Unit 5- Cell Interactions and Signal Transduction.ppt

UNIT 5- CELL
INTERACTIONS AND SIGNAL
TRANSDUCTION
Dr. P.
Deepak

CELL INTERACTIONS AND SIGNAL
TRANSDUCTION
Junction between cells – desmosomes,
plasmodesmata; synapse, gap and tight
junctions; Cell to cell adhesion; Chemical
signalling in unicellular organisms and between
cells in higher organisms; Cell surface and
intracellular receptors and their role in signal
mediation (general information); Importance of
second messengers in signal transduction
(cAMP, cGMP, calcium ions, phosphatidyl
inositol, phytohormones).

CELL INTERACTIONS
Cell signalling, also known as cell
communication or signal transduction,
is the process by which cells
communicate with each other to
coordinate their activities and respond
to their environment. It is a crucial
aspect of cellular function, allowing
cells to sense changes in their
surroundings and appropriately adjust

Cell signaling involves the transmission of
molecular signals from one cell to another,
it can occur through various mechanisms,
including direct cell-to-cell contact, secretion
of signaling molecules, and the detection of
extracellular matrix components.
These signals are received and interpreted by
specialized proteins known as receptors on the
surface or inside the target cells.

THERE ARE THREE MAIN TYPES OF
CELL SIGNALLING:

AUTOCRINE SIGNALLING:
In autocrine signalling, a cell produces
signalling molecules that bind to receptors
on its own surface, leading to a response
within the same cell. This process is often
associated with the regulation of cell
growth and differentiation.

Unit 5- Cell Interactions and Signal Transduction.ppt

PARACRINE SIGNALLING
Paracrine signalling: In paracrine
signalling, cells release signalling
molecules into the extracellular fluid,
where they only affect nearby target cells.
These target cells have specific receptors
for the signalling molecules and respond
accordingly. Paracrine signalling is
common in the nervous system and in the
immune response.

ENDOCRINE SIGNALLING
Endocrine signalling: In endocrine signalling,
specialized cells release signalling molecules
(hormones) into the bloodstream.
These hormones travel through the
bloodstream to distant target cells, which
possess the appropriate receptors for the
specific hormones.
Endocrine signalling plays a crucial role in
coordinating processes throughout the entire

THE KEY COMPONENTS OF CELL
SIGNALLING INCLUDE
Signalling molecules
Receptors

SIGNAL TRANSDUCTION
Signal transduction is a fundamental process
in biology that refers to the transmission of
signals from the external environment or
neighboring cells to the interior of a cell,
ultimately leading to a cellular response.
This mechanism allows cells to sense changes
in their surroundings and adapt their behavior
accordingly.
Signal transduction is critical for various

THE PROCESS OF SIGNAL
TRANSDUCTION GENERALLY
INVOLVES THE FOLLOWING STEPS:

JUNCTION BETWEEN CELLS
The junctions between cells are specialized
structures that facilitate cell-to-cell
communication, adhesion, and coordination of
cellular activities. These junctions are essential
for the organization and function of tissues in
multicellular organisms.
There are three main types of cell junctions:
Tight junctions, (Occluding Junction)
Adherens junctions, (Anchoring Junction) &
Gap junctions (Communicating Junction).

TIGHT JUNCTIONS:
Tight junctions, also known as occluding junctions, form
a barrier between adjacent cells, preventing the movement
of substances between the cells.
They are commonly found in epithelial tissues, such as
the lining of the intestines and blood vessels.
Tight junctions consist of transmembrane proteins, such
as claudins and occludins, that interact with
complementary proteins on adjacent cells.
The tight junctions effectively seal the space between
cells, controlling the passage of ions, water, and
molecules, and ensuring the proper functioning of
selective permeability.
Apical surface –carrier
proteins
Basolateral surface-carrier
proteins

ADHERENS JUNCTIONS:
Adherens junctions play a role in cell adhesion and
tissue integrity.
They are typically found just below tight junctions in
epithelial tissues.
Adherens junctions are formed by cadherin proteins
that interact with cadherins on neighboring cells.
The cadherins are linked to the actin cytoskeleton
inside the cell, providing structural support and
allowing the transmission of mechanical forces between
cells.
Adherens junctions contribute to the overall strength

GAP JUNCTIONS:
Gap junctions facilitate direct communication
between adjacent cells.
They are composed of connexin proteins,
which form small channels called connexons.
These channels allow the passage of ions, small
molecules, and signalling molecules between
the cytoplasm of connected cells.
Gap junctions are essential for coordinating
activities among cells in tissues like cardiac
muscle and nervous tissue. They enable
electrical and biochemical coupling, enabling
rapid communication and synchronization of

Connexin 43- cataract in mice
Connexin 37- signals to reach eggs to
mature
Connexin 46 – lens transparancy

DESMOSOMES
Desmosomes, also known as macula adherens, are
specialized cell junctions that provide strong mechanical
adhesion between cells in tissues subjected to mechanical
stress.
These junctions are commonly found in tissues such as
the skin, heart, and uterus, where cells experience
significant stretching or shearing forces.
Desmosomes contribute to the overall integrity and
strength of tissues, helping them resist tearing or
disruption during physical movements or stresses. Here
are detailed notes on desmosomes:

STRUCTURE OF DESMOSOMES:
Transmembrane Proteins: Desmosomes consist of
transmembrane proteins that extend across the
plasma membrane of adjacent cells. The primary
transmembrane proteins found in desmosomes are
desmogleins and desmocollins.
Cadherin Family: Desmogleins and desmocollins
belong to the cadherin family of cell adhesion
molecules. These cadherins interact homophilically,
meaning desmogleins on one cell bind to
desmogleins on an adjacent cell, and desmocollins
on one cell bind to desmocollins on an adjacent

