AP Bio Cell Communication

1. The series of steps by which a signal on a cells surface is converted into a specific cellular response is called the

signal transduction pathway

2. Are the general aspects of cell signaling pathways similar in various organisms? What is the basis for your yes or no answer?

Yes. Those in yeast and mammals are strikingly similar and there are other similarities between signaling systems in plants and bacteria.

3. What two types of Local Signaling in multicellular animals are described in your textbook?

Paracrine signaling and Synaptic signaling

4. Plants and animals use this type of signaling for signaling at greater distances and over a diffuse area.

Hormonal Signaling - also called Endocrine Signaling

5. What System in an animal's body transports hormones?

Circulatory System

6. How can cells communicate when they are in direct contact with one another?

Gap Junctions in animal cells; Plasmodesmata in plant cells; Interaction of cell surface molecules (glycoproteins) in cell-cell recognition

7. What are the three stages of the cell signaling process?

Reception - Transduction - Response

8. Define the term Ligand.

A small molecule that specifically binds to a larger molecule

9. Name the three main types of membrane bound receptors.

G - protein-linked receptors, Tyrosine-kinase receptors, and ligand-gated ion-channel receptors

10. What is the molecular structure of all G-protein linked receptors?

They are protein with seven alpha-helices spanning the cell membrane.

11. Write a sequence of steps whereby a signal molecule binding to a G-protein receptor can cause an intracellular response.

- signal molecule binds to cell membrane receptor

- receptor binds inactive G-protein and causes GTP to replace GDP on protein thereby activating G-protein

- activated G-protein binds another protein (usually an enzyme) and alters the activity of the enzyme

- get cell resposne

12. What is the structure of a Tyrosine-kinase receptor?

Before the signal binds to the receptor the receptors exist as individual polypeptides. Each polypeptide has an intracellular tail containing a number of tyrosine amino acids, and a single alpha-helix in the cell membrane.

13. How is a Tyrosine-kinase receptor activated?

When the signal binds to the receptor two receptor polypeptides aggregate forming a dimer, this aggregation activates the tyrosine-kinase parts of both polypeptides and these phosphorylate the tyrosines on the tail of the other polypeptide.

14. What does a Tyrosine-kinase receptor do once it is activated?

Inactive proteins within the cell bind to the phosphorylated tyrosine residues, the phosphate is transferred to the proteins, and the proteins become active.

15. What is a ligand-gated ion channel?

Protein pores in the membrane that open or close in response to a chemical signal, allowing or blocking the flow of specific ions, such as Na+ or Cl-.

16. Are all receptors located on the cell membrane? Explain your answer and if no, indicte which molecules may interact with these non-membrane receptors.

No, some are proteins located in the cytoplasm or the nucleus of the cell. Ones that are small enough to pass between membrane phospholipids or ones that are lipid soluable.

17. What two sex hormones does your book indicate bind to receptors located in the cytoplasm or nucleus?

Testosterone and Estrogen

18. When testosterone binds to a receptor in the cytoplasm the complex goes into the nucleus of the cell and turns on

genes

19. Special proteins that control which genes are turned on are called

transcription factors

20. What action is a widespread mechanism for regulating protein activity?

phosphorylation of proteins

21. What is a Protein Kinase and why are they important?

A molecule that transfers phosphate groups from ATP to proteins. Many of the molecules in signal-transduction pathways are protein kinases.

22. What is a "phosphorylation cascade"?

A series of different molecules in a pathway phosphorylated in each turn, each molecule adding a phosphate group to the next

23. What is a Protein Phosphatase and why are they important?

These are enzymes that remove phosphate groups from proteins; they reverse the action of protein kinases.

24. What are Second Messangers

These are small, nonprotein, water-soluble molecules that are components of the signal-transduction pathway.

25. Why can second messengers spread rapidly throughout the cell?

Because they are small and water-soluble.

26. What types of signaling pathways use second messengers?

G-protein-linked receptors and tyrosine-kinase receptors

27. What are the two most common second messengers?

Cyclic-AMP and Calcium ions

28. How is cyclic-AMP formed?

A membrane-bound enzyme, adenylyl cyclase, converts ATP to cAMP

29. What is the immediate effect of cyclic-AMP once it is produced?

Cyclic-AMP activates protein kinase A. This activated protein kinase then activates other proteins within the cell depending upon the cell type.

Why doesn't cyclic-AMP keep working all of the time the cell is alive

Another enzyme, Phosphodiesterase, converts cAMP to AMP which is not active.

What role does the second messenger cyclic-AMP play in the disease Cholera?

Bacteria obtained from contaminated water gets into our intestine. A protein produced by the bacteria modifies the G-protein so that the G-protein does not function properly. Salt and water exit the cells into the lumen of the digestive tract and this material is lost from the body in profuse diarrhea.

32. Can some first messengers decrease cyclic-AMP production? If yes, how?

Yes, by interacting with an inhibitory G-protein instead of a stimulatory G-protein. This causes a decrease in the amount of cAMP within the cell.

Calcium is used as a second messenger in these signaling pathways.

G-protein-linked pathways and tyrosine-kinase pathways.

Why can calcium ions be used as a second messenger? Where are most of the calcium ions in a cell?

Because the concentration of Calcium ions within the cytoplasm of the cell is very low. Small increases in the level of cytoplasmic Calcium can stimulate certain processes.

Most of the Calcium ions are inside the Endoplasmic Reticulum.

Describe the pathways that can lead to the release of calcium from the endoplasmic reticulum.

- signal molecule binds to membrane receptor

- G-protein is activated

- Phospholipase C enzyme converts a membrane lipid into DAG (diacylglycerol) and IP3 (inositol triphosphate)

- IP3 binds to ion-gated protein in ER membrane, which allows release of Calcium ions from the ER.

- Calcium ions interact with proteins to activate them

How could a signaling pathway be involved in activating genes on the chromosomes?

Special proteins within the nucleus called Transcription Factors can be phosphorylated (activated) by protein kinases. The Transcription Factor can then bind to a specific region of the DNA and initiate the process of mRNA production from the DNA strand.

What is Signal Amplification?

At each catalytic step the number of activated products is much greater than in the preceding step. The amplification effect depends on the fact that these proteins persist in active form long enough o process numerous molecules of substrate before they become inactive again.

What determines the specificity of a particular cell for a specific cellular response?

Different kinds of cell have different collections of proteins.

- signal proteins

- relay proteins

- proteins needed to carry out the response