definition, types of cell signaling, steps of cell signaling, basic terms related to cell signaling, receptors, ligands, paracrine cell signaling, eocrine cell signaling, autocrine cell signaling, direct signaling across gap junctions.

1. Cell Signaling
Saima Shafique
BS III
Semester V
Roll No. 1825138049
Session: 2018 - 2022
Submitted to: Dr. Zakia Kanwal
General Biochemistry (Maj/Zoo-302)
DEPARTMENT OF ZOOLOGY, LCWU1/10/2021 1

2. Definition
Course of cellular communication within the body
Network of communication between and inside each cell of our
body.
Driven by:
Cells releasing and receiving hormones
Other signaling molecules.
Intra-cellular and Extra-cellular signaling
1-Intra-cellular signaling: occurs within the cell
2-Inter-cellular signaling: occurs between cells
Also called extracellular signaling1/10/2021 2

3. Basic terms used in cell signaling
1-Signaling cell:
oThe cell that release signaling molecule
2- Ligand:
oSignaling molecule
oSmall, soluble molecules.
Types of Ligands:
There are two main types of ligands
a. Small hydrophobic ligands
b. Large hydrophilic ligands
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4. a. Small hydrophobic ligands:
Lipids in nature
Pass easily through cell membrane
Must bind with the carrier protein to dissolve in blood
Examples:
Steroid hormones (which are lipids)
Estrogen and Cholesterol.
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5. b. Large hydrophilic ligands:
Polar substances(Water soluble)
Easily soluble in blood
Cannot pass through plasma membrane
Bind to cell surface receptors
Examples:
Proteins and Peptides.
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6. Receptor:
Proteins present on target cells.
Bind with the ligands
Receptor confirmation (shape) changes.
Types of receptors
There are two types of receptors
a. Internal receptors
b. Cell surface receptors
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7. a. Internal receptors:
Intracellular or cytoplasmic receptors
Respond to hydrophobic ligands
Ligand receptor complex formed
Binding results in conformational change in receptor
Exposes DNA binding site on receptor
mRNA copy is synthesized in nucleus
Copy move out of nucleus
Enters ribosome for synthesizing protein
Protein modified in Golgi apparatus
Protein released out of the cell for response.
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8. https://study.com/cimages/multimages/16/estrogen_signaling.jpg
Fig.01 Internal receptor
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9. b. Cell surface receptors:
These are also called Trans-membrane receptors.
Surface membrane bound proteins
Bind to external hydrophilic ligand molecules.
Cell specific proteins or markers
Particular to each cell types
Three main components of cell surface receptors are:
External Ligand binding domain
Hydrophobic membrane spanning region
Intracellular domain
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10. https://study.com/cimages/videopreview/videopreview-full/79naapzob7.jpg
Fig.02 Cell surface receptor shown in green
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11. 4-Target cell:
Cells attacked by ligands
Three Steps of cell signaling
I. Reception
II. Transduction
III. Response
I. Reception: Cell detects a signaling molecule
II. Transduction: Receptor protein conformation changes
Extracellular signal is converted into intracellular signal.
III. Response: Triggering of specific cell response.
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12. https://www.cellapplications.com/sites/default/files/Signal%20Trasduction%202.jpg
Fig.03 Three steps of chemical signaling
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13. Types of signaling mechanism
i. Paracrine signaling
ii. Endocrine signaling
iii. Autocrine signaling
iv. Direct signaling across gap junctions
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14. https://s3-us-west-2.amazonaws.com/courses-images/wp- content/uploads/sites/1842/2017/05/26154741/figure-09-
01-01.jpeg
Fig.04 Four Types of chemical signaling1/10/2021 14

15. i. Paracrine signaling:
Definition:
Act between cells that are close together
Quick responses
Quickly degraded by enzymes or removed by neighbouring cells.
Restore the concentration gradient
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16. Example:
The transfer of signals across synapses between nerve cells
Neuron
Synapse
Neurotransmitters
Synaptic cleft
Neurotransmitters de-activation
Recovery of nerve cell
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17. https://s3-usws.com/coures/wp-content/uploads/sites/110/2016/05/02233201/Figure_09_01_02.jpg
Fig.05 Paracrine signaling pathway in Nervous System
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18. ii. Endocrine Signaling
Definition:
Ligands have to travel a long distance
Endocrine cells in endocrine glands, (the thyroid gland)
Slower response
Longer-lasting effect.
Hormones.
Passage through Blood
Dilution of hormones
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19. https://player.slideplayer.com/24/7091458/data/images/img5.jpg
Fig.06 Pathway of Endocrine Signaling
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20. Example:
Balancing actions of insulin
Hyperglycemia state
Pancreas
Insulin in blood
Insulin in body cells
Insulin receptors
Glucose channels to open.
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21. https://www.newsmedical.net/image.aicture=2015%2f12%2fHow_does_insulinwork590.jpg&ts=20151214075022&ri=590
Fig.07 The mechanism of insulin action
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22. iii. Autocrine Signaling
Definition:
Cell targets itself
Example
Virus infected cell
Signal itself
Undergo programmed cell death
An example of an autocrine agent
Cytokine interleukin-1 in monocytes
https://simply.wordpress.com/2013/10/autosig1.png
Fig.08 Autocrine Signaling Pathway
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23. iv. Direct Signaling Across Gap Junctions
Definition
Signaling occurs between the two touching cells
Gap junctions
Plasmodesmata
Small ligands diffusion
Calcium ions (Ca2+)
No passage for larger molecules
Rapidly and simply transmit signals
Examples:
Signal propagation in cardiac muscles
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24. https://upload.wikimedia.org/wikipedia/commons/0/02/Figure_04_06_05.jpg
Fig.09 Gap junctions between two neighbouring cells
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