1. Chapter 11
Cell Communication

2. Evolution of Cell Signaling
A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response
Each cell type secretes a mating factor that binds to receptors on the other cell type. Binding of the factors to receptors induces changes in the cell that lead to their fusion.
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3. Yeast cell, mating type a Yeast cell, mating type 
Figure 11.2
 factor
Receptor 1
Exchange of mating factors a 
a factor
Yeast cell,
mating type a
Yeast cell,
mating type  2
Mating a 
Figure 11.2 Communication between mating yeast cells. 3
New a/ cell
a/

4. Gap junctions between animal cells Plasmodesmata between plant cells
Figure 11.4
Plasma membranes
Gap junctions between animal cells
Plasmodesmata between plant cells
(a) Cell junctions
Figure 11.4 Communication by direct contact between cells.
(b) Cell-cell recognition

5. 3
Paracrine signaling- a secreting cell acts on nearby target cells by discharging molecules of local regulators
-ex. Growth factor (stimulates cell growth and division)
Synaptic signaling- nerve cell releases neurotransmitter molecules into a synapse stimulating target cell
-ex. Nervous system

6. Neurotransmitter diffuses across synapse. Secreting cell
Figure 11.5a
Local signaling
Target cell
Electrical signal along nerve cell triggers release of neurotransmitter.
Neurotransmitter diffuses across synapse.
Secreting cell
Secretory vesicle
Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals.
Local regulator diffuses through extracellular fluid.
Target cell is stimulated.
(a) Paracrine signaling
(b) Synaptic signaling

7. 4
Specialized cells release hormone molecules into vessels of the circulatory system to travel to target cells in other parts of the body.

8. Long-distance signaling
Figure 11.5b
Long-distance signaling
Endocrine cell
Blood vessel
Hormone travels in bloodstream.
Target cell specifically binds hormone.
Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals.
(c) Endocrine (hormonal) signaling

9. Target cell detects signaling molecule from outside of cell.
Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane 1
Reception
Target cell detects signaling molecule from
outside of cell.
Signaling molecule binds to receptor protein on cell’s surface or inside.
Receptor
Figure 11.6 Overview of cell signaling.
Signaling molecule

10. Relay molecules in a signal transduction pathway
Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane 1
Reception 2
Transduction
Receptor
Relay molecules in a signal transduction pathway
Figure 11.6 Overview of cell signaling.
Receptor protein changes in some way, initiating process of transduction. Converts a signal into a from that cause a response.
Signaling molecule

11. Relay molecules in a signal transduction pathway
Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane 1
Reception 2
Transduction 3
Response
Receptor
Activation of cellular response
Relay molecules in a signal transduction pathway
Figure 11.6 Overview of cell signaling.
Transduced molecules trigger a specific response.
Ensures crucial activities occur in right cells at right time and proper conditions.
Signaling molecule

12. Concept 11.2: Reception: A signaling molecule binds to a receptor protein, causing it to change shape
A molecule that specifically binds to another molecule, often a larger one.
Generally causes a receptor protein to undergo a shape change.
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13. 7
G protein functions as a switch
inactive when GDP is attached

14. Signaling molecule binds to extracellular side of protein receptor activating it and causing a shape change. This allows G protein to bind and GDP is displaced by GTP.

15. Activated G protein dissociates from receptor, binds to enzyme, alters enzyme’s shape and activity. Triggers next step in pathway.

16. G protein also functions as GTPase enzyme, hydorlyzes bond GTP to GDP to leave enzyme and return to original state, available for reuse.

17. Binding to activated receptor and replacing GDP with GTP.
to rapidly shut down pathway
catalyzes the transfer of phosphate groups

18. 14. Receptor tyrosine kinases (RTKs) are membrane receptors that attach phosphates to tyrosines
15. A receptor tyrosine kinase can trigger multiple signal transduction pathways at once
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19. 16
Receptors exist as individual polypeptides before signaling molecule binds.

20. 17
Binding causes formation of dimer.

21. 18
Dimerization activates tyrosine kinase region, each tyrosine kinase adds a phosphate group from an ATP to tyrosine.

22. 19
Specific relay proteins recognize fully activated receptor. Each protein binds to a specific phosphorylated tyrosine resulting in structural change Each triggers a transduction pathway leading to a cellular response.

23. 21
Gate remains closed until a ligand binds to the receptor.

24. 22
Ligand binds, gate opens, specific ions flow in and rapidly change ion concentration inside cell.

25. Ligand dissociates from receptor, gate closes.
nervous system

26. Intracellular Receptors
25. Intracellular receptor proteins are found in the cytosol or nucleus of target cells
Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors
Examples of hydrophobic messengers are the steroid and thyroid hormones of animals
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27. Testosterone passes through plasma membrane. Binds to receptor
Figure
Hormone (testosterone)
EXTRACELLULAR FLUID
Testosterone passes
through plasma
membrane.
Plasma membrane
Receptor protein
Hormone- receptor complex
Binds to receptor
protein and
activates it.
DNA
Figure 11.9 Steroid hormone interacting with an intracellular receptor.
NUCLEUS
CYTOPLASM

28. Enters nucleus and binds to specific genes. Hormone (testosterone)
Figure
Hormone (testosterone)
EXTRACELLULAR FLUID
Plasma membrane
Receptor protein
Hormone- receptor complex
Enters nucleus
and binds to
specific genes.
DNA
Figure 11.9 Steroid hormone interacting with an intracellular receptor.
NUCLEUS
CYTOPLASM

