Final answer:
The central cavities of the Golgi complex's flattened, membranous sacs are called cisternae. These structures play a critical role in processing and packaging proteins and lipids within the cell.
Step-by-step explanation:
The central cavities of the flattened, membranous sacs of the Golgi complex are called cisternae. The cisternae are cup-shaped, membrane-covered compartments that function to modify, sort, and package proteins and lipids for either secretion out of the cell or for use within the cell. The Golgi apparatus acts as an intracellular post office, processing and packaging materials to be transported throughout the cell. Proteins and lipids are tagged with specific signals that determine their final destination, whether it be secretion out of the cell, delivery to lysosomes, or integration into the cell membrane. This organelle is found in both animal and plant cells, and its structure facilitates the essential modification and distribution of cellular products.