We are all familiar with the specialized structures our bodies possess that enable us to meet our various needs. We know that our jaws and teeth help us to process the food we eat and that our stomachs continue the digestive process; we know that our hearts pump blood and that our lungs enable us to absorb oxygen and release carbon dioxide. We are able to function because each of these specialized structures does its job.

Specialized structures are not unique to higher organisms like ourselves. In fact, every living cell—including single-celled organisms and the individual cells that make up higher organisms—contains its own set of structures, generally called organelles, that perform specific jobs and enable cells to serve their various functions. All cells must perform three basic functions: They must obtain and process energy; they must synthesize a wide variety of proteins; and they must replicate their genetic material and divide.

Mitochondria, a type of organelle, provide cells with the energy they need to drive all other functions. Because of this they are often referred to as the "power stations" of cells. The energy that mitochondria produce comes from a process called aerobic respiration, in which they burn food molecules in the presence of oxygen and release the chemical energy the food contains.

Other cell structures are dedicated to protein synthesis. These include ribosomes, which synthesize proteins, endoplasmic reticula, which store and transport proteins and other compounds, and Golgi bodies, which process and package proteins destined for other organelles or for transport outside the cell. Without any one of these structures, cells would be unable to perform their vital functions.

Although all of the organelles serve critical functions, one organelle—the nucleus—is arguably the most important. This structure contains a cell's genetic material, its DNA. DNA provides the instructions for building proteins, and, thus, dictates the structure and function of the cell, and the organism of which the cell is a part, throughout its life. DNA also provides a mechanism for passing genetic information on to the next generation. Through mitosis, cells replicate their DNA and then pass these complete sets of genetic material to their daughter cells when they divide. Structures called centrosomes play a role in this process by generating microtubules, which help to pull duplicate chromosomes apart.

Although most cells have many types of organelles, a cell's role in part determines the type and number of organelles it contains. For example, a cell that is heavily involved in protein synthesis will possess large numbers of protein-synthesizing ribosomes and an extensive endoplasmic reticulum. Cells that do a great deal of work, such as muscle cells, have many mitochondria to keep up with their high energy demands. And many plant cells contain chloroplasts that enable the cells to capture and convert the Sun's energy through photosynthesis.

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