Cells contain organelles that carry out specific functions. The cytoplasm contains organelles like mitochondria, which produce energy, and the endoplasmic reticulum and Golgi apparatus, which process and package proteins and lipids. The cytoskeleton is made of microtubules, microfilaments, and intermediate filaments that help maintain cell shape and allow movement. The nucleus houses the cell's DNA and directs protein production. Organelles are surrounded by membranes and transport materials throughout the cell.