Alveolar cell. Coloured transmission electron micrograph (TEM) of a section through a type II alveolar cell. This cell is found in the alveoli (air sacs) of the lungs. At upper centre is the cell nucleus (brown), which contains the cell's genetic information. In the cell's cytoplasm (pink) are mitochondria (brown), which provide the cell with energy, and lamellar bodies (purple), which contain surfactant. The lamellar bodies are secreted onto the epithelial cell surface of the alveoli where the surfactant decreases the surface tension, allowing the alveoli to inflate during inspiration and preventing their collapse during expiration.