2. TYPES OF CELLULAR JUNCTIONS:
Tight Junctions –
Belts around epithelial cells that serve as a seal beneath the cell’s surface that prevent “leaks”.
They limit the passage of molecules and ions through the space between cells. 
Found in organs such as the bladder to prevent urine from escaping the organ.

3. Desmosome –
Intercellular junctions that provide strong adhesion between cells.
They link intracellularly to the cytoskeleton so they form an “adhesive bond” giving mechanical strength and stretch to tissues.
Desmosomes help to resist shearing forces found in epithelial and muscle tissue.

4. Gap Junctions:
 A specialized intercellular connection between animal cells.
They directly connect the cytoplasm of two cells and allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells.
Does not have interference from a plasma membrane.

5. Plasmodesmata
 Microscopic channels which pass through the cell walls of plants.
  Allows communication between adjacent plant cells.

6. Cellular Signaling Reception – The first step in Cell Signaling.
Process occurs in 3 stages:
Reception
Transduction
Response
Reception –
The first step in Cell Signaling.
The signaling molecule binds to the receptor protein in the cellular membrane.

8. RESPONSE:
Signaling pathways share a common goal: to produce some kind of cellular response.
A signal is released by the sending cell in order to make the receiving cell change in a particular way.
Helps to insure the correct processes occur in the right cells.
Coordinates activities of many cells within the organism.

9. TARGET CELLS
Cells that are meant to respond to the communications released are known as target cells .

10. AUTOCRINE SIGNALING:
 A form of cell signaling in which a cell secretes a hormone or chemical messenger that binds to receptors on that same cell, leading to changes in the cell.

11. PARACRINE SIGNALING:
A form of cell-cell communication in which a cell produces a signal to induce changes in nearby cells.
This type signaling alters the behavior or differentiation of affected cells.

12. ENDOCRINE SIGNALING
Cells that need to communicate a message over a long distance, use the endocrine system.
Endocrine signaling uses chemicals called hormones to send messages throughout the body.
Hormones are released from the cell into the bloodstream and can travel around the entire body.

13. Negative Feedback System :

14. Positive Feedback System :

15. G Protein-Coupled Receptors

16. Receptor Tyrosine Kinase

17. Ion Channel Receptors