Section 2- New idea (electrochemical gradients) In the images below, the gray ba

Question

Section 2- New idea (electrochemical gradients) In the images below, the gray band represents the plasma membrane of a cell. The plus and minus signs near the membrane represent the relative charge on the two sides. The plusses and minuses in circles represent ions. 1. Which side of the membrane do you think represents the interior of the cell? Why do you think this? The green arrows indicate the strength of the electro-chemical gradient in each scenario (note: the drawings are not meant to show that the ions are crossing the membrane) Why do you think the gradient is so much weaker for the negative ions than for the positive ions? 2. 3. In which of the cases illustrated below would transport of an ion from Side 1 to Side 2 (downward) be LEAST energetically favorable? Why? Which scenario below most likely represents an electrochemical gradient at equilibrium? Explain your choice 4. Side 1 K+ CI- K+ Side 2

Explanation / Answer

1. Negatively charged (-ve) side is the interior of the cell. The overall charge on a side is a relative to the charge on other side. Cell interior can be negatively charged either due to more cation outside the membrane or more anion inside the cell. Usually, the negative charge inside the cell is a result of chloride ion, carboxyl group of proteins or other macromolecules. Also, Na+/K+ pump plays important role in membrane potential as it transports three Na+ outside the cell and two K+ inside the cell simultaneously, ultimately making the cell i terior more negative.

2. It is due to the fact that cell interior is negatively charged so cations can easily be transported down the gradient while anions would face repulsion from the negative charge of the cell interior.

3. B is correct answer, because chloride ion has to be transported against the gradient. In other cases, Na+ or K+ can still be transported because of the overall negative charge at the interior surface of cell membrane.

4. A is correct answer, because there are enough K+ at negatively charged interior surface of cell membrane. In other case (B), chloride ion inside the cell increases the gradient by keeping it more negative. In case (C) there are enough Na+ at positively charged side of the membrane.

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