Desmosomal Plaque: Inside the cell, the
cytoplasmic tails of desmogleins and
desmocollins are linked to the
desmosomal plaque, a dense region
containing various proteins. Plakoglobin
and desmoplakin are major proteins in the
desmosomal plaque.
Intermediate Filaments: Desmoplakin
connects the desmosomal plaque to the
intermediate filament network within the
cell. Intermediate filaments, primarily
composed of keratins, provide structural

FUNCTION OF DESMOSOMES
Mechanical Adhesion: The
main function of desmosomes
is to provide mechanical
adhesion between cells. This
adhesion allows cells to resist
mechanical stress, such as
stretching, compression, or
shear forces, within tissues.
For example, in the skin,
desmosomes between
epidermal cells help maintain

Tissue Integrity: Desmosomes contribute
to the structural integrity of tissues,
preventing the separation of cells during
physical movement or external forces.
Cell-to-Cell Communication: While
desmosomes primarily function as
mechanical adhesion points, they also play
a role in cell-to-cell communication.

REGULATION AND PATHOLOGICAL
SIGNIFICANCE
Regulation: The formation and
disassembly of desmosomes are tightly
regulated processes. Various signalling
pathways and protein interactions control
the assembly and stability of
desmosomes. For instance, calcium ions
play a crucial role in regulating the
adhesive properties of cadherins.

REGULATION AND PATHOLOGICAL
SIGNIFICANCE
Pathological Significance: Mutations or
dysregulation of desmosomal proteins can lead
to diseases known as desmosomal
cardiomyopathies and blistering skin disorders.
In desmosomal cardiomyopathies, the
mechanical integrity of cardiac tissue is
compromised, leading to heart dysfunction. In
blistering skin disorders, weakened
desmosomal adhesion causes the epidermal
layers to separate, resulting in blister

PLASMODESMATA
Plasmodesmata (singular: plasmodesma) are
specialized channels that facilitate
communication and transport between plant
cells.
These microscopic structures create
cytoplasmic connections, allowing adjacent
plant cells to share various molecules, including
water, ions, nutrients, and signalling molecules.
Plasmodesmata play a crucial role in
coordinating growth, development, and defense

STRUCTURE OF PLASMODESMATA
Channels: Plasmodesmata are narrow channels
that traverse the cell walls of neighboring plant
cells. They create a direct cytoplasmic
continuum between adjacent cells, allowing for
direct communication.
Plasma Membrane: The plasmodesmata are
lined with the plasma membrane, which
encases the cytoplasmic sleeve connecting two
cells. The plasma membrane controls the
movement of molecules between cells through
these channels.

STRUCTURE OF PLASMODESMATA
Cytoplasmic Sleeve: The cytoplasmic sleeve is
the space within the plasmodesmata that
contains cytoplasm and cell organelles. It
provides a direct pathway for the movement of
substances between connected cells.
Desmotubule: In the center of the
plasmodesma, there is a strand-like structure
called the desmotubule. The desmotubule is a
remnant of the endoplasmic reticulum (ER) that
passes through the plasmodesma and helps
maintain its structure.

FUNCTION OF PLASMODESMATA
Intercellular Communication: Plasmodesmata
allow plant cells to communicate directly with
each other.
This communication enables the coordinated
response of cells within a tissue to various
developmental and environmental signals.
Important signalling molecules, such as
hormones and defense-related molecules, can
move through plasmodesmata, propagating
responses to neighboring cells.

Transport of Nutrients and Hormones:
Plasmodesmata facilitate the exchange of
various essential molecules, including
nutrients (sugars, amino acids), ions (e.g.,
potassium, calcium), and hormones (e.g.,
auxins, cytokinins), between adjacent
plant cells.
This transport is vital for supporting the
growth and development of plant tissues.

Cell-to-Cell Movement of Macromolecules:
Plasmodesmata also enable the movement of
larger macromolecules, such as proteins and
nucleic acids, between plant cells.
This feature plays a crucial role in
coordinating plant responses to infections and
other stress conditions.

REGULATION OF PLASMODESMATA
Size Exclusion Limit: Plasmodesmata are
regulated by size exclusion limits.
The size of molecules that can pass
through the channels depends on the
diameter of the plasmodesmata and
associated proteins. Smaller molecules can
pass more freely, while larger molecules
may require specific transport
mechanisms.

Callose Deposition: The movement of
molecules through plasmodesmata can be
controlled by the deposition of callose, a
polysaccharide.
Callose can temporarily block the
channels, restricting molecular exchange
during certain developmental or stress-
related processes.

Plant Development and Environmental
Cues: Plasmodesmata regulation is
influenced by plant developmental signals
and environmental cues.
Hormones and other signalling
molecules can modulate the permeability
of plasmodesmata, ensuring that specific
responses occur in specific cells or
tissues.

SYNAPSES
Synapses are specialized junctions that enable
communication between neurons or between
neurons and other cells in the nervous system.
They are fundamental to the transmission of
nerve impulses and play a crucial role in various
cognitive, sensory, and motor functions.
Understanding the structure and function of
synapses is essential for comprehending how
information is processed and transmitted within
the nervous system. :

STRUCTURE OF SYNAPSE
Presynaptic Terminal: The presynaptic terminal
is the end of the axon where neurotransmitters
are released. When an action potential reaches
the presynaptic terminal, it triggers the release
of neurotransmitters into the synaptic cleft.
Synaptic Cleft: The synaptic cleft is a small
fluid-filled gap that separates the presynaptic
terminal from the postsynaptic cell.
Neurotransmitters diffuse across this gap to
transmit the signal from the presynaptic neuron
to the postsynaptic neuron.

