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Chapter 9: Cell Membrane and Transport Mechanisms

9.4. Bulk (or Vesicular) Transport

Learning Objectives

By the end of this section, you will be able to:

  • Explain endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
  • Explain the process of exocytosis

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles. Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that when a cell uptakes and releases large particles, it requires energy (ATP). A large particle, however, cannot pass through the membrane, even with energy that the cell supplies.

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 variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created vacuole that is formed from the plasma membrane.

Three types of endocytosis are shown: (a) phagocytosis, (b) pinocytosis, and (c) receptor-mediated endocytosis. Part a shows the plasma membrane forming a pocket around a particle in the extracellular fluid. The membrane subsequently engulfs the particle, which becomes trapped in a vacuole. Part b shows a plasma membrane forming a pocket around fluid in the extracellular fluid. The membrane subsequently engulfs the fluid, which becomes trapped in a vacuole. Part c shows a part of the plasma membrane that is clathrin-coated on the cytoplasmic side and has receptors on the extracellular side. The receptors bind a substance, then pinch off to form a coated vesicle.
Figure 9.4.1: Three variations of endocytosis are shown. (a) In one form of endocytosis, phagocytosis, the cell membrane surrounds the particle and pinches off to form an intracellular vacuole. (b) In another type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. (c) In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds at the receptor on the external cell membrane. (credit: modification of work by Mariana Ruiz Villarreal)

Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 9.4.1.(a)).

Pinocytosis “cell drinking”, is a misleading term. A cell is not purposefully taking in extracellular fluid. Instead, the process allows needed small soluble solutes to be taken in from the extracellular fluid (Figure 9.4.1.(b)).

Receptor-mediated endocytosis. Receptors are target-specific membrane proteins, binding only with select molecules. An extracellular substance, therefore, can only be taken into the cell if there is a receptor to recognizes it (Figure 9.4.1.(c)). Iron is transported into the cell using receptor-mediated endocytosis. If there is a defect in the receptor, the substance remains outside the cell to accumulate in tissue or blood. The most physiological relevant example of a receptor protein genetic defect relates to the resulting accumulation of ‘bad cholesterol’ in the blood which in turn is associated with an increased risk of cardiovascular disease. 

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. A particle enveloped in membrane fuses with the interior of the plasma membrane. This fusion opens the membranous envelope to the exterior of the cell, and the particle is expelled into the extracellular space (Figure 9.4.2.).


This illustration shows vesicles fusing with the plasma membrane and releasing their contents to the extracellular fluid.
Figure 9.4.2: In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents then release to the cell’s exterior. (credit: modification of work by Mariana Ruiz Villareal)

License and attributions:

  • Concepts of Biology, 2013, Fowler, S. et al. License: CC BY 4.0. Located at https://openstax.org/books/concepts-biology/pages/3-6-active-transport
  • Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/5-4-bulk-transport

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BIO130: Introduction to Physiology Copyright © 2024 by Dinor Dhanabala; Sandra Fraley; and Gordon Lake is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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