1. Chapter 11
Cell Communication

2. Overview: The Cellular Internet
Cell-to-cell communication is essential for multicellular organisms.
Biologists have discovered some universal mechanisms of cellular regulation.
The combined effects of multiple signals determine cell response.
For example, the dilation of blood vessels is controlled by multiple molecules.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. 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.
Signal transduction pathways convert signals on a cell’s surface into cellular responses.
Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes.
The concentration of signaling molecules allows bacteria to detect population density.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Local and Long-Distance Signaling
Cells in a multicellular organism communicate by chemical messengers.
Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells.
In local signaling, animal cells may communicate by direct contact, or cell-cell recognition.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. Cell Signaling 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

6. In many other cases, animal cells communicate short distances using local regulators, messenger molecules that travel only.
In long-distance signaling, plants and animals use chemicals called hormones delivered via the blood.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. Long-distance signaling Local signaling
Local and Long-Distance Signaling
Long-distance signaling
Local signaling
Target cell
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Endocrine cell
Blood
vessel
Neurotransmitter
diffuses across
synapse
Secreting
cell
Secretory
vesicle
Hormone travels
in bloodstream
to target cells
Local regulator
diffuses through
extracellular fluid
Target cell
is stimulated
Target
cell
Figure 11.5 Local and long-distance cell communication in animals
(a) Paracrine signaling
(b) Synaptic signaling
(c) Hormonal signaling

8. The Three Stages of Cell Signaling: A Preview
Earl W. Sutherland discovered how the hormone epinephrine acts on cells.
Sutherland suggested that cells receiving signals went through three processes:
Reception
Transduction (relay / cascade … amplifies)
Response
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. Amplification of signal
Signal Transduction Pathway
EXTRACELLULAR
FLUID
CYTOPLASM
Plasma membrane 1
Reception 2
Transduction
Amplification of signal 3
Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Figure 11.6 Overview of cell signaling
Signaling
molecule: ligand

10. Protein Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane.
There are three main types of membrane receptors:
G protein-coupled receptors
Receptor tyrosine kinases
Ion channel receptors
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. Signaling molecule (ligand) response Cellular 1 Gate closed Ions
Plasma
membrane
Ligand-gated
ion channel receptor 2
Gate open
Cellular
response
Figure 11.7 Membrane receptors—ion channel receptors 3
Gate closed

12. Signal transduction usually involves multiple steps.
Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell
Signal transduction usually involves multiple steps.
The molecules that relay a signal from receptor to response are mostly proteins.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Phosphorylation cascade
Amplification -- Cascade
Signaling molecule
Receptor
Activated relay
molecule
Inactive
protein kinase 1
Active
protein
kinase 1
Inactive
protein kinase 2
ATP
Phosphorylation cascade
ADP
Active
protein
kinase 2 P PP P i
Figure 11.9 A phosphorylation cascade
Inactive
protein kinase 3
ATP
ADP
Active
protein
kinase 3 P PP P i
Inactive
protein
ATP
ADP P
Active
protein
Cellular
response PP P i

14. Small Molecules and Ions as Second Messengers
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.
Cyclic AMP and calcium ions are common second messengers.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Calcium Ions -- Second Messengers
Calcium ions (Ca2+) act as a second messenger in many pathways.
Calcium is an important second messenger because cells can regulate its concentration.
A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. EXTRACELLULAR FLUID Plasma membrane Ca2+ pump ATP Mitochondrion
Ca+ Ions as Second Messengers
EXTRACELLULAR
FLUID
Plasma
membrane
Ca2+ pump
ATP
Mitochondrion
Nucleus
CYTOSOL
Ca2+
pump
Endoplasmic
reticulum (ER)
Figure The maintenance of calcium ion concentrations in an animal cell
Ca2+
pump
ATP
Key
High [Ca2+]
Low [Ca2+]

17. Calcium and IP3 in signaling pathways
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 11.13
Endoplasmic
reticulum (ER)
Various
proteins
activated
Cellular
responses
Ca2+
Ca2+
(second
messenger)
CYTOSOL

18. Nuclear and Cytoplasmic Responses
Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities.
The response may occur in the cytoplasm or may involve action in the nucleus.
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 may function as a transcription factor.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Nuclear responses to a signal: the activation of a specific gene by a
growth factor
Growth factor
Reception
Receptor
Phosphorylation
cascade
Transduction
CYTOPLASM
Inactive
transcription
factor
Active
transcription
factor
Figure 11.14
Response P
DNA
Gene
NUCLEUS
mRNA

20. Other signaling pathways regulate the activity of enzymes.
Signaling pathways can also affect the physical characteristics of a cell, for example, cell shape.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. Fine-Tuning of the Response
Signal Transduction multi-step pathways have two important benefits:
Amplifying the signal (and thus the response)
Contributing to the specificity of the response
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

22. 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. 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

24. Specificity of Cell 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.

25. 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. Termination of the Signal
Inactivation mechanisms are an essential aspect of cell signaling.
When signal molecules leave the receptor, the receptor returns to its inactive state.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

27. Apoptosis is programmed or controlled cell suicide.
Apoptosis (programmed cell death) integrates multiple cell-signaling pathways
Apoptosis is programmed or controlled cell suicide.
Apoptosis can be triggered by:
An extracellular death-signaling ligand
DNA damage in the nucleus
Protein misfolding in the endoplasmic reticulum.
For example, apoptosis in the cells that form webbing between fetal human fingers and toes is a normal part of development.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. Reception Transduction Response Receptor Activation of cellular
Review: Signal Transduction Pathway 1
Reception 2
Transduction 3
Response
Receptor
Activation
of cellular
response
Relay molecules
Signaling
molecule

29. You should now be able to:
Describe the nature of a ligand-receptor interaction and state how such interactions initiate a signal-transduction system.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. 6. Describe the role of apoptosis in normal development.
4. Define the term second messenger ; briefly describe the role of these molecules in signaling pathways.
5. Explain why different types of cells may respond differently to the same signal molecule.
6. Describe the role of apoptosis in normal development.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings