Cells are the basic building blocks of all living organisms. They are tiny structures that can only be seen under a microscope. Each cell has different parts, like the nucleus, which acts like a control center, and mitochondria, which provide energy. Cells can be specialized for various functions, such as muscle cells that help us move or nerve cells that transmit signals. There are two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells, like those found in bacteria, are simpler and do not have a nucleus. Eukaryotic cells, found in plants and animals, are more complex and contain a nucleus and other organelles. Together, these cells work to sustain life.