C. testable and observable
Ost proteins that function in the cytosol (such as actin) or in the nucleus (such as DNA polymerase) are synthesized by free ribosomes. Proteins that function within the endomembrane system (such as lysosomal enzymes) or those that are destined for secretion from the cell (such as insulin) are synthesized by bound ribosomes.
<span>As a protein destined for the endomembrane system is being synthesized by a ribosome, the first amino acids in the growing polypeptide chain act as a signal sequence. That signal sequence ensures that the ribosome binds to the outer membrane of the ER and that the protein enters the ER lumen.</span>
On the islands of the Galapagos lives a group of birds called finches. There are 13 species in this group. These species have diverse ecologies--some eat seeds, some eat insects, some eat fruit, and some even eat cactus. Although the bodies of these species look fairly similar, their beaks are very different. That is because it is their beak that is adapted to these diverse ways of feeding (Figure 8.14).
