The document discusses the structure and functions of mitochondria and smooth endoplasmic reticulum (SER). Mitochondria are located in the cytoplasm and have their own DNA. They have a double membrane structure and use proton gradients to drive ATP synthesis. Mitochondria are important for muscle cells, sperm cells, and brain cells that require energy. SER is located in the cytoplasm near the nucleus and functions to store lipids and synthesize steroids. Cells that produce oils or hormones like testosterone have many SER.