Answer 1:
That is an excellent question! The lipid
bilayer of the cell membrane is selectively
permeable to some small molecules, including
carbon dioxide and oxygen. These molecules can
cross the membrane passively via diffusion.
Ions in solution, however, can’t pass through the
membrane because they are hydrated. Although the
bare radii of the ions are smaller than
O2 and CO2, the charge of
the ions causes water molecules to arrange around
them in hydration shells. The arrangement of water
molecules around an ion occurs because water is
a dipolar molecule, with a positively charged
end and a negatively charged end. Around a
positively charged ion in solution, water arranges
itself so that the negative end of the water
molecule is close to the positive charge of the
ion. This arrangement of bound water molecules
around an ion is called solvation, and
greatly increases the effective size of the ion.
For example, a water molecule
has an approximate radius of 0.14 nanometers and
O2 has an approximate radius of 0.18
nanometers (approximate because these are not
spherical molecules). For comparison, a sodium
ion, Na+, has a bare radius of only
0.095 nanometers, smaller than both water and
O2. A bare hydrogen ion is even
smaller, 1 femtometer, or 0.000001 nanometers.
However, hydrogen ions in solution exist as
hydronium ions (H3O+),
which are thought to be solvated by between 2 and
4 water molecules, yielding a hydrated radius of
0.28 nanometers. Sodium is thought to be solvated
by 3-5 water molecules, yielding a hydrated radius
of 0.36 nanometers. More highly charged ions
like are solvated by even more water molecules and
are even larger. For example,
Ca2+ is solvated by 5-7
water molecules and has a hydrated radius of 0.41
nanometers (much larger than its bare ionic radius
of 0.099 nanometers). Therefore, the solvated
sizes of the ions are greater than the sizes of
water molecules and oxygen molecules, and so
the ions cannot freely pass through the
membrane.
Sincerely,
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