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Intro to Anatomy 4: Cell Structure and Function
Intro to Anatomy 4: Cell Structure and Function
The Lone Ranger
Published by The Lone Ranger
01-07-2007
Default Active Transport of Substances Across Cellular Membranes


Active Transport of Substances Across Cellular Membranes:
When substances are transported across cellular membranes actively, the cells must expend energy in the process. Usually, it’s because the cells are transporting substances against the concentration gradient. Active transport involves either the use of membrane proteins to transport substances, or movement of the cellular membrane itself. Glucose is a good example of a molecule that is actively transported into cells against the concentration gradient.


Active Transport:
It may be a little confusing, but when carrier proteins temporarily bind to molecules and physically transport them across cellular membranes against the concentration gradient, the process is called “active transport.” In other words, “active transport” is a kind of active transport.


Endocytosis and Exocytosis:
Endocytosis and exocytosis are the other form of active transport. Endocytosis and exocytosis are basically the same process, but run in different directions. Both involve movement of the cellular membranes themselves, and movement of vesicles or vacuoles within the cytoplasm.


“Endo” means “inside,” and “cyto” means “cell.” So endocytosis means to bring something inside of a cell. Specifically, endocytosis occurs when cells engulf substances with their plasma membranes and bring them inside the cytoplasm in vacuoles or vesicles. For instance, when an amoeba engulfs a victim, it is performing endocytosis. Substances brought into a cell through endocytosis are enclosed within a vacuole or vesicle, which can travel through the cytoplasm. Lysosomes may fuse with the vesicle and release digestive enzymes into it.

If the substance engulfed by the cell is relatively large, and especially if it’s solid, endocytosis is referred to as phagocytosis (“cell eating”). For example, some white blood cells can engulf and destroy cells nearly as large as themselves.

If the substance engulfed by the cell is relatively small, and especially if it’s a liquid, endocytosis is referred to as pinocytosis (“cell drinking”).

Some substances will be brought into a cell through endocytosis only if they first bind to receptor proteins in the cell’s membrane. This is known as receptor-mediated endocytosis.


“Exo” means “outside,” so exocytosis is precisely the opposite of endocytosis. In exocytosis, a vacuole or vesicle containing some substance to be eliminated is transported to the plasma membrane, where it fuses with the membrane. When a vesicle or fuses with the plasma membrane, it is incorporated into it and therefore ceases to exist. This leaves the substance formerly contained within the vacuole or vesicle on the outside of the cell.


Endocytosis (top) and Exocytosis (bottom)

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