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12.6Endocytosis

Fig. 12-14 Material absorption into the cell and intracellular digestion

The cell absorbs large-sized substances such as nutrients and foreign bodies by a method called endocytosis (pinocytosis and phagocytosis) and processes them via hydrolysis. Some absorbed membrane proteins are recycled.

Cells absorb important nutrients from the outside, but to this end, they uptake only low-molecular-weight molecules such as monosaccharides and amino acids through a dedicated carrier found in the cell membrane. They occasionally need to uptake nutrients with a massive molecular weight as well. For example, when uptaking cholesterol, which is a major structural component of biological membranes, into the cell, a lipid called low-density lipoprotein (LDL) is absorbed in the form of a protein complex (particles with a diameter of 20–30 nm found in the blood). Cells also occasionally absorb large bodies into the cell. For example, macrophages, which are a type of leukocyte, absorb foreign bodies such as bacteria and viruses invading from the outside or dead cells and aging erythrocytes in order to degrade them. The uptake of this type of large molecules into the cell is generally called endocytosis (Fig. 12-14). Endocytosis can be classified into pinocytosis (small-sized vesicles) and phagocytosis (large-sized vesicles), depending on the size of the vesicle being formed. Vesicles absorbed into the cell by pinocytosis aggregate to form structures called endosomes. When a vesicle absorbed in LDL forms an endosome, acid hydrolase transported from the Golgi body penetrates into the endosome. As a result, it changes to a lysosome, and LDL on the inside undergoes hydrolysis to release cholesterol. In case of phagocytosis, acid hydrolase is transported from the Golgi body into a phagosome that has absorbed a foreign body to form a lysosome in order to degrade the foreign body.
The membrane of a transport vesicle (composed of the cell membrane) transported into an endosome by endocytosis contains various types of membrane proteins such as growth factors or LDL receptors.

Some of these proteins return to the cell membrane for recycling. For example, an LDL receptor binds LDL and transports it to the vesicle, which is a cell membrane invagination. The vesicle absorbed in LDL forms an endosome. Thus, the proteins are degraded and returned to the cell membrane.

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Carrying Transport Vesicles and Organelles

Column Fig. 12-3 Transport pathway of transport vesicles

Transport vesicles transfer substances along the cytoskeleton distributed within the cell. Here, the example of a transport pathway by microtubules is shown. The arrows indicate the directions of movement of ER and transport vesicles. In a cell that performs active material transport in a set direction, such as an epithelial cell, the cytoskeleton is distributed along that transport direction.

The cytoplasm has developed a mechanism for carrying transport vesicles and organelles. It contains the cytoskeleton, which is arranged in the cell to have a certain directionality, and motor proteins (see Selection 1 of Chapter17, Fig. 17-2B, Fig. 17-3), which travel along the filaments of the cytoskeleton. In the cell, organelles such as the rough ER and Golgi body are functionally arranged in fixed positional relationships. Furthermore, microtubules and actin filaments are positioned to conform to these positional relationships, and the cytoskeleton acts as a network for organelles to bind to each other or for them to bind to the cell membrane. Here motor proteins bind to loaded transport vesicles or organelles and move them along the cytoskeleton. By this mechanism, a transport vesicle or organelle is efficiently transported to the target site.

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