circulation, reproduction, excitability, and movement. Protoplasm, thus, has often been called “the secret of life.” A typical cell is made up of the plasma membrane, the nucleus, and the cytoplasm. The plasma membrane is a selectively permeable membrane surrounding the cell. In addition to holding the cell together, the membrane selectively controls the exchange   of   materials   between   the   cell   and   its environment by physical and chemical means. Gases (such   as   oxygen)   and   solids   (such   as   proteins, carbohydrates,   and   mineral   salts)   pass   through   the plasma membrane by a process known as  diffusion. The  nucleus  is  a  small,  dense,  usually  spherical body that controls the chemical reactions occurring in the  cell.  The  substance  contained  in  the  nucleus  is called  nucleoplasm. The nucleus is also important in the cell's reproduction, since genetic information for the cell is stored there. Every human cell contains 46 chromosomes, and each chromosome has thousands of genes that determine the cell's function. The   cytoplasm   is   a   gelatinous   substance surrounding the nucleus and is contained by the plasma membrane. The cytoplasm is composed of all of the cell protoplasm except the nucleus. The simplest living organism consists of a single cell.  The  amoeba  is  a  unicellular  animal.  The  single cell of such a one-celled organism must be able to carry on all processes necessary for life. This cell is called a simple   or   undifferentiated   cell,   one   that   has   not acquired distinguishing characteristics. In   multicellular   organisms,   cells   vary   in   size, shape, and number of nuclei. When stained, the various cell structures can be more readily recognized under a microscope. Other differences such as the number and type of cells can be seen with the aid of a microscope. Many  cells  are  highly  specialized.  Specialized  cells perform  special  functions  (e.g.,  muscle  cells,  which contract, and epithelial cells, which protect the skin). TISSUES LEARNING  OBJECTIVES: Identify   the types  of  tissues  in  the  human  body  and  their functions. Tissues are groups of specialized cells similar in structure  and  function.  They  are  classified  into  four main   groups:   epithelial,   connective,   muscular,   and nervous. EPITHELIAL TISSUE The lining tissue of the body is called epithelium. It forms the outer covering of the body known as the free surface of the skin. It also forms the lining of the digestive,  respiratory,  and  urinary  tracts;  blood  and lymph vessels; serous cavities (cavities which have no communication   with   the   outside   of   the   body,   and whose lining membrane secretes a serous fluid), such as the peritoneum or pericardium; and tubules (small tubes which convey fluids) of certain secretory glands, such  as  the  liver  and  kidneys.  Epithelial  tissues  are classified according to their shape, arrangement, and the   function   of   their   cells.   For   example,   epithelial tissues that are composed of single layers of cells are called “simple,” while cells with many layers are said to be “stratified.” In the following paragraphs we will discuss   the   three   categories   of   epithelial   tissue: columnar, squamous, and cuboidal. Columnar Epithelial Tissue Epithelial cells of this type are elongated, longer than they are wide. Columnar tissue is composed of a single layer of cells whose nuclei are located at about the same level as the nuclei in their neighboring cells (fig. 1-3). These cells can be located in the linings of the uterus, in various organs of the digestive system, and in the passages of the respiratory system. In the digestive system, the chief function of columnar tissue is the secretion of digestive fluids and the absorption of nutrients from digested foods. In certain areas (such as the nostrils, bronchial tubes, and trachea), this tissue has a crown of microscopic hairlike processes known as cilia. These cilia provide motion to move secretions 1-3 HM3F0103 Figure 1-3.—Columnar epithelial tissue.