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Revolutionary advances in the understanding of cell structure and function have been made through use of the techniques of modern cellular and molecular biology. There have also been major advances in the study of embryology and development at the cellular level. Developmental biology and the details of cell biology are beyond the scope of this book. However, a basic knowledge of cell biology is essential to an understanding of the organ systems in the body and the way they function. The specialization of the cells in the various organs is very great, and no cell can be called "typical" of all cells in the body. However, a number of structures (organelles) are common to most cells. These structures are shown in Figure 1-4. Many of them can be isolated by ultracentrifugation combined with other techniques. When cells are homogenized and the resulting suspension is centrifuged, the nuclei sediment first, followed by the mitochondria. High-speed centrifugation that generates forces of 100,000 times gravity or more causes a fraction made up of granules called the microsomes to sediment. This fraction includes organelles such as the ribosomes and peroxisomes. Cell Membrane The membrane that surrounds the cell is a remarkable structure. It is made up of lipids and proteins and is semipermeable, allowing some substances to pass through it and excluding others. However, its permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. It is generally referred to as the plasma membrane. The nucleus is also surrounded by a membrane of this type, and the organelles are surrounded by or made up of a membrane. Although the chemical structure of membranes and their properties vary considerably from one location to another, they have certain common features. They are generally about 7.5 nm (75 Angstrom units) thick. They are made up primarily of protein and lipids. The chemistry of proteins and lipids is discussed in Chapter 17. The major lipids are phospholipids such as phosphatidylcholine and phosphatidylethanolamine. The shape of the phospholipid molecule is roughly that of a clothespin (Figure 1-5). The head end of the molecule contains the phosphate portion and is relatively soluble in water (polar, hydrophilic). The tails are relatively insoluble (nonpolar, hydrophobic). In the membrane, the hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane. In prokaryotes (cells such as bacteria in which there is no nucleus), the membranes are relatively simple, but in eukaryotes (cells containing nuclei), cell membranes contain various glycosphingolipids, sphingomyelin, and cholesterol. There are many different proteins embedded in the membrane. They exist as separate globular units and many pass through the membrane (integral proteins), whereas others (peripheral proteins) stud the inside and outside of the membrane (Figure 1-5). The amount of protein varies with the function of the membrane but makes up on average 50% of the mass of the membrane; ie, there is about one protein molecule per 50 of the much smaller phospholipid molecules. The proteins in the membranes carry out many functions. Some are cell adhesion molecules that anchor cells to their neighbors or to basal laminas. There are proteins that function as pumps, actively transporting ions across the membrane. Other proteins function as carriers, transporting substances down electrochemical gradients by facilitated diffusion. Still others are ion channels, which, when activated, permit the passage of ions into or out of the cell. The role of the pumps, carriers, and ion channels in transport across the cell membrane is discussed below. Proteins in another group function as receptors that bind neurotransmitters and hormones, initiating physiologic changes inside the cell. Proteins also function as enzymes, catalyzing reactions at the surfaces of the membrane. In addition, some glycoproteins function in antibody processing and distinguishing self from nonself (see Chapter 27).
# Endoplasmic Reticulum (ER): Introduction - The ER is a system of membranous tubules and sacs - Within the cytoplasm of most animals cells in an extensive network of membrane – limited channels, collectively called Endoplasmic reticulum. - The outer surface of rough endoplasmic reticulum has attached ribosomes, where as smooth endoplasmic reticulum don‘t have. - Functions of smooth ER include lipid metabolism (Both catabolism and anabolism) glycogenolysis (degradation of glycogen) and drug detoxification. On their membranes, rough ER contain certain ribosome‘s – specific, transmembrane glycoprotein‘s called ribophorins I & II, to which are attached the ribosome‘s while engaged in polypeptide synthesis. Rough ER pinches off certain tiny protein- filled vesicles which ultimately get fused to Cis-Golgi. RER also synthesize membrane and glycoprotein‘s which are co translationally inserted into rough ER membranes. - Thus ER is the site of biogenesis of cellular membranes‘.
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