2-13. MOVEMENT OF MATERIALS INTO AND OUT OF THE CELL
We noted earlier that all substances that enter or leave the cell must pass
through the cell membrane in some way.
a. Semipermeability. The permeability of a membrane is its capacity to allow
materials to move through it. Since the cell membrane of animal cells is selective and
does not allow all materials to pass through it, we say that it is semipermeable (noun:
b. Diffusion. Some materials readily pass through the membrane from an area
of higher concentration to an area of lower concentration. This process is called
diffusion. When materials require help to pass through the cell membrane, the process
is referred to as facilitated diffusion.
c. Active Transport. In certain situations, materials pass through the cell
membrane against the concentration gradient. In this case, an expenditure of energy is
required. The process is called active transport. An example is the sodium/potassium
pump, in which the sodium ions are forced out of the cytoplasm of the cell and into the
surrounding tissue fluid and potassium ions are pumped back into the cell cytoplasm.
d. Osmosis. Sometimes a substance is not able to pass through the cell
membrane. When the concentration of this substance is greater on one side of the cell
membrane than the other, water will tend to pass through the membrane to the area of
greater concentration. This process is called osmosis. This process involves the
concept of tonicity, discussed in paragraph 2-12.
e. Pinocytosis and Phagocytosis. Sometimes, the cell membrane will engulf a
minute amount of tissue fluid and its contents. This process is called pinocytosis.
During pinocytosis, the cell membrane produces a vacuole to contain the engulfed
material. When the cell membrane engulfs larger particles, such as bacteria or other
cells, the process is called phagocytosis. After either pinocytosis or phagocytosis,
digestive fluids may pass from the cytoplasm into the vacuole. The end products of
digestion are absorbed from the vacuole into the cell cytoplasm.
2-14. MEMBRANE POTENTIALS
In living cells, there is generally a higher concentration of positively charged ions
on the outside of the cell and a higher concentration of negatively charged ions on the
inside of the cell. Thus, there is a concentration gradient (an electrical potential or
polarity) across the membrane that we call the membrane potential that creates an
a. Resting Potential. When the cell is in a resting state, the membrane
potential is maintained by the sodium/potassium pump. The sodium/potassium pump
actively transports 3 positive sodium ions (Na+) to the outside of the cell membrane and