Biomedical Engineering Reference
In-Depth Information
glucose transport. Once glucose passes through the intercellular cleft (see
Section 7.3
), a
carrier protein is required to move glucose into individual cells, so that the glucose can be
used as an energy source. In most cases, glucose transport is mediated by a coupled pro-
tein that moves some molecule in the direction of its electrochemical gradient and at the
same time moves glucose against its electrochemical gradient. The energy gained by mov-
ing the first molecule down its concentration gradient is used to move glucose against its
gradient. Remember that glucose is typically in a higher concentration within the cells
than outside of the cell. In the most common transport mechanism for glucose, sodium is
moved in the direction of its concentration gradient (i.e., into cells) at the same time that
glucose is moved into the cell. The energy that is gained from sodium movement is used
to transport glucose against its gradient. This transporter only works after both sodium
and glucose have interacted with the protein, which most likely occurs when the protein
is open to the extracellular space (because sodium is in a much higher concentration in the
extracellular space). Once sodium and glucose bind to the protein, there is a conforma-
tional change that closes the protein to the extracellular space and opens it to the intracel-
lular space. The conformational change also causes the release of sodium, and glucose
molecules into the cell after the conformational change. Many of these types of transpor-
ters exist for various compounds coupled to glucose movement.
In many tissues, this asymmetric distribution is used to move glucose through cells that
line a cavity/lumen, for example, the epithelial cells that line the intestines. There is a
high concentration of glucose within the epithelial cells but a low concentration of glucose
on either the intestinal side or the basal side of the cells. To effectively transport glucose
through the intestinal wall, the sodium-glucose co-transporter brings glucose and sodium
from the intestine into the epithelial cells. A carrier protein specific for glucose is also
found on the basal side of the cell and uses the glucose concentration gradient to move
glucose out of the cell. There is a similar mechanism within the kidneys. Glucose transport
is difficult to model because of the coupling of the proteins conformational changes and
binding kinetics. The affinity of the protein for glucose is dependent on the presence of
other ions (such as sodium and its concentration gradient), the pH of the solution, the
quantity of proteins within the cell membrane, and the concentration gradient of glucose.
Therefore, for most molecules that use this mechanism of transport, many assumptions
must be made to conduct some form of analysis.
7.3 VASCULAR PERMEABILITY
We will now move our discussion to generalizations about how molecules move out of
and into the vascular system. Diffusion accounts for the majority of exchange between the
vascular system and the cells throughout the body. The amount of water and dissolved
solutes that move allow for a constant mixing between these two compartments. As we
know, diffusion is a result of random thermal motion of molecules. If the molecule is lipid
soluble (i.e., hydrophobic), then it can diffuse directly through the endothelial cell mem-
brane. These molecules are typically small and uncharged. There is no direct transport
mechanism (e.g., channels, transporters, among others) for these types of molecules within
the cell membrane. The permeability of these molecules is typically much larger than other
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