17.2: Endocytosis

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different endocytosis variations, but all share common characteristics:

1. The cellʼs plasma membrane invaginates, forming a pocket around the target particle.
2. The pocket pinches off.
3. This results in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane.

Phagocytosis

Phagocytosis (the condition of “cell eating”) is the process by which a cell takes in large particles, such as other cells or relatively large particles. For example, when microorganisms invade the human body, a type of white blood cell, a neutrophil, will remove the invaders through this process, surrounding and engulfing the microorganism, which the neutrophil then destroys (figure 17.4).

Figure 17.4: General process of phagocytosis. In phagocytosis, the cell membrane surrounds the particle and engulfs it.

In preparation for phagocytosis, a portion of the plasma membraneʼs inward-facing surface becomes coated with the protein clathrin, which stabilizes this membraneʼs section. The membraneʼs coated portion then extends from the cellʼs body and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane ,and the vesicle merges with a lysosome for breaking down the material in the newly formed compartment (endosome). When accessible nutrients from the vesicular contentsʼ degradation have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Receptor-mediated endocytosis

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances (figure 17.5).

Figure 17.5: Receptor-mediated endocytosis; LDL receptor is a classic example of this process.

In receptor-mediated endocytosis, the cellʼs uptake of substances targets a single type of substance that binds to the receptor on the cell membraneʼs external surface.

Clathrin attaches to the plasma membraneʼs cytoplasmic side. If a compoundʼs uptake is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. The failure of receptor-mediated endocytosis causes some human diseases.

For example, receptor-mediated endocytosis removes low-density lipoprotein or LDL from the blood. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood because their cells cannot clear LDL particles. See chapter 6.

Exocytosis

Exocytosis is the opposite of the processes we discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the plasma membraneʼs interior. This fusion opens the membranous envelope on the cellʼs exterior, and the waste material expels into the extracellular space. Other examples of cells releasing molecules via exocytosis include extracellular matrix protein secretion and neurotransmitter secretion into the synaptic cleft by synaptic vesicles (figure 17.6).

Figure 17.6: Exocytosis: vesicles containing substances fuse with the plasma membrane. The contents then release to the cell’s exterior.

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 8: Cytoplasmic Membrane Systems: Structure, Function, and Membrane Trafficking.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 46–47.

Figures

Alberts B, Johnson A, Lewis J, et al. Figure 17.4 General process of phagocytosis... Adapted under Fair Use from Cell and Molecular Biology. 6th Ed. pp 308. Figure 8.46 A summary of phagocytic pathway. 2014.

Alberts B, Johnson A, Lewis J, et al. Figure 17.5 Receptor mediated endocytosis, LDL-receptor is a classic example of this process. Adapted under Fair Use from Cell and Molecular Biology. 6th Ed. pp 306. Figure 8.42 The endocytic pathway. 2014.

Alberts B, Johnson A, Lewis J, et al. Figure 17.6 Exocytosis: vesicles containing substances fuse with the plasma membrane... Adapted under Fair Use from Cell and Molecular Biology. 6th Ed. pp 299. Figure 35 A summary of the autophagic pathway. 2014.

Additional resources

• Some helpful animations related to this content

  • Translocation into ER: https://youtu.be/pxG8-BWbpnU

  • Proteasome: https://youtu.be/Gp8lhKghckY

  • Clathrin: https://youtu.be/o_EUHu4OJus