1. Fine-Tuning of the Response
Multistep pathways have two important benefits:
Amplifying the signal (and thus the response)
Contributing to the specificity of the response
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2. Enzyme cascades amplify the cell’s response
Signal Amplification
Enzyme cascades amplify the cell’s response
At each step, the number of activated products is much greater than in the preceding step
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3. The Specificity of Cell Signaling and Coordination of the Response
Different kinds of cells have different collections of proteins
These different proteins allow cells to detect and respond to different signals
Even the same signal can have different effects in cells with different proteins and pathways
Pathway branching and “cross-talk” further help the cell coordinate incoming signals
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Fig. 11-17 Figure 11.17 The specificity of cell signaling Signaling
molecule
Receptor
Relay
molecules
Response 1
Response 2
Response 3
Cell A. Pathway leads
to a single response.
Cell B. Pathway branches,
leading to two responses.
Figure The specificity of cell signaling
Activation
or inhibition
Response 4
Response 5
Cell C. Cross-talk occurs
between two pathways.
Cell D. Different receptor
leads to a different response.

5. Cell B. Pathway branches, leading to two responses.
Fig a
Signaling
molecule
Receptor
Relay
molecules
Figure The specificity of cell signaling
Response 1
Response 2
Response 3
Cell A. Pathway leads
to a single response.
Cell B. Pathway branches,
leading to two responses.

6. Cell C. Cross-talk occurs between two pathways.
Fig b
Activation
or inhibition
Figure The specificity of cell signaling
Response 4
Response 5
Cell C. Cross-talk occurs
between two pathways.
Cell D. Different receptor
leads to a different response.

7. Signaling Efficiency: Scaffolding Proteins and Signaling Complexes
Scaffolding proteins are large relay proteins to which other relay proteins are attached
Scaffolding proteins can increase the signal transduction efficiency by grouping together different proteins involved in the same pathway
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8. Signaling Plasma molecule membrane Receptor Three different protein
Fig
Signaling
molecule
Plasma
membrane
Receptor
Three
different
protein
kinases
Figure A scaffolding protein
Scaffolding
protein

9. Termination of the Signal
Inactivation mechanisms are an essential aspect of cell signaling
When signal molecules leave the receptor, the receptor reverts to its inactive state
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10. Apoptosis is programmed or controlled cell suicide
Concept 11.5: Apoptosis (programmed cell death) integrates multiple cell-signaling pathways
Apoptosis is programmed or controlled cell suicide
A cell is chopped and packaged into vesicles that are digested by scavenger cells
Apoptosis prevents enzymes from leaking out of a dying cell and damaging neighboring cells
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11. Fig
Figure Apoptosis of human white blood cells
2 µm

12. Apoptosis in the Soil Worm Caenorhabditis elegans
Apoptosis is important in shaping an organism during embryonic development
The role of apoptosis in embryonic development was first studied in Caenorhabditis elegans
In C. elegans, apoptosis results when specific proteins that “accelerate” apoptosis override those that “put the brakes” on apoptosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Figure 11.20 Molecular basis of apoptosis in C. elegans
Ced-9
protein (active)
inhibits Ced-4
activity
Mitochondrion
Ced-4
Ced-3
Receptor
for death-
signaling
molecule
Inactive proteins
(a) No death signal
Ced-9
(inactive)
Cell
forms
blebs
Death-
signaling
molecule
Figure Molecular basis of apoptosis in C. elegans
Active
Ced-4
Active
Ced-3
Other
proteases
Nucleases
Activation
cascade
(b) Death signal

14. Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4
Fig a
Ced-9
protein (active)
inhibits Ced-4
activity
Mitochondrion
Figure Molecular basis of apoptosis in C. elegans
Ced-4
Ced-3
Receptor
for death-
signaling
molecule
Inactive proteins
(a) No death signal

15. Ced-9 (inactive) Cell forms blebs Death- signaling molecule Active
Fig b
Ced-9
(inactive)
Cell
forms
blebs
Death-
signaling
molecule
Active
Ced-4
Active
Ced-3
Other
proteases
Nucleases
Figure Molecular basis of apoptosis in C. elegans
Activation
cascade
(b) Death signal

16. Apoptotic Pathways and the Signals That Trigger Them
Caspases are the main proteases (enzymes that cut up proteins) that carry out apoptosis
Apoptosis can be triggered by:
An extracellular death-signaling ligand
DNA damage in the nucleus
Protein misfolding in the endoplasmic reticulum
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17. Apoptosis evolved early in animal evolution and is essential for the development and maintenance of all animals
Apoptosis may be involved in some diseases (for example, Parkinson’s and Alzheimer’s); interference with apoptosis may contribute to some cancers
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Interdigital tissue 1 mm Fig. 11-21
Figure Effect of apoptosis during paw development in the mouse

19. Reception Transduction Response Receptor Activation of cellular
Fig. 11-UN1 1
Reception 2
Transduction 3
Response
Receptor
Activation
of cellular
response
Relay molecules
Signaling
molecule

20. Fig. 11-UN2

21. You should now be able to:
Describe the nature of a ligand-receptor interaction and state how such interactions initiate a signal-transduction system
Compare and contrast G protein-coupled receptors, tyrosine kinase receptors, and ligand-gated ion channels
List two advantages of a multistep pathway in the transduction stage of cell signaling
Explain how an original signal molecule can produce a cellular response when it may not even enter the target cell
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22. Define the term second messenger; briefly describe the role of these molecules in signaling pathways
Explain why different types of cells may respond differently to the same signal molecule
Describe the role of apoptosis in normal development and degenerative disease in vertebrates
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings