NCERT Textbook: Cellular Processes | Biotechnology for Class 11 - NEET PDF Download

 Page 1


Cellular 
Processes
5.1 Cell Signaling
5.2 Metabolic Pathways
5.3 Cell Cycle
5.4 Programmed Cell 
Death (Apoptosis)
5.5 Cell Differentiation
5.6 Cell Migration
5.1 Cell Signaling We are aware that cells (both prokaryotic and eukaryotic) 
constantly receive and interpret environmental signals and 
respond to them in real time. These signals include light, 
heat, sound, and touch. The cell fates during development 
are speci??ed by signaling pathways in response to such 
extracellular signals. Cells interact with their neighbouring 
cells by transmitting and receiving signals. These signals 
are synthesised by the cells in the form of chemicals and 
released in the extracellular milieu. However, cells can also 
respond to ‘external’ signals which are not synthesised 
by the cells of our body. Therefore, one can assume that 
the cells are capable of sensing a wide variety of signals. 
It is important to note that a cell can only respond to a 
particular signal if it possesses the corresponding receptor 
for it. A receptor is a protein located either on the cell 
surface or inside the cytoplasm or the nucleus. A chemical 
messenger to which a receptor responds is a ligand. The 
association between a receptor and its corresponding 
ligand is highly speci??c, which means that a cell will only 
Chapter 5
Chapter 5 Cellular Processes.indd   103 09/01/2025   15:21:28
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Page 2


Cellular 
Processes
5.1 Cell Signaling
5.2 Metabolic Pathways
5.3 Cell Cycle
5.4 Programmed Cell 
Death (Apoptosis)
5.5 Cell Differentiation
5.6 Cell Migration
5.1 Cell Signaling We are aware that cells (both prokaryotic and eukaryotic) 
constantly receive and interpret environmental signals and 
respond to them in real time. These signals include light, 
heat, sound, and touch. The cell fates during development 
are speci??ed by signaling pathways in response to such 
extracellular signals. Cells interact with their neighbouring 
cells by transmitting and receiving signals. These signals 
are synthesised by the cells in the form of chemicals and 
released in the extracellular milieu. However, cells can also 
respond to ‘external’ signals which are not synthesised 
by the cells of our body. Therefore, one can assume that 
the cells are capable of sensing a wide variety of signals. 
It is important to note that a cell can only respond to a 
particular signal if it possesses the corresponding receptor 
for it. A receptor is a protein located either on the cell 
surface or inside the cytoplasm or the nucleus. A chemical 
messenger to which a receptor responds is a ligand. The 
association between a receptor and its corresponding 
ligand is highly speci??c, which means that a cell will only 
Chapter 5
Chapter 5 Cellular Processes.indd   103 09/01/2025   15:21:28
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Biotechnology 104
be able to respond to a chemical messenger, if it bears the 
corresponding receptor for it and not otherwise. 
Transmission of chemical messages from one cell to 
another cell requires binding of a ligand to its receptor, 
which results in conformational changes in the receptor. 
These changes then initiate a message relay system and 
bring about further important changes in the activities 
inside the cell. 
It should be noted that cells send and receive signals in 
different ways. Depending on the proximity of sender and 
recipient cells, signaling can be broadly classi??ed in the 
following categories:
1. Paracrine signaling: In this form of signaling, 
communication between cells occurs over relatively 
short distances. A chemical message released in the 
extracellular space by the sender cells is sensed by the 
recipient cells instantly.  This type of signaling is seen 
commonly in the communication of neurons.
2. Autocrine signaling: Many times, a cell which secretes 
a ligand, also possesses receptors speci??c for that 
ligand. This type of signaling is referred to as autocrine 
signalling. For instance, cancer cells are characterised 
by uncontrollable growth. Therefore, they require a 
greater amount of growth factors for their proliferation. 
Unlike normal cells, cancer cells do not depend on 
external growth factors for their growth. Instead, they 
are capable of synthesising their own growth factors 
and also possess the receptors speci??c for them. 
