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When a lysosome combines with a vesicle that contains a phagocytized particle, its digestive enzymes may destroy the particle. The products of this intracellular digestion diffuse into the cytoplasm. Any residue may be expelled from the cell by exocytosis.

membrane surrounding the solid detaches from the cell's surface, forming a vesicle containing the particle (fig. 3.30). Such a vesicle may be several micrometers in diameter.

Usually, a lysosome soon combines with such a newly formed vesicle, and lysosomal digestive enzymes decompose the contents (fig. 3.31). The products of this decomposition may then diffuse out of the lysosome and into the cytoplasm, where they may be used as raw materials in metabolic processes. Exocytosis may expel any remaining residue. In this way, phagocytic cells dispose of foreign objects, such as dust particles, remove damaged cells or cell parts that are no longer functional, or destroy bacteria that might otherwise cause infections.

Shier-Butler-Lewis: I. Levels of Organization 3. Cells © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Molecules outside cell

Molecules outside cell

Receptor site protein

Cell membrane

Receptor-ligand combination

Receptor site protein

Cell membrane

Cytoplasm

Cell membrane indenting

Figure 3.32

Receptor-mediated endocytosis. (a) A specific substance binds to a receptor site protein. (b and c) The combination of the substance with the receptor site protein stimulates the cell membrane to indent. (d) The resulting vesicle transports the substance into the cytoplasm.

Phagocytosis is an important line of defense against invasion by disease-causing microorganisms.

Pinocytosis and phagocytosis engulf nonspecifi-cally. In contrast is the more discriminating receptor-mediated endocytosis, which moves very specific kinds of particles into the cell. In this mechanism, protein molecules extend through the cell membrane and are exposed on its outer surface. These proteins are receptors to which specific substances from the fluid surroundings of the cell can bind. Molecules that can bind to the receptor sites selectively enter the cell; other kinds of molecules are left outside (fig. 3.32). (Molecules that bind specifically to receptors are called ligands.)

Entry of cholesterol molecules into cells illustrates receptor-mediated endocytosis. Cholesterol molecules synthesized in liver cells are packaged into large spherical particles called low-density lipoproteins (LDL). An LDL particle has a coating that contains a binding protein called apoprotein-B. The membranes of various body cells have receptors for apoprotein-B. When the liver releases LDL particles into the blood, cells with apoprotein-B receptors can recognize the LDL particles and bind them. Formation of such a receptor-ligand combination stimulates the cell membrane to indent and form a vesicle around the LDL particle. The vesicle carries the LDL particle to a lysosome, where enzymes digest it and release the cholesterol molecules for cellular use.

Receptor-mediated endocytosis is particularly important because it allows cells with the appropriate re ceptors to remove and process specific kinds of substances from their surroundings, even when these substances are present in very low concentrations. In short, this mechanism provides specificity.

As a toddler, Stormie Jones already had a blood serum cholesterol level six times normal. Before she died at age ten, she had suffered several heart attacks and had undergone two cardiac bypass surgeries, several heart valve replacements, and finally a heart-liver transplant. The transplant lowered her blood cholesterol to a near-normal level, but she died from the multiple traumas suffered over her short lifetime.

Stormie had the severe form of familial hypercholesterolemia (FH), meaning simply too much cholesterol in the blood. Her liver cells lacked LDL receptors. Blocked from entering cells, cholesterol accumulated in her bloodstream, forming the plaques that caused her heart disease.

Stormie Jones was one in a million. Far more common are the one in 500 people who have the milder form of FH, in which liver cells have half the normal number of LDL receptors. These individuals are prone to suffer heart attacks in early adulthood. However, they can delay symptom onset by taking precautions to avoid cholesterol buildup, such as exercising, eating a low-fat diet, and not smoking. (These precautions may also benefit individuals not suffering from FH.)

Golgi apparatus

Endoplasmic reticulum

Golgi apparatus

Endoplasmic reticulum

Nucleus

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