562 MHR • Appendix 5
Small, non-polar molecules such as oxygen
and carbon dioxide easily cross the cell
membrane through diffusion. Because they are
small, water molecules and other small polar
molecules may also cross the membrane
through minute imperfections between
individual phospholipid molecules. Large polar
and non-polar molecules and particles with
charges are unable to cross the membrane on
their own. The cell membrane, therefore, is
selectively permeable, and certain essential
substances (such as glucose) require assistance
to cross it. In Chapter 1, you reviewed one
method for transporting certain molecules, such
as glucose, across the membrane, with the help
of a carrier protein. Since carrier proteins
accept only non-charged molecules with a
specific shape, a different membrane protein,
called a channel protein, is used to facilitate
the movement of charged particles (ions) into
and out of cells. The diagram on the right
shows how a channel protein accomplishes this.
Keep in mind that channel proteins and
carrier proteins involve the movement of
substances along their concentration gradient.
In other words, the cell provides no energy to
assist in the transport of these substances. To
move substances against their concentration
gradient, the cell must expend energy through
the process of active transport.Figure A5.5Dissolved ions (charged particles) cross
the cell membrane by means of water-filled passages
supplied by channel proteins.negatively charged
channel protein