A lysosome36 (LY- so-some) (fig. 3.28a) is a package of enzymes bounded by a single unit membrane. Although often round or oval, lysosomes are extremely variable in shape. When viewed with the TEM, they often exhibit dark gray contents devoid of structure, but sometimes show crystals or parallel layers of protein. At least 50 lyso-somal enzymes have been identified. They hydrolyze proteins, nucleic acids, complex carbohydrates, phospho-lipids, and other substrates. In the liver, lysosomes break down stored glycogen to release glucose into the blood
36lyso = loosen, dissolve + some = body stream. White blood cells use their lysosomes to digest phagocytized bacteria. Lysosomes also digest and dispose of worn-out mitochondria and other organelles; this process is called autophagy37 (aw-TOFF-uh-jee). Some cells are meant to do a certain job and then die. The uterus, for example, weighs about 900 g at full-term pregnancy and shrinks to 60 g within 5 or 6 weeks after birth. This shrinkage is due to autolysis,38 the digestion of surplus cells by their own lysosomal enzymes. Such programmed cell death is further discussed in chapter 5.
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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.