The Cell PDF

The cell The Cell The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. Although the many cells of the b...

The cell The Cell The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. Although the many cells of the body often differ markedly from one another, all of them have certain basic characteristics that are alike. Almost all cells also have the ability to reproduce additional cells of their own kind.  About 60 per cent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid The extracellular fluid contains the ions and nutrients needed by the cells to maintain cell life.  Cells are capable of living, growing, and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this environment (i.e. extracellular fluid). Structure of the Cell Cell Structure consists of two major parts: the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the surrounding fluids by a cell membrane, also called the plasma membrane. The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates Water The principal fluid medium of the cell is water, which is present in most cells, except for fat cells, in a concentration of 70 to 85 per cent. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates Ions Ions are the most important ions in the cell are potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium. The ions provide inorganic chemicals for cellular reactions. ions acting at the cell membrane are required for transmission of electrochemical impulses in nerve and muscle fibers Proteins Proteins. After water, the most abundant substances in most cells are proteins, which normally constitute 10 to 20 per cent of the cell mass. These can be divided into two types: structural proteins and functional proteins.  Structural proteins Structural proteins are present in the cell mainly in the form of long filaments that themselves are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form microtubules that provide the “cytoskeletons” of such cellular organelles as cilia, nerve axons, the mitotic spindles of mitosing cells, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Extracellularly, fibrillar proteins are found especially in the collagen and elastin fibers of connective tissue and in blood vessel walls, tendons, ligaments, and so. Structural proteins Functional proteins The functional proteins are mainly the enzymes of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell.  The enzymes come into direct contact with other substances in the cell fluid and thereby catalyze specific intracellular chemical reactions. For instance, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes. Lipids Lipids. Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids are phospholipids and cholesterol, which together constitute only about 2 per cent of the total cell mass. The significance of phospholipids and cholesterol is that they are mainly insoluble in water and, therefore, are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments.  In addition to phospholipids and cholesterol, some cells contain large quantities of triglycerides, also called neutral fat. In the fat cells, triglycerides often account for as much as 95 per cent of the cell mass. The fat stored in these cells represents the body’s main storehouse of energy-giving nutrients that can later be dissoluted and used to provide energy wherever in the body it is needed. Carbohydrates Carbohydrates. Carbohydrates have little structural function in the cell except as parts of glycoprotein molecules, but they play a major role in nutrition of the cell. Carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell.  Also, a small amount of carbohydrate is always stored in the cells in the form of glycogen, which is an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cells’ energy needs. Cell Membrane Cell Membrane The cell membrane (also called the plasma membrane), which envelops the cell, is a thin elastic structure only 7.5 to 10 nanometers thick. It is composed mostly of proteins and lipids. The approximate composition is proteins, 55 per cent; phospholipids, 25 per cent; cholesterol, 13 per cent; other lipids, 4 per cent; and carbohydrates, 3 per cent.  the structure of the cell membrane. Its basic structure is a lipid bilayer, which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule is soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic  The hydrophilic phosphate portions constitute the two surfaces of the complete cell membrane, in contact with intracellular water on the inside of the membrane and extracellular water on the outside surface. The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.  The cholesterol molecules in the membrane are also lipid in nature because their steroid nucleus is highly fat soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well.  Cell Membrane Proteins Proteins are globular masses floating in the lipid bilayer. Most of these membrane proteins are glycoproteins. Two types of proteins occur: integral proteins that protrude all the way through the membrane, and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.  Many of the integral proteins provide structural channels (or pores) through which water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others.  Other integral proteins act as carrier proteins for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their natural direction of diffusion, which is called “active transport.” Still others act as enzymes.  Integral membrane proteins can also serve as receptors for water- soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell.  In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. Peripheral protein molecules are often attached to the integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through the cell membrane “pores.” Cell Membrane Carbohydrates Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral proteins are glycoproteins, and about one tenth of the membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude to the outside of the cell, hanging outward from the cell surface.  Many other carbohydrate compounds, called proteoglycans— which are mainly carbohydrate substances bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the glycocalyx. Functions of membrane carbohydrates The carbohydrate moieties attached to the outer surface of the cell have several important functions: (1) Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negative objects. (2) The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.  (3) Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes. (4) Some carbohydrate moieties enter into immune reactions, play a key role in cell-cell recognition (ability of a cell to distinguish one cell from another) that represent basis for rejection of foreign cells by immune system. (5) Carbohydrates are important in organ & tissue development.