After a phagocytic cell has engulfed the proteins, polysaccharides, and lipids present in a particle of "food" (such as a bacterium), these molecules are still kept isolated from the cytoplasm by the membranes surrounding the food vacuole. The large molecules of proteins, polysaccharides, and lipids must first be digested into their smaller subunits (including amino acids, monosaccharides, and fatty acids) before they can cross the vacuole membrane and enter the cytoplasm.
Cell Structure and Genetic Control
Cell Structure and Genetic Control
Clinical Investigation Clues
■ Figure 3.9 An electron micrograph of lysosomes. This photograph shows primary and secondary lysosomes, mitochondria, and the Golgi complex.
The digestive enzymes of a cell are isolated from the cytoplasm and concentrated within membrane-bound organelles called lysosomes (fig. 3.9). A primary lysosome is one that contains only digestive enzymes (about forty different types) within an environment that is considerably more acidic than the surrounding cytoplasm. A primary lysosome may fuse with a food vacuole (or with another cellular organelle) to form a secondary lysosome in which worn-out organelles and the products of phagocytosis can be digested. Thus, a secondary lysosome contains partially digested remnants of other organelles and ingested organic material. A lysosome that contains undigested wastes is called a residual body. Residual bodies may eliminate their waste by exocytosis, or the wastes may accumulate within the cell as the cell ages.
Partly digested membranes of various organelles and other cellular debris are often observed within secondary lysosomes. This is a result of autophagy, a process that destroys worn-out organelles so that they can be continuously replaced. Lysosomes are thus aptly characterized as the "digestive system" of the cell.
Lysosomes have also been called "suicide bags" because a break in their membranes would release their digestive enzymes and thus destroy the cell. This happens normally in programmed cell death (or apoptosis), described later in the discussion of the cell cycle. An example is the loss of tissues that must accompany embryonic development, when earlier structures (such as gill pouches) are remodeled or replaced as the embryo matures.
Most, if not all, molecules in the cell have a limited life span. They are continuously destroyed and must be continuously replaced. Glycogen and some complex lipids in the brain, for example, are normally digested at a particular rate by lysosomes. If a person, because of some genetic defect, does not have the proper amount of these lysosomal enzymes, the resulting abnormal accumulation of glycogen and lipids could destroy the tissues. Examples of such defects include Tay Sach's disease and Gaucher's disease.
Remember that Timothy has large amounts of glycogen granules, with many intact granules seen within his secondary lysosomes.
Could his apparent liver disease be caused by another disorder? What condition may Timothy have that would explain the presence of intact glycogen granules in his lysosomes?
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.