A cell is a system. Its parts each do a job, and the parts work together to keep the whole cell alive. The cell membrane is the boundary that controls what enters and leaves. The nucleus holds the DNA, the instructions. Mitochondria convert energy from food into a form the cell can use. The cytoplasm is the jelly-like fluid that fills the cell and holds the other parts. This week the crew builds a bag-of-water cell model with beads and string.