STRUCTURE OF SYNAPSE
Postsynaptic Membrane: The postsynaptic membrane
is the specialized region on the receiving neuron
where neurotransmitter receptors are located. These
receptors bind to neurotransmitters released by the
presynaptic neuron, initiating a response in the
postsynaptic neuron.
Neurotransmitters: Neurotransmitters are chemical
messengers that carry signals across the synapse.
They are stored in vesicles within the presynaptic
terminal and are released in response to an action
potential. The most common neurotransmitters
include acetylcholine, dopamine, serotonin, glutamate,

TYPES OF SYNAPSES
Chemical Synapses: The majority of
synapses in the nervous system are
chemical synapses.
In a chemical synapse, neurotransmitters
are released by the presynaptic neuron
and bind to receptors on the postsynaptic
neuron, generating a response.
This type of synapse allows for signal
amplification and modulation.

TYPES OF SYNAPSES
Electrical Synapses: Electrical synapses, also known
as gap junctions, are less common but allow for direct
electrical communication between neurons.
In electrical synapses, connexin proteins form gap
junction channels that permit the direct flow of ions
and small molecules between adjacent cells,
facilitating rapid and synchronized transmission of
signals.

GAP JUNCTIONS
Gap junctions are specialized cell-to-cell
communication channels found in various
tissues throughout the body.
They facilitate direct communication and
exchange of ions, small molecules, and
signalling molecules between adjacent cells.
Gap junctions are crucial for coordinating
cellular activities and maintaining tissue
homeostasis. Here are the key features and

STRUCTURE:
Gap junctions consist of connexons,
which are formed by hexameric
assemblies of connexin proteins.
Connexins are transmembrane proteins
that span the cell membrane and create
channels for communication.
Each connexon is made up of six
connexin subunits, and two connexons
from adjacent cells align to form a gap
junction channel.

TIGHT JUNCTIONS
Tight junctions are specialized cell-to-cell
junctions that form a tight seal between
adjacent cells in epithelial and endothelial
tissues.
Also known as occluding junctions or zonulae
occludentes, tight junctions play a critical role
in regulating the passage of molecules and ions
between cells and creating a selectively
permeable barrier.

Structure: Tight junctions are cell junctions
found primarily in epithelial and
endothelial tissues.
They create a seal or barrier between
adjacent cells, preventing the passage of
substances between the cells.
Tight junctions consist of
transmembrane proteins, such as claudins
and occludins, that interact with
complementary proteins on adjacent cells.

Function: Tight junctions play a crucial
role in maintaining the selective
permeability of epithelial and endothelial
tissues.
By restricting the movement of
molecules through the intercellular space,
tight junctions help regulate the passage
of ions, water, and solutes and maintain
tissue integrity.

Barrier Function:
The main function of tight junctions is to
create a physical barrier that separates the
apical and basolateral domains of
epithelial and endothelial cells.
This barrier prevents the leakage of
substances between cells and allows for
the establishment of concentration
gradients essential for various
physiological processes.

Cell Polarity:
Tight junctions contribute to the
establishment and maintenance of cell
polarity in epithelial cells.
They help maintain the distinct molecular
composition and functional differences
between the apical and basolateral
surfaces of the cells.

Regulation:
The permeability of tight junctions can
be dynamically regulated in response to
various physiological and pathological
cues.
Signalling pathways and environmental
factors can modulate tight junction
proteins, influencing their barrier
properties.

Clinical Significance:
Dysfunction of tight junctions is
implicated in various diseases, including
inflammatory bowel diseases, leaky gut
syndrome, and certain kidney disorders.

COMPARISON BETWEEN GAP AND
TIGHT JUNCTIONS
Gap junctions and tight junctions are two
different types of cell junctions found in various
tissues, and they serve distinct functions.
Here's a comparison between gap junctions and
tight junctions:

1. STRUCTURE:
Gap Junctions: Gap junctions are formed by
connexons, which are hexameric protein complexes
made up of connexin proteins. These connexons align
between adjacent cells, creating channels that allow
the direct exchange of ions, small molecules, and
signalling molecules.
Tight Junctions: Tight junctions are composed of
transmembrane proteins, such as claudins and
occludins, that interact with complementary proteins
on adjacent cells. Tight junctions create a barrier
between cells, sealing the intercellular space and
preventing the passage of substances between cells.

2. FUNCTION:
Gap Junctions: Gap junctions facilitate direct
communication between cells, allowing the passage of
ions, small molecules, and second messengers. This
enables synchronized electrical and chemical
signalling between connected cells, contributing to
coordinated cellular responses.
Tight Junctions: Tight junctions create a physical
barrier between cells, regulating the passage of ions,
water, and solutes across epithelial and endothelial
tissues. They maintain tissue integrity and selectively
control the movement of substances to establish
concentration gradients.

3. PERMEABILITY:
Gap Junctions: Gap junctions have high
permeability, allowing the direct exchange of
ions and small molecules between adjacent
cells.
Tight Junctions: Tight junctions have low
permeability, restricting the movement of
molecules through the intercellular space. They
establish a selective barrier to control the
passage of substances.

4. LOCATION:
Gap Junctions: Gap junctions are found in
various tissues, including cardiac muscle,
smooth muscle, neurons, and some
epithelial tissues.
Tight Junctions: Tight junctions are
primarily found in epithelial and
endothelial tissues, forming the apical
junctional complex that separates the
apical and basolateral domains of cells.

5. CELLULAR FUNCTION:
Gap Junctions: Gap junctions play a role in
coordinating electrical and chemical signals,
contributing to functions like cardiac
rhythmicity, neural synchronization, and
coordinated smooth muscle contractions.
Tight Junctions: Tight junctions are crucial for
maintaining the barrier function of epithelial
and endothelial tissues, regulating nutrient
absorption, waste elimination, and protecting
against pathogens.

