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(cell biology) A membrane-bound cytoplasmic structure through which molecules that are endocytosed pass en route to the lysosome
Endocytosis is a process in which cell takes in particulates from the outside by engulfing and fusing them with its plasma membrane. The particulate is taken into the cell and then surrounded by a portion of the cell membrane. The resulting vesicle containing the particulate is taken first to the endosome and then to the lysosome (for degradation).
An endosome is a cytoplasmic sac. It is where particulates that have been endocytosed are taken to. It is associated with the endocytotic membrane transport pathway. Some references consider it as an organelle; others do not consider them as such. The latter defines an organelle as a structure surrounded by double lipid bilayers. As such, endosomes are not considered as an organelle in the same way with the lysosome. Nevertheless, there are references that are less restrictive. Accordingly, an organelle is one that acts as a specialized subunit inside the cell that performs a specific function. In this case, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles. In this regard, the endosome would, therefore, be included in the membrane-bound organelles together with the lysosomes, endoplasmic reticulum, Golgi apparatus, nucleus, mitochondria, plastids, lysosomes, and vacuoles. The endosome originates from the trans-Golgi network.
Endosomes can be typified as follows: (1) early endosome, (2) late endosome, and (3) recycling endosome. An early endosome is its initial form and then matures into the late endosome. Maturation entails a decreasing pH and an increasing size. Also, it is characterized by the loss of one of its markers, RAB5A while gaining a new marker, RAB7A. This change implicates the capacity of the late endosome to now fuse with the lysosome. An early endosome has a tubular-vesicular network, which is not seen in late endosome. The tubules are of 50 nm in diameter and they connect the vesicles whose diameters can reach up to 1 µm. The early endosome is often seen in the periphery of the cell where it receives the vesicles from the cell surface. A recycling endosome is a form of endosome characterized by being comprised largely of a tubular network, with a marker RAB11, and concentrated at the microtubule-organizing center. The late endosomes may also originate from the phagosomes (in the phagocytic pathway) apart from the early endosomes (in the endocytic pathway).1 The early endosome, in turn, originates from the trans-Golgi network.
The endosomes are involved in the endocytic pathway. In mammals, the principal components of this pathway are the early endosomes, the late endosomes, and the lysosomes. The early endosomes and the recycling endosomes play a part in processing and then recycling the internalized molecules back to the surface of the cell. The late endosomes, in turn, are involved in the sorting and shuttling of the internalized molecules to the lysosome for degradation.
The cell takes in particulates from the outside by engulfing and fusing them with its plasma membrane. Examples are phagocytosis, which literally means cell-eating, and pinocytosis, which literally means cell-drinking. The particulate that is taken inside the cell will be surrounded by a portion of the cell membrane. The resulting vesicle containing the particulate is next shuttled to the endosome and eventually to the lysosome for degradation. In the lysosome, the particulate is broken down into simpler biomolecular components that may be recycled into re-usable materials for the cell. The process is catalyzed by the lysosomal enzymes.
- Stoorvogel, W., Strous, G. J., Geuze, H. J., Oorschot, V., & Schwartz, A. L. (May 1991). “Late endosomes derive from early endosomes by maturation”. Cell. 65 (3): 417–27.
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