1. Signaling into cells Communication between cells

2. Cell Recognition and Adhesion
Cells are able to arrange themselves into groups through
Cell recognition: one cell specifically binds to another cell of a certain type.
Cell adhesion, the relationship between the two cells is “cemented”.
Tissue-specific aggregation occur because of plasma membrane recognition proteins.

3. Cell Recognition and Adhesion
Cell-Cell and Cell-Matrix Physical interaction
Extra Cellular Matrix (ECM)

4. Cell Recognition and Adhesion
Specialized cell junctions form between cells in a tissue.
Animals have three types of cell junctions: tight junctions, desmosomes, and gap junctions.

5. Figure 5.6 Junctions Link Animal Cells Together (Part 1)

6. Cell Recognition and Adhesion
Desmosomes act like spot welds on adjacent cells, holding them together.
Desmosomes have dense plaques that are attached both to cytoplasmic fibers and to membrane cell adhesion proteins.
The membrane cell adhesion proteins bind to the proteins of an adjacent cell.
(Cadherins)
The cytoplasmic fibers are
intermediate filaments of the
cytoskeleton.

7. Figure 5.6 Junctions Link Animal Cells Together (Part 3)
Hemidesmosome

8. Adhesion to surface during cell movement
Adhesion and cell movement
Adhesion to surface during cell movement

9. Cell Recognition and Adhesion
Gap junctions are connections that facilitate communication between cells.
Gap junctions are made up of specialized protein channels called connexons.
Connexons span the plasma membranes of two adjacent cells and protrude from them slightly.
Connexons are made of proteins called connexins, which snap together to generate a pore.

10. Figure 5.6 Junctions Link Animal Cells Together (Part 4)

11. Direct Intercellular Communication
Some cells send signals directly from their interior to the interior of adjacent cells.
This transfer occurs by way of specialized structures called gap junctions in animal cells, and plasmodesmata in plant cells.

12. Cell Recognition and Adhesion
Plasmodesmata in plant cells allow connections between ER compartment of adjacent cells

13. Cell Recognition and Adhesion
Tight junctions are specialized structures at the plasma membrane that link adjacent epithelial cells.
They have two primary functions:
To restrict the migration of membrane proteins and phospholipids from one region of the cell to another
To prevent substances from moving through the intercellular space

14. Figure 5.6 Junctions Link Animal Cells Together (Part 2)
Freeze Fracture

15. Figure 5.6 Junctions Link Animal Cells Together (Part 2)
Tight junctions restrict the migration of membrane proteins and phospholipids from one region of the cell to another
Cell Polarity

16. Cell Signaling and Communication
Signals
Receptors
Signal Transduction
Effectors
Effects on Cell Function

17. Signals
Both prokaryotic and eukaryotic cells must process information from their environment and respond appropriately.
Signals may be chemical molecules or physical stimuli such as light.
Cells must be set up to interpret signals—not all cells can interpret all signals.
To interpret a signal, a cell must have the appropriate receptor protein.
Membrane

18. Signal transduction
The entire signaling process, from signal detection to final response, is called a signal transduction pathway.
Membrane

19. Signal transduction
A signal transduction pathway involves a signal, a receptor, transducers, effectors and effects.
Signal
Receptor
Transducers
Effector
Effect

20. Signals
Multicellular organisms’ internal cells are exposed to extracellular fluids and other cells, from which they receive information.
A few of the many types of signals in animal cells are hormones, neurotransmitters, chemical messages from the immune system, CO2, and H+.

21. Signals
In large animals, local signals reach targets via diffusion (autocrine or paracrine signals )when the target is close.
Paracrine signals diffuse to and affect nearby cells.

22. Signals
Cells can sense signals only if they express the right receptor
Autocrine signals are signals generated by the same cells upon which they act.
Autocrine
Paracrine

23. Signals
When the target is distant, signals travel by circulation in the blood.(hormones, Pheromones)

24. Signals
A review of the steps in this signal transduction pathway (Cascade): one signal can induce several coordinated effects
Receptor
Target/Effector

25. Receptors
The type of receptors each cell makes is genetically determined.
A cell responds to only a few of the many signals it receives.
Receptors have specific binding sites for their signals.

26. Figure 15.3 A Signal Bound to Its Receptor

27. Ligands
A ligand is the signaling molecule that binds the receptor.
Binding of the ligand causes the receptor to change shape.
The ligand has no further involvement in the pathway.
Ligand binding is reversible.
Inhibitors can bind (reversibly or irreversibly) to the ligand binding sites on receptor molecules and block it.
Natural and artificial inhibitors are important in medicine.

28. Ligands
There are two classes of signaling molecules:
Ligands with plasma membrane receptors: large and/or polar molecules that can not cross, such as insulin. Receptors are usually transmembrane proteins.
Ligands with cytoplasmic receptors: small and/or nonpolar molecules that can cross the plasma membrane, such as steroids.