3. Endocrine signaling: Endocrine signaling or long-
distance signaling requires the ligand to be synthesised 
by the cell and released into the bloodstream to travel to 
the recipient or target cell. Hormones generally exhibit 
this form of signaling. 
5.2 Metaboli C Pathway S
Metabolism is the process through which living organisms 
take and utilise the free energy required to carry out their 
life processes. Living organisms are of two types on the 
basis of taking free energy: phototrophs and chemotrophs. 
Chapter 5 Cellular Processes.indd   104 09/01/2025   15:21:28
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Page 3


Cellular 
Processes
5.1 Cell Signaling
5.2 Metabolic Pathways
5.3 Cell Cycle
5.4 Programmed Cell 
Death (Apoptosis)
5.5 Cell Differentiation
5.6 Cell Migration
5.1 Cell Signaling We are aware that cells (both prokaryotic and eukaryotic) 
constantly receive and interpret environmental signals and 
respond to them in real time. These signals include light, 
heat, sound, and touch. The cell fates during development 
are speci??ed by signaling pathways in response to such 
extracellular signals. Cells interact with their neighbouring 
cells by transmitting and receiving signals. These signals 
are synthesised by the cells in the form of chemicals and 
released in the extracellular milieu. However, cells can also 
respond to ‘external’ signals which are not synthesised 
by the cells of our body. Therefore, one can assume that 
the cells are capable of sensing a wide variety of signals. 
It is important to note that a cell can only respond to a 
particular signal if it possesses the corresponding receptor 
for it. A receptor is a protein located either on the cell 
surface or inside the cytoplasm or the nucleus. A chemical 
messenger to which a receptor responds is a ligand. The 
association between a receptor and its corresponding 
ligand is highly speci??c, which means that a cell will only 
Chapter 5
Chapter 5 Cellular Processes.indd   103 09/01/2025   15:21:28
Reprint 2025-26
Biotechnology 104
be able to respond to a chemical messenger, if it bears the 
corresponding receptor for it and not otherwise. 
Transmission of chemical messages from one cell to 
another cell requires binding of a ligand to its receptor, 
which results in conformational changes in the receptor. 
These changes then initiate a message relay system and 
bring about further important changes in the activities 
inside the cell. 
It should be noted that cells send and receive signals in 
different ways. Depending on the proximity of sender and 
recipient cells, signaling can be broadly classi??ed in the 
following categories:
1. Paracrine signaling: In this form of signaling, 
communication between cells occurs over relatively 
short distances. A chemical message released in the 
extracellular space by the sender cells is sensed by the 
recipient cells instantly.  This type of signaling is seen 
commonly in the communication of neurons.
2. Autocrine signaling: Many times, a cell which secretes 
a ligand, also possesses receptors speci??c for that 
ligand. This type of signaling is referred to as autocrine 
signalling. For instance, cancer cells are characterised 
by uncontrollable growth. Therefore, they require a 
greater amount of growth factors for their proliferation. 
Unlike normal cells, cancer cells do not depend on 
external growth factors for their growth. Instead, they 
are capable of synthesising their own growth factors 
and also possess the receptors speci??c for them. 
3. Endocrine signaling: Endocrine signaling or long-
distance signaling requires the ligand to be synthesised 
by the cell and released into the bloodstream to travel to 
the recipient or target cell. Hormones generally exhibit 
this form of signaling. 
5.2 Metaboli C Pathway S
Metabolism is the process through which living organisms 
take and utilise the free energy required to carry out their 
life processes. Living organisms are of two types on the 
basis of taking free energy: phototrophs and chemotrophs. 
Chapter 5 Cellular Processes.indd   104 09/01/2025   15:21:28
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c ellular Pro c esses 105
Phototrophs use the energy of sunlight to convert simple 
molecules (less energy containing) into more complex 
molecules (energy rich) that serve as fuel to perform life 
processes. Phototrophs are photosynthetic organisms 
(such as plants and some bacteria); they transform light 
energy into chemical energy. Heterotrophs such as 
animals, obtain energy indirectly from plants through 
their food. In chemotrophs, the energy is obtained by 
oxidising chemical compounds (organic or inorganic). 
