CH.YBW.CTS.C04.Final.q 6/21/03 1:04 PM Page 47
cells and glycogen granules in liver cells are both examples of inclusions that store energy. Mucus and pigments, such as melanin in skin and hair cells, are inclusions that contain cell products. Organelles (“little organs�) are the structures that actually carry out particular functions for the cell as a whole. Loosely speaking, they are analogous to organs, which carry out complex and specific functions for the entire organism. Some organelles, such as the cytoskeleton, ribosomes, and centrioles, lack cell membranes. However, most organelles are surrounded by a cell membrane, similar in composition and function to the plasma membrane. These include the mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes. Ribosomes Ribosomes are tiny, round, non-membranous structures
made of proteins and RNA. They are the actual sites of protein synthesis within cells. In a sense, they can be thought of as a zipper, allowing appropriate amino acids to be linked
DID YOU KNOW? Cell membranes permit organelles to maintain an internal environment different from the surrounding cytosol. In fact, this compartmentalization is absolutely crucial to cell functioning and is arguably the major difference between biology and chemistry. For example, if all the chemicals found in an organelle, such as the mitochondria, were placed in vitro (in a test tube), only a small number of appropriate metabolic reactions would occur. This is because cell membranes are necessary for the constituents of an organelle to be appropriately separated or appropriately mixed in order to function as they do in vivo (in the living cell). Further, membrane compartmentalization is necessary to prevent the thousands of cellular enzymes and chemicals from randomly mixing, which would cause chaos.
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