Questions: Chapter 4: A Tour of the Cell 2. The nuclear envelope has passages for substances moving into and out of the nucleus. These passages are called nuclear pores and they are made by proteins that are inserted into the plasma membrane that makes up the nuclear envelope. These proteins would be assembled by: Ribosomes A) Free-floating ribosomes B) The nucleus C) Ribosomes bound to the endoplasmic reticulum D) Nuclear pores

The answer is C: Ribosomes bound to the endoplasmic reticulum.

Explanation for each option:

A) Free-floating ribosomes: These ribosomes are responsible for synthesizing proteins that function within the cytosol. They are not typically involved in synthesizing proteins that are inserted into membranes or exported from the cell.

B) The nucleus: The nucleus contains the cell's genetic material and is the site of transcription (the process of making RNA from DNA). However, it does not assemble proteins. Protein synthesis occurs in the cytoplasm.

C) Ribosomes bound to the endoplasmic reticulum: These ribosomes are specifically involved in synthesizing proteins that are destined for insertion into membranes, secretion, or for use in lysosomes. Since the nuclear envelope is continuous with the endoplasmic reticulum, the proteins that form nuclear pores are synthesized by these ribosomes.

D) Nuclear pores: Nuclear pores are structures that allow the passage of molecules between the nucleus and the cytoplasm. They are not involved in the synthesis of proteins.

Summary: The proteins that form nuclear pores are assembled by ribosomes bound to the endoplasmic reticulum.