This energy uptake in organisms is done by coupling the 
exergonic reactions of nutrient oxidation to the endergonic 
processes required to maintain the living state. Central 
to all these energy transactions is the energy currency 
called ATP (detail is given in section 4.2 bioenergetics). In 
metabolism, there are interlinked biochemical reactions 
that begin with a particular molecule and convert it into 
some other molecule or molecules in a carefully de??ned 
fashion. The energy is utilised for various processes within 
the cell such as, the creation of gradient, movement of 
molecules across membranes, conversion of chemical 
energy into mechanical energy and powering of reactions 
that result in the synthesis of biomolecules.
The synthesis and breakdown of biomolecules is 
accomplished through a number of steps inside the living 
system. These steps collectively constitute metabolic 
pathway. Metabolic pathways can broadly be classi??ed into 
two classes; anabolic pathways and catabolic pathways.
(i) Anabolic pathways
In these pathways, larger and more complex molecules 
are synthesised from small molecules. Anabolic pathways 
are endergonic (consumption of energy). Reactions that 
require energy such as synthesis of glucose, fats, protein 
or DNA are called anabolic reactions or anabolism.
Chapter 5 Cellular Processes.indd   105 09/01/2025   15:21:28
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Page 4


Cellular 
Processes
5.1 Cell Signaling
5.2 Metabolic Pathways
5.3 Cell Cycle
5.4 Programmed Cell 
Death (Apoptosis)
5.5 Cell Differentiation
5.6 Cell Migration
5.1 Cell Signaling We are aware that cells (both prokaryotic and eukaryotic) 
constantly receive and interpret environmental signals and 
respond to them in real time. These signals include light, 
heat, sound, and touch. The cell fates during development 
are speci??ed by signaling pathways in response to such 
extracellular signals. Cells interact with their neighbouring 
cells by transmitting and receiving signals. These signals 
are synthesised by the cells in the form of chemicals and 
released in the extracellular milieu. However, cells can also 
respond to ‘external’ signals which are not synthesised 
by the cells of our body. Therefore, one can assume that 
the cells are capable of sensing a wide variety of signals. 
It is important to note that a cell can only respond to a 
particular signal if it possesses the corresponding receptor 
for it. A receptor is a protein located either on the cell 
surface or inside the cytoplasm or the nucleus. A chemical 
messenger to which a receptor responds is a ligand. The 
association between a receptor and its corresponding 
ligand is highly speci??c, which means that a cell will only 
Chapter 5
Chapter 5 Cellular Processes.indd   103 09/01/2025   15:21:28
Reprint 2025-26
Biotechnology 104
be able to respond to a chemical messenger, if it bears the 
corresponding receptor for it and not otherwise. 
Transmission of chemical messages from one cell to 
another cell requires binding of a ligand to its receptor, 
which results in conformational changes in the receptor. 
These changes then initiate a message relay system and 
bring about further important changes in the activities 
inside the cell. 
It should be noted that cells send and receive signals in 
different ways. Depending on the proximity of sender and 
recipient cells, signaling can be broadly classi??ed in the 
following categories:
1. Paracrine signaling: In this form of signaling, 
communication between cells occurs over relatively 
short distances. A chemical message released in the 
extracellular space by the sender cells is sensed by the 
recipient cells instantly.  This type of signaling is seen 
commonly in the communication of neurons.
2. Autocrine signaling: Many times, a cell which secretes 
a ligand, also possesses receptors speci??c for that 
ligand. This type of signaling is referred to as autocrine 
signalling. For instance, cancer cells are characterised 
by uncontrollable growth. Therefore, they require a 
greater amount of growth factors for their proliferation. 
Unlike normal cells, cancer cells do not depend on 
external growth factors for their growth. Instead, they 
are capable of synthesising their own growth factors 
and also possess the receptors speci??c for them. 
