2-42 Eukaryotic cells absorb things by endocytosis

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• The major pathway into eukaryotic cells is by endocytosis. This process takes two forms, phagocytosis and pinocytosis.
• The Golgi apparatus, ER, lysosomes and endocytosis work as a coordinated whole to move particles into and out of the cell.

Lysosomes are an important part of endocytosis, a process that eukaryotic cells use to take up particles from the environment. Endocytosis is illustrated in Figure 2-60. The process creates membrane-bound cavities filled with fluid and solid materials. Larger membrane-enclosed cavities are called vacuoles while smaller ones are called vesicles. Endocytosis comes in two forms, phagocytosis and pinocytosis. Phagocytosis involves the engulfment of large particles, even microorganisms, into membrane-bound compartments. It is a process used most often in the immune system and is described in detail in the chapter on infection and immunity. Pinocytosis involves the recognition of specific particles in the environment as described below. The process is used by unicellular eukaryotic microbes to ingest food and by multicellular organisms to take in certain macromolecules traveling from other parts of the organism.

Pinocytosis begins when protein receptors on the cell surface bind to the target molecule to be ingested. The bound particle migrates to an area of the membrane that is rich in a protein called clathrin. This protein forms a matrix and causes an indentation in the membrane called a clathrin-coated pit. The entrance of a receptor with a bound particle begins a process of invagination at the pit that results in the internalization of the pit and the engulfment of the incoming particle inside a membrane vesicle. This vesicle is called an endosome and once inside the cell, proteins in the endosome membrane begin to pump protons inside, dropping the internal pH of the endosome. The clathrin on the membrane of the endosome then migrates back to the plasma membrane to repeat the cycle. The endosome fuses with a lysosome to form an endolysosome, which causes the ingested particle to be degraded. The valuable breakdown products are transported out of the endolysosome into the cell. The spent endolysosome is called a residual body and sometimes fuses with the plasma membrane to release remaining compounds into the environment.

The ER, Golgi apparatus, lysosomes and endosomes seem to operate as a coordinated whole, functioning in the import and export of materials. It is probably correct to think of these structures as a functional unit and the term vacuome has been coined to describe them. Ribosomes in the ER manufacture proteins and these are modified in the ER and Golgi. The mature proteins eventually find their way to the plasma membrane for export or become part of lysosomes. Lysosomes serve in the endocytotic pathway to take up materials and process them for cell use. They also digest spent cell constituents. All of these processes occur within membrane structures and this carefully controls the import and export of materials from the cell. Considering the extent of these structures in the cell it is remarkable that the membranes of these structures, especially lysosomes, never rupture. If they did it would be catastrophic to the cell and would rapidly lead to cell death.

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