Single-celled organisms are organized using specialized cell organelles, while several cells may organize into larger structures, such as tissues and organs. The single-celled organism has all it needs to live independently. Cells organized into more complex structures work together to perform a common function.
An example of a single-celled organism is the amoeba. It is made up of the nucleus, which controls reproduction and growth, a food vacuole that digests food, pseudopods that propel the amoeba, and a contractile vacuole that excretes water and waste. These systems are organized within the cytoplasm, a jelly-like substance. The entire animal is surrounded by the cell membrane. Amoebas reproduce by cell division, a process that is fairly uncomplicated.
Cells that organize in groups to form tissues and organs are remarkably similar to the amoeba. The main difference is that the like cells depend on a trigger, such as a hormone, to allow those cells to work together. One illustration is the reproductive system. The organized cells that make up the pituitary gland generate a hormone called LH, which travels to the cells of the ovary. This causes the cells of the ovary to create progesterone, which controls the thickness of the uterine lining and governs impregnation.