Page 88 - The Miracle in the Cell Membrane

The Miracle in the Cell Membrane

pro teins and sugar can not pass through with out assist -
ance. Proteins with in the mem brane serve to carry these
sub stan ces into and out of the cell.
The cell mem brane lip ids are not per me a ble to elec tri -
cal ly-charged mol e cu les, no mat ter how small they may
be, because phos phol i pid mol e cu les con sist of an elec tri -
cal ly-charged polar "head" and two non-polar fatty-acid
"tails." As in water, the lipid parts repel ions and other
polar sub stan ces, and so many sub stan ces are able to
enter and leave the cell only by means of spe cial pro tein
mol e cu les with in the cell mem brane. As Gerald
Schroeder asks, "Who or what decides what should enter
and leave?" 32
Viewed from the out side, the cell mem brane con sist -
ing of fat mol e cu les can be com pared to a sphere made
out of mar bles. Once you enter the "wall" around this
sphere, the wall's con tents resem ble pota toes and rod-
like objects. These are the pro tein mol e cu les that per form
the cell mem bra ne's func tions, iden ti fy ing those sub stan -
ces out side the cell that need to be car ried inside. They
allow these sub stan ces in and, depend ing on their prop -
er ties, per form func tions such as trans por ta tion.
The pro teins assume a most crit i cal respon si bil i ty.
The super vi sion of entry and exit in the cell mem brane is
com pa ra ble to the advanced secu ri ty checks at the
entrance to an impor tant build ing. Anyone want ing to
enter is first searched, and any bags or pack a ges he may
be car ry ing are passed through an X-ray machine. If nec -
es sa ry, his iden ti ty is con firmed with opti cal scan ners or

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