29. Bound protein acts as transcription factor. Hormone (testosterone)
Figure
Hormone (testosterone)
EXTRACELLULAR FLUID
Plasma membrane
Receptor protein
Hormone- receptor complex
DNA
Figure 11.9 Steroid hormone interacting with an intracellular receptor.
mRNA
Bound protein
acts as
transcription
factor.
NUCLEUS
CYTOPLASM

30. mRNA is translated into a specific protein Hormone (testosterone)
Figure
Hormone (testosterone)
EXTRACELLULAR FLUID
Plasma membrane
Receptor protein
Hormone- receptor complex
DNA
Figure 11.9 Steroid hormone interacting with an intracellular receptor.
mRNA
NUCLEUS
mRNA is
translated into
a specific
protein
New protein
CYTOPLASM

31. 27. controls which genes are turned on or transcribed at a particular time in particular cells

32. Concept 11.3: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell
28. Multistep pathways can amplify a signal: A few molecules can produce a large cellular response
Multistep pathways provide more opportunities for coordination and regulation of the cellular response
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33. Protein Phosphorylation and Dephosphorylation
29. Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation
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34. Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation
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35. Relay molecule activated protein kinase 1.
Figure 11.10
Signaling molecule
Receptor
Relay molecule activated protein kinase 1.
Activated relay molecule
Active pk1 transfers phosphate from
ATP to pk 2, activating it
Inactive protein kinase 1
Active protein kinase 1
Inactive protein kinase 2
ATP
pk 2 phosphorylates pk 3
Phosphorylation cascade
ADP
Active protein kinase 2 P PP
P i
Figure A phosphorylation cascade.
Inactive protein kinase 3
ATP
ADP
pk 3 brings about
cell’s response to
signal P
Protein phosphatases
remove phosphates, making
proteins inactive and
available for reuse
Active protein kinase 3 PP
P i
Inactive protein
ATP
ADP P
Active protein
Cellular response PP
P i

36. Small Molecules and Ions as Second Messengers
31. The extracellular signal molecule (ligand) that binds to the receptor is a pathway’s “first messenger”
Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion
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37. 32. Adenylyl cyclase catalyzes ATP to cAMP which activates
Figure 11.12
First messenger (signaling molecule such as epinephrine)
Adenylyl cyclase
G protein
G protein-coupled receptor
GTP
ATP
32. Adenylyl cyclase catalyzes
ATP to cAMP which activates
another protein leading to cellular
responses
Second messenger
cAMP
Figure cAMP as a second messenger in a G protein signaling pathway.
Protein kinase A
Cellular responses

38. cAMP usually activates protein kinase A, which phosphorylates various other proteins
A different signaling molecule activates a different receptor which activates an inhibitory G protein.
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39. 35
Bacteria colonizes the lining of small intestine and produces toxins.
The toxin is an enzyme that modifies G protein involved in regulating salt and water secretion.
Unable to hydrolyze GTP to GDP, it remains active continuously making cAMP.
High concentrations of cAMP causes secretion of large amounts of salt and water.
Leads to development of profuse diarrhea.

40. 36. muscle contraction, secretion of certain substances, and cell division

41. Signaling molecule (first messenger)
Figure
EXTRA- CELLULAR FLUID
Signaling molecule (first messenger)
G protein
DAG
GTP
G protein-coupled receptor
PIP2
Phospholipase C
IP3 (second messenger)
IP3-gated calcium channel
Figure Calcium and IP3 in signaling pathways.
Various proteins activated
Cellular responses
Endoplasmic reticulum (ER)
Ca2
Ca2 (second messenger)
CYTOSOL

42. Nuclear and Cytoplasmic Responses
38. Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus
The final activated molecule in the signaling pathway may function as a transcription factor
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43. Inactive transcription factor Active transcription factor
Figure 11.15
Growth factor
Reception
Receptor
Phosphorylation cascade
Transduction
CYTOPLASM
Inactive transcription factor
Active transcription factor
Figure Nuclear responses to a signal: the activation of a specific gene by a growth factor.
Response P
DNA
Gene
NUCLEUS
mRNA

44. Other pathways regulate the activity of enzymes rather than their synthesis
Signaling pathways can also affect the overall behavior of a cell, for example, changes in cell shape
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45. Glucose 1-phosphate (108 molecules)
Figure 11.16
Reception
Binding of epinephrine to G protein-coupled receptor (1 molecule)
Transduction
Inactive G protein
Active G protein (102 molecules)
Inactive adenylyl cyclase
Active adenylyl cyclase (102)
ATP
Cyclic AMP (104)
Inactive protein kinase A
Active protein kinase A (104)
Figure Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine.
Inactive phosphorylase kinase
Active phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
Response
Glycogen
Glucose 1-phosphate (108 molecules)

46. Cell A. Pathway leads to a single response.
Figure 11.18a
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.

47. Activation or inhibition
Figure 11.18b
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.

48. Signaling Efficiency: Scaffolding Proteins and Signaling Complexes
42. 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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49. Three different protein kinases
Figure 11.19
Signaling molecule
Plasma membrane
Receptor
Three different protein kinases
Figure A scaffolding protein.
Scaffolding protein

50. Concept 11.5: Apoptosis integrates multiple cell-signaling pathways
43. Components of the cell are chopped up 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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51. Figure 11.20
Figure Apoptosis of a human white blood cell.
2 m

52. Apoptotic Pathways and the Signals That Trigger Them
44. 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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53. Cells undergoing apoptosis
Figure 11.22
Cells undergoing apoptosis
Space between digits
1 mm
Interdigital tissue
Figure Effect of apoptosis during paw development in the mouse.