6. REGULATION:
Gap Junctions: Gap junction permeability
can be regulated by various factors, such
as calcium concentration, pH, and
phosphorylation status of connexin
proteins.
Tight Junctions: Tight junctions can be
dynamically regulated in response to
physiological and pathological cues,
altering their barrier properties.

7. CLINICAL SIGNIFICANCE:
Gap Junctions: Dysfunction of gap
junctions can lead to various diseases,
including cardiac arrhythmias,
neuropathies, and certain developmental
disorders.
Tight Junctions: Tight junction
dysfunction is associated with conditions
like inflammatory bowel diseases, leaky
gut syndrome, and certain kidney
disorders.

CELL-TO-CELL ADHESION: THE
IMPORTANCE OF CELL ADHESION IN
MULTICELLULAR ORGANISMS
Cell-to-cell adhesion is a critical process that
enables cells to stick together and form tissues and
organs in multicellular organisms.
It plays a fundamental role in various physiological
processes, including development, tissue
homeostasis, immune responses, wound healing,
and organ function.
The ability of cells to adhere to each other is
essential for maintaining the structural integrity of

MECHANISMS OF CELL-TO-CELL
ADHESION:
Cell-to-cell adhesion involves complex
molecular interactions between specific
proteins and carbohydrates on the surfaces of
neighboring cells.
The process can be broadly categorized into
two main mechanisms: direct cell adhesion
and indirect cell adhesion.

MECHANISMS OF CELL-TO-CELL
ADHESION:

DIRECT CELL ADHESION:
Cadherin Family: Cadherins are a major class
of transmembrane proteins responsible for
direct cell adhesion.
They are calcium-dependent adhesion
molecules that mediate strong and specific
homophilic interactions between cells of the
same type.
For example, E-cadherin promotes adhesion
between epithelial cells, while N-cadherin is

https://www.researchgate.net/publication/47791127_Prowse_ABJ_Chong_F_Gray_PP_Munro_TP
_Stem_cell_integrins_implications_for_ex-
vivo_culture_and_cellular_therapies_Stem_Cell_Res_6_1-

DIRECT CELL ADHESION:
Integrins: Integrins are another group of
transmembrane proteins that mediate cell
adhesion, particularly in cell-extracellular
matrix interactions.
Integrins can bind to specific extracellular
matrix proteins like fibronectin and collagen,
allowing cells to anchor to the surrounding
extracellular matrix and participate in tissue
organization.

https://www.youtube.com/watch?v=Ayz7kuEAQac

INTEGRIN STRUCTURE
Integrins are proteins that function mechanically, by
attaching the cell cytoskeleton to the extracellular
matrix (ECM), and biochemically, by sensing whether
adhesion has occurred.
The integrin family of proteins consists of alpha and beta
subtypes, which form transmembrane heterodimers.
Integrins function as adhesion receptors for extracellular
ligands and transduce biochemical signals into the cell,
through downstream effector proteins.
Remarkably, they function bidirectionally, meaning they
can transmit information both outside-in and inside-out

Integrin α subunit domains: Top: Linear domain arrangement. Middle: The globular
structure formed by protein domains. Bottom: simplified version of the integrin α subunit.
The αI domain is present in some subtypes of the α subunit.
EF-hand

Integrin β subunit domains: Top: Linear domain arrangement. Middle: The
globular structure formed by protein domains. Bottom: simplified version of the
integrin β subunit.
EF-hand

INDIRECT CELL ADHESION:
Selectins: Selectins are a family of cell adhesion
molecules involved in cell interactions with
carbohydrates on the surface of other cells or the
extracellular matrix. They mediate the initial
tethering and rolling of leukocytes during
inflammatory responses.
Immunoglobulin Superfamily: Immunoglobulin
superfamily cell adhesion molecules, such as ICAMs
and VCAMs, play crucial roles in immune cell
adhesion and interactions between leukocytes and
endothelial cells during inflammation.

https://www.mdpi.com/2073-4409/10/3/577

TYPES OF CELL-TO-CELL
ADHESION:
Cell-to-cell adhesion can occur in various forms,
each tailored to specific functions and contexts.
Some of the prominent types of cell adhesion
include:
Desmosomes: Desmosomes are strong adhesion
junctions found in tissues subjected to mechanical
stress, like skin and cardiac muscle.
They rely on cadherins, particularly desmogleins
and desmocollins, to connect the cytoskeletons of
adjacent cells, providing structural support and
resistance to mechanical forces.

ADHESION:
Adherens Junctions: Adherens junctions are
cadherin-based adhesion junctions that are
crucial for maintaining tissue integrity and
cell polarity.
They form belts around epithelial cells,
linking their actin cytoskeletons and
promoting cell cohesion.

ADHESION:
Tight Junctions: Tight junctions create barriers
between cells in epithelial and endothelial
tissues, regulating the passage of ions and
solutes. They are essential for establishing
selective permeability and maintaining tissue
homeostasis.
Gap Junctions: Gap junctions enable direct
communication and molecular exchange
between adjacent cells through connexon
channels. They play a role in coordinating

ADHESION:
Hemidesmosomes: Hemidesmosomes are
specialized adhesion structures that anchor
epithelial cells to the underlying basement
membrane.
They involve integrins connecting to
components of the extracellular matrix.

BIOLOGICAL SIGNIFICANCE OF
CELL-TO-CELL ADHESION:
Cell-to-cell adhesion is vital for several biological
processes and physiological functions:
Tissue Formation: During embryonic development, cell
adhesion is essential for tissue formation and
morphogenesis. The adhesion of cells allows them to
organize into specific tissues and structures, such as the
formation of germ layers during gastrulation.
Tissue Homeostasis: In adult organisms, cell-to-cell
adhesion maintains tissue integrity and prevents the
disintegration of tissues. The presence of adherens
junctions, tight junctions, and desmosomes in epithelial

Immune Response: Cell adhesion molecules are
critical for immune responses. They facilitate
leukocyte adhesion to the endothelium during
inflammation, enabling leukocytes to migrate to
sites of infection or injury.
Wound Healing: During wound healing, cell
adhesion is essential for the migration and
proliferation of cells to repair damaged tissues.
Integrins and other adhesion molecules play a
crucial role in this process.

Cell Differentiation: Cell adhesion is involved in cell
differentiation and tissue specialization. For
example, E-cadherin is crucial for the
differentiation of epithelial cells during embryonic
development.
Metastasis: In cancer, altered cell adhesion can
contribute to metastasis. Dysregulation of adhesion
molecules can enable cancer cells to detach from
the primary tumor and invade other tissues.

WHAT IS A CHEMICAL SIGNALLING
Chemical signalling is a form of communication
that occurs between cells, tissues, or organisms
through the release and detection of chemical
substances.
These chemical signals, also known as signalling
molecules or ligands, can travel short or long
distances to interact with specific receptors on
target cells.
The process of chemical signalling allows cells to
coordinate their activities, regulate physiological

Chemical signalling is fundamental to various
biological processes in both unicellular and
multicellular organisms. It plays a crucial role in:
Cellular communication: Cells use chemical signals
to relay information to neighboring cells or distant
target cells, enabling them to respond to
environmental cues or coordinate their actions in
complex multicellular organisms.

Cell-to-cell coordination: Within multicellular
organisms, chemical signalling helps different cell
types work together harmoniously to form tissues,
organs, and systems.
Homeostasis: Chemical signalling is involved in
maintaining the internal balance and stability
(homeostasis) of an organism by regulating
physiological processes like hormone release,
metabolic activity, and immune responses.

Development and growth: During embryonic
development and tissue regeneration, chemical
signals guide the differentiation and growth of cells
into specific tissues and structures.
Immune responses: Immune cells use chemical
signalling to communicate and coordinate their
actions, facilitating the recognition and elimination
of pathogens or abnormal cells.
Behavior and social interactions: In animals,
chemical signals known as pheromones play a

CHEMICAL SIGNALLING
MECHANISMS CAN BE BROADLY
CATEGORIZED INTO TWO TYPES:
Endocrine signalling: In this form of signalling, cells
release hormones into the bloodstream. These
hormones travel throughout the body and affect
distant target cells equipped with specific receptors
for that particular hormone.
Paracrine and Autocrine signalling: In paracrine
signalling, cells release chemical signals that act
locally on nearby target cells. Autocrine signalling is a
special case of paracrine signalling, where the cell
releases signals that also act on its own receptors.

CHEMICAL SIGNALLING IN
UNICELLULAR ORGANISMS
Chemical signalling in unicellular organisms plays a
vital role in their survival, growth, and response to
environmental changes.
These single-celled organisms have developed various
mechanisms to communicate with each other and their
surroundings using chemical signals.
Here are some common forms of chemical signalling
in unicellular organisms:

Quorum sensing: Quorum sensing is a prevalent form of
chemical signalling among unicellular organisms,
particularly bacteria. It involves the release and detection
of small signalling molecules called autoinducers. As the
population of bacteria increases, the concentration of
autoinducers rises.
When the autoinducer concentration reaches a certain
threshold (quorum), it triggers specific responses in the
bacterial community, such as biofilm formation,
virulence factor production, or cooperative behavior.

Pheromones: Unicellular organisms, especially
some algae and protozoa, release pheromones to
communicate with individuals of the same
species. Pheromones are chemical substances that
can evoke specific behaviors or physiological
responses in nearby cells or organisms. For
example, certain types of algae release
pheromones to attract gametes during sexual
reproduction.

Chemotaxis: Chemotaxis is a movement response of
unicellular organisms to gradients of chemical substances in
their environment. They can detect and respond to varying
concentrations of specific chemicals by either moving
towards or away from the source. This behavior allows them
to seek out beneficial environments or avoid harmful ones.
Response to environmental cues: Unicellular organisms can
respond to changes in their environment by altering their
gene expression and physiological functions. For instance,
when exposed to stressors like heat, toxins, or nutrient
availability, they may activate specific genes that help them

Secondary metabolites: Some unicellular organisms
produce and release secondary metabolites, which are
small molecules not directly involved in growth or
reproduction. These metabolites can function as
signalling molecules, influencing the behavior or
physiology of other organisms in the vicinity.
Horizontal gene transfer: Unicellular organisms can
exchange genetic material with their neighbors
through processes like conjugation, transduction, and
transformation. These exchanges can involve the
transfer of genes responsible for signalling pathways,

ROLE OF CELL SURFACE
RECEPTORS IN SIGNAL MEDIATION
Cell surface receptors play a crucial role in signal
mediation, acting as molecular intermediaries that
facilitate communication between the extracellular
environment and the cell's interior.
They are responsible for detecting and binding
specific signalling molecules (ligands) and initiating

LIGAND BINDING:
The process begins when a signalling molecule,
known as the ligand, encounters and binds to the
extracellular domain of its specific cell surface
receptor.
The binding event is highly specific, like a lock-
and-key mechanism, ensuring that only the

RECEPTOR ACTIVATION:
Ligand binding induces conformational
changes in the receptor, triggering its
activation.
This activation can involve dimerization
(receptor pairs coming together) or
conformational shifts that expose or hide

INITIATION OF INTRACELLULAR
SIGNALLING:
Once activated, the cell surface receptor acts as a
molecular switch, initiating intracellular signalling
pathways.
Different types of cell surface receptors activate
distinct downstream signalling pathways that lead
to diverse cellular responses.

