1. Introduction to Cell Communication
Packet #17
Chapter #11

2. Why Communication Between Cells is Important
A typical free-living cell must be able to
Sniff out nutrients
Sense the difference between light and dark
Detect and avoid poisons and predators
Cells must also be able to communicate to provide long range integration of metabolism.
Therefore, cells must be able to communicate with each other.
Example {Plants}
Respond to temperature, conditions of light and dark, guide the cycle of growth, flowering and fruiting

3. Players of Cell Signaling

4. The Players The Signal Signaling Cell Target Cell Ligand
Cell sending the message
Target Cell
Cell receiving the message
Ligand
Molecule such as a hormone or a neurotransmitter that binds to a specific site on a protein (receptor protein)
Receptor Protein*
Recognizes and responds specifically to the signal molecule
Performs the first step in a series of transduction processes at the receiving end

5. Molecules Used As Signals
Proteins
Peptides
Amino Acids
Nucleotides
Steroids
Fatty Acid Derivatives
Dissolved Gases

6. Target Cell Contain receptor proteins in the cell membrane
Recognizes and responds specifically to the signal molecule
Performs the first step in a series of transduction processes at the receiving end

7. General Steps of Cell Signaling

8. Three Steps
Three Steps
Signal reception
Transduction
Response

9. Signal Reception & Types of Receptors

10. Reception Cells are stimulated by an extracellular signal
Signal binds to, and activates, a receptor protein
Each receptor protein recognizes a specific signal molecule
Once signal is recognized, a new intracellular signal is generated.
This is the first step of transduction

11. Receptors I
There are two basic mechanisms by which chemical signals cause a biologic effect within the cell
Hence there are two types of receptors
Intracellular receptors
Inside the cell
Cell surface receptors
Found on the surface of the cell

12. Classes of Cell-Surface Receptors
Three classes
Ion-channel linked receptor
G-protein linked receptor
Enzyme-linked receptor

13. Intracellular Receptors
Used for molecules that are sufficiently small and hydrophobic to diffuse across the membrane
Best known hydrophobic signal molecules
Steroid hormones
Cortisol
Estradiol
Testosterone
Thyroid hormones
Thyroxine
Vitamin D
Retinoic Acid

14. Intracellular Receptors
Receptors lie in the interior of the target cell
Cytosol or nucleus
When steroid hormone binds, receptor undergoes large conformational change that enables it to bond to its corresponding sequence in the DNA
Promotes or inhibits transcription of a set of genes
Generally regulatory proteins or enzymes
More details to come later.

15. Cell Surface Receptors
Largest class of receptors
Used for signals that are too large or too hydrophillic to cross the plasma membrane
Lie in the plasma membrane of the cell
Relays message across the membrane

16. Signal Transduction & Transduction Pathways

17. Signal Transduction
Signal transduction is the continuation and/or conversion of signals from one form to the next.
The signals, although sometimes in different forms, represent the same information.

18. Signal Transduction II
After reception, when the original signal molecule binds on the receptor protein, the message is passed through the cell membrane through a set of intracellular signaling molecules—transduction.

19. Pathways Two classifications Catabolic pathway Anabolic
Degradable pathway
Reactions serve to capture chemical energy {ATP} from the degradation of energy-rich fuel molecules
Reactions that break down complex molecules, such as proteins, polysaccharides, and lipids, to a few simple molecules
Anabolic
Synthetic pathway
Combine small molecules, such as amino acids to form complex molecules such as proteins
Require energy
ATP is broken down to ADP + P

20. Introduction of Pathways
Multistep Pathways
Can amplify a signal
Provide more opportunities for coordination and regulation
At each step in a pathway
The signal is transduced into a different form, commonly a conformational change in a protein
Many signal pathways
Include phosphorylation cascades
In this process
A series of protein kinases add a phosphate to the next one in line, activating it
Phosphatase enzymes then remove the phosphates

