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, cells use endocytosis and exocytosis to transport material in and out of the cell, respectively.

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.

And large, polar molecules, like glucose, as well as highly polar, charged ions like Na+, K+, Cl-, or molecules that possess a charge, like amino acids are 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 - a water channel and chloride channels which let chloride ions get across membranes, or 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 that 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.