Biology 111B
Study Questions 7

CELL COMMUNICATION
ANSWERS

 

1. Define or describe the following:

ligand - the signal molecule that comes from outside the cell

receptor protein - the protein to which the ligand binds; this can be in the cell membrane or inside the cell if the ligand is a lipid.

relay molecule - the molecule that the receptor protein activates

signal transduction - the series of activations that lead to cellular response; this is very often a phosphorylation cascade.

protein kinase - the molecules most often involved in a phosphorylation cascade.

 

2. What determines which signal molecules a cell will respond to?

 The kinds of receptor proteins they have on their membranes or inside the cells (for lipid signals)

3. What is the typical mechanism of response to a steroid hormone signal?

 Typically, a steroid hormone signal will bind to a receptor that, once activated, will bind to DNA to control its expression.

4. How can the information that a signal has been received be passed from a membrane-bound receptor to an interior target cell?

 The membrane-bound receptor protein changes shape upon binding with the ligand. This allosteric shape change causes the interior portion of the protein to catalyze the activation of some other molecule, often a relay molecule.

5. How does the reception of a single signal molecule get amplified to cause a response in millions of target molecules?

 Because an activated molecule at any step can often activate many target molecules before it is deactivated, and each of those target molecules can activate many of their targets before activation, the original single signal gets amplified many times.

6. Protein phosphorylation is commonly involved with which of the following? (choose all that apply)

a. regulation of DNA expression by lipid signal molecules

b. regulation of DNA expression by non-lipid signal molecules

c. activation of ligand-gated ion channels

d. activation of protein kinase molecules

 

7. Amplification of a chemical signal would likely occur when… (choose all that apply)

a. a receptor in the plasma membrane activates relay molecules while a signal is bound to it

b. a relay molecule activates one protein kinase molecule before being deactivated

c. the enzymes that activate and deactivate protein kinase "1" are in equal concentration

d. a single ligand causes many transport channels to open in the ER

 

8. Lipid-soluble signal molecules, such as testosterone, cross the plasma membranes of all cells but affect only their target cells because… (choose all correct answers)

a. only target cells have the appropriate DNA segments

b. intracellular receptors are present only in target cells

c. only target cells have the relay proteins required to transduce the signal

d. membrane receptors are found only in target cells

 

9. Multi-step signal-transduction pathways benefit cells for which of the following reasons?

a. they allow cells to respond to signal molecules that are too large or too polar to cross the cell membrane

b. they enable different cells to respond appropriately to the same signal

c. variations in the signal-transduction pathways can enhance response specificity

d. they can amplify a signal

 

10. Very rarely, a genetic male is physically female. This is known as "testosterone insensitivity" because testosterone is produced but it has no effect. Describe a simple mechanism that would cause this condition based on the mechanism of testosterone action.

 One possible mechanism would be that the individual has a defective gene for the testosterone receptor protein such that testoterone cannot bind to the receptor and cause the receptor to bind to DNA and control expression. Another possibility is that testosterone can bind, but the defective receptor protein still can't bind to DNA because the DNA binding site is defective.

11. Cholera is an often-fatal disease with symptoms of severe diarrhea and salt loss. It is caused by a bacterium, Vibrio cholerae, which colonizes the small intestine. This bacterium secretes a toxin that interferes with the ability of a relay molecule to be "turned off." The relay molecule normally activates membrane channels that actively transport salts out of the small intestine epidermal cells into the interior of the small intestine. Why would this toxin cause the symptoms described?

Given that the normal function of the relay protein is to open membrane channels to actively transport salts into the small intestine, then that function should happen only when salt needs to be exported. When the cholera bacterium interferes with the "off" signal, the channels stay open, exporting more salt into the small intestine than should be there. That causes salt loss directly, and indirectly causes diarrhea because the salt pulls too much water into the small intesting by osmosis.