Plasma Membrane

Quiz 4 (aka work from home assignment 1)


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Before you attempt the following questions, please review chapter 12 lecture slides 1 through 30. Feel free to read the textbook and other sources as necessary. The videos help too.



The plasma membrane acts as a selectively permeable barrier, keeping cellular contents in, and the other things out.


  1. What are the properties of something that can easily cross the plasma membrane without help? (1 point)



  1. Provide an example of such a thing. (1 point)



  1. What are the properties of something that cannot cross the plasma membrane without help? (1 point)



  1. Provide an example of something that cannot cross the plasma membrane without assistance. (1 point)



Transporter proteins facilitate movement of certain molecules across the plasma membrane.

  1. What are the three classes of transporter protein? (3 points)





Thanks to selective permeability, many molecules are found at differing concentrations across the plasma membrane when a cell is at rest.

  1. Complete the following table using the words “high” or “low” to summarize the concentration gradients of Na+, Ca2+, K+, Cl and glucose. (5 points)


Solute Extracellular Intracellular


  1. Draw a diagram to show how the cell uses differential concentration gradients of ions to move glucose from the outside to the inside of a cell. What kind of transporter is utilized in this process? (2 points)









  1. The sodium/potassium ATPase pump sets up the concentration gradient of sodium and potassium cations respectively across the plasma membrane. Is this an example of an active or a passive transporter? (1 point).



  1. How many sodium and potassium cations are moved per pump cycle? (1 point).



  1. Is there an even or uneven distribution of charge across the plasma membrane per pumping cycle? (1 point)


Ion channels are another class of protein that can selectively allow ions to cross the plasma membrane.

  1. When a specific ion channel opens, can ions move against their concentration gradient through that channel? (1 point)



  1. Potassium leak channels are found in the plasma membrane of all cells. Considering that these channels are open all the time, why is the concentration of potassium inside the cell different from outside the cell, when your scientific spidey sense tells you that the concentrations should be equal (ignore the role of the Na+/K+ ATPase pump when you answer this question)? (1 point)





Neurons are a specialized class of cells that can transmit information from one place to another by selectively changing the concentration gradient of certain ions in response to a stimulus.

  1. When a neuron is at rest, is the intracellular concentration of sodium cations higher or lower, relative to the outside? (1 point)



  1. When a neuron is stimulated, what class of ion channel opens first? (1 point)



Immediately after a neuron is stimulated, a second class of ion channel opens, called a potassium rectifying channel.

  1. What happens when this channel opens? (1 point)



  1. What happens to the cell’s membrane potential as a consequence of this channel opening? (1 point)



When an electrode is stuck inside an intact squid giant axon, the membrane potential registers

-70mV. When the axon, suspended in a bath of seawater, is stimulated to conduct an impulse, the membrane potential changes transiently to +40mV. For univalent ions at 20oC, the Nernst equation reduces to:


V = 58mV x log10(Co/Ci)


Where Co and Ci are the extra and intracellular concentrations of the ion respectively (see table below).


Ion [cytoplasm] [seawater]
Na+ 65mM 430mM
K+ 344mM 9mM


Using this equation, calculate the potential across the resting membrane;


  1. Assuming that it is solely due to K+. (2 points)




  1. Assuming that it is solely due to Na+. (2 points)




(Make sure to show how you rearranged the equation to solve for the unknowns).



  1. Which calculation is closer to the measured resting potential and what conclusion do you draw from this? (2 points)





  1. Which calculation is closer to the measured action potential? What does this mean? (2 points)