1. Chapter 11
Cell Communication

2. Overview: The Cellular Internet Cell-to-cell communication
Is absolutely essential for multicellular organisms

3. Concept 1: Reception: External signals are converted into responses within the cell

4. 1. Signal transduction pathways convert signals on a cell’s surface into cellular responses
Are similar in microbes and mammals. What does this mean?

5. Local and Long-Distance Signaling
2. Cells in a multicellular organism communicate via chemical messengers

6. 3. Gap junctions in animal cells and plasmodesmata in plant cells directly connect the cytoplasm of adjacent cells
Plasma membranes
Plasmodesmata
between plant cells
Gap junctions
between animal cells
Figure 11.3
(a) Cell junctions. Both animals and plants have cell junctions that allow molecules
to pass readily between adjacent cells without crossing plasma membranes.

7. Type 1: Autocrine Cell Signaling
Cells communicate with themselves

8. Type 2: Juxtacrine Cell Signaling
Signaling depends on direct contact
Figure 11.3
(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.

9. Type 3: Paracrine cell signaling
local cells communicate with each other
(a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid.
(b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.
Local regulator
diffuses through
extracellular fluid
Target cell
Secretory
vesicle
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Neurotransmitter
diffuses across synapse
is stimulated
Local signaling

10. Type 4: Endocrine Cell Signaling
long-distance signaling, hormones are used
Hormone travels
in bloodstream
to target cells
(c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood Hormones may reach virtually all body cells.
Long-distance signaling
Blood
vessel
Target
cell
Endocrine cell

11. 3 Step Process
Reception
Transduction
Response

12. Relay molecules in a signal transduction pathway
Overview of cell signaling
EXTRACELLULAR
FLUID
Receptor
Signal
molecule
Relay molecules in a signal transduction pathway
Plasma membrane
CYTOPLASM
Activation
of cellular
response
Figure 11.5
Reception 1
Transduction 2
Response 3

13. 4. The binding between ligand and receptor is highly specific
5. When ligand bonds to a receptor, receptor changes shape, resulting in a response from the cell

14. Signal molecules that are small or hydrophobic
And can readily cross the plasma membrane use these receptors

15. Receptors in the Plasma Membrane
There are three main types of membrane receptors
G-protein-linked
Tyrosine kinases
Ligand Gated Ion channel

16. Ion channel receptors Figure 11.7 Cellular response Gate open
Gate close
Ligand-gated
ion channel receptor
Plasma Membrane
Signal molecule (ligand)
Figure 11.7
Gate closed
Ions

17. Concept 2: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell
Multistep pathways
Can amplify a signal
Provide more opportunities for coordination and regulation

18. Signal Transduction Pathways
6. Can be multiple steps in a pathway
a. At each step in a pathway the signal is transduced into a different form

19. b. This is called a cascade
Signal molecule
Active
protein
kinase 1 2 3
Inactive
protein kinase
Cellular
response
Receptor P
ATP
ADP PP
Activated relay
molecule i
Phosphorylation cascade P 
A relay molecule
activates protein kinase 1. 1 2
Active protein kinase 1
transfers a phosphate from ATP
to an inactive molecule of
protein kinase 2, thus activating
this second kinase.
Active protein kinase 2
then catalyzes the phos-
phorylation (and activation) of
protein kinase 3. 3
Enzymes called protein
phosphatases (PP)
catalyze the removal of
the phosphate groups
from the proteins,
making them inactive
and available for reuse. 5
Finally, active protein
kinase 3 phosphorylates a
protein (pink) that brings
about the cell’s response to
the signal. 4
Figure 11.8

20. b. A cascade is basically a large chain reaction
Signal molecule
Active
protein
kinase 1 2 3
Inactive
protein kinase
Cellular
response
Receptor P
ATP
ADP PP
Activated relay
molecule i
Phosphorylation cascade P 
A relay molecule
activates protein kinase 1. 1 2
Active protein kinase 1
transfers a phosphate from ATP
to an inactive molecule of
protein kinase 2, thus activating
this second kinase.
Active protein kinase 2
then catalyzes the phos-
phorylation (and activation) of
protein kinase 3. 3
Enzymes called protein
phosphatases (PP)
catalyze the removal of
the phosphate groups
from the proteins,
making them inactive
and available for reuse. 5
Finally, active protein
kinase 3 phosphorylates a
protein (pink) that brings
about the cell’s response to
the signal. 4
Figure 11.8

21. Fine-Tuning of the Response
7. Cascades with multiple steps can amplify the signal

22. Small Molecules and Ions as Second Messengers
8. Second messengers are small molecules or ions that become messenger after cell is activated by first messenger

23. Cyclic AMP

24. Many G-proteins
Trigger the formation of cAMP, which then acts as a second messenger in cellular pathways
ATP
GTP
cAMP
Protein
kinase A
Cellular responses
G-protein-linked
receptor
Adenylyl
cyclase
G protein
First messenger
(signal molecule
such as epinephrine)
Figure 11.10

25. 9. Some pathways regulate genes turning them on or off
Reception
Transduction
Response
mRNA
NUCLEUS
Gene P
Active
transcription
factor
Inactive
DNA
Phosphorylation
cascade
CYTOPLASM
Receptor
Growth factor
Figure 11.14

26. Concept 3: Response: Cell signaling leads to regulation of cytoplasmic activities or transcription

27. Protein Phosphorylation and Dephosphorylation
10. Many signal pathways include phosphorylation cascades

28. 11. In this process a series of protein kinases add a phosphate to the next one in line, activating it

29. A phosphorylation cascade
Signal molecule
Active
protein
kinase 1 2 3
Inactive
protein kinase
Cellular
response
Receptor P
ATP
ADP PP
Activated relay
molecule i
Phosphorylation cascade P 
A relay molecule
activates protein kinase 1. 1 2
Active protein kinase 1
transfers a phosphate from ATP
to an inactive molecule of
protein kinase 2, thus activating
this second kinase.
Active protein kinase 2
then catalyzes the phos-
phorylation (and activation) of
protein kinase 3. 3
Enzymes called protein
phosphatases (PP)
catalyze the removal of
the phosphate groups
from the proteins,
making them inactive
and available for reuse. 5
Finally, active protein
kinase 3 phosphorylates a
protein (pink) that brings
about the cell’s response to
the signal. 4
Figure 11.8