Lysosomes are roughly spherical bodies bounded by a single membrane. They are manufactured by the Golgi apparatus (pathway 2 in the figure).
|Link to a discussion of how proteins synthesized in the endoplasmic reticulum are sent to the appropriate destinations.|
The pH within the lysosome is about pH 5, substantially less than that of the cytosol (~pH 7.2). All the enzymes in the lysosome work best at an acid pH. This reduces the risk of their digesting their own cell if they should escape from the lysosome.
|At one time, it was thought that lysosomes were responsible for killing cells scheduled to be removed from a tissue; for example, the resorption of its tail as the tadpole metamorphoses into a frog. This is incorrect. These examples of programmed cell death (PCD) or apoptosis take place by an entirely different mechanism. Link to a discussion of apoptosis.|
Materials within the cell scheduled for digestion are first deposited within lysosomes. These may be:
|Some tricks that intracellular parasites use to avoid destruction by lysosomes.|
|Link to discussion of how antigens are taken up by macrophages and B cells and degraded in their lysosomes.|
The exocytosis of lysosomes provides the additional membrane needed to quickly seal wounds in the plasma membrane.
Peroxisomes are about the size of lysosomes (0.5-1.5 µm) and like them are bound by a single membrane.
They also resemble lysosomes in being filled with enzymes.
The enzymes and other proteins destined for peroxisomes are synthesized in the cytosol. Each contains a peroxisomal targeting signal (PTS) that binds to a receptor molecule that takes the protein into the peroxisome and then returns for another load.
Two peroxisomal targeting signals have been identified:
Some of the functions of the peroxisomes in the human liver:
Example: X-linked adrenoleukodystrophy (X-ALD). This disorder results from a failure to metabolize fatty acids properly. One result is deterioration of the myelin sheaths of neurons. The disorder occurs in young boys because the gene is X-linked. An attempt to find an effective treatment was the subject of the 1992 film Lorenzo's Oil.
Example: Zellweger syndrome. This disorder results from the inheritance of two mutant genes for one of the receptors (PXR1) needed to import proteins into the peroxisome.
Peroxisomes are also called microbodies.
|Link to schematic showing lysosomes and peroxisomes in a typical animal cell.|