Just as the human body has individual organs the carry out specific functions and work together, the body's cells have parts dedicated to specific functions. These parts are called organelles, which means "little organs." Many of the organelles can be thought of as analogous to organs: For example, the plasma membrane is like the cell's skin. There are several types of organelles, each with a particular structure, function, method of interacting with other organelles, and potential problems.
Types
The covering that wraps around the cell is called the plasma membrane. Within the plasma membrane is the liquid of the cell, called the cytosol or cytoplasm. The control center is called the nucleus. Other types of organelles found in most cells include ribosomes, the cytoskeleton, smooth and rough endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes and peroxisomes.
Structure
There is great variability in the size, complexity and composition of organelles. For example, the nucleus is one of the larger structures at 5 to 10 micrometers; as a point of reference, one thousand micrometers make up a centimeter. In contrast, a ribosome is about 300 times smaller. Each of the other organelles has a structure that is specific to its function.
Function
The nucleus is the control center of the cell. DNA in the nucleus is transcribed into RNA, and then enzymes shuttle the RNA out of the nucleus and into the cytoplasm through a nuclear pore. The ribosomes translate the RNA into proteins, which have structures and functions specific to the cell. Proteins can remain in the cytoplasm or go to the rough endoplasmic reticulum and Golgi apparatus for further folding and modification. The function of the smooth endoplasmic reticulum depends on the cell type, but it generally contains enzymes to modify molecules in the cell. Liver cells have extensive smooth endoplasmic reticulum to process ingested molecules of food and drugs. The peroxisomes and lysosomes digest old parts of the cell and can play a role in fighting germs. All this activity is fueled by energy supplied by the mitochondria.
Interaction
According to "Molecular Biology of the Cell" by Bruce Alberts, the organelles have specific functions and coordinate their activities as needed. They may communicate from nearby or across the cell. When ribosomes are making protein from RNA, they may touch the rough endoplasmic reticulum, delivering the proteins directly into the organelle. If a virus enters the cell, the nucleus can release factors that tell the organelles to fight off the infection, or even signal the mitochondria to activate lysosomes and enzymes, which kill the cell in order to destroy the infection in the body.
Problems
Various problems occur in the organism or cell when organelles do not function properly. For example, according to the Merck Manual, lysosomal storage diseases are a class of disorders in which the lysosomes fail to break down and digest materials. The result is that unwanted molecules build up and damage the cells. Similarly, as they are supply of fuel for the cell, problems in the mitochondria can also cause disease. For example, in a condition called Leber's hereditary optic neuropathy, the mitochondria within the nerves of the eye do not supply enough energy to the cells and the person loses vision.
References
- National Institutes of Health: National Center for Biotechnology Information: What is a Cell
- Chart of Actual Organelle Sizes
- "Molecular Biology of the Cell"; Bruce Alberts; 5th Ed. 2007
- The Merck Manuals: Lysosomal Storage Disorders
