A cell is a highly organized unit. Just as your body as a whole has important organs that carry out specific tasks, each cell also has individual parts-called organelles-for different functions. The most important organelle is the nucleus. This is the control centre for all that goes on in the cell. Another type of organelle, a sausage-shaped structure called a metochondrion, is like a power station. It transforms nutrients from food into energy that the cell can use. The cell's outer boundary, called the membrane, regulates the flow of substances into and out of the cell by processes such as osmosis. Inside the cell, many substances move around by a spreading-out process called diffusion (opposite).