They are the site of protein synthesis in the cytoplasm. Smooth ER produces different compounds in different cells. Peroxisomes house drug-detoxifying enzymes. The Golgi apparatus, a stack of flattened vacuoles with vesicles near the edges, packages, stores, and distributes the proteins the ER produces. The Golgi apparatus produces lysosomes, which contain hydrolytic enzymes that digest unwanted materials inside the cell, including worn-out cell parts.
Mitochondria are organelles bounded by a double membrane.
The inner membrane is folded into cristae. The gel-like material between the cristae is matrix. Mitochondria convert the energy stored in glucose into ATP molecules in a process called aerobic cellular respiration. Filamentous protein structures, microtubules, and actin filaments form a cytoskeleton that serves as a framework for the cell's interior. They occur in pairs.