A typical cell, as seen by the light microscope has 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: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. Also, they are necessary for operation of some of the cellular control mechanisms For instance, ions acting at the cell membrane are required for transmission of electrochemical
impulses in nerve and muscle fibers.
Proteins: After water, the most abundant substances in most cells are proteins, which normally constitute 10 to 20 % of the cell mass.
These can bedivided into two types: structural proteins and functional proteins.
Structural proteins are present in the cell mainly in the form of long filaments that themselves are polymers of many individual protein molecules.
The functional proteins are an entirely different type of protein, usually composed of combinations of a few molecules in tubular-globular form.
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 %of the total cell mass. 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 %of the 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.
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.
However, carbohydrate in the form of dissolved glucose is always present in surrounding extracellular fluid so that it is readily available to the cell. Also, 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 only contain 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. For instance, without one of the organelles, the mitochondria, more than 95 per cent of the cell’s energy release from nutrients would cease immediately.
These membranes include the cell membrane, nuclear membrane, membrane of the endoplasmic reticulum, and membranes of the mitochondria, lysosomes, and Golgi apparatus.
The lipids of the membranes provide a barrier that impedes the movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules in the membrane often do penetrate all the way through the membrane, thus providing specialized pathways, often organized into actual pores, for passage of specific substances through the membrane.
The cell membrane (also called the plasma membrane), which envelops the cell, is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids.The approximate composition is proteins, 55 %; phospholipids, 25 %; cholesterol, 13 %; other lipids, 4 %; and carbohydrates, 3 %.
Lipid Barrier of the Cell Membrane Impedes Water Penetration
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 lipid layer in the middle of the membrane is impermeable to the usual