Cells can be considered the smallest LIVING unit because they are able to grow and multiply, but there are many things smaller than cells. Inside cells, there are smaller compartments, called organelles. Each organelle contains even smaller molecules, like proteins and DNA. Each organelle has a specific function. For example, the nucleus is the organelle that holds DNA, which is the genetic code that tells the cells what kind of proteins to make. Each protein also has a specific function. For example, some proteins help the cells turn food into energy. All of the organelles, proteins, DNA and other molecules work together in one big compartment (the cell), creating a living unit. Taking it one step further, different types of cells come together to form organs like heart, lungs, and brain.