29. Cytoplasmic Receptors
Cytoplasmic receptors which are located inside the cell bind with ligands that can cross the plasma membrane.
The receptor changes shape and can then enter the nucleus where it acts as a transcription factor.
Steroid hormones
are an example of
such signal molecules.
Bind to promoter and regulate transcription

30. Transmembrane Receptors
Three well-studied types of transmembrane receptors in complex eukaryotes:
Ion channel receptors
Protein kinases
G protein-linked receptors

31. Receptors
Some ion channel proteins, acting as “gates,” are signal receptors.
Channel proteins can open to let certain ions in or out, or close to restrict them.
The signal to open or close the channel can be chemical, light, sound, pressure, or voltage.
An example of a gated ion channel is the acetylcholine receptor of muscle cells.

32. Nicotine = Agonist = increased blood pressure
Figure A Gated Ion Channel
Nicotine = Agonist = increased blood pressure
Curare = Antagonist = Paralysis

33. Receptors
Some eukaryotic receptor proteins become kinases when activated.
A phosphate is transferred from ATP to a protein, the target protein, changing its shape or activity.
Sometimes the protein kinase phosphorylates itself. This is called autophosphorylation.
Insulin receptors are examples of Tyrosine kinase receptors.

34. Figure 15.6 A Protein Kinase Receptor

35. Figure 15.9 A Protein Kinase Cascade

36. Signal Transduction
There are at least three advantages to having many kinase steps in signal transduction:
A signal at the cell membrane is transferred to the nucleus.
Each activated protein kinase can phosphorylate many target proteins, so amplification of the signal occurs at each step.
Having many steps affecting different target proteins allows for a variety of responses by different cells to the same signal.

37. Receptors
The G protein-linked receptors are proteins with seven regions that pass through the lipid bilayer.
A ligand binds to the extracellular side and changes the shape of the protein on the cytoplasmic side. This exposes a binding site for the G protein.
G protein also has a binding site for GTP. The GTP-bound subunit separates and moves along the membrane until it finds an effector protein.
The effector protein may catalyze many reactions, amplifying the signal.

38. Figure 15.7 A G Protein-Linked Receptor (Part 1)

39. Figure 15.7 A G Protein-Linked Receptor (Part 2)
Ga,Gi

40. Signal transduction pathway
Adrenaline
Transcription Factor

41. Signal Transduction
Transducers convert signals from one form to another.
Direct transduction results from the action of the receptor itself on effector proteins. Direct transduction occurs at the plasma membrane.
Indirect transduction uses a second messenger to mediate the interaction between receptor binding and cellular reaction.
In both direct and indirect transduction the signal initiates a series of events that eventually lead to a final response.

42. Signal Transduction
Indirect transduction is more common than direct transduction.
Scientists investigating the effects of epinephrine on the liver enzyme phosphorylase discovered cyclic AMP (cAMP) as a second messenger.
The second messenger carries the signal from the membrane receptor to the cytoplasm (does not act by itself).
Second messengers affect many cell processes, amplifying the signal.

43. Signal Transduction
The cAMP molecule is a small cyclic nucleotide generated from ATP.
The enzyme adenylyl cyclase produces cAMP using ATP as a substrate. Adenylyl cyclase is activated by an activated G protein subunit.
Like other second messengers, cAMP is not an enzyme. Second messengers act as cofactors or allosteric regulators of target proteins.
cAMP has two major kinds of targets: ion channels and protein kinases.

44. Figure 15.10 The Formation of Cyclic AMP

45. Signal Effects: Changes in Cell Function
Sensory nerve cells of the sense organs are stimulated through the opening of ion channels.
Each of the thousands of nerve cells in the nose expresses just one of these receptors.
When an odorant molecule binds to its receptor, a G protein becomes activated, which leads to formation of the second messenger, cAMP.
The cAMP binds to ion channels, causing them to let in Na+.
The change in Na+ ion concentration stimulates the neuron to send a signal to the brain.

46. Figure 15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels (Part 1)

47. Figure 15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels (Part 2)

48. Membrane potential
Animal cells use ATP to pump Na+ and K+ against their concentration.
Na+-K+ ATPase

49. Membrane potential
Animal cells use ATP to pump Na+ and K+ against their concentration.
Na+-K+ ATPase

50. Signal Effects: Changes in Cell Function
Neuronal cell communication.

51. Signal Effects: Changes in Cell Function
Voltage gated Na+ Channels have three states

52. Signal Effects: Changes in Cell Function
Voltage gated Na+ Channels have three states

53. Signal Effects: Changes in Cell Function
Electric signal in cells is propagated by transient opening of Na+ channels.

54. Signal Effects: Changes in Cell Function
Electric signal in cells is propagated by transient opening of Na+ channels.

55. Signal Effects: Changes in Cell Function
Electric signal is translated into secreted chemical signal in the synapse