Chapter 2

• Each of the 100 trillion cells in a human being is a living structure that can survive for months or many years, provided its surrounding fluids contain appropriate nutrients.
• Understanding the function of organs and other structures of the body requires understanding the basic organization of the cell and the functions of its component parts.
• A typical cell, as seen by the light microscope, is composed 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 is the principal fluid medium of the cell, present in most cells, except for fat cells, in a concentration of 70 to 85 percent.
• Many cellular chemicals are dissolved in the water, while others are suspended in the water as solid particulates.
• Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes.
• Important ions in the cell include potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium.
• These ions provide inorganic chemicals for cellular reactions and are necessary for operation of some of the cellular control mechanisms.
• Electrochemical impulses in nerve and muscle fibers are mediated by proteins.
• Proteins can be divided into two types: structural proteins and functional proteins.
• 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.
• Enzymes, which are the main proteins in the cell, are mainly the enzymes of the cell and are often mobile in the cell fluid.
• Many of the enzymes are adherent to membranous structures inside the cell and come into direct contact with other substances in the cell fluid, catalyzing specific intracellular chemical reactions.
• Structural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules.
• Nucleoplasm, cytoplasm, nucleus, nucleolus, cell membrane, nuclear membrane, and cell are the structures of the cell.
• Proteins are the most abundant substances in most cells, constituting 10 to 20 percent of the cell mass.
• The functional proteins are an entirely different type of protein, usually composed of combinations of a few molecules in tubular-globular form.
• Microtubules, which provide the “cytoskeletons” of 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, are a prominent use of such intracellular filaments.
• Fibrillar proteins, which are found especially in the collagen and elastin fibers of connective tissue and in blood vessel walls, tendons, ligaments, and so forth, are also extracellularly.
• 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 percent 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 percent 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 have little structural function in the cell except as parts of glycoprotein molecules, but they play a major role in nutrition of the cell.
• Most human cells do not maintain large stores of carbohydrates; the amount usually averages about 1 percent of their total mass but increases to as much as 3 percent in muscle cells and, occasionally, 6 percent in liver cells.
• Carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell.
• A small amount of carbohydrate is 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.
• The cell is not merely a bag of fluid, enzymes, and chemicals; it also contains highly organized physical structures, called intracellular organelles.
• The physical nature of each organelle is as important as the cell’s chemical constituents for cell function.
• Without one of the organelles, the mitochondria, more than 95 percent of the cell’s energy release from nutrients would cease immediately.
• Cholesterol molecules in the membrane are lipid in nature because their steroid nucleus is highly fat soluble.
• Cholesterol molecules in the membrane are dissolved in the lipid bilayer and 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.
• Membrane proteins, most of which are glycoproteins, occur in two types: 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 watersoluble substances, especially ions, can diffuse between the extracellular and intracellular fluids.