C H A P T E R 7
Introduction to cell physiology
77
materials with no ionic charge move most freely through
the channels. Substances with a negative charge move
more freely than substances with a positive charge. Sub-
stances that move into and out of a cell by diffusion
include sodium, potassium, calcium, carbonate, oxygen,
bicarbonate and water.
When a cell is very active and is using energy and
oxygen, the concentration of oxygen within the cell
decreases. The concentration of oxygen outside the
cell remains relatively high, so oxygen moves across
the cell membrane (down the concentration gradient)
to supply needed oxygen to the inside of the cell. Cells
use this process to maintain homeostasis during many
activities that occur during their life.
Osmosis
Osmosis
, a special form of diffusion, is the movement of
water across a semipermeable membrane from an area
that is low in dissolved solutes to one that is high in dis-
solved solutes. The water is attempting to equalise the
dilution of the solutes. This diffusion of water across a cell
membrane from an area of high concentration (of water)
to an area of low concentration creates pressure on the
cell membrane called
osmotic pressure.
The greater the
concentration of solutes in the solution to which the water
is flowing, the higher is the osmotic pressure.
A fluid that contains the same concentration of
solutes as human plasma is called an
isotonic
solution.
A fluid that contains a higher concentration of solutes
than human plasma is a
hypertonic
solution; it draws
water from cells. A fluid that contains a lower concen-
tration of solutes than human plasma is
hypotonic
;
it
loses water to cells. If a human red blood cell, which
has a cytoplasm that is isotonic with human plasma, is
placed into a hypertonic solution, it shrinks and shrivels
because the water inside the cell diffuses out of the cell
into the solution. If the same cell is placed into a hypo-
tonic solution, the cell swells and bursts because water
moves from the solution into the cell (see Figure 7.5).
Facilitated diffusion
Sometimes a substance cannot move freely on its own
into or out of a cell. Such a substance may attach to
another molecule, called a carrier, to be diffused.
This form of diffusion, known as
facilitated diffu-
sion
, does not require energy, just the presence of the
carrier. Carriers may be hormones, enzymes or proteins.
Because the carrier required for facilitated diffusion is
usually present in a finite amount, this type of diffusion
is limited.
Active transport
Sometimes a cell requires a substance in greater con-
centration than is found in the environment around it
or needs to maintain its cytoplasm in a situation that
would normally allow chemicals to leave the cell. When
this happens, the cell must move substances against the
concentration gradient using active transport, which
requires energy. When a cell is deprived of oxygen
because of a blood supply problem or insufficient oxy-
genation of the blood, systems of active transport begin
to malfunction, placing the cell’s integrity in jeopardy.
One of the best-known systems of active transport is
the sodium–potassium pump. Cells use active transport
to maintain a cytoplasm with a higher level of potassium
and a lower level of sodium than the extracellular fluid
contains. This allows the cell to maintain an electrical
charge on the cell membrane, which gives many cells the
electrical properties of excitation (the ability to generate
a movement of electrons) and conduction (the ability to
send this stimulus to other areas of the membrane). Some
drugs use energy to move into cells by active transport.
Drugs are frequently bonded with a carrier when they
are moved into the cell. Cells in the kidney use active
transport to excrete drugs from the body, as well as to
maintain electrolyte and acid–base balances.
Hypertonic solution
A red blood cell placed
in hypertonic solution will
shrink and shrivel up
as water moves out of
the cell
Hypotonic solution
A red blood cell placed
in hypotonic solution will
swell and burst as water
moves into the cell
Isotonic solution
A red blood cell placed
in isotonic solution is
stable and will retain
its shape
FIGURE 7.5
Red blood cell, showing the cell’s response to hypertonic,
isotonic and hypotonic solutions.
