AssessmentsEndocytosis and exocytosis
Endocytosis and exocytosis
USMLE® Step 1 style questions USMLE
An investigator is studying virulence factors that assist bacteria to evade host immune responses. Human white blood cells and group A streptococci are inoculated together, with observed uptake of group A streptococci into the cytoplasm of macrophages. Which of the following structures are unique to this endocytosis pathway?
Content Reviewers:Rishi Desai, MD, MPH
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, 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.
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”.
Then the edges of the cup come together, forming a vesicle.
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.