4 3 6C H A P T E R 2 0 ■ S c i e n t i f i c R e a s o n i n gThe coated vesicle then leaves the ER, travels through the cytoplasm, and
binds to the lysosome through another specific receptor protein. Finally, in
a maneuver involving several more proteins, the vesicle fuses with the lyso-
some and the protein arrives at its destination.
During its travels our protein interacted with dozens of macromolecules
to achieve one purpose: its arrival in the lysosome. Virtually all components
of the transport system are necessary for the system to operate, and therefore
the system is irreducible. And since all of the components of the system are
comprised of single or several molecules, there are no black boxes to invoke.
The consequences of even a single gap in the transport chain can be seen
in the hereditary defect known as I-cell disease. It results from a deficiency of
the enzyme that places the mannose-6-phosphate on proteins to be^ targeted
to the lysosomes. I-cell disease is characterized by progressive retardation,
skeletal deformities, and early death.The Study of “Molecular Evolution”
Other examples of irreducible complexity abound, including aspects of
protein transport, blood clotting, closed circular DNA, electron transport,
the bacterial flagellum, telomeres, photosynthesis, transcription regulation,
and much more. Examples of irreducible complexity can be found on virtu-
ally every page of a biochemistry textbook. But if these things cannot be ex-
plained by Darwinian evolution, how has the scientific community regarded
these phenomena of the past forty years?StepIIIIII IV VFigure 20.3 Transport of a protein from the ER to the lysosome. Step I: A specific enzyme (gray
oval) places a marker on the protein (black sphere). This takes place within the ER, which is
delimited by a barrier membrane (cross-hatched bar with ends curving to the left). Step II:
The marker is specifically recognized by a receptor protein and the clathrin vesicle (hexagonal
shape) begins to form. Step III: The clathrin vesicle is completed and buds off from the ER
membrane. Step IV: The clathrin vesicle crosses the cytoplasm and attaches through another
specific marker to a receptor protein (dark gray box) on the lysosomal membrane and releases
its cargo.97364_ch20_ptg01_423-448.indd 436 15/11/13 12:09 3M
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