Membrane Bound Organelles
Eukaryotes Have Compartments for Different Cellular Functions
The major difference between a eukaryotic cell and a prokaryotic cell is the presence of membrane bound organelles. Derived from the Greek “eu” meaning “true” and “karyon” meaning “nut”, the “true nut” of a eukaryotic cell is the nucleus, the cellular storehouse for DNA. A eukaryotic cell maintains its chromosomes and many supporting proteins in its nucleus. And besides the nucleus there are several more membrane bound, or membrane delimited, organelles that serve specialized functions. Some of the major ones are described below.
The Mitochondrion
Cells need energy in the form of ATP (adenosine triphosphate) to survive and carry out all of their routine functions. They must generate their energy in this specialized, double membrane bound organelle. Unlike other organelles, mitochondria (the plural form) have both an outer membrane and an inner membrane. These different membranes are laden with enzyme complexes that use an electrochemical gradient to help produce ATP for the cell to use.
The Lysosome
When certain classes of proteins (most often proteins that are inserted in the membranes of the cell), and other complex molecules such as carbohydrates and lipids, are finished serving their specific function or have just simply worn out they are degraded so that the cell can recycle some of their component parts. This is largely undertaken in the lysosomes. Lysosomes are cytoplasmic vacuoles that contain numerous hydrolases (enzymes that break things down). They are also sensitive to pH. The enzymes work at acidic pH, a feature that keeps the enzymes inactive until the lysosome is activated by hydrogen ions being pumped across its membrane. White blood cells use lysosomes to digest bacteria and viruses that they have engulfed.
The Endoplasmic Reticulum and Golgi Apparatus
These organelles are heavily involved with the processing and readying of cellular proteins that either need to be secreted from the cell (such as peptide hormones) or will reside in one of the cell membranes (like cell surface receptors). Attached to the nuclear membrane, the endoplasmic reticulum (or ER), has the machinery to help complex membrane proteins fold to the correct shape and it can begin the addition of sugars and other molecules to proteins. From the ER, membrane and secreted proteins are moved to the Golgi apparatus (this is relatively easy as the Golgi is attached to the ER), where final processing and modification can take place before the protein heads to its final site for use or secretion.
Endosomes
This class of organelle can be involved in several different processes. They can help in the sorting of proteins taken in from the cell surface by endocytosis. Endosomes can also fuse with lysosomes to deliver a cargo of proteins or other molecules for degradation. They can also serve to direct newly made membrane or vesicular proteins to their site of action.
To learn more about the function of organelles and their behaviors visit the University of Texas Medical Branch Graduate Program in Cell Biology pages or for informative, fun, interactive cartoons of organelles visit Cells Alive.
A Photograph of a Nerve Cell
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