Endocytosis and exocytosis

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 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 do so very slowly. Large, nonpolar 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 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 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, 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, Literraly 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 as well. 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 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.