Page 38 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 23

respond to stresses placed upon them by
changes in their environment. Their
VetBooks.ir responses are attempts to maintain a state Phagocytosis

of homeostasis.
Growth is increase in size. Increase in
size of a cell or organ beyond normal is ENDOCYTOSIS
called hypertrophy. An increase in the size
of a structure due to an increase in the Pinocytosis
number of cells is called hyperplasia. A
decrease in size from normal is called
atrophy. Failure of a tissue or organ to
develop is called aplasia, while incom­ EXOCYTOSIS
plete development or defective develop­
ment of a tissue or organ is called
hypoplasia.
Reproduction of a cell or of an organ­
ism implies the ability to produce more Figure 2-1. Endocytosis (phagocytosis and
cells or more organisms that are essentially pinocytosis) and exocytosis.
the same as the original. Some fully differ­
entiated cells, for instance nerve cells, do
not normally retain the ability to repro­ termed phagocytosis (Fig. 2‐1), and cells
duce in the adult. capable of taking in large amounts of
Cells may be found in solutions whose material are called phagocytes. This ability
composition is quite different from that is characteristic of some white blood cells,
of the fluid within the cells. To maintain which engulf large particulate matter, tis­
intracellular homeostasis in these condi­ sue debris, or bacteria. After a phagocytic
tions, the passage of particles and water vesicle enters the substance of a cell, it may
in and out of the cell must be regulated. fuse with a different type of membrane
Absorption is the process of taking vesicle, a lysosome that was produced
dissolved materials or water through within the cell. Lysosomes are specialized
the cell membrane into the substance of membrane vesicles that contain enzymes,
the cell. This can be a passive process also produced within the cell, that must be
dependent on the forces of diffusion and contained within their membraned vesicle
osmosis, or an active process requiring or else they would enzymatically disrupt
the expenditure of energy from adeno­ the remainder of the cell. Fusion of the
sine triphosphate, or the result of electro­ phagocytic vesicle with the lysosome
chemical ionic forces and affinities that allows the lysosomal enzymes to act upon
require no direct expenditure of energy. the contents of the phagocytic vesicle in
All three can occur at the same time a small, local area that is isolated from
across the same cell membrane. the cytosol. Most types of cells are capa­
Endocytosis is another means by which ble of endocytosing small amounts of
extracellular materials may enter a cell. fluid containing dissolved particles. This
In endocytosis the exterior cell membrane type of endocytosis is termed pinocytosis
moves to surround extracellular materials (Fig. 2‐1).
in a membrane pocket (Fig. 2‐1). This mem­ Metabolism refers to the sum total of
brane vesicle detaches from the inner the physical and biochemical reactions
surface of the cell membrane and moves occurring in each cell, and therefore in
into the interior of the cell. the entire animal. Reactions that build
If a large amount of particulate material and maintain cellular components are
is endocytosed by ameboid movements called anabolic, and those that break down
of a cell, the process is more specifically cellular components or constituents are