ACTIVATION OF SECOND
MESSENGER SYSTEMS:
Some receptors, like G protein-coupled receptors
(GPCRs), activate intracellular second messenger
systems.
Upon activation, GPCRs interact with intracellular G
proteins, leading to the production of second
messengers such as cyclic AMP (cAMP) or inositol
trisphosphate (IP3).

KINASE CASCADES:
Other receptors, such as receptor tyrosine kinases
(RTKs), possess intrinsic kinase activity.
Ligand binding to RTKs leads to receptor
dimerization and autophosphorylation of specific
tyrosine residues in the receptor's cytoplasmic
domain.
These phosphorylated tyrosines serve as docking
sites for downstream signalling proteins, initiating
kinase cascades that transmit the signal.

GENE EXPRESSION CHANGES:
Many signalling pathways activated by cell surface
receptors ultimately lead to changes in gene
expression.
Transcription factors are often involved, controlling
the expression of specific genes in response to the
signal received by the receptor.

CELLULAR RESPONSE:
The culmination of the intracellular signalling events
is a cellular response, which can vary widely
depending on the specific receptor and downstream
pathways involved.
Cellular responses may include changes in
metabolism, proliferation, differentiation, secretion,
cell movement, or apoptosis.

SIGNAL TERMINATION AND
REGULATION:
To maintain cellular homeostasis, signal termination
mechanisms come into play.
Receptor internalization through endocytosis can
remove the activated receptors from the cell surface,
attenuating the signal.
Negative feedback loops and other regulatory
mechanisms ensure that the cellular response is

INTRACELLULAR RECEPTORS:
Introduction to Intracellular
Receptors:
Intracellular receptors are a class of receptors
located inside the cell, typically in the
cytoplasm or nucleus.
They are distinct from cell surface receptors,

Ligand Types:
Intracellular receptors primarily bind to
hydrophobic ligands, such as steroid
hormones (e.g., estrogen, testosterone) and
thyroid hormones.
These ligands can diffuse through the cell

Mechanism of Activation:
When a hydrophobic ligand enters the cell and
binds to the intracellular receptor, it induces a
conformational change, activating the
receptor.
Nuclear Localization:
Activated intracellular receptors can
translocate to the nucleus, where they act as
transcription factors.

Gene Expression Regulation:
In the nucleus, intracellular receptors directly bind to
specific DNA sequences called hormone response
elements (HREs) on target genes.
Binding of the receptor to the HREs influences gene
transcription, leading to changes in protein synthesis.
Cellular Response:
By regulating gene expression, intracellular receptors

Example of Action:
 The activation of estrogen receptors by estrogen hormone leads to
gene expression changes that influence female reproductive
processes.
Physiological Importance:
 Intracellular receptors play a critical role in mediating the effects of
steroid hormones and other hydrophobic ligands.
 They contribute to maintaining homeostasis and coordinating the
response to changing internal environments.
Pharmaceutical Significance:
 Intracellular receptors are important targets for pharmacological
interventions.
 Hormone-based therapies, such as estrogen replacement therapy,
target intracellular receptors to treat hormonal imbalances.

ROLE OF INTRACELLULAR
The role of intracellular receptors in signal mediation is to
translate specific signals carried by hydrophobic ligands
into changes in gene expression, which ultimately leads to
various cellular responses.
These receptors are located inside the cell, typically in the
cytoplasm or nucleus, and primarily interact with lipophilic
(hydrophobic) signalling molecules, such as steroid

ROLE OF INTRACELLULAR
Ligand Binding and Activation:
Nuclear Translocation:
DNA Binding and Gene Regulation:
Gene Expression Changes:
Cellular Responses:
Physiological Significance

IMPORTANCE OF
SECOND MESSENGERS IN SIGNAL
TRANSDUCTION
cAMP,
cGMP,
calcium ions,
phosphatidyl inositol,
phytohormones.

IMPORTANCE OF cAMP IN SIGNAL
TRANSDUCTION
Amplification of Signals:
cAMP acts as a potent signal amplifier. When an
extracellular ligand binds to a G protein-coupled receptor
(GPCR), it activates a G protein that stimulates adenylyl
cyclase to produce cAMP.
Each activated adenylyl cyclase can generate many cAMP
molecules, leading to a significant amplification of the
initial signal.
This amplification step ensures that even small changes in
the extracellular environment can trigger robust cellular
responses.

TRANSDUCTION
Activation of Protein Kinase A (PKA):
cAMP activates the intracellular enzyme protein
kinase A (PKA).
PKA is a serine/threonine kinase that
phosphorylates specific target proteins in response
to elevated cAMP levels.
Phosphorylation by PKA alters the activity of target
proteins, leading to diverse cellular responses.

TRANSDUCTION
Regulation of Metabolism:
cAMP-PKA signalling regulates various metabolic
processes, such as glycogen breakdown and
glucose metabolism.
In response to signals like adrenaline, cAMP-PKA
activation stimulates glycogen breakdown into
glucose, providing energy for the "fight or flight"
response.

TRANSDUCTION
Control of Cellular Proliferation:
cAMP-PKA signalling influences cell growth
and proliferation by modulating gene
expression and cell cycle regulators.
Dysregulation of cAMP signalling pathways is
associated with several diseases, including
cancer.

TRANSDUCTION
Neurotransmission and Neuronal
Plasticity:
In neurons, cAMP plays a role in neurotransmission
and synaptic plasticity.
It modulates the activity of ion channels and
neurotransmitter receptors, affecting synaptic
strength and information processing in the brain.

TRANSDUCTION
Regulation of Ion Channels:
cAMP-PKA signalling can modulate the activity of
various ion channels, influencing cellular
excitability and electrical signalling.
For example, in the heart, cAMP increases the
activity of calcium channels, leading to enhanced
contractility and heart rate.

TRANSDUCTION
Immune Response Modulation:
cAMP-PKA signalling is involved in regulating
immune responses.
It can inhibit the activation of certain immune
cells, dampening inflammation and immune
reactions.

