Biomedical Engineering Reference
In-Depth Information
As described earlier, transport of glucose through the cell membrane is a capacity-
limited reaction because of the enzyme carrier. Glucose concentration in the blood varies
from a typical value of 90
mg
100 mL
, up to 200
mg
100 mL
after eating, and down to 40
mg
100 mL
three
hours after eating. Thus, the input,
J
o
, is a function of eating. The body uses the following
two mechanisms to control glucose concentration:
1.
Automatic feedback involving insulin secretion by the pancreas
2.
The liver
The pancreas, in addition to digestive functions, secretes insulin directly into the blood.
Insulin facilitates diffusion of glucose across the cell membrane. The rate of insulin secre-
tion is regulated so glucose is maintained at a constant level.
The liver acts as a storage vault for glucose. When excess amounts of glucose are present,
almost immediately two-thirds is stored in the liver. Conversely, when the glucose level in
the blood falls, the stored glucose in the liver replenishes the blood glucose. Insulin has a
moderating effect on the function of the liver. The binding of the carrier enzyme with glu-
cose is a function of the transfer rate,
1
, and is a function of the insulin level, which can
increase the transport of glucose by as much 20 times the base rate without insulin. We will
ignore the aspects of insulin and liver storage in our model.
K
8.4.4 Active Transport
Now consider active transport, which is similar to carrier-mediated transport but oper-
ates against the concentration gradient and uses energy to move the substrate across the cell
membrane. Active transport uses an enzyme carrier in the cell membrane that has a selec-
tive binding site for a substrate, which, when bound, transports the substrate through the
membrane to be released inside the cell.
Active transport uses energy to run, typically by the hydrolysis of ATP. Here, the energy
from ATP is used in the transport of the substrate, leaving ADP and an inorganic phosphate
PO
2
4
in the cytosol. The ADP is then recycled in the mitochondria to create more ATP
using glucose as described in the next section. Active transport is capacity-limited like car-
rier-mediated diffusion: as the quantity of the substrate increases, the transport reaction rate
increases and then saturates, as shown in Figure 8.20.
The Na-K pump is the most important active transport process, which pumps
Na
þ
out of
K
þ
against the
concentration gradient. This pump is used to maintain the ion gradients and resting mem-
brane potential, as described in Chapter 12, and is also required to maintain cell volume, as
described in the previous chapter.
Another important active transport process is the Na-Ca ATP-ase pump that keeps
the cell against the concentration gradient and replaces it inside the cell with
Ca
þ2
Ca
þ2
be kept
levels low inside the cell. It is vitally important that the concentration of
as compared to the outside concentration
approximately 10
7
M
low inside the cell
. The concentration gradient drives
approximately 10
3
M
Ca
þ2
into the cell, and the Na-Ca
Ca
þ2
out of the cell.
ATP-ase pump drives
Na-K Pump
The Na-K pump is an integral part of the cell membrane that exists in all cells in the body.
Approximately 70 percent of all ATP in the neuron and 25 percent of all ATP in all other cells
