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Organelles are well defined large scale structures that have a particular set of functions in the cell. Some organelles such as cilia, centrioles, and ribosomes and the membrane infoldings of some prokaryotes are not isolated from the cytoplasm of the cell. But in eukaryotes, many of the organelles are what are called "membrane bound" organelles: that is, organelles completely surrounded by a plasma membrane, or even a double membrane.
Indeed, the concept of membrane bound organelles is so important that many texts restrict the definition of organelles to mean membrane bound structures within a cell. However, this leaves out important structures in prokaryotes such as the bacterial flagella and membrane infoldings found in bacteria.* Also, people who study bacteria usually consider such non membrane bound structures as the ribosome to be organelles. So here we will not restrict organelles to membrane bound structures.
Be that as it may, membrane bound organelles are extremely important in the organization of eukaryotic cells. These organelles allow different sets of chemical reactions to be separated from each other so that they do not interfere. Its much like a chemical factory where the different chemicals are kept in separate vats and the different reaction pathways involved in manufacturing compounds are kept isolated from each other. In the compartmentalization of the cytoplasm by membrane bound organelles not only prevents interference between different reaction pathways, but allows the cell to provide radically different environments that allow each reaction to operate most efficiently.
*If your instructor insists on defining organelles as being membrane bound, humor him or her: the concept is what's important and membrane bound organelles are extremely important from a eukaryote point of view.
pgd revised 02/02/00