and functional unit of all living organisms.
Anton Von Leeuwenhoek first saw and described a live cell. Robert
Brown later discovered the nucleus. The invention of the microscope and
its improvement leading to the electron microscope revealed all the
structural details of the cell.
8.2 CELL THEORY
In 1838, Matthias Schleiden, a German botanist, examined a large number
of plants and observed that all plants are composed of different kinds of
cells which form the tissues of the plant. At about the same time, Theodore
CELL: THE UNIT OF LIFE
CHAPTER  8
8.1 What is a Cell?
8.2 Cell Theory
8.3 An Overview of
Cell
8.4 Prokaryotic Cells
8.5 Eukaryotic Cells
2024-25
88 BIOLOGY
Schwann (1839), a British Zoologist, studied different types of animal cells
and reported that cells had a thin outer layer which is today known as the
‘plasma membrane’. He also concluded, based on his studies on plant
tissues, that the presence of cell wall is a unique character of the plant
cells. On the basis of this, Schwann proposed the hypothesis that the bodies
of animals and plants are composed of cells and products of cells.
Schleiden and Schwann together formulated the cell theory. This theory
however, did not explain as to how new cells were formed. Rudolf Virchow
(1855) first explained that cells divided and new cells are formed from
pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of
Schleiden and Schwann to give the cell theory a final shape. Cell theory
as understood today is:
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
8.3 AN OVERVIEW OF CELL
You have earlier observed cells in an onion peel and/or human cheek
cells under the microscope. Let us recollect their structure. The onion cell
which is a typical plant cell, has a distinct cell wall as its outer boundary
and just within it is the cell membrane. The cells of the human cheek
have an outer membrane as the delimiting structure of the cell. Inside
each cell is a dense membrane bound structure called nucleus. This
nucleus contains the chromosomes which in turn contain the genetic
material, DNA. Cells that have membrane bound nuclei are called
eukaryotic whereas cells that lack a membrane bound nucleus are
prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix
called cytoplasm occupies the volume of the cell. The cytoplasm is the
main arena of cellular activities in both the plant and animal cells. V arious
chemical reactions occur in it to keep the cell in the ‘living state’.
Besides the nucleus, the eukaryotic cells have other membrane bound
distinct structures called organelles like the endoplasmic reticulum (ER),
the golgi complex, lysosomes, mitochondria, microbodies and vacuoles.
The prokaryotic cells lack such membrane bound organelles.
Ribosomes are non-membrane bound organelles found in all cells –
both eukaryotic as well as prokaryotic. Within the cell, ribosomes are
found not only in the cytoplasm but also within the two organelles –
chloroplasts (in plants) and mitochondria and on rough ER.
Animal cells contain another non-membrane bound organelle called
centrosome which helps in cell division.
Cells differ greatly in size, shape and activities (Figure 8.1). For example,
Mycoplasmas, the smallest cells, are only 0.3 µm in length while bacteria
2024-25
Page 5


UNIT 3
Biology is the study of living organisms. The detailed description of
their form and appearance only brought out their diversity. It is the
cell theory that emphasised the unity underlying this diversity of forms,
i.e., the cellular organisation of all life forms. A description of cell
structure and cell growth by division is given in the chapters comprising
this unit. Cell theory also created a sense of mystery around living
phenomena, i.e., physiological and behavioural processes. This mystery
was the requirement of integrity of cellular organisation for living
phenomena to be demonstrated or observed. In studying and
understanding the physiological and behavioural processes, one can
take a physico-chemical approach and use cell-free systems to
investigate. This approach enables us to describe the various processes
in molecular terms. The approach is established by analysis of living
tissues for elements and compounds. It will tell us what types of organic
compounds are present in living organisms. In the next stage, one can
ask the question: What are these compounds doing inside a cell? And,
in what way they carry out gross physiological processes like digestion,
excretion, memory, defense, recognition, etc. In other words we answer
the question, what is the molecular basis of all physiological processes?
It can also explain the abnormal processes that occur during any
diseased condition. This physico-chemical approach to study and
understand living organisms is called ‘Reductionist Biology’. The
concepts and techniques of physics and chemistry are applied to
understand biology. In Chapter 9 of this unit, a brief description of
biomolecules is provided.
CELL: STRUCTURE AND FUNCTIONS
Chapter 8
Cell: The Unit of Life
Chapter 9
Biomolecules
Chapter 10
Cell Cycle and
Cell Division
2024-25
G.N. RAMACHANDRAN, an outstanding figure in the field of protein
structure, was the founder of the ‘Madras school’ of
conformational analysis of biopolymers. His discovery of the triple
helical structure of collagen published in Nature in 1954 and his
analysis of the allowed conformations of proteins through the
use of the ‘Ramachandran plot’ rank among the most outstanding
contributions in structural biology. He was born on October 8,
1922, in a small town, not far from Cochin on the southwestern
coast of India. His father was a professor of mathematics at a
local college and thus had considerable influence in shaping
Ramachandran’s interest in mathematics. After completing his
school years, Ramachandran graduated in 1942 as the top-
ranking student in the B.Sc. (Honors) Physics course of the
University of Madras. He received a Ph.D. from Cambridge
University in 1949. While at Cambridge, Ramachandran met
Linus Pauling and was deeply influenced by his publications on
models of the a-helix and ß-sheet structures that directed his
attention to solving the structure of collagen. He passed away at
the age of 78, on April 7, 2001. G.N. Ramachandran
(1922 – 2001)
2024-25
When you look around, you see both living and non-living things. You
must have wondered and asked yourself – ‘what is it that makes an
organism living, or what is it that an inanimate thing does not have which
a living thing has’ ? The answer to this is the presence of the basic unit of
life – the cell in all living organisms.
