What is an organelle? a small structure within the cell that serves specialized functions. a specialized protein that speeds up chemical reactions. packets within the cell cytoplasm that contain powerful enzymes. the semipermeable membrane surrounding a cell.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.