Final answer:
In eukaryotic cells, the structures that occupy the cytoplasm and perform specific functions are known as organelles, which include the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus among others.
Step-by-step explanation:
The cytoplasmic space in eukaryotic cells is filled with various structures that are essential for the cell to function. These are typically membrane-bound and are known as organelles. Each organelle in a eukaryotic cell has a specific role, much like organs within a body. The nucleus contains the cell's hereditary material (DNA) and is considered one of the most critical organelles. Other important organelles include the mitochondria, which are involved in energy production through cellular respiration, the endoplasmic reticulum (ER), which is involved in the synthesis and transport of proteins and lipids, and the Golgi apparatus, which modifies, sorts, and packages proteins and lipids for storage or transport. While ribosomes are not membrane-bound, they are often referred to as organelles because they are crucial for protein synthesis.