All organisms are composed of cells. Some are composed of a single
cell and are called unicellular organisms while others, like us, composed
of many cells, are called multicellular organisms.
8.1 WHAT IS A CELL?
Unicellular organisms are capable of (i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete
structure of a cell does not ensure independent living. Hence, cell is the
fundamental structural and functional unit of all living organisms.
Anton Von Leeuwenhoek first saw and described a live cell. Robert
Brown later discovered the nucleus. The invention of the microscope and
its improvement leading to the electron microscope revealed all the
structural details of the cell.
8.2 CELL THEORY
In 1838, Matthias Schleiden, a German botanist, examined a large number
of plants and observed that all plants are composed of different kinds of
cells which form the tissues of the plant. At about the same time, Theodore
CELL: THE UNIT OF LIFE
CHAPTER  8
8.1 What is a Cell?
8.2 Cell Theory
8.3 An Overview of
Cell
8.4 Prokaryotic Cells
8.5 Eukaryotic Cells
2024-25
88 BIOLOGY
Schwann (1839), a British Zoologist, studied different types of animal cells
and reported that cells had a thin outer layer which is today known as the
‘plasma membrane’. He also concluded, based on his studies on plant
tissues, that the presence of cell wall is a unique character of the plant
cells. On the basis of this, Schwann proposed the hypothesis that the bodies
of animals and plants are composed of cells and products of cells.
Schleiden and Schwann together formulated the cell theory. This theory
however, did not explain as to how new cells were formed. Rudolf Virchow
(1855) first explained that cells divided and new cells are formed from
pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of
Schleiden and Schwann to give the cell theory a final shape. Cell theory
as understood today is:
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
8.3 AN OVERVIEW OF CELL
You have earlier observed cells in an onion peel and/or human cheek
cells under the microscope. Let us recollect their structure. The onion cell
which is a typical plant cell, has a distinct cell wall as its outer boundary
and just within it is the cell membrane. The cells of the human cheek
have an outer membrane as the delimiting structure of the cell. Inside
each cell is a dense membrane bound structure called nucleus. This
nucleus contains the chromosomes which in turn contain the genetic
material, DNA. Cells that have membrane bound nuclei are called
eukaryotic whereas cells that lack a membrane bound nucleus are
prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix
called cytoplasm occupies the volume of the cell. The cytoplasm is the
main arena of cellular activities in both the plant and animal cells. V arious
chemical reactions occur in it to keep the cell in the ‘living state’.
Besides the nucleus, the eukaryotic cells have other membrane bound
distinct structures called organelles like the endoplasmic reticulum (ER),
the golgi complex, lysosomes, mitochondria, microbodies and vacuoles.
The prokaryotic cells lack such membrane bound organelles.
Ribosomes are non-membrane bound organelles found in all cells –
both eukaryotic as well as prokaryotic. Within the cell, ribosomes are
found not only in the cytoplasm but also within the two organelles –
chloroplasts (in plants) and mitochondria and on rough ER.
Animal cells contain another non-membrane bound organelle called
centrosome which helps in cell division.
Cells differ greatly in size, shape and activities (Figure 8.1). For example,
Mycoplasmas, the smallest cells, are only 0.3 µm in length while bacteria
2024-25
CELL: THE UNIT OF LIFE 89
Red blood cells
(round and biconcave)
White blood cells
(Branched and long)
Columnar epithelial cells
(long and narrow)
(amoeboid)
Nerve cell
Mesophyll cells
(round and oval)
A tracheid
(elongated)
Figure 8.1 Diagram showing different shapes of the cells
could be 3 to 5 µm. The largest isolated single cell is the egg of an ostrich.
Among multicellular organisms, human red blood cells are about 7.0
µm in diameter. Nerve cells are some of the longest cells. Cells also vary
greatly in their shape. They may be disc-like, polygonal, columnar , cuboid,
thread like, or even irregular. The shape of the cell may vary with the
function they perform.
8.4 PROKARYOTIC CELLS
The prokaryotic cells are represented by bacteria, blue-green algae,
mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are
generally smaller and multiply more rapidly than the eukaryotic cells
(Figure 8.2). They may vary greatly in shape and size. The four basic
shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma
shaped) and spirillum (spiral).
The organisation of the prokaryotic cell is fundamentally similar even
though prokaryotes exhibit a wide variety of shapes and functions. All
2024-25