equal amount enters and leaves the cell. In the medical setting, red blood cells can be kept
intact in a solution that is isotonic to the blood cells. If the blood cells were put in pure
water, the solution would be hypotonic to the blood cells, so the blood cells would swell and
burst. This is represented in theFigure 4.3.
Figure 4.3: Osmosis causes these red blood cells to change shape by losing or gaining water.
( 1 )
Sometimesdiffusionacrossthemembraneissloworevenimpossibleforsomechargedorlarge
molecules. These molecules need the help of special helper proteins that are located in the
plasma membrane. Ion channel proteinsmove ions across the plasma membrane. Other
molecules, suchasglucose, moveacrossthecellmembranebyfacilitated diffusion,inwhich
a carrier protein physically moves the molecule across the membrane (Figure 4.4). Both
channel proteins and carrier proteins are specific for the molecule transported. Movement
by ion channel proteins and facilitated diffusion are still considered passive transport,
meaning they move molecules down the cell’s concentration gradient and do not require any
energy input.
Figure 4.4: Facilitated Diffusion is a type of passive transport where a carrier protein aids
in moving the molecule across the membrane. ( 11 )