When the cell is in a resting state, the membrane potential is maintained by the
sodium/potassium pump. This membrane potential is called the resting potential. The
sodium/potassium pump actively transports three positive sodium ions to the outside of
the cell membrane and two potassium ions to the inside of the cell membrane. This
results in a negative charge inside the cell and a positive charge outside the cell.
When a neuron or muscle fiber is stimulated, the resulting activity is called the
action potential. This involves depolarization and subsequent repolarization. First,
sodium ions move into the cell by diffusion. This reverses the polarity. Second,
potassium moves out of the cell by diffusion that causes repolarization. (para 2-14)
A condition in which tissue mass increases without an increase in the number of
cells is called hypertrophy. When tissue mass increases due to an increase in the
number of cells, it is hyperplasia. Another word for cell multiplication is mitosis.
The epithelial tissue lining a serous cavity secretes a serous fluid to act as a
lubricant. The secretion of sebaceous glands is formed as portions of the cells are
In FCT, the fibroblast moves up and down the fiber. During this movement, it
keeps the fiber in repair and restructures it in response to the stresses applied to the
body. The collagen fibers are limited in stretchability, particularly when compared to the
elastic fibers. As a collagen fiber lengthens, the tension increases considerably. As an
FCT becomes warmer, it becomes more stretchable and more resistant to damage; this
is the basis of warm-up exercises before more strenuous activities.
Fats, oils, and fatty acids are types of lipids. Such substances are stored mostly
as neutral fat, which consists of triglycerides. A molecule of triglyceride is formed from
a glycerol and three fatty acids. Within the body, triglycerides are kept in a liquid state
by adjusting the length of each fatty acid. Within a 3-week period, there is a complete
turnover of the triglycerides. Fats may be taken in as fats or converted from other
substances, such as carbohydrates. When the diet contains more fats and
carbohydrates than necessary for body activities, the result may be obesity.
End of Lesson 2