Essential Cell Biology 5th edition

528 CHAPTER 15 Intracellular Compartments and Protein Transport
Figure 15–37 Materials destined for
degradation in lysosomes follow different
pathways to the lysosome. Each pathway
leads to the intracellular digestion of
materials derived from a different source.
Early endosomes, phagosomes, and
autophagosomes can fuse with either
lysosomes or late endosomes, both of which
contain acid-dependent hydrolytic enzymes.
Where the membrane fragments that form
the autophagosome originate is still actively
investigated.
bacterium
early endosome
PHAGOCYTOSIS
phagosome
hydrolytic enzymes
ENDOCYTOSIS
late
endosome
lysosomes
autophagosome
AUTOPHAGY
mitochondrion
sorted and packaged into transport vesicles, which bud off and deliver
their contents to lysosomes via endosomes (see Figure 15–19).
Depending on their source, ECB5 materials e15.36/15.37 follow different paths to lysosomes.
We have seen that extracellular particles are taken up into phagosomes,
which fuse with lysosomes, and that extracellular fluid and macromolecules
are taken up into smaller endocytic vesicles, which deliver their
contents to lysosomes via endosomes.
Cells have an additional pathway that supplies materials to lysosomes;
this pathway, called autophagy, is used to degrade obsolete parts of the
cell: as the term suggests, the cell literally eats itself. In electron micrographs
of liver cells, for example, one often sees lysosomes digesting
mitochondria, as well as other organelles. The process involves the
enclosure of the organelle by a double membrane, creating an autophagosome,
which then fuses with a lysosome (Figure 15–37). Autophagy
of organelles and cytosolic proteins—some of which are marked for
destruction by the attachment of ubiquitin tags (as discussed in Chapter
4)—increases when eukaryotic cells are starved or when they remodel
themselves extensively during development. The amino acids generated
by this cannibalistic form of digestion can then be recycled to allow continued
protein synthesis.
ESSENTIAL CONCEPTS
• Eukaryotic cells contain many membrane-enclosed organelles,
including a nucleus, an endoplasmic reticulum (ER), a Golgi apparatus,
lysosomes, endosomes, mitochondria, chloroplasts (in plant
cells), and peroxisomes. The ER, Golgi apparatus, peroxisomes,
endosomes, and lysosomes are all part of the endomembrane system.
• Most organelle proteins are made in the cytosol and transported into
the organelle where they function. Sorting signals in the amino acid
sequence guide the proteins to the correct organelle; proteins that
function in the cytosol have no such signals and remain where they
are made.