circulation, reproduction, excitability, and movement.Protoplasm, thus, has often been called “the secret oflife.”A typical cell is made up of the plasma membrane,the nucleus, and the cytoplasm.The plasma membrane is a selectively permeablemembrane surrounding the cell. In addition to holdingthe cell together, the membrane selectively controls theexchange of materials between the cell and itsenvironment by physical and chemical means. Gases(such as oxygen) and solids (such as proteins,carbohydrates, and mineral salts) pass through theplasma membrane by a process known as diffusion.The nucleus is a small, dense, usually sphericalbody that controls the chemical reactions occurring inthe cell. The substance contained in the nucleus iscalled nucleoplasm. The nucleus is also important inthe cell's reproduction, since genetic information forthe cell is stored there. Every human cell contains 46chromosomes, and each chromosome has thousands ofgenes that determine the cell's function.The cytoplasm is a gelatinous substancesurrounding the nucleus and is contained by the plasmamembrane. The cytoplasm is composed of all of thecell protoplasm except the nucleus.The simplest living organism consists of a singlecell. The amoeba is a unicellular animal. The singlecell of such a one-celled organism must be able to carryon all processes necessary for life. This cell is called asimple or undifferentiated cell, one that has notacquired distinguishing characteristics.In multicellular organisms, cells vary in size,shape, and number of nuclei. When stained, the variouscell structures can be more readily recognized under amicroscope. Other differences such as the number andtype of cells can be seen with the aid of a microscope.Many cells are highly specialized. Specialized cellsperform special functions (e.g., muscle cells, whichcontract, and epithelial cells, which protect the skin).TISSUESLEARNING OBJECTIVES:Identify thetypes of tissues in the human body and theirfunctions.Tissues are groups of specialized cells similar instructure and function. They are classified into fourmain groups: epithelial, connective, muscular, andnervous.EPITHELIAL TISSUEThe lining tissue of the body is called epithelium.It forms the outer covering of the body known as thefree surface of the skin. It also forms the lining of thedigestive, respiratory, and urinary tracts; blood andlymph vessels; serous cavities (cavities which have nocommunication with the outside of the body, andwhose lining membrane secretes a serous fluid), suchas the peritoneum or pericardium; and tubules (smalltubes which convey fluids) of certain secretory glands,such as the liver and kidneys. Epithelial tissues areclassified according to their shape, arrangement, andthe function of their cells. For example, epithelialtissues that are composed of single layers of cells arecalled “simple,” while cells with many layers are saidto be “stratified.” In the following paragraphs we willdiscuss the three categories of epithelial tissue:columnar, squamous, and cuboidal.Columnar Epithelial TissueEpithelial cells of this type are elongated, longerthan they are wide. Columnar tissue is composed of asingle layer of cells whose nuclei are located at aboutthe same level as the nuclei in their neighboring cells(fig. 1-3). These cells can be located in the linings ofthe uterus, in various organs of the digestive system,and in the passages of the respiratory system. In thedigestive system, the chief function of columnar tissueis the secretion of digestive fluids and the absorption ofnutrients from digested foods. In certain areas (such asthe nostrils, bronchial tubes, and trachea), this tissuehas a crown of microscopic hairlike processes knownas cilia. These cilia provide motion to move secretions1-3HM3F0103Figure 1-3.—Columnar epithelial tissue.
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