21. Phosphorylation Cascades

22. Cascade Utilizing G-proteins

23. Regulation of Pathways
How does the organism regulate metabolism?
Regulatory signals inform individual cells of the metabolic state of the body as a whole
Hormones
Nervous system
Availability of nutrients
These influence signals that are generated within the cell itself

24. Signals Within the Cell
Regulatory signals
Substrates
Allosteric activators
Allosteric inhibitors
Product
Cause a rapid response

25. Signaling Cascades The relay chain during transduction
Physically transfer the point of reception to the end point where the cell machinery will make the response
Transform the signal into a form that stimulates the response
Amplify the signal received
Distribute the signal so that many processes are influenced
Each step is open to interference from other factors so that the signal of the cell can me modulated according to conditions prevailing inside or outside the cell

26. Questions How do cells receive the signal?
Where do the cells receive the signal?
What do cells do once they have received the signal?
How do cells respond to the signals received?
Among all the hundreds of signals being sent, and received, how do cells determine which signal was sent to them?

27. “Types” of Communication

28. Private vs. Public Private Messages Public Messages
Only a select few cells receive the message
Public Messages
Message sent throughout entire body

29. Long Distance vs. Short Distances
Long Distance Messages
Are messages that are sent over a long path from the point of origin location up until the signal's final spot.**
Short Distance Messages
Are messages that are sent over a short distance from the point of origin up until the signal’s final stop.**
Sometimes within the immediate area.

30. Example Public & Long Distance
Hormonal Signaling
Hormones may be sent throughout the bodies of plants and animals
In animals, hormonal signals are secreted into the bloodstream
In plants, hormonal signals are secreted into plant sap.
Hormones, produced in animals, are produced in endocrine cells and help make up the Endocrine System.

31. Example Public and Short Distance
Paracrine Signaling
Signal molecules diffuse locally through the extracellular medium
Signal remains in the neighborhood of the cell that secretes them
Signal acts as local mediators
Regulate inflammation at sites of infection
Regulate cell proliferation in wound healing
Growth factors
Stimulate target cells to grow and multiply
Part of the bigger Immune System
Used in the healing process

32. Private & Short Distance Synaptic Signaling
Signals sent via the nervous system are sent over long distances. However, the signal must travel along multiple individual nerve cells and the communication between individual nerve cells is called synaptic signaling.

33. Private & Short Distance Intimate Messaging
Most short range of all
Face to face conversation
No molecules are released
Different in comparison to the other three.
Cells make direct contact, through signaling molecules, in their plasma membrane
Message is delivered by the binding of a signal molecule anchored in the plasma membrane of the signaling cell to a receptor molecule embedded in the plasma membrane of the target cell
Cell to cell connection
Remember from earlier packet the functions of proteins in cell membrane.

34. Private & Short Distance Intimate Messaging II
In embryonic development, this type of signaling has an important role in tissues where adjacent cells that are initially similar have to become specialized in different ways
Plays role in determining what type of cell a stem cell becomes.

35. Types of Cell to Cell Connection
Gap Junctions
Tight Junctions
Desomosomes
Plasmodesmata*

36. Cell to Cell Connections
Intimate Messaging
Cell to Cell Connections

37. Gap Junctions
Communicating cell to cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next
Found in animal cells

38. Tight Junctions
Cell to cell junction that seals adjacent epithelial cells together, preventing dissolved molecules in the extracellular medium from passing from one side of the epithelial sheet to the other
Found in animal cells.

39. Desmosomes
Specialized to cell to cell junction, usually formed between two epithelial cells, mediated by cadherin molecules and characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert

40. Plasmodesmata
Organelle found in plant cells that allow adjacent cells to communicate, relay signals and move materials between them

41. Private & Short Distance

42. Yeast Cells
When a yeast cell is ready to mate, it secretes a small protein called a mating factor to which other yeast cells are sensitive
They detect the mating factor
They put out a protrusion toward the source of the factor
Halt the cell cycle