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What is Endocytosis?

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  • Written By: Katriena Knights
  • Edited By: A. Joseph
  • Last Modified Date: 05 December 2016
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In order to function properly and remain healthy, cells must absorb certain nutrients and other substances. Many of the molecules necessary for cell health, including proteins and other nutrients, are too large to pass through the cell membrane, so the cell depends on a process called endocytosis to absorb these substances. Endocytosis allows the cell to surround vital molecules with its cell membrane, thus absorbing them into the body of the cell.

During endocytosis, a portion of the cell membrane surrounds the substance to be absorbed, forming a pocket that projects into the interior of the cell. This membrane then closes off around the substance, creating a vesicle called an endosome or phagosome that then migrates into the cytoplasm of the cell. The endosome delivers its contents to another structure within the cell called a lysosome. Endosomes and lysosomes fuse together, allowing a digestive process to occur within the lysosome. A similar process, called exocytosis, occurs in reverse to excrete substances from the interior of the cell.

Depending on the type of substance being absorbed, endocytosis can be referred to by different terms. Phagocytosis involves cell absorption of a solid material, and in pinocytosis, the cell absorbs a liquid. Macropinocytosis absorbs liquid, as well, but also often brings in solid material at the same time. Many viruses exploit macropinocytosis in order to enter cells. Viruses stimulate the cell to absorb extracellular liquids, then travel into the cell along with the liquid.

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Clathrin-mediated endocytosis uses specialized receptors to attract and absorb specific materials such as lipoproteins and antibodies. In clathrin-mediated endocytosis, the cell forms a pit, or a caveolae, on the exterior membrane. This pit is coated with receptors. When the pit becomes filled with the lipoproteins or other molecules that are intended to attach to the existing receptors, the pitted area forms a vesicle and is absorbed into the cell. Most cells have caveolae on their membrane surfaces, but some, most notably neurons, or nerve cells, have none.

Endocytosis also serves as a defense mechanism against bacteria by absorbing bacteria into the cell, then destroying them within the liposome. Bacteria such as salmonella bacteria short-circuit this process by producing a protein that makes it impossible for the phagosome to fuse with the liposome. Tuberculosis spreads through cells in a similar manner. Under normal conditions, the phagosome is coated with tryptophan-aspartate-containing protein, which is removed when the phagosome and lysosome fuse. The tuberculosis bacterium prevents the coat protein from being removed so the phagosome cannot fuse to the lysosome.

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pastanaga
Post 3

@Fa5t3r - I wonder if there are any scientists studying the way some bacteria can disguise themselves as human cells, so that they can start applying the same methods to transplanted human tissue, like organs and skin. If they could fool the human immune system into not thinking that kind of tissue is an invader, they wouldn't have to put the recipient onto anti-rejection drugs.

The funny thing is, those same tissues are probably being fed by the body through endocytosis and other means, even while defense systems are trying to destroy it.

Fa5t3r
Post 2

@Mor - There's a few computer games where they take advantage of that same imagery in order to encourage sick children to feel like they have some power over their own disease.

I actually think someday they will be able to increase the amounts and the potency of the cells we have fighting diseases with endocytosis, but it's important to realize that it isn't the only disease fighting mechanism we have, and in fact it's a weakness in some cases. AIDS for example, works by exploiting cells who swallow it, by incorporating itself into their DNA once it's inside the cell.

Other bacteria and viruses manage to disguise themselves as human cells in other ways so that they won't be digested. It's almost like a tiny ecosystem with predators and prey who have different survival strategies.

Mor
Post 1

This process always makes me think of those people who try to imagine their cancer (or other disease) cells being eaten by their own blood cells in order to try and get better.

They go through meditation sessions where they think of their white blood cells as an army who is fighting invaders and try to send strength to that army. I'm a bit skeptical that it would do any good, but on the other hand, the placebo effect can be very strong, so if someone really believes that kind of meditation can help them, it may actually help them.

I guess the idea of endocytosis is really visceral for humans, so it's something we can easily get behind.

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