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. Some molecules can diffuse across the membrane, or be transported across with the help of membrane-bound proteins. For transport of larger cargo, however, cells use endocytosis and exocytosis to transport material in and out of the cell, respectively. There are roughly five categories of molecule 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 do so very slowly. Large non-polar molecules like vitamin A are also very slow to cross the cell membrane. And large polar molecules like glucose and highly polar charged ions like sodium, potassium, and chloride, 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 one very big molecule, it resorts to bulk transport, which comes in two styles, endocytosis and exocytosis.
Endocytosis is a process that cells use to engulf extracellular material, and exocytosis is the opposite of that. 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, lysosome heads over to the cell membrane to expel the leftovers out into the extracellular space, like a cellular burp. Pinnocytosis, on the other hand, literally 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 not 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, penocytosis 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 as well. And once inside the cell, motor proteins like kinesin or dynion carry the penocytosis vesicle using ATP deeper into the cytosol. At the same time, the vesicle slowly releases the extracellular fluid and the solutes into the cytosol as well.
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're also called coded 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 receptor. At that point, the vesicle splits into two vesicles, one which has all of the LDL that's been brought into the cell, and the other that has all of the LDL receptors. The LDL-filled vesicle goes to the lysosome for digestion, while the one containing the 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. It's kind of like washing and reusing a dish instead of buying new plates for every meal. Now exocytosis on the other hand starts deep within the cell in an organelle called the Golgi apparatus. This organelle takes the proteins, lipids, and hormones that are generated in both the rough and smooth endoplasmic reticulum 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 the 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 toward 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 in order to happen. Helping current and future clinicians focus, learn, retain, and thrive.
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