TRANSDUCTION
Therapeutic Targets:
The importance of cAMP in cellular processes
makes it an attractive target for drug
development.
Drugs that affect cAMP levels or PKA activity
have therapeutic potential in various
diseases, such as heart failure, asthma, and
mental disorders.

IMPORTANCE OF cGMP IN SIGNAL
TRANSDUCTION
Activation of Protein Kinase G (PKG):
Similar to cAMP and protein kinase A (PKA),
cGMP activates protein kinase G (PKG), also
known as cGMP-dependent protein kinase.
PKG is a serine/threonine kinase that
phosphorylates specific target proteins in
response to elevated cGMP levels.
Phosphorylation by PKG modulates the activity
of these target proteins, leading to diverse
cellular responses.

TRANSDUCTION
Regulation of Smooth Muscle
Relaxation:
One of the most well-known roles of cGMP is
in smooth muscle relaxation.
In vascular smooth muscle cells, cGMP-PKG
signalling is stimulated by nitric oxide (NO),
leading to vasodilation and improved blood
flow.
Similarly, in the gastrointestinal tract, cGMP
signalling mediates relaxation of smooth

TRANSDUCTION
Vision and Phototransduction:
In photoreceptor cells of the retina, cGMP is
involved in the process of vision and
phototransduction.
Light causes the activation of rhodopsin,
leading to a decrease in cGMP levels, which
closes cGMP-gated ion channels in the
photoreceptor cells.
This closure of ion channels hyperpolarizes the
cell, initiating the neural signal for vision.

TRANSDUCTION
Regulation of Ion Channels:
cGMP-PKG signalling can modulate
the activity of certain ion channels in
various cell types.
For example, in cardiac myocytes,
cGMP can enhance the activity of
potassium channels, leading to
cardiac relaxation.

TRANSDUCTION
Neurotransmission and Synaptic
Plasticity:
In the nervous system, cGMP is involved
in neurotransmission and synaptic
plasticity.
It can modulate synaptic strength and
influence the release of
neurotransmitters.

TRANSDUCTION
Regulation of Cell Growth and
Proliferation:
cGMP signalling can impact cell growth and
proliferation by regulating gene expression
and cell cycle control.
It has been implicated in processes like cell
differentiation and wound healing.

TRANSDUCTION
Phototransduction in Plants:
In plants, cGMP is involved in
phototransduction and the response to
light.
It regulates processes like stomatal
opening and chloroplast movement.

TRANSDUCTION
Therapeutic Relevance:
Dysregulation of cGMP signalling is
associated with various diseases,
including cardiovascular disorders,
pulmonary hypertension, and erectile
dysfunction.
Drugs targeting cGMP signalling
pathways are used for treating some of
these conditions.

IMPORTANCE OF CALCIUM IONS IN
SIGNAL TRANSDUCTION
Ubiquitous Second Messengers:
Calcium ions act as second messengers in
response to a wide range of extracellular
signals, including neurotransmitters,
hormones, and mechanical stimuli.
Their versatility allows cells to respond to
diverse signalling inputs and coordinate
various physiological responses.

SIGNAL TRANSDUCTION
Intracellular Calcium Concentration
Regulation:
Calcium signalling involves tightly regulated
changes in intracellular calcium concentration
([Ca2
+])i.
Normally, resting [Ca2
+]i is kept low by active
pumping of calcium out of the cell or into
intracellular compartments.
Upon stimulation, specific signalling pathways

SIGNAL TRANSDUCTION
Activation of Calmodulin and Protein
Kinases:
Increased [Ca2+]i leads to the binding of
calcium ions to calmodulin, a calcium-binding
protein.
The calcium-calmodulin complex can activate
various enzymes, including protein kinases,
such as calmodulin-dependent protein kinases
(CaMKs).

SIGNAL TRANSDUCTION
Muscle Contraction:
In muscle cells, calcium ions play a
crucial role in excitation-contraction
coupling.
During muscle activation, an action
potential triggers the release of calcium
ions from the sarcoplasmic reticulum,
which initiates muscle contraction.

SIGNAL TRANSDUCTION
Neurotransmitter Release:
In neurons, calcium signalling is essential
for neurotransmitter release at synapses.
The influx of calcium ions into the
presynaptic terminal triggers vesicle
fusion and the release of
neurotransmitters into the synaptic cleft.

SIGNAL TRANSDUCTION
Cell Migration and Cytoskeletal
Dynamics:
Calcium signalling is involved in cell
migration and cytoskeletal dynamics.
Calcium ions can regulate actin and
myosin filaments, influencing cell shape
changes and movement.

SIGNAL TRANSDUCTION
Gene Expression Regulation:
Calcium signalling can modulate gene
expression by activating specific transcription
factors.
Transcription factors activated by calcium may
regulate the expression of genes involved in
various cellular processes, including
development and immune responses.

SIGNAL TRANSDUCTION
Immune Response Modulation:
Calcium signalling plays a role in immune
responses, including activation of T cells
and the release of cytokines.
Calcium signals are crucial for the proper
functioning of immune cells.

SIGNAL TRANSDUCTION
Cell Death and Apoptosis:
Aberrant calcium signalling can lead to
cell death through apoptosis.
Dysregulation of calcium homeostasis has
been linked to neurodegenerative
diseases and other pathological
conditions.

SIGNAL TRANSDUCTION
Calcium signalling pathways are potential
targets for drug development.
Calcium channel blockers, for instance, are
used to treat hypertension and heart-related
conditions by modulating calcium entry into
cells.

