Biomedical Engineering Reference
In-Depth Information
•
Solutions
: are the mixtures of solutes and solvents in different proportions giv-
ing different final concentrations in chemical systems. In a physiological, