30 Chapter 2
health AlertAcid RainWe are all aware of the term “acid rain.”
Excess industrial pollutants emitted into the air
from coal-fired power plants and automobiles
can change the pH of our en-vironment. These
pollutants fall back to the earth as acid
precipitation (rain, snow, or fog). This acid rain
can cause respiratory problems when
breathed in or gastric prob-lems when
reaching the stomach and diges-tive systems.
We are aware of how serious this problem can
be when we see lakesMany biologic membranes are selectively per-meable,
such as the membranes of cells. The effects of osmosis on
red blood cells can easily be demon-strated (see Figure 2-
14). If a red blood cell is placed in a normal saline solution
(an isotonic solution) where the salt concentration
outside the red blood cell equals the salt concentration
inside the red blood cell, water molecules will pass into and
out of the red blood cell at an equal rate, and there will be
no observed change in the shape of the red blood cell (see
Figure 2-14A). If, however, the red blood cell is placed in
pure distilled water (a hypotonic solution) where the
water molecules are in a higher concentra-tion outside the
red blood cell, water will move into the red blood cell,
causing it to swell and eventu-ally rupture (see Figure 2-
14B). If the red blood cell is placed in a 5% salt solution (a
hypertonic solution) where there is more water inside
the red blood cell than in the solution, the red blood cell
will lose water to the solution and will shrivel up or
crenulate (see Figure 2-14C).
Because blood in the circulatory system is under
pressure due to the beating of the heart, much blood plasma
(the fluid part of blood, which is predominantly water with
dissolved and colloidal suspended materials in it) is lost
into surrounding tissues in the highly perme-able one-cell-
thick capillaries. Colloidally suspended proteins in the
blood cannot pass through the capillary cell membranes;
thus, they cause an osmotic pressure
whose fish have all been killed or whole
forests destroyed near industrial plants with
high pollution rates. Acid precipitation is of
global concern.
Buildings and monuments made of
limestone (calcium carbonate) are eas-ily
eroded by even weak acids. The United States
passed and implemented the Clean Air Act of
1990 to help lower the levels of acid
precipitation to protect our health and our
environment.large enough to reabsorb most of the fluid that escapes
from the capillaries.^
Although water and a few other substances with small
molecular weights can osmose into the cells that need them,
osmotic transportation is insufficient for most of the cell’s
needs. Sugars, amino acids, larger proteins, and fats are
needed by the cell to produce ATP and to maintain and
create structure. Cells obtain these nonosmotic or
nondiffusable materials by a special mechanism called
active transport. This mechanism, however, needs
energy in the form of ATP to overcome the
osmotic/diffusional barriers—another major reason that
ATP is so important to a cell’s survival. Active transport is
the transportation of materials against a concentration
gradient or in opposition to other factors that would
normally keep the material from entering the cell.
Molecules move from an area of low concentration to an
area of high concentration (like a food vacuole).pH
pH is defined as the negative logarithm of the hydrogen ion
concentration in a solution: pH 5 2log [H^1 ]. Pure water has
a pH of 7. Remember that when distilled water (H 2 O)
dissociates, for every H^1 ion formed, an OH^2 ion is also
formed. Or, in other words, the dissociation of water
produces H^1 and OH^2 in equal amounts. Therefore, a pH of
7 indicates neutrality on the pH scale. Figure 2-15 shows
the pH of various solutions.