1. Chapter 11
Cell Communication

2. Learning Objectives
Understanding why cell-to-cell communication is essential for both multicellular and unicellular organisms
Discovering the universal mechanisms of cellular regulation
Apply the concepts on a day-to-day basis

3. The yeast, Saccharomyces cerevisiae, have two mating types, a and 
 factor
The yeast, Saccharomyces cerevisiae, have two mating types, a and 
Cells of different mating types locate each other via secreted factors specific to each type
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
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
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. Neurotransmitter diffuses across synapse. Secreting cell
Local signaling
Target cell
Electrical signal along nerve cell triggers release of neurotransmitter.
Neurotransmitter diffuses across synapse.
Secreting cell
Secretory vesicle
Local regulator diffuses through extracellular fluid.
Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals.
Target cell is stimulated.
(a) Paracrine signaling
(b) Synaptic signaling

6. 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

7. Which of the following best describes a signal transduction pathway?
binding of a signal molecule to a cell protein
catalysis mediated by an enzyme
sequence of changes in a series of molecules resulting in a response
binding of a ligand on one side of a membrane that results in a change on the other side
the cell’s detection of a chemical or mechanical stimulus
Answer: c

8. The Three Stages of Cell Signaling
Earl W. Sutherland suggested that cells receiving signals went through three processes
Reception
Transduction
Response
A movie showing the three steps -

9. Relay molecules in a signal transduction pathway
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane 1
Reception 2
Transduction 3
Response
Receptor
Activation of cellular response
Relay molecules in a signal transduction pathway
Signaling molecule
EACH STUDENT - Place the following correctly in the above box:
Reception, Transduction, Response, Receptor, Signaling Molecule, Activation of cellular response, Extracellular fluid, Cytoplasm, Relay Molecules
Figure 11.6 Overview of cell signaling.
Where would you place epinephrine, cyclic AMP and Glucose from the movie that you saw?
Which would be reception, transduction and response

10. Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane
There are three main types of membrane receptors
G protein-coupled receptors
Receptor tyrosine kinases
Ion channel receptors

11. G-protein-coupled receptor (GPCRs) are the largest family of cell-surface receptors
A GPCR is a plasma membrane receptor that works with the help of a G protein
The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive

12. Receptor tyrosine kinases (RTKs) are membrane receptors that attach phosphates to tyrosines
A receptor tyrosine kinase can trigger multiple signal transduction pathways at once
Abnormal functioning of RTKs is associated with many types of cancers

13. A ligand-gated ion channel receptor acts as a gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor

14. Activated relay molecule
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
Inactive protein kinase 3
ATP
Figure A phosphorylation cascade.
ADP P
Active protein kinase 3 PP
P i
Inactive protein
ATP
ADP P
Active protein
Cellular response PP
P i

15. In reactions mediated by protein kinases, what does phosphorylation of successive proteins do to drive the reaction?
make functional ATP
change a protein from its inactive to its active form
change a protein from its active to its inactive form
alter the permeability of the cell’s membranes
produce an increase in the cell’s store of inorganic phosphates
Answer: b
Note: some responses would be true if the question were not specifically about driving a reaction.

16. 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
Second messengers participate in pathways initiated by GPCRs and RTKs
Cyclic AMP and calcium ions are common second messengers

17. Which of the following is an example of signal amplification?
catalysis of many cAMP molecules by several simultaneously binding signal molecules
activation of 100 molecules by a single signal binding event
activation of a specific gene by a growth factor
activation of an enzyme molecule
utilization of a second messenger system
Answer: b

18. Cyclic AMP
Cyclic AMP (cAMP) is one of the most widely used second messengers
Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal
Adenylyl cyclase
Phosphodiesterase
Pyrophosphate
H2O P
P i
ATP
cAMP
AMP

19. Many signal molecules trigger formation of cAMP
Other components of cAMP pathways are G proteins, G protein-coupled receptors, and protein kinases
cAMP usually activates protein kinase A, which phosphorylates various other proteins
Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase

20. Calcium Ions and Inositol Triphosphate (IP3)
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
Pathways leading to the release of calcium involve inositol triphosphate (IP3) and diacylglycerol (DAG) as additional second messengers

21. Endoplasmic reticulum (ER)
EXTRACELLULAR FLUID
Plasma membrane
Ca2 pump
ATP
Mitochondrion
Nucleus
CYTOSOL
Ca2 pump
Figure The maintenance of calcium ion concentrations in an animal cell.
Endoplasmic reticulum (ER)
Ca2 pump
ATP
Key
High [Ca2 ]
Low [Ca2 ]

22. Signaling molecule (first messenger)
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

23. open channels in the membrane for other substances to enter
A steroid hormone binds to an intracellular receptor. When it does, the resulting complex is able to do which of the following? Why?
open channels in the membrane for other substances to enter
open channels in the nuclear envelope for cytoplasmic molecules to enter
mediate the transfer of phosphate groups to/from ATP
act as a transcription factor in the nucleus
make water-soluble molecules able to diffuse across membranes
Answer: d

24. Inactive transcription factor Active transcription factor
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

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

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

27. One of the important outcomes of apoptosis is protection of neighboring cells. Which of the following is responsible?
cell shrinkage and blebbing
destruction of the cell’s DNA
formation of numerous vesicles to be digested
action of tyrosine kinases
activation of specific proteins
Answer: c

28. 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
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

29. Cells undergoing apoptosis
Space between digits
1 mm
Interdigital tissue
Figure Effect of apoptosis during paw development in the mouse.