Active Transport
Slide 2
While passive transport processes make use of concentration and electrochemical gradients to move substances across biological membranes without any energy
input, active transport processes move substances against gradients and require an energy
input. In this module we will see how cells make use of ATP and electro-chemical
gradients to move substances into and out of the cell against their concentration gradients.
We will also look at the processes of exocytosis and endocytosis, ways in which large
molecules are moved across the membrane by vesicle-mediated transport.
Slide 3
As with facilitated diffusion, active transport makes use of very specific
proteins that are embedded in the phospholid membrane. These proteins may be
uniports, symports, or antiports. Uniports move one specific type of molecule in one
direction. Symports move two substances in the same direction at the same time. In
some cases symports must bind both substances at the same time in order to function
properly. Antiports move two different substances in opposite directions.
Slide 4
Active transport systems can be classified as either primary or secondary.
Primary active transport requires the hydrolysis of ATP to drive the movement of the
different substances across a membrane. Remember that these substances are moving
against their concentration gradients, so an energy input is required. The sodium (Na+)potassium (K+) pump found in animal cells provides an example of primary active
transport. This pump is an integral membrane protein that uses the energy released by the
hydrolysis of ATP to pump Na+ ions out of the cell and K+ ions into the cell against their
concentration gradients. The cell needs to maintain these gradients for various reasons,
one of which we will see in the next slide.
In this illustration the sodium ions and ATP bind to the protein. Then the hydrolysis of
ATP causes the protein to change shape and transport the sodium ions to the other side of
the cell membrane. As the potassium ions are bound to the protein on the outside of the
cell, the sodium ions are released and the potassium ions are moved to the inside of the
cell.
Slide 5
Rather than directly using ATP as an energy source, secondary active
transport makes use of concentration gradients set up by primary active transport
processes. Let’s expand on the example of the sodium-potassium pump we just looked
at. After this pump has done its work, there is a high concentration of sodium ions
outside of the cell. These sodium ions may now passively diffuse back across the
membrane by way of carrier proteins. As these ions “fall” back down their concentration
gradient, they provide a small amount of energy that is used to transport glucose
molecules into the cell against their concentration gradient through the same carrier
proteins. You might think of this process as being similar to water flowing over a
waterfall and dragging a tree trunk along with it. Note that both antiport and symport
proteins are involved in this process.
Slide 6
Not everything that a cell needs to bring inside or move outside can
diffuse through the membrane or fit through a membrane protein. Larger particles or
volumes of water are moved into cells by a process called endocytosis. In endocytosis,
the cell membrane folds around a particle, engulfing it, and then pinches off to form a
vesicle that brings the particle into the cell. There are several specific types of
endocytosis. In phagocytosis, vesicles fuse with a lysosome to digest the particle that they
contain. In pinocytosis the vesicle encloses water with dissolved substances in it.
Receptor-mediated endocytosis acts to bring specific macromolecules into the cell.
Receptor proteins on the outer surface of the cell membrane recognize and bind to
specific molecules outside the cell. When the correct molecule binds to the receptors, the
molecule is brought into the cell by endocytosis as we just described. The process of
receptor-mediated endocytosis has been well studied in the case of cholesterol uptake
from the bloodstream. Cholesterol, which is an important component of cell membranes,
moves through the blood and is taken up by cells via receptor-mediated endocytosis.
People with a deficient receptor for this type of endocytosis have too much cholesterol in
their blood because the cholesterol cannot bind to and be taken up by the cell.
Exocytosis is the opposite process to endocytosis. Substances such as waste or materials
to be exported from the cell are moved in vesicles to the plasma membrane. The vesicle
then fuses with the membrane and opens to the outside of the cell, releasing its contents.
Exocytosis acts to export many kinds of substances, from waste to proteins to
neurotransmitters.