3. Endocrine signaling: Endocrine signaling or long-
distance signaling requires the ligand to be synthesised 
by the cell and released into the bloodstream to travel to 
the recipient or target cell. Hormones generally exhibit 
this form of signaling. 
5.2 Metaboli C Pathway S
Metabolism is the process through which living organisms 
take and utilise the free energy required to carry out their 
life processes. Living organisms are of two types on the 
basis of taking free energy: phototrophs and chemotrophs. 
Chapter 5 Cellular Processes.indd   104 09/01/2025   15:21:28
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c ellular Pro c esses 105
Phototrophs use the energy of sunlight to convert simple 
molecules (less energy containing) into more complex 
molecules (energy rich) that serve as fuel to perform life 
processes. Phototrophs are photosynthetic organisms 
(such as plants and some bacteria); they transform light 
energy into chemical energy. Heterotrophs such as 
animals, obtain energy indirectly from plants through 
their food. In chemotrophs, the energy is obtained by 
oxidising chemical compounds (organic or inorganic). 
This energy uptake in organisms is done by coupling the 
exergonic reactions of nutrient oxidation to the endergonic 
processes required to maintain the living state. Central 
to all these energy transactions is the energy currency 
called ATP (detail is given in section 4.2 bioenergetics). In 
metabolism, there are interlinked biochemical reactions 
that begin with a particular molecule and convert it into 
some other molecule or molecules in a carefully de??ned 
fashion. The energy is utilised for various processes within 
the cell such as, the creation of gradient, movement of 
molecules across membranes, conversion of chemical 
energy into mechanical energy and powering of reactions 
that result in the synthesis of biomolecules.
The synthesis and breakdown of biomolecules is 
accomplished through a number of steps inside the living 
system. These steps collectively constitute metabolic 
pathway. Metabolic pathways can broadly be classi??ed into 
two classes; anabolic pathways and catabolic pathways.
(i) Anabolic pathways
In these pathways, larger and more complex molecules 
are synthesised from small molecules. Anabolic pathways 
are endergonic (consumption of energy). Reactions that 
require energy such as synthesis of glucose, fats, protein 
or DNA are called anabolic reactions or anabolism.
Chapter 5 Cellular Processes.indd   105 09/01/2025   15:21:28
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Biotechnology 106
(ii) Catabolic pathways
These pathways involve the breakdown of larger molecules. 
These are exergonic (release of energy) reactions and 
produce reducing equivalents and ATP. The useful forms 
of energy that are produced in catabolism are utilised in 
anabolism, to generate complex structures from simple 
ones or energy-rich states from energy poor ones. 
5.2.1 Overview of carbohydrate metabolism
In animals, the metabolic fuel for most of the tissues is 
glucose. Glucose is metabolised into pyruvate through 
glycolysis. In aerobic condition (in presence of oxygen) 
pyruvate enters into mitochondrial matrix, where it 
is converted into acetyl CoA and take part in the citric 
acid cycle to complete oxidation of glucose to CO
2
 and 
H
2
O (Fig. 5.1). This oxidation is linked to the formation 
of ATP through the process of oxidative phosphorylation. 
In anaerobic (in absence/lack of O
2
) condition pyruvate 
is converted into lactic acid. The metabolic intermediates 
of glycolysis also take part in other metabolic processes, 
such as
(i) In synthesis of glycogen and its storage in animals.
(ii) In pentose phosphate pathway which is source of 
reducing equivalent (NADPH) for fatty acid synthesis, 
and source of ribose for nucleotides and nucleic acid 
synthesis.
(iii) The triose phosphate generates glycerol moiety of 
triacylglycerol.
(iv) Acetyl CoA is the precursor for synthesis of fatty 
acids and cholesterol. Cholesterol then synthesises 
all other steroids in animals.
(v) Pyruvate and intermediates of citric acid cycle give 
rise to carbon skeleton for amino acid synthesis.
