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Answers to Text Questions
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Inquiry Questions
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Self Test

1). Which of the following techniques has recently aided the study of receptor proteins?
    a). protein purification
    b). monoclonal antibodies
    c). isolation of cell signal molecules
    d). all of these
Answer:b

2). Which of the following describes autocrine signaling?
    a). Signal molecules released by cells diffuse through the extracellular fluid to other cells.
    b). Signal molecules enter the organism's circulatory system and travel throughout the body.
    c). Signal molecules are released from a cell and bind to receptors on its own plasma membrane.
    d). Signal molecules are released into a narrow space between cells called a synapse.
Answer:c

3). Intracellular receptors usually bind
    a). water-soluble signals.
    b). large molecules that act as signals.
    c). signals on the cell surface.
    d). lipid-soluble signals.
Answer:d

4). Which of the following is not a type of cell surface receptor?
    a). chemically gated ion channels
    b). intracellular receptors
    c). enzymic receptors
    d). G-protein-linked receptors
Answer:b

5). Which of the following is not a second messenger?
    a). adenylyl cyclase
    b). cyclic adenosine monophosphate
    c). calcium ions
    d). cAMP
Answer:a

6). The amplification of a cellular signal requires all but which of the following?
    a). a second messenger
    b). DNA
    c). a signal molecule
    d). a cascade of protein kinases
Answer:b

7). MHC proteins are
    a). molecules that determine a person's blood type.
    b). large molecules that pass through the membrane many times.
    c). identity markers present on the surface of an individual's cells.
    d). different for each type of tissue in the body.
Answer:c

8). Sheets of cells are formed from which type of cell junctions?
    a). tight junctions
    b). anchoring junctions
    c). communication junctions
    d). none of these
Answer:a

9). Cadherin can be found in which of the following?
    a). tight junctions
    b). anchoring junctions
    c). communication junctions
    d). adherens junctions
Answer:b

10). Plasmodesmata are a type of
    a). gap junction.
    b). anchoring junction.
    c). communicating junction.
    d). tight junction.
Answer:c


Test Your Visual Understanding

1). Match the following labels with the appropriate structures in the figure, and explain where each type of junction is found.
adherens junction
desmosome
gap junction
hemidesmosome
tight junction

Answer:
1). Tight junction with #1, Adherens junction with #2, Desmosomes with #3, Gap junction with #4 and Hemidesmosome with #5
    1--Tight junction
    Tight junctions hold cells together in a sheet, restricting the movement of substances between the cells. Tight junctions are found in cells that line the digestive tract.
    2--Adherens junction
    Adherens junctions are anchoring junctions that use integrin to hold cells together by anchoring to the cells' actin networks. Adherens junctions, as other anchoring junctions are found in tissues that are subject to mechanical stress such as muscle and epithelium.
    3--Desmosomes
    Desmosomes are anchoring junctions that use cadherin to hold cells together by anchoring to the cells' actin networks, or for more stable cell-to-cell connections by anchoring to the cells' intermediate fibers. Desmosomes, as other anchoring junctions are found in tissues that are subject to mechanical stress such as muscle and epithelium.
    4--Gap junction
    Gap junctions are communicating junctions that use protein channels called connexons to create passageways between cells. When the connexons of two adjacent cells are lined up, a direct connection between the cytoplasms of the two cells is made. Small molecules and ions can pass freely between the cells. Gap junctions are found in animal cells.
    5--Hemidesmosome
    Hemidesmosomes are anchoring junctions that use cadherin to hold cells to an extracellular matrix. Hemidesmosomes are found in epithelial cells, holding them to a basement membrane.


Apply Your Knowledge
1). In paracrine signaling, the signal molecule is destroyed by enzymes in 6 milliseconds. The diffusion rate of the signal through the extracellular fluid is 2 nanometer/1 millisecond. How long will the signal last in the extracellular fluid, and what is the farthest distance a target cell can be from the releasing cell to be affected by the signal?
Answer: Enzymes breakdown the signal molecule in 6 milliseconds so the signal can only last for 6 milliseconds in the extracellular fluid. Therefore, the signal has only 6 milliseconds to travel to its target cell. If the signal can travel 2 nanometers/millisecond then:
2 nanometers x 6 milliseconds = 12 nanometers
The farthest distance a target cell can be from the releasing cell is 12 nanometers.

2). At first glance, the signaling systems that involve cell surface receptors may appear rather complex and indirect, with their use of G proteins, second messengers, and often multiple stages of enzymes. What are the advantages of such seemingly complex response systems?
Answer: There are several advantages to response system that is organized in multiple levels. The first is obviously the ability to amplify the signal. With the multiple stages of enzymes, a small amount of a signal is all that is needed to get the response going. This allows for a very high level of sensitivity.

Another advantage is the ability of the cell to use the same "machinery" for different signals--not having to produce new cascade machines to elicit the same response or a related response. If two signals produce the same response, two different G proteins could activate the same adenylyl cyclase and activate the same cascade. Similarly, the same second messenger, for example cAMP could be used in different cascade responses, offering a certain redundancy to the system.

3). Shigella flexneri is one of several species of bacteria that cause shigellosis, or bacillary dysentery. Recent evidence has shown that S. flexneri cannot spread between the epithelial cells of the intestines without the expression of cadherin by those cells. Why do you suppose it cannot?
Answer: Shigella flexneri invades one cell, multiplies and then spreads to contiguous epithelial cells. Cells that are expressing cadherin are closely adhering to their neighboring cells, which makes it easier for Shigella flexneri to spread.







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