A Typical Cell

To discover what each part of the cell does, move your mouse pointer over it.

A fertilized egg is the result of a women's ovum uniting with a male sperm. While there are unique features of a fertilized egg, this first cell of life is also typical of the billions of cells that will eventually make up the human body. The typical cell has a variety of parts, many of which are involved in the formation of cancer.

Components of the Typical Cell

The cell membrane surrounds the cytoplasm of the cell, separating the cell from its surroundings. The cell membrane allows:

The cell membrane is primarily a double layer of lipids (fats) with proteins within the membrane, some of which protrude through one side or the other (external or internal to the cell itself). Some proteins at the surface, protruding out from the cell, participate in communications between cells or serve as receptors, receiving chemical signals that are passed to the internal portion of the cell. Receptors are a class of transmembrane proteins that have an active site that recognizes a ligand on the exterior side of the membrane. When a ligand binds to the receptor, the protein changes its shape or the protein moves to the interior, triggering other changes in the cell. This process is called signal transduction and often serves as a part of a cascade of cellular events that contribute to the cell cycle, an essential aspect of both normal and cancerous cells.

Ribosomes are granular-appearing bodies that line the endoplasmic reticulum; they are comprised primarily of RNA and some proteins. They are the site of translation, the process of decoding mRNA into proteins based on genetic information passed from DNA to mRNA.

Endoplasmic reticulum, or ER, is a convoluted sheet of membranes comprised of lipids (fats) and some proteins that extends from near the nuclear membrane and winds throughout the cell. The ER is often lined with ribosomes, where the translation of mRNA takes place. When ribosomes are present, the ER is referred to as rough endoplasmic reticulum; when they are absent, it is termed smooth endoplasmic reticulum. The inner portion of the ER, called the lumen, serves as a "tunnel" throughout the interior of the cell; proteins that are synthesized at the ribosomes traverse the lumen before some are modified or packaged in vesicles for "export" out of the cell.

The nuclear membrane surrounds a dense body within the cell, the nucleus, and allows some molecules within the nucleus to pass through to the cytoplasm while keeping a variety of molecules in the cytoplasm excluded from the nucleus.

The Golgi apparatus is found between the endoplasmic reticulum and the cell membrane. It functions to modify and package proteins for exit from the cell to the bloodstream for use by other cells or distribution within the same cell. This process of directing proteins to their final destination is called protein sorting or trafficking.

Lysosomes are membrane-bound organelles within the cell that hold enzymes (specialized proteins) that digest and break down cellular materials; lysosomes have been described as the "garbage bags" of the cell.

The nucleus is a dense, membrane-bound body within the cell where the chromosomes reside as well as the molecular apparatus that participates in the replication of DNA, the first steps in the processing of genetic information, transcription, and cell division. The nucleus is surrounded by a nuclear membrane that maintains the integrity of the nucleus.

The nucleolus is a vesicle within the nucleus, associated with specific parts of chromosomes; particular forms of RNA are transcribed within the nucleoli.

Centrioles are spindle-like structures that guide the separation of replicated chromosomes during the latter stages of mitosis in the process of cell duplication and division.

Mitochondria are complex organelles in the cytoplasm that are the sites for the production of cellular energy, primarily in the form of a molecule called ATP (adenosine triphosphate). Mitochondria are surrounded by a double membrane and contain their own, circular DNA that codes for some of the proteins that function in energy production. Mitochondria are passed only from mother to offspring via the mother's oocytes (eggs) and are also believed to have been derived from free-living organisms in the course of evolution.

Vacuoles are membrane-bound vesicles in the cytoplasm that are formed by "in-pocketing" of the cell's membrane, particularly as part of an immune response to foreign substances, such as bacteria. Vacuoles may fuse with lysosomes as part of the process to destroy bodies and substances that the cell recognize as foreign.

Peroxisomes are membrane-enclosed bodies that contain enzymes, such as catalases, that are responsible for a series of oxidizing (energy-producing) reactions.


Last updated Wednesday, February 21, 2001.
© 2001 by PRR, Inc. All rights reserved.