(vi) When glycogen reserves are depleted such as in 
starvation conditions the non-carbohydrate precursors 
such as lactic acid, amino acids, and glycerol can 
Fuel (carbohydrate, protein, fats)
CO +  HO  +  Useful energy
22
Catabolism
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Page 5


Cellular 
Processes
5.1 Cell Signaling
5.2 Metabolic Pathways
5.3 Cell Cycle
5.4 Programmed Cell 
Death (Apoptosis)
5.5 Cell Differentiation
5.6 Cell Migration
5.1 Cell Signaling We are aware that cells (both prokaryotic and eukaryotic) 
constantly receive and interpret environmental signals and 
respond to them in real time. These signals include light, 
heat, sound, and touch. The cell fates during development 
are speci??ed by signaling pathways in response to such 
extracellular signals. Cells interact with their neighbouring 
cells by transmitting and receiving signals. These signals 
are synthesised by the cells in the form of chemicals and 
released in the extracellular milieu. However, cells can also 
respond to ‘external’ signals which are not synthesised 
by the cells of our body. Therefore, one can assume that 
the cells are capable of sensing a wide variety of signals. 
It is important to note that a cell can only respond to a 
particular signal if it possesses the corresponding receptor 
for it. A receptor is a protein located either on the cell 
surface or inside the cytoplasm or the nucleus. A chemical 
messenger to which a receptor responds is a ligand. The 
association between a receptor and its corresponding 
ligand is highly speci??c, which means that a cell will only 
Chapter 5
Chapter 5 Cellular Processes.indd   103 09/01/2025   15:21:28
Reprint 2025-26
Biotechnology 104
be able to respond to a chemical messenger, if it bears the 
corresponding receptor for it and not otherwise. 
Transmission of chemical messages from one cell to 
another cell requires binding of a ligand to its receptor, 
which results in conformational changes in the receptor. 
These changes then initiate a message relay system and 
bring about further important changes in the activities 
inside the cell. 
It should be noted that cells send and receive signals in 
different ways. Depending on the proximity of sender and 
recipient cells, signaling can be broadly classi??ed in the 
following categories:
1. Paracrine signaling: In this form of signaling, 
communication between cells occurs over relatively 
short distances. A chemical message released in the 
extracellular space by the sender cells is sensed by the 
recipient cells instantly.  This type of signaling is seen 
commonly in the communication of neurons.
2. Autocrine signaling: Many times, a cell which secretes 
a ligand, also possesses receptors speci??c for that 
ligand. This type of signaling is referred to as autocrine 
signalling. For instance, cancer cells are characterised 
by uncontrollable growth. Therefore, they require a 
greater amount of growth factors for their proliferation. 
Unlike normal cells, cancer cells do not depend on 
external growth factors for their growth. Instead, they 
are capable of synthesising their own growth factors 
and also possess the receptors speci??c for them. 
3. Endocrine signaling: Endocrine signaling or long-
distance signaling requires the ligand to be synthesised 
by the cell and released into the bloodstream to travel to 
the recipient or target cell. Hormones generally exhibit 
this form of signaling. 
5.2 Metaboli C Pathway S
Metabolism is the process through which living organisms 
take and utilise the free energy required to carry out their 
life processes. Living organisms are of two types on the 
basis of taking free energy: phototrophs and chemotrophs. 
Chapter 5 Cellular Processes.indd   104 09/01/2025   15:21:28
Reprint 2025-26
c ellular Pro c esses 105
Phototrophs use the energy of sunlight to convert simple 
molecules (less energy containing) into more complex 
molecules (energy rich) that serve as fuel to perform life 
processes. Phototrophs are photosynthetic organisms 
(such as plants and some bacteria); they transform light 
energy into chemical energy. Heterotrophs such as 
animals, obtain energy indirectly from plants through 
their food. In chemotrophs, the energy is obtained by 
oxidising chemical compounds (organic or inorganic). 
