What is phagocytosis endocytosis and exocytosis?

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion.

Marion S, Hoffmann E, Holzer D, Le Clainche C, Martin M, Sachse M, Ganeva I, Mangeat P, Griffiths G. Marion S, et al. Traffic. 2011 Apr;12(4):421-37. doi: 10.1111/j.1600-0854.2011.01158.x. Epub 2011 Feb 8. Traffic. 2011. PMID: 21210911

Table of Contents

• What is Endocytosis?
• A Final Word
• Suggested Reading

Endocytosis is the process by which a cell takes in molecules from the extracellular fluid. Phagocytosis is a form of endocytosis in which a cell consumes an entire other cell or macromolecule. Pinocytosis is a form of endocytosis in which a cell takes in a pocket of extracellular fluid and micro molecules at random.

The human body is packed with more mysteries than we will ever understand, but we do understand the crucial processes that allow each of our cells, organs, tissues and muscles to function. The microscopic movement of material between cellular factories and the parts of the body where complex molecules are needed is an incredibly important process for human beings. This movement of molecules consists of numerous different processes, including endocytosis, phagocytosis and pinocytosis.

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What is Endocytosis?

Have you ever watched bubbles floating on top of water? When they collide, the smaller bubbles are often consumed or swallowed up the larger ones. On an even smaller scale, at the cellular level, the cells in our body can do a similar thing – taking up molecules from the extracellular fluid. This process is called endocytosis, and is currently happening in millions of your cells at this very instant!

While it may seem strange to think of a cell needing to consume something, there are many reasons that a cell requires processes like endocytosis, such as absorbing essential nutrients from the extracellular fluid, consuming entire microorganisms, or transporting dead or dying cells to where they can be eliminated.

The actual process of endocytosis is simple and straightforward, although there are a number of different forms. Basically, once a molecule or other substance is encountered by the cell, the cell’s plasma membrane folds inwards surrounding the material, forming a pocket. Eventually, that pocket of membrane closes, and a vesicle is formed, which can separate from the membrane in the inside of the cell and then move to the organelle where it can be used.

Endocytosis is the opposite process to exocytosis, which is the means by which a cell can export molecules, whether they are proteins, neurotransmitters, waste products or any other cellular material. We’ve covered all the details of exocytosis in this article.

Three Forms of Endocytosis. (Photo Credit: OpenStax/Wikimedia Commons)

There are two main types of endocytosis, phagocytosis and pinocytosis, which are defined by the types of materials being taken up by the cell. Furthermore, there are two less common forms of endocytosis – caveolae and receptor-mediated endocytosis – which we will also briefly discuss.

Phagocytosis

When certain types of cells (phagocytes) consume an entire other cell or macromolecule, this form of endocytosis is called phagocytosis. This process can occur in various cells, including amoeba and white blood cells. For example, when a white blood cell is sent to the site of inflammation or infection, it can then consume unwanted bacteria, viruses, foreign particles or even pieces of dust. At that point, the unwanted material can be broken down into simple, usable molecules, or discarded from the body as waste.

Depending on the size of the material being consumed, the speed of this process may vary. Since the molecules being consumed are typically quite large, the vesicles that must be formed require a moderate amount of energy and resources, in the form of ATP. Similar to the process of exocytosis, this is a form of active transport, as the molecules cannot passively be consumed through the cell membrane. It is important to remember that only certain specialized cells are able to perform phagocytosis, whereas pinocytosis, explained below, is something that all cells must perform.

Pinocytosis

The second main form of endocytosis is pinocytosis, by which a cell takes in a pocket of extracellular fluid and micro molecules at random. This process is also informally called “cell drinking”, which is an accurate description of this fluid uptake. Similar to phagocytosis, the plasma membrane of the cell extends to form a pocket or a bud, and then closes the pocket. This forms a vesicle within the cell, which is then pinched off and can be moved to other areas of the cell in the cytosol. This form of endocytosis is also important because it can transport extracellular liquid through the cell without it interacting or affecting the cytoplasm. As mentioned, this cell drinking occurs in every type of cell.

Receptor-Mediated Endocytosis – Clathrin-Mediated and Caveolae-Mediated Endocytosis

The last two types of endocytosis are slightly more specialized, and require different cellular technology to occur. In clathrin-mediated endocytosis, larger macromolecules are taken into a cell through concentrated areas of clathrin-coated vesicles. Clathrin is a coat protein on the pits with a bristle-like action. These pits on a plasma membrane facilitate the easier movement of large molecules, and are found on almost all cell types. Certain molecules have different receptors, and the cathrin-coated pits are able to provide the necessary “keys” for the membrane “lock”.

Caveolae is similar to receptor-mediated endocytosis, but the vesicles are not coated in clathrin, but another transport membrane protein called caveolin. They are small bottle-shaped vesicles waiting below the cellular membrane and serve a similar function as the clathrin-coated pits. Caveolae are extremely abundant in certain cell types, often occupying more than 20% of the surface area of the plasma membrane.

(Photo Credit: OpenStax/Wikimedia Commons)

A Final Word

These processes are essential to our survival every day, and yet we have no conscious control over them. They are yet another example of how impressive and intricate our bodies are, and how even the simplest processes at the smallest level of life can have measurable impacts on health. As is so often seen and commented on in science, the microcosm reflects the macrocosm. Just as we consume food from our surroundings, so too do our cells, and while we don’t capture our hamburgers with a plasma membrane, there are certainly parallels if you pay attention!

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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 the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.

There are two primary mechanisms that transport these large particles: endocytosis and exocytosis.

Learning Objectives

• Describe endocytosis and identify different varieties of import, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
• Identify the steps of exocytosis

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 intracellular vesicle formed from the plasma membrane.

Phagocytosis

Figure 1. In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: Mariana Ruiz Villareal)

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 1).

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell 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 the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents 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.

Pinocytosis

Figure 2. In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. (credit: Mariana Ruiz Villareal)

A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome (Figure 2).

A variation of pinocytosis is called potocytosis. This process uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin.

The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-Mediated Endocytosis

Figure 3. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane. (credit: modification of work by Mariana Ruiz Villareal)

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances (Figure 3).

In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound 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.

Some human diseases are caused by the failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. 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 from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed in the last section 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 interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure 4). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

Figure 4. In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal)

A summary of the cellular transport methods discussed is contained in Table 1, which also includes the energy requirements and materials transported by each.

Cells perform three main types of endocytosis. Phagocytosis is the process by which cells ingest large particles, including other cells, by enclosing the particles in an extension of the cell membrane and budding off a new vacuole. During pinocytosis, cells take in molecules such as water from the extracellular fluid. Finally, receptor-mediated endocytosis is a targeted version of endocytosis where receptor proteins in the plasma membrane ensure only specific, targeted substances are brought into the cell.

Exocytosis in many ways is the reverse process from endocytosis. Here cells expel material through the fusion of vesicles with the plasma membrane and subsequent dumping of their content into the extracellular fluid.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.