Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 9

Charged or Not: Does It Make a Difference Many solutes that must move into and out of cells exist in either a protonated or ionized form, or they have functional groups that can be either protonated or ionized. Simple molecules such as CO 2 , H 3 PO 4 (phosphoric acid), and NH3 (ammonia)are in this category, as are organic molecules with carboxylic acid groups, phosphate groups, and/or amino groups.
(a)Consider ammonia as a simple example of such compounds. What is the charged form of ammonia called What is its chemical formula
(b)Which of these two forms will predominate in a solution with a highly acidic pH Explain your answer.
(c)For which of these two forms will the uptake across the plasma membrane of a cell be affected by the concentration gradient of that form on the inside versus the outside of the cell For which, if either, of these two forms will uptake be affected by the membrane potential of the plasma membrane Explain your answers.
(d)For a cell that must take up ammonia from its environment, will uptake of the charged form require more or less energy than uptake of the uncharged form, assuming that the cell has a negative membrane potential Explain your answer.
(e)Instead of ammonia, consider the uptake of acetic acid, CH 3 COOH, an important intermediate in several biological pathways. What is the charged form in this case For which of the two forms will the uptake across the plasma membrane of a cell be affected by the concentration gradient of that form on the inside versus the outside of the cell For which, if either, of these two forms will uptake be affected by the membrane potential of the plasma membrane Explain your answers.
(f)For a cell that must take up acetic acid from its environment, will uptake of the charged form require more or less energy than uptake of the uncharged form, assuming that the cell has a negative membrane potential Explain your answer.