Page 108 - The Cell in 40 Topics

very cell in every human being contains more than a million
protein molecules, of thousands of different kinds. 16
In addition, these proteins are constantly renewed. Once every month,
they are separated into the amino acids that comprise them and reproduced
17
according to the cells’ needs. At the end of complex processes known as
protein synthesis, they are combined together again within the cell.
Here, we wish to concentrate on the protein traffic flow that results
from the newly produced proteins changing places inside the cell. Because
the cell immediately begins using part of these proteins, they must be trans-
ported to the sites where they will be used.
One portion is sent to the cell’s “protein warehouse,” to be used at a
later date. Proteins to be used outside the cell membrane are removed from
it under the supervision of the cell membrane itself. At this point, proteins
entering the cell from the outside—again under the cell membrane’s super-
vision, represent a significant part of this heavy protein traffic. In short,
there is considerable activity in the cell, despite its microscopically small
size (Figure 86).

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