Page 170 - The Cell in 40 Topics

pon reaching the cell membrane, a substance in the blood-
stream does not immediately enter the cell. It is met in differ-
ent ways, depending upon its size, chemical properties, and
whether or not it is beneficial. Any substance about to enter a cell is sub-
jected to tight controls, just like visitors to the borders of another country. If
it is a foreign substance, its identity is established and, if it is determined
that it represents a threat to security, it is deported. However, the entry and
departure of some substances has been made easier, in the same way that
countries do for their own citizens. These substances are able to enter and
leave the cell without being subjected to any precautionary measures. In
short, substances approaching the cell membrane are greeted with different
forms of welcome, depending on their identity.
In order for a substance to be able to pass through the cell membrane, it
must be able to mix with the cell membrane—in other words, to be soluble in
fat. In the same way that you can never mix oil and water, no matter how
hard you try, so it is impossible for a substance that iso insoluble in fat to mix
with the cell membrane. A special method is employed for the passage of
such substances, and in this, proteins in the cell membrane play a crucial role.

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