Chapter 6: Tour of a Cell HOMEWORK
Part A – Organelles of the endomembrane system
The endomembrane system is critical for the synthesis, processing, and movement of
proteins and lipids in the cell. The smooth ER functions mainly in lipid synthesis and
processing. The rough ER is the site of secretory protein synthesis. These proteins are
processed further in the Golgi apparatus, from where they are dispatched in vesicles to
the plasma membrane. Lysosomes, whose enzymes and membranes are made and
processed by the rough ER and Golgi apparatus, function in the hydrolysis of
macromolecules, such as in phagocytosis and autophagy.
, Part B – Sites of protein synthesis
All proteins are synthesized by ribosomes in the cell. Some ribosomes float freely in the
cytosol, while others are bound to the surface of the endoplasmic reticulum. Most
proteins made by free ribosomes function in the cytosol. Proteins made by bound
ribosomes either function within the endomembrane system or pass through it and are
secreted from the cell.
Which of the following proteins are synthesized by bound ribosomes?
Most 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.
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.
Part C – The pathway of secretory proteins
As they are being synthesized, secretory proteins enter the lumen of the endoplasmic
reticulum. From the ER, vesicles transport these proteins to the Golgi, where they are
sequentially modified and concentrated in a cis-to-trans direction. Secretory vesicles
bud from the Golgi and move along cytoskeletal filaments to eventually fuse with the
plasma membrane, secreting their protein cargo. Each of these transport steps requires
specialized proteins to ensure that the cargo is sent to the proper location and is able to
fuse with the target membrane.
Part A – Organelles of the endomembrane system
The endomembrane system is critical for the synthesis, processing, and movement of
proteins and lipids in the cell. The smooth ER functions mainly in lipid synthesis and
processing. The rough ER is the site of secretory protein synthesis. These proteins are
processed further in the Golgi apparatus, from where they are dispatched in vesicles to
the plasma membrane. Lysosomes, whose enzymes and membranes are made and
processed by the rough ER and Golgi apparatus, function in the hydrolysis of
macromolecules, such as in phagocytosis and autophagy.
, Part B – Sites of protein synthesis
All proteins are synthesized by ribosomes in the cell. Some ribosomes float freely in the
cytosol, while others are bound to the surface of the endoplasmic reticulum. Most
proteins made by free ribosomes function in the cytosol. Proteins made by bound
ribosomes either function within the endomembrane system or pass through it and are
secreted from the cell.
Which of the following proteins are synthesized by bound ribosomes?
Most 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.
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.
Part C – The pathway of secretory proteins
As they are being synthesized, secretory proteins enter the lumen of the endoplasmic
reticulum. From the ER, vesicles transport these proteins to the Golgi, where they are
sequentially modified and concentrated in a cis-to-trans direction. Secretory vesicles
bud from the Golgi and move along cytoskeletal filaments to eventually fuse with the
plasma membrane, secreting their protein cargo. Each of these transport steps requires
specialized proteins to ensure that the cargo is sent to the proper location and is able to
fuse with the target membrane.
