Question 1

Protein synthesis isn’t finished when its amino acid sequence (primary structure) has been assembled.

Accepted Answer

To be functional, it must coil or fold into precise secondary and tertiary structures; in some cases, it associates with other protein chains (quaternary structure) or binds with a nonprotein such as a vitamin or carbohydrate.

Question 2

As a new protein is assembled by a ribosome, it is often bound by an older protein called a chaperone.

Accepted Answer

The chaperone guides the new protein in folding into the proper shape and helps to prevent improper associations between different proteins.

Question 3

As in the colloquial sense of the word, a chaperone is an older protein that escorts and regulates the behavior of the “youngsters.”

Accepted Answer

Some chaperones are also called stress proteins or heat shock proteins because they’re produced in response to heat or other stress on a cell and help damaged proteins fold back into their correct functional shapes.

Question 4

If a protein is going to be used in the cytosol (for example, the enzymes of glycolysis), it is likely to be made by free ribosomes in the cytosol.

Accepted Answer

However, if it is going to be packaged into a lysosome or secreted from the cell (for example, insulin), the entire polyribosome migrates to the rough ER and docks on its surface.

Question 5

Assembly of the amino acid chain is then completed on the rough ER, and the protein is sent to the Golgi complex for final modification.

Accepted Answer

Thus, we turn to the functions of the ER and Golgi complex in the processing and secretion of a protein.

Question 6

1) As a protein is assembled on the ER surface, it threads itself through a pore in the ER membrane and into the cistern.

Accepted Answer

1) Enzymes in the cistern modify the new protein in a variety of ways—removing some amino acid segments, folding the protein and stabilizing it with disulfide bridges, adding carbohydrates, and so forth. Such changes called posttranslational modification

Question 7

1) Insulin, for example, is first synthesized as a protein 86 amino acids long. In posttranslational modification, the chain folds back on itself, 3 disulfide bridges are formed, and 35 amino acids are removed from the middle of the protein

Accepted Answer

1) . The final insulin molecule is therefore made of 2 chains of 21 and 30 amino acids held together by disulfide bridges

Question 8

2) When the rough ER is finished with a protein, it pinches off bubblelike transport vesicles coated with a protein called clathrin. Clathrin apparently helps to select the proteins to be transported

Accepted Answer

2) in the vesicles, and as a basketlike cage, it helps to mold the forming vesicles. Soon after the vesicles detach from the ER, they fuse into irregularly shaped clusters that carry their cargo to the Golgi complex.

Question 9

3) As they reach the complex, these clusters fuse and form a new Golgi cistern,

Accepted Answer

3) called the cis cistern because it is the one closest to the ER.

Question 10

4) This new cistern migrates through the complex toward the opposite (trans) face. It matures as it travels, producing new enzymes that modify the cargo in different ways.

Accepted Answer

4) For example, it may add carbohydrate chains to the proteins, producing the glycoproteins mentioned in chapter 2. Thyroid-stimulating hormone is one such glycoprotein made in this way.

Question 11

5) Finally, the trans cistern—the one farthest away from the ER—

Accepted Answer

5) breaks up into Golgi vesicles laden with the cell product.

Question 12

6) Some of the Golgi vesicles become lysosomes, while others become secretory vesicles that migrate to the plasma membrane and fuse with it, releasing the cell product by exocytosis.

Accepted Answer

6) This is how a cell of a salivary gland, for example, secretes mucus and digestive enzymes, and how a cell of the pituitary gland releases thyroid-stimulating hormone.

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