This energy uptake in organisms is done by coupling the 
exergonic reactions of nutrient oxidation to the endergonic 
processes required to maintain the living state. Central 
to all these energy transactions is the energy currency 
called ATP (detail is given in section 4.2 bioenergetics). In 
metabolism, there are interlinked biochemical reactions 
that begin with a particular molecule and convert it into 
some other molecule or molecules in a carefully de??ned 
fashion. The energy is utilised for various processes within 
the cell such as, the creation of gradient, movement of 
molecules across membranes, conversion of chemical 
energy into mechanical energy and powering of reactions 
that result in the synthesis of biomolecules.
The synthesis and breakdown of biomolecules is 
accomplished through a number of steps inside the living 
system. These steps collectively constitute metabolic 
pathway. Metabolic pathways can broadly be classi??ed into 
two classes; anabolic pathways and catabolic pathways.
(i) Anabolic pathways
In these pathways, larger and more complex molecules 
are synthesised from small molecules. Anabolic pathways 
are endergonic (consumption of energy). Reactions that 
require energy such as synthesis of glucose, fats, protein 
or DNA are called anabolic reactions or anabolism.
Chapter 5 Cellular Processes.indd   105 09/01/2025   15:21:28
Reprint 2025-26
Biotechnology 106
(ii) Catabolic pathways
These pathways involve the breakdown of larger molecules. 
These are exergonic (release of energy) reactions and 
produce reducing equivalents and ATP. The useful forms 
of energy that are produced in catabolism are utilised in 
anabolism, to generate complex structures from simple 
ones or energy-rich states from energy poor ones. 
5.2.1 Overview of carbohydrate metabolism
In animals, the metabolic fuel for most of the tissues is 
glucose. Glucose is metabolised into pyruvate through 
glycolysis. In aerobic condition (in presence of oxygen) 
pyruvate enters into mitochondrial matrix, where it 
is converted into acetyl CoA and take part in the citric 
acid cycle to complete oxidation of glucose to CO
2
 and 
H
2
O (Fig. 5.1). This oxidation is linked to the formation 
of ATP through the process of oxidative phosphorylation. 
In anaerobic (in absence/lack of O
2
) condition pyruvate 
is converted into lactic acid. The metabolic intermediates 
of glycolysis also take part in other metabolic processes, 
such as
(i) In synthesis of glycogen and its storage in animals.
(ii) In pentose phosphate pathway which is source of 
reducing equivalent (NADPH) for fatty acid synthesis, 
and source of ribose for nucleotides and nucleic acid 
synthesis.
(iii) The triose phosphate generates glycerol moiety of 
triacylglycerol.
(iv) Acetyl CoA is the precursor for synthesis of fatty 
acids and cholesterol. Cholesterol then synthesises 
all other steroids in animals.
(v) Pyruvate and intermediates of citric acid cycle give 
rise to carbon skeleton for amino acid synthesis.
(vi) When glycogen reserves are depleted such as in 
starvation conditions the non-carbohydrate precursors 
such as lactic acid, amino acids, and glycerol can 
Fuel (carbohydrate, protein, fats)
CO +  HO  +  Useful energy
22
Catabolism
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c ellular Pro c esses 107
synthesise glucose through the process of 
gluconeogenesis.
5.2.2 Overview of lipid metabolism
Some vital tissues such as brain, heart and red blood 
cells are exclusively dependent on glucose. In the 
fasting state when glucose is limiting, then less glucose-
dependent tissues such as muscles, liver and other tissues 
alternatively use fuel other than glucose (Fig. 5.2). This fuel 
is long chain fatty acids which are either taken from diet 
or synthesized from acetyl CoA derived from carbohydrate 
or amino acids. Fatty acids may be oxidized to acetyl CoA 
through the ß-oxidation pathway or esteri??ed with glycerol 
Fig. 5.1: Overview of carbohydrate metabolism
Glycogen
Glucose
Glucose phosphate
Triose phosphate
Pyruvate Amino acids
Proteins
Amino acids
Acetyl CoA
Ribose
phosphate
DNA
RNA
Cholesterol
Citric
acid
cycle
CO
2
Diet
Pentose
phosphate
pathway
Triacylglycerol
Lactate
Fatty acids
Diet
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