1. Cell Communication
Chapter 11

2. Importance
Cell-to-cell signaling is important for multicellular organisms
Our trillions of cells must communicate with each other to coordinate their activities in a way that enables us to develop from a fertilized egg, grow and survive, and finally reproduce

3. Signal Transduction Pathway
The series of steps that occur when a signal on a cell’s surface is converted into a specific cellular response
For example) yeast cells recognize their mates by chemical signaling

4. Chemical Messengers
Cells in a multicellular organism communicate by chemical messengers
Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells
Animal cells may communicate by direct contact or cell-cell recognition

5. Local Signaling
In many other cases, animal cells communicate using local regulators
Local regulators are “chemical messengers” made and secreted by the signaling cell
Ex) Growth factors, which regulate target cells to grow and divide
Ex) Neurotransmitters released by nerve cells
Typically the local regulators travel short distances to the target cells, hence “local” signaling

6. Long-Distance Signaling
In long-distance signaling, plants and animals use chemicals called hormones
In animals, long-distance signaling = hormonal signaling = endocrine signaling
Specialized cells of the endocrine system release hormone molecules, which travel via the circulatory system to target cells in other parts of the body
In plants, hormones may travel in vessels but more often reach their targets by moving through cells (plasmodesmata) or by diffusing through the air as a gas

7. Ex) the pancreas secretes insulin into the blood stream and it regulates sugar levels

8. 3 Stages of Cell Signaling
Reception
The target cell’s detection of a signaling molecule (called a ligand)
Occurs outside the cell on the cell membrane
Signaling molecule binds to a receptor protein

9. Transduction The receptor protein is activated and changes shape
This stage converts the signal to a form that can bring about a specific cellular response

10. Response The desired cellular response
Can be a cellular activity such as activating an enzyme, rearranging the cytoskeleton, or activating specific genes in the nucleus

11. Reception
The binding between a signal molecule (ligand) and receptor is highly specific
A shape change in a receptor is often the initial transduction of the signal
Most signal receptors are plasma membrane proteins
There are three main types of membrane receptors:
G protein-coupled receptors
Receptor tyrosine kinases
Ion channel receptors

12. G Protein-Coupled Receptors
Involved in vision, smell, embryonic development and more. Also, involved in many diseases such as cholera, pertussis, and botulism. The bacteria produce toxins that interfere with G-protein pathways

13. Receptor Tyrosine Kinases
Involved in cell growth and cell division. One receptor can activate 10 or more different transduction pathways and cellular responses. Abnormal receptors may lead to cancer…

14. Ion Channel Receptors
Important for the nervous system. Neurotransmitters sent from one neuron to the next bind to ion channels opening them initiating an electrical signal that travels down the neuron, “sending” the message

15. Reception Not all receptor proteins are embedded in the cell membrane
Intracellular receptors are found in the cytoplasm of a cell or in the nucleus of target cells
Ligand must diffuse through the cell membrane to activate the receptor

16. Transduction: Protein Phosphorylation
In many pathways, the signal is transmitted by a cascade of protein phosphorylations
Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation
Protein kinases are extremely important – about 2% of our genes code for them
Example) regulates cell reproduction

17. Transduction: Protein Dephosphorylation
Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation
This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off

18. Transduction: Second Messengers
Second messengers are small, nonprotein, water-soluble molecules or ions
Second messengers can readily spread throughout cells by diffusion
Second messengers participate in pathways initiated by G-protein-linked receptors and receptor tyrosine kinases
2 most common second messengers:
cyclic AMP
Calcium (Ca2+)

19. cyclic AMP cyclic AMP = cAMP
Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal
cAMP usually activates protein kinase A, which phosphorylates various other proteins
Many signal molecules trigger formation of cAMP

20. cAMP Examples
Involved in the breakdown of glycogen caused by ephinepherine
Uses a G protein-coupled receptor
Cholera, a disease acquired by drinking water that is contaminated with human feces
The toxin produced by the cholera bacterium modifies the shape of a G protein so that it remains active causing the production of cAMP
Causes intestinal cells to secrete salt into the intestine and water follows
Gives a person explosive diarrhea

21. 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 pathways regulate the activity of enzymes
Many other signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus

22. Apoptosis Programmed cell death
Signal often originates outside of the cell from a neighboring cell
Signals may also occur from inside the cell such as when the DNA has suffered irreparable damage or when numerous proteins are folded improperly
Necessary for development of fingers and toes