concentration gradients (used in secondary active transport)
Exhibit a maximum level like carrier-mediated facilitated diffusion
1.
Primary active transport
Energy derived from ATP, changes shape of carrier protein which pumps substance across plasma membrane against
concentration gradient, 40% of ATP is expended
Most prevalent is
sodium-potassium pump
because part of it acts as ATPase – it must work non-stop to keep K+ and Na+
from leaking down their concentration gradients, the pump helps maintain normal tonicity on each side of the plasma
membrane ensuring the cells never shrink or explode due to osmosis, it works in these steps
i.
Thee Na+ in cytosol binds to the pump protein
ii.
Binding of Na+ triggers hydrolysis of ATP into ADP which attaches a phosphate group to the pump protein changing
its shape and expelling the three Na+ into the extracellular fluid, now the shape favors binding to two K+
iii.
The binding of the K+ triggers release of the phosphate group again changing the shape of the pump protein again
iv.
As the pump protein reverts to its original shape it releases the K+ into the cytosol and is read to bind to the Na+
again
2.
Secondary Active Transport
Energy stored in a Na+ or H+ concentration gradient is used to drive other substances across the membrane against their own
concentration gradients, these gradients are established by primary transport and thus secondary active transport indirectly
uses energy obtained from the hydrolysis of ATP (the steep concentration gradient of Na+ results in storage of potential
energy, and when there is a leak for Na+ to come back in this energy is converted to kinetic energy and is used to transport
other substances). Secondary active transport harnesses the energy of other cells by providing routes for Na+ to leak through
into cells – the carrier protein binds to both Na+ and the other substance changing shape so that they both may enter the cell
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Symporters
move substances in the same direction,
antiporters
move two substances in opposite directions
3.
Vesicle Transport
A vesicle is a small, spherical sac that transport a variety of substance across the membrane and import materials from and
release materials into extracellular fluid
Endocytosis
process in which material move into a cell in a vesicle formed from the plasma membrane
-
three types:
i.
receptor-mediated endocytosis
– a highly selective type in which cells take up specific ligands (molecules that
bind to specific receptors). The vesicle is formed after a receptor protein recognized and binds to a particular particle
in the extracellular fluid. The process of endocytosis of LDL’s is as follows:
a.
binding
LDL particle binds to specific receptor (integral potein located in the clathrin-coated pits) to form a
receptor-LDL complex, here the protein clathrin attaches to the membrane on its cytoplasmic end, many clathrin
molecules come together forming a basketlike structure around the complexes causing the membrane to
invaginate or fold inward
b.
vesicle formation
