Active Transport
During active transport, molecules move against the concentration gradient, toward the
area of higher concentration. This is the opposite of diffusion. Active transport requires
both an input of energy, in the form of ATP, and a carrier protein to move the molecules.
These proteins are often called pumps, because, as a water pump uses energy to force water
against gravity, proteins involved in active transport use energy to move molecules against
their concentration gradient.
There are many examples of why active transport is important in your cells. One example
occurs in your nerve cells. In these cells, the sodium-potassium pump(Figure4.5)
moves sodium outside the cell and potassium into the cell, both against their concentration
gradients.
Figure 4.5: The sodium-potassium pump moves sodium ions to the outside of the cell and
potassium ions to the inside of the cell. ATP is required for the protein to change shape. As
ATP adds a phosphate group to the protein, it leaves behind adenosine diphosphate (ADP).
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Transport Through Vesicles
Some large molecules are just too big to move across the membrane, even with the help of a
carrier protein. These large molecules must be moved through vesicle formation, a process
by which the large molecules are packaged in a small bubble of membrane for transport.
This process keeps the large molecules from reacting with the cytoplasm of the cell. Vesicle
formation does require an input of energy, however.
There are several kinds of vesicle formation that allow large molecules to move across the
plasmamembrane.Exocytosismoveslargemoleculesoutsideofthecell. Duringexocytosis,
the vesicle carrying the large molecule fuses with the plasma membrane. The large molecule
is then released outside of the cell, and the vesicle is absorbed into the plasma membrane.
Endocytosisistheprocessbywhichcellstakeinlargemoleculesbyvesicleformation. Types