Functional Systems of the Cell
We discuss
several representative functional systems of the cell that make it a living
organism.
Ingestion by the
Cell—Endocytosis
If a cell is
to live and grow and reproduce, it must obtain nutrients and other substances from
the surrounding fluids. Most substances pass through the cell membrane by
diffusion and active transport. Diffusion involves simple movement through the
membrane caused by the random motion of the molecules of the substance;
substances move either through cell membrane pores or, in the case of lipid soluble
substances, through the lipid matrix of the membrane. Active transport involves
the actual carrying of a substance through the membrane by a physical protein
structure that penetrates all the way through the membrane. These active
transport mechanisms are so important to cell function that they are presented
in detail in other article. Very large particles enter the cell by a
specialized function of the cell membrane called endocytosis. The principal
forms of endocytosis are pinocytosis and phagocytosis. Pinocytosis means
ingestion of minute particles that form vesicles of extracellular fluid and
particulate constituents inside the cell cytoplasm. Phagocytosis means
ingestion of large particles, such as bacteria, whole cells, or portions of
degenerating tissue.
Pinocytosis
Pinocytosis
occurs continually in the cell membranes of most cells, but it is especially
rapid in some cells. For instance, it occurs so rapidly in macrophages that
about 3 per cent of the total macrophage membrane is engulfed in the form of
vesicles each minute. Even so, the pinocytotic vesicles are so small usually
only 100 to 200 nanometers in diameter that most of them can be seen only with
the electron microscope. Pinocytosis is the only means by which most large
macromolecules, such as most protein molecules, can enter cells. In fact, the
rate at which pinocytotic vesicles form is usually enhanced when such
macromolecules attach to the cell membrane. The given diagram demonstrates the successive
steps of pinocytosis, showing three molecules of protein attaching to the
membrane. These molecules usually attach to specialized protein receptors on
the surface of the membrane that are specific for the type of protein that is to
be absorbed. The receptors generally are concentrated in small pits on the
outer surface of the cell membrane, called coated pits. On the inside of the
cell membrane beneath these pits is a latticework of fibrillar protein called
clathrin, as well as other proteins, perhaps including contractile filaments of
actin and myosin. Once the protein molecules have bound with the receptors, the
surface properties of the local membrane change in such a way that the entire
pit invaginates inward, and the fibrillar proteins surrounding the invaginating
pit cause its borders to close over the attached proteins as well as over a
small amount of extracellular fluid. Immediately thereafter, the invaginated
portion of the membrane breaks away from the surface of the cell, forming a
pinocytotic vesicle inside the cytoplasm of the cell. What causes the cell
membrane to go through the necessary contortions to form pinocytotic vesicles
remains mainly a mystery. This process requires energy from within the cell;
this is supplied by ATP, a high energy. Also, it requires the presence of
calcium ions in the extracellular fluid, which probably react with contractile
protein filaments beneath the coated pits to provide the force for pinching the
vesicles away from the cell membrane.
Phagocytosis
Phagocytosis
occurs in much the same way as pinocytosis, except that it involves large
particles rather than molecules. Only certain cells have the capability of
phagocytosis, most notably the tissue macrophages and some of the white blood
cells. Phagocytosis is initiated when a particle such as a bacterium, a dead
cell, or tissue debris binds with receptors on the surface of the phagocyte. In
the case of bacteria, each bacterium usually is already attached to a specific
antibody, and it is the antibody that attaches to the phagocyte receptors, dragging
the bacterium along with it. This intermediation of antibodies is called
opsonization.
Phagocytosis
occurs in the following steps:
1. The cell
membrane receptors attach to the surface ligands of the particle.
2. The edges
of the membrane around the points of attachment evaginate outward within a
fraction of a second to surround the entire particle; then, progressively more
and more membrane receptors attach to the particle ligands. All this occurs
suddenly in a zipper-like manner to form a closed phagocytic vesicle.
3. Actin and
other contractile fibrils in the cytoplasm surround the phagocytic vesicle and
contract around its outer edge, pushing the vesicle to the interior.
4. The contractile
proteins then pinch the stem of the vesicle so completely that the vesicle
separates from the cell membrane, leaving the vesicle in the cell interior in
the same way that pinocytotic vesicles are formed.
Digestion of Pinocytotic and
Phagocytic Foreign Substances Inside the Cell—Function of the Lysosomes
Almost
immediately after a pinocytotic or phagocytic vesicle appears inside a cell,
one or more lysosomes become attached to the vesicle and empty their acid
hydrolases to the inside of the vesicle. Thus, a digestive vesicle is formed
inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing
the proteins, carbohydrates, lipids, and other substances in the vesicle. The
products of digestion are small molecules of amino acids, glucose, phosphates,
and so forth that can diffuse through the membrane of the vesicle into the
cytoplasm. What is left of the digestive vesicle, called the residual body,
represents indigestible substances. In most instances, this is finally excreted
through the cell membrane by a process called exocytosis, which is essentially
the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles
containing lysosomes can be called the digestive organs of the cells.
Regression of Tissues and Autolysis
of Cells
Tissues of
the body often regress to a smaller size. For instance, this occurs in the
uterus after pregnancy, in muscles during long periods of inactivity, and in
mammary glands at the end of lactation. Lysosomes are responsible for much of
this regression. The mechanism by which lack of activity in a tissue causes the
lysosomes to increase their activity is unknown. Another special role of the
lysosomes is removal of damaged cells or damaged portions of cells from
tissues. Damage to the cell caused by heat, cold, trauma, chemicals, or any
other factor induces lysosomes to rupture. The released hydrolases immediately
begin to digest the surrounding organic substances. If the damage is slight,
only a portion of the cell is removed, followed by repair of the cell. If the
damage is severe, the entire cell is digested, a process called autolysis. In
this way, the cell is completely removed, and a new cell of the same type
ordinarily is formed by mitotic reproduction of an adjacent cell to take the
place of the old one. The lysosomes also contain bactericidal agents that can
kill phagocytized bacteria before they can cause cellular damage.
These agents
include
(1)
Lysozyme, which dissolves the bacterial cell membrane;
(2)
Lysoferrin, which binds iron and other substances before they can promote
bacterial growth; and
(3) Acid at
a pH of about 5.0, which activates the hydrolases and inactivates bacterial
metabolic systems.
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PROCESS OF ENDOCYTOSIS |
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PROCESS OF PINOCYTOSIS |
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