IMPORTANCE OF PHOSPHATIDYL
INOSITOL IN SIGNAL
TRANSDUCTION
Phosphoinositide Signalling Network:
Phosphatidylinositol is a phospholipid present
in the cell membrane, serving as a precursor
for several phosphorylated derivatives,
collectively known as phosphoinositides.
Phosphoinositides form a dynamic signalling
network that regulates numerous cellular
processes, including cell growth,
differentiation, migration, and survival.

INOSITOL IN SIGNAL
TRANSDUCTION
Phospholipid Kinases and
Phosphatases:
The levels of phosphoinositides are tightly
regulated by the action of specific kinases and
phosphatases.
Phospholipid kinases add phosphate groups to
phosphatidylinositol, generating different
phosphoinositides with distinct cellular
functions.
Phospholipid phosphatases remove phosphate

INOSITOL IN SIGNAL
TRANSDUCTION
Phosphoinositide Binding Proteins:
Phosphoinositides act as docking sites for
various proteins containing specific domains,
such as pleckstrin homology (PH) domains or
FYVE domains.
These phosphoinositide-binding proteins
serve as effectors, recruiting and regulating
signalling proteins at specific cellular
locations.

INOSITOL IN SIGNAL
TRANSDUCTION
Formation of Second Messengers:
Upon activation of cell surface receptors,
phosphoinositides are rapidly phosphorylated
to generate second messengers, such as
inositol 1,4,5-trisphosphate (IP3) and
diacylglycerol (DAG).
IP3 triggers calcium release from intracellular
stores, while DAG activates protein kinase C
(PKC), both contributing to downstream
signalling events.

INOSITOL IN SIGNAL
TRANSDUCTION
Regulation of Protein Kinases and
Signalling Cascades:
Phosphoinositides regulate the activity of
several protein kinases, including PKC, Akt
(protein kinase B), and PIP3-dependent kinase-
1 (PDK1).
These kinases are involved in crucial cellular
processes, including cell survival, metabolism,
and proliferation.

INOSITOL IN SIGNAL
TRANSDUCTION
Actin Cytoskeleton Dynamics:
Phosphoinositides influence actin cytoskeleton
dynamics and cell motility through interactions
with actin-binding proteins.
They contribute to processes like cell
migration, shape changes, and membrane
remodeling.

INOSITOL IN SIGNAL
TRANSDUCTION
Endocytosis and Membrane
Trafficking:
Phosphoinositides play a role in endocytosis
and vesicle trafficking, regulating membrane
fusion, and internalization of cell surface
receptors.
They are essential for the dynamic
organization of membrane compartments
within the cell.

INOSITOL IN SIGNAL
TRANSDUCTION
Neuronal Signalling:
In neurons, phosphoinositides are involved in
synaptic vesicle exocytosis and recycling,
regulating neurotransmitter release and
synaptic plasticity.

INOSITOL IN SIGNAL
TRANSDUCTION
Pathological Implications:
Dysregulation of phosphoinositide
signalling is associated with various
diseases, including cancer,
neurodegenerative disorders, and
immune dysfunctions.

INOSITOL IN SIGNAL
TRANSDUCTION
Components of the phosphoinositide signalling pathway
are potential targets for drug development.
Modulating phosphoinositide levels or targeting specific
phosphoinositide-binding proteins may have therapeutic
benefits in certain diseases including cancer, viral
infection, neurodegenerative diseases, developmental
disorders, diabetes and inflammatory diseases.

IMPORTANCE OF
PHYTOHORMONES IN SIGNAL
TRANSDUCTION
Coordination of Plant Growth and
Development:
Phytohormones regulate plant growth and
development by influencing cell division,
elongation, and differentiation.
They control processes like seed germination,

IMPORTANCE OF
TRANSDUCTION
Response to Environmental Stimuli:
Phytohormones enable plants to respond to various
environmental cues, such as light, gravity,
temperature, and stress factors.
They modulate adaptive responses, such as
phototropism, gravitropism, and responses to biotic

IMPORTANCE OF
TRANSDUCTION
Seed Development and Dormancy:
Phytohormones are involved in seed
development, dormancy, and germination.
They regulate the transition of seeds from
dormancy to active growth in response to
favorable environmental conditions.

IMPORTANCE OF
TRANSDUCTION
Flowering and Reproduction:
Phytohormones control flowering processes,
including the initiation and development of
flowers.
They also influence the formation and

IMPORTANCE OF
TRANSDUCTION
Leaf Senescence and Abscission:
Phytohormones regulate leaf senescence, the
natural process of aging and deterioration in
leaves.
They also play a role in leaf abscission, the
shedding of leaves in response to changing

IMPORTANCE OF
TRANSDUCTION
Signal Transduction Pathways:
Phytohormones activate intracellular signalling
pathways that mediate their effects on gene expression
and cellular responses.
These pathways involve receptor proteins, kinases, and
transcription factors.

IMPORTANCE OF
TRANSDUCTION
Interactions and Crosstalk:
Phytohormones often interact with each other
and engage in crosstalk, leading to complex
and integrated responses.
This interplay allows plants to fine-tune their
responses to changing environmental

IMPORTANCE OF
TRANSDUCTION
Role in Plant Defense and Immunity:
Some phytohormones, like salicylic acid and jasmonic acid,
are involved in plant defense responses against pathogens
and pests.
They trigger the expression of defense-related genes and
the production of antimicrobial compounds.

IMPORTANCE OF
TRANSDUCTION
Role in Symbiotic Relationships:
Phytohormones play a role in establishing and
maintaining symbiotic relationships between
plants and beneficial microorganisms, such as
mycorrhizal fungi and nitrogen-fixing bacteria.

IMPORTANCE OF
TRANSDUCTION
Agricultural and Horticultural
Applications:
Understanding phytohormone signalling has
significant implications in agriculture and
horticulture.

Unit 5- Cell Interactions and Signal Transduction.ppt

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Unit 5- Cell Interactions and Signal Transduction.ppt