Cells are some of the most diverse structures in the world. A cell may exist as a lone organism, free-floating and independent. Or a cell may be one small part of a complex, integrated system. In a multicellular organism, cells can vary widely in size and structure. That's because the structure of a cell is suited to its function, and different types of cells have different functions. For example, nerve cells have long thread-like projections that receive and transmit electrical signals throughout the body. Some skin cells, by contrast, look somewhat like shingles and perform a similar protective function for the body.
For all their differences, however, most cells have three critical parts in common: the nucleus, the cytoplasm, and the membrane. The nucleus is arguably the most important structure for many cells. While some single-celled organisms including bacteria have no nucleus (their single chromosome floats freely in the cytoplasm), nearly all other cells do. The nucleus contains the cell's DNA. This genetic material provides the instructions for building proteins and, thus, dictates the structure and function of the cell throughout its life. Even more important, the DNA provides a means of passing genetic information to the next generation of cells. Through mitosis, cells replicate their DNA and then pass these complete sets of genetic material to their offspring, the daughter cells created when they divide.
The fluid-filled region between the nucleus and the cell membrane is called the cytoplasm. For many years scientists thought that very little happened there; they couldn't have been more wrong. Despite its modest appearance, cytoplasm provides the stage for some of the cell's most important functions, including protein synthesis, energy production, and cell growth and replication. This is because cytoplasm is packed with organelles that are critical to a cell's survival, including mitochondria, ribosomes, Golgi bodies, and the endoplasmic reticulum.
In some types of cells, cytoplasm is also used to move the cell itself or to move organelles within the cell. In plants this process is called cytoplasmic streaming. It allows plants to circulate their choroplasts and position them for optimal exposure to sunlight. Amoebas and certain types of white blood cells use a similar process, squeezing their cytoplasm in one direction or another as a means of locomotion.
Finally, the cell membrane gives the cell its structure. However, it has another function that is far more complex. In order for a cell to survive, it needs to take in water, food, oxygen, and other materials. It also needs to expel excess water, wastes, and the substances it produces that other cells in the body may need. The membrane controls what materials pass into and out of the cell.