The human body is made up of trillions of cells. The website Nature's Scitable explains that cells are the basic structural and functional unit of life and come in many different shapes and sizes depending on the job that they are supposed to do. Tissues and organs are made up of aggregates of cells that all perform the same task. Cells are able to function because they contain specialized structures called organelles. The organelles found in most animal cells include the plasma membrane, the nucleus, the endoplasmic reticulum, the golgi apparatus, and mitochondria.

The plasma membrane is what separates the inside of a cell from its surrounding environment. It houses the cell's other organelles and its liquid, known as cytoplasm. "Molecular Cell Biology" explains that the plasma membrane is semi-permeable, meaning that certain ions and small molecules are able to cross in and out of the cell while others cannot. This property allows the cell to regulate its internal conditions such as salt concentration and pH.

The nucleus is the control center of the cell and it is where the genetic information or DNA is stored. Nature's Scitable notes that the nucleus is surrounded by its own membrane, the nuclear envelope. Like the plasma membrane, the nuclear envelope is semi-permeable, allowing the passage of only certain ions and proteins. Inside the nucleus is chromatin, DNA associated with proteins. The functions of the cell are carried out by transcription of the DNA within the nucleus to messenger RNA. The mRNA then exits the nucleus into the cytoplasm, where it is translated into protein by ribosomes. Ribosomes themselves are manufactured by a specialized organelle within the nucleus called the nucleolus.

According to "The Cell: A Molecular Approach," the endoplasmic reticulum, or ER, is an organelle that forms a membranous, interconnected network of tubules and sac-like structures called cisternae. Endoplasmic reticulum comes in two types. The rough enoplasmic reticulum has protein-synthesizing ribosomes bound to its membrane. Proteins synthesized in the RER are secreted by the cell for use elsewhere in the body. The smooth endoplasmic reticulum does not have ribosomes bound to its surface. The function of the SER is to synthesize lipids and steroids, as well as to detoxify potentially harmful molecules. The SER is also important for carbohydrate metabolism.

"The Cell: A Molecular Approach" notes that the Golgi apparatus is a stacked, membranous structure that functions to modify and package proteins in order to prepare them for transport out of the cell. Proteins manufactured in the rough endoplasmic reticulum enter the Golgi apparatus and are packed into vesicles capable of fusing with the plasma membrane in order to facilitate transport of the protein out of the cell. The Golgi apparatus also synthesizes lysosomes. Lysosomes are vesicles packed with enzymes needed to digest proteins and sugar within the cell.

Nature's Scitable explains that mitochondria are the energy sources of a cell. These small membrane-bound organelles are the site of nutrient breakdown and adenosine triphosphate, or ATP, synthesis. ATP is a molecule sometimes referred to as the “energy currency” of a cell. It is a co-enzyme required for many of the metabolic functions of a cell. The number of mitochondria found in a cell can vary greatly depending on the cell's function.