Figure 16. Diagram of the establishment of osmotic equilibrium when cells
are placed in a hypotonic A or a hypertonic B solution.
112. ACTIVE TRANSPORT
Active transport means the movement of materials through the cell membrane by
energyrequiring chemical processes rather than by simple diffusion.
a. Energy Expenditure. Active transport requires a considerable amount of
energy for two reasons:
Energy is required to initiate the carrier system and begin the chemical
(2) A considerable amount of energy is used to transport a substance from
a medium of low concentration to one of high concentration. An example is the
transport of potassium from the extracellular fluid to the intracellular fluid and sodium
from intracellular fluid to extracellular fluid by the sodium/potassium ion pump.
b. Concentration Control. There exists a control mechanism that determines
how much of a substance should be concentrated. This process also is genetically
113. TISSUE FLUID
a. General. Fluid outside the vascular system, bathing the cells, is interstitial
fluid. This extracellular fluid that constitutes the liquid environment of cells is composed
of the interstitial fluid and the circulating blood plasma. Capillaries provide a continuous