Nuclear import and export is tightly regulated by importin and exportin proteins. These proteins interact with target proteins to carry them into or out of the nucleus, respectively. Proteins cannot passively diffuse into the nucleus and they are not translocated into the nucleus during translation.

All of these choices are correct regarding the nucleus. Nuclear pore complexes are incredibly important for regulating nuclear import and export. Ribosomal subunits are assembled in the nucleolus, which is a portion of the nucleus. The nucleus is covered by two lipid bilayers, which constitute the nuclear envelope.

The correct answer is lamina. This is a fibrous network of filaments within the nucleus that provides architectual support similar to the cytoskeleton. Chromatin also is held in place by association with the lamina. 

ATP synthase and cytochromes stud the inner membrane, and more surface area means that more of them can be present in each mitochondrion. This increases the capacity to generate ATP. 

Chloroplasts are found primarily in the mesophyll cells. Mesophyll cells are the green colored tissue that is found within the leaf. It is the abundance of chloroplasts containing chlorophyll that give the mesophyll cells their green coloring. Parenchyma, sclerenchyma, and collenchyma cells are all found within the shoot and root of the plant, not in the leaves. These cell types have little to no chloroplasts within their membranes.

Membrane-bound ribosomes are found on the surface of the rough endoplasmic reticulum. Proteins synthesized by membrane-bound ribosomes are translocated into the endoplasmic reticulum while they are being translated.

These proteins generally undergo modification in the rough endoplasmic reticulum and are packaged into vesicles by the Golgi apparatus. Vesicles from the Golgi apparatus then interface with the plasma membrane of the cell or the membranes of other organelles. Proteins from the interior of the vesicle can be released into the extracellular space or interior of other organelles. Proteins in the membranes of the vesicle become embedded in the plasma membrane of the cell or the organelle.

Proteins synthesized on free ribosomes commonly remain in the cytosol and are modified by different processes than protein synthesized on the rough endoplasmic reticulum.

The smooth endoplasmic reticulum serves several different purposes in the cell, one of which is lipid synthesis. This structure is responsible for producing phospholipids and cholesterol, as well as some steroid hormones and other lipid molecules. It also helps to eliminate foreign toxins from the cell.

The rough endoplasmic reticulum houses ribosomes that synthesize proteins that are destined for the membrane or extracellular environment. The Golgi apparatus is responsible for packaging proteins in vesicles, allowing for transport out of the cell. Ribosomes are involved in protein synthesis (translation).

Eukaryotic ribosomes are 80S with one large 60S subunit and one small 40S subunit. Prokaryotic ribosomes are 70S, with one large 50S subunit and one small 30S subunit. "S" refers to the sedimentation coefficient (Svedberg), which is a particles sedimentation velocity for a given applied acceleration.

Ribosomes can be classified as ribonucleoproteins because the proteins are associated with ribonucleic acids (RNA). Thus, RNA-protein complex is the best descriptor. 

Interestingly, ribosomes are one of the only examples of an RNA structure that has enzymatic activity. The primary enzymatic function of the ribosome is one of a peptidyl transferase; that is, the catalysis of peptide bond formation between amino acids as those acids are brought to the nascent strand.