18.1
内吞作用
真核细胞能够获取生长和增殖所需的营养物质。需要降解的营养物质和其他分子会通过一种称为内吞作用的过程从细胞外的空间内化。“内吞作用”一词是由克里斯汀·德·迪夫（Christian de Duve）在 1963 年首次提出的。
内吞作用开始时，细胞质膜通常会将进入的分子包裹起来，从而会形成一个运输囊泡，…
真核细胞通过一种称为内吞作用的过程从细胞外空间摄取分子。
在内吞作用中，细胞质膜的一部分向内弯曲，包裹进入的分子，随后出芽形成一个运输囊泡。
运输囊泡将分子带入细胞质，以便与其他细胞内细胞器（如内体或溶酶体）融合。与溶酶体融合后，摄入的物质可在溶酶体酶的作用下降解。另一方面，某些通过内吞作用进入细胞的分子必须在细胞内释放以供利用。
内吞作用使细胞能够摄取营养物质、某些蛋白质以及多种可溶性分子。
某些特化的细胞，例如吞噬细胞，还会吞噬细胞碎片、外来颗粒或生物体（如细菌或原生动物），作为宿主防御机制的一部分。这种类型的内吞作用被称为吞噬作用。
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Q1: How does endocytosis allow cells to bring molecules into the cytosol?
In endocytosis, the plasma membrane bends inward to surround an incoming molecule, then buds off to form a transport vesicle. This vesicle carries molecules into the cytosol for fusion with intracellular organelles like endosomes or lysosomes. The process enables cells to uptake nutrients, proteins, and soluble molecules from the extracellular space.
Q2: What happens to endocytosed material after it fuses with a lysosome?
Once endocytosed material fuses with a lysosome, lysosomal enzymes degrade the ingested material. However, some endocytosed molecules must be released in the cell for utilization rather than degradation. This sorting determines whether material is broken down or used by the cell for growth and proliferation.
Q3: What is phagocytosis and what role does it play in host defense?
Phagocytosis is a specialized type of endocytosis where phagocytes engulf pathogenic microorganisms, parasites, and cell debris. The internalized pathogens are contained in a vesicle called a phagosome, which fuses with a lysosome for degradation. This process is vital for providing immunity to the host organism.
Q4: How does receptor-mediated endocytosis differ from other endocytic pathways?
Receptor-mediated endocytosis is a highly specific process where an incoming molecule is internalized only after binding to its associated receptor. The coated vesicle containing the endocytosed molecule fuses with the early endosome. This selectivity allows cells to regulate which molecules enter through targeted receptor recognition.
Q5: What is pinocytosis and how does it differ from phagocytosis?
Pinocytosis is endocytosis where cells engulf dissolved substances from the extracellular space, unlike phagocytosis which targets large particles and pathogens. In pinocytosis, a coated vesicle containing the endocytosed fluid fuses with an early endosome. This process allows cells to internalize small soluble molecules for nutrient acquisition.
Q6: What role do clathrin-coated vesicles play in endocytosis?
Transport vesicles formed during endocytosis can be coated with a protein called clathrin. These clathrin-coated vesicles protect and transport endocytosed material as they bud from the plasma membrane. The coating helps organize and facilitate the movement of molecules into the cytosol for subsequent sorting and processing.
Q7: How can pathogenic organisms exploit endocytosis to infect host cells?
Some pathogenic organisms take advantage of endocytosis to enter cells and establish infection. By mimicking or binding to cellular receptors, these pathogens trigger endocytic uptake into the host cell. Once internalized, they can evade immune defenses or manipulate cellular machinery for replication and survival.