Endocytosis and exocytosis

Definitions & Key takeaways

Endocytosis is the process of taking in material by enclosing it in a vesicle. Exocytosis is the opposite process, where the vesicle fuses with the plasma membrane and expels its contents to the outside of the cell. Endocytosis and exocytosis work together to allow cells to take in nutrients from their environment and to excrete waste products.

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Cells transport material in and out across their cell membrane, which is a barrier made up of a double layer of lipids with embedded protein and carbohydrate components.
And there are roughly five categories of molecules that try to get across the cell membrane. Small non-polar molecules, like oxygen or carbon dioxide, are able to diffuse rapidly through the cell membrane.
Small, polar molecules, like water, can cross as well, but very slowly. Large, nonpolar molecules like Vitamin A, are also very slow to cross the cell membrane.
Large, polar molecules, like glucose; and highly polar, charged ions like Na+, K+, Cl-; as well as molecules that possess a charge, like amino acids; are all highly unlikely to get across a cell membrane on their own.
So many of these molecules - some common ones being water, glucose, and ions, pass through the membrane using transport proteins.
Examples of transport proteins include channels, like aquaporins, which is a water channel; and chloride channels, which let chloride ions cross membranes; as well as carriers - such as the glucose transporter.
However, when the cell needs to transport a lot of molecules, or a very big molecule, it resorts to bulk transport, which comes in two flavors: endocytosis and exocytosis.
Endocytosis is a process that cells use to engulf extracellular material. And exocytosis is the opposite process, during which cells expel material into the extracellular space.
Both endocytosis and exocytosis need energy in the form of adenosine triphosphate or ATP, used in the movement of the substances in and out of the cell.
There are three types of endocytosis - phagocytosis, pinocytosis and receptor-mediated endocytosis. Phagocytosis - where phago- means to eat - is used by white blood cells like macrophages and neutrophils which patrol the body looking for debris, bacteria and dead cells to eat.
So let’s imagine that a macrophage comes across a particularly bothersome Streptococcus - first, the Strep attaches to macrophage receptors on its cell surface.
The macrophage then extends arm-like projections called pseudopods around the Strep - like a death hug. Then the Strep is slowly engulfed by the cell membrane, which invaginates to form a vesicle on its inner side.
The vesicle then separates from the cell membrane forming a phagosome. During this step, an electron pump uses ATP to pump protons into the phagosome, lowering the pH inside.
In the cytoplasm, the phagosome encounters an organelle called a lysosome, which contains digestive enzymes. The lysosome and the phagosome fuse together, merging their contents forming a structure known as the phagolysosome.
Inside the phagolysosome, lysosomal enzymes start destroying the bacteria with the help of an acidic pH. After it’s all over, the lysosome heads over to the cell membrane to expel the leftovers out into the extracellular space - like a cellular burp.
Pinocytosis, on the other hand, means “the cell drinks”. In pinocytosis, the cell’s plasma membrane invaginates to form a small cup around the portions of extracellular fluid and solutes that are dissolved in it.
Then the edges of the cup come together, forming a vesicle. Since the cell is not really “eating” anything other than the occasional solute, the result is not a phagosome, but merely a vesicle.
The pinocytosis vesicle is much smaller than a phagosome. Also, unlike phagocytosis, pinocytosis is a non-specific way for cells to take in solutes - so whatever solutes are hanging around in the extracellular fluid get pulled inside the cup.
Once inside the cell, motor proteins like kinesin, or dynein carry the pinocytosis vesicle using ATP deeper into the cytosol.
At the same time, the vesicle slowly releases the extracellular fluid and the solutes into the cytosol. Now, finally, sometimes endocytosis involves special receptor proteins on the cell membrane - so it’s called receptor-mediated endocytosis.
Some molecules are taken in this way, like transferrin, which is an iron-binding protein, or low density lipoproteins - or LDL, which contain cholesterol.
As an example, let’s see how this goes for LDL. Now, on the surface of the cell membrane, there are indented pits that have specific receptors for molecules like LDL.
These pits are covered on the intracellular side of the cell membrane by a layer of clathrin proteins - so they are also called coated pits.
Now, let's say LDL binds to its receptor in one of these pits. The edges of the pit start coming together.At the same time, the clathrin proteins inside the cell link up to one another like a sturdy shell around the forming vesicle.
Once the vesicle pinches off from the cell membrane, the clathrin proteins detach from it and go back to the cell membrane.
Inside the cell, the vesicle merges with an organelle called the endosome. Endosomes are similar to lysosomes, in that they also fuse with ingested vesicles, but they can also do something else: endosomes can separate the LDL particle from the LDL receptor it bound to.
This is because the endosome has a proton pump that uses ATP to generate a low pH within, which causes the LDL to separate from the LDL receptor.
At that point, the vesicle splits into two vesicles, one vesicle that has all of the LDL that’s been brought into the cell, and the other vesicle that has all of the LDL receptors.
The LDL-filled vesicle goes to the lysosome for digestion, while the one containing the LDL receptors goes back and releases the receptor back on the surface of the cell membrane.
This is called receptor recycling - because now the LDL receptor can bind to another LDL molecule, and the cycle can repeat itself.
Kinda like washing a dirty dish and using it again, instead of using new plates for every meal. Now, exocytosis, on the other hand, starts deep within the cell, in an organelle called the Golgi apparatus, that takes the proteins, lipids, and hormones that are generated in the rough ER and smooth ER, and packages them into a vesicle that can be ziplined around the cell using the cytoskeleton.
The cytoskeleton is made out of proteins like microfilaments, microtubules, and intermediate filaments, which all provide structural stability.
The cytoskeleton is also very dynamic, allowing the cell to change shape by selectively extending and contracting filaments - which is important in some cell functions, like muscle contraction, cell division, and even cell movement!
The cytoskeleton also helps structures within the cell move from one area to another. Now, there are secretory vesicles which move molecules out of the cell with help of motor proteins like kinesin, or dynein, which pick up the vesicle and carry it towards the cell membrane along microtubules using ATP as fuel.
The vesicle moves towards the cell surface, fuses with the cell membrane, and ruptures on its external side, spilling its contents into the extracellular space.
Alright, as a quick recap, endocytosis refers to the process in which cells engulf extracellular material, and there are different forms of endocytosis like phagocytosis, pinocytosis and receptor-mediated endocytosis.
Exocytosis refers to the process in which cells expel material into the extracellular space. Both endocytosis and exocytosis require ATP to happen.