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. To understand
the function of organs and other structures of the body, it
is essential that we first understand the basic organization
of the cell and the functions of its component parts.
Organization of the Cell
A typical cell, as seen by the light microscope, is shown
in Figure 2-1. Its two major parts are 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 percent. Many cellular chemicals are
dissolved in the water. 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.
Ions. Important ions in the cell include potassium, magnesium, phosphate, sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium. These are all discussed
in more detail in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids.
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...