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Endocytosis and exocytosis
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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.
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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