Question

A certain channel has been isolated from a cell membrane, that allows cyanide (CN-) ions, but not nitrate (NO3) ions, to cross a phospholipid-bilayer membrane. a) Briefly, explain how the structure of the channel might allow the channel to discriminate between these two anions. HINT: You may find it helpful to consult the three-dimensional CPK representations in the on-line table of polyatomic ions. b) Which, if any, of the following ions would you expect to be able to pass through this channel: bicarbonate (HCO3-), hydroxide (OH-), or permanganate (MnO4-)? Briefly, explain your reasoning. HINT: You may find it helpful to consult the three-dimensional CPK representations in the on-line table of polyatomic ions. c) Neglecting size considerations, tell whether you expect methane (CH4) to pass through the K+ channel depicted in Figure 4. Briefly, explain your reasoning.

All of 1

Answer 1

a)

For the channel to allow passage of polar or charged particles between two aqueous sides of the membrane it should be lined with hydrophilic amino acid residues. Such channels may left open all the time or they may open and close by cellular gating mechanisms. They have certain shape to fit only certain molecules. Thus, the channel isolated can allow only small sized ions such as cyanide and not large sized anions such as nitrates. Cyanide contains only two atoms (1 C and 1 N), while nitrate anion contains four atoms (3 oxygen and 1 nitrogen).

b)

Of the three anions bicarbonate (HCO₃^-), hydroxide (OH^-), and permanganate (MnO₄^-), only hydroxide can pass through this channel, since it’s shape is similar to that of cyanide and contains only two atoms (oxygen and hydrogen) similar to cyanide.

c)

The space present in the K+ channel depicted in figure 4 is very small to fit methane molecule, which contains five atoms (1 C and 4 H). Therefore, methane cannot enter such small space and cannot be passed through the channel.