Final answer:
The rough endoplasmic reticulum (RER) most closely resembles the Golgi apparatus in appearance under an electron microscope, with both featuring membrane-bound sac-like structures essential for protein modification and transport.
Step-by-step explanation:
The organelle that most resembles the Golgi apparatus when a eukaryotic cell is viewed under an electron microscope is the rough endoplasmic reticulum (RER). Both the Golgi apparatus and the RER are comprised of membrane-bound structures, but the Golgi appears as a series of stacked, cup-shaped, and often semicircular flattened compartments or cisternae. The RER also has flattened sacs but is covered with ribosomes on the cytoplasmic side, making it appear rough, unlike the smooth appearance of the Golgi apparatus. Both play key roles in the modification and transport of proteins within the cell.
The Golgi modifies, sorts, and packages different substances for secretion out of the cell or for use within the cell. It is sometimes likened to a post office due to its role in packaging and labeling cellular materials. In contrast, the RER is primarily involved in the synthesis of proteins that are destined to be secreted by the cell, incorporated into the cell’s plasma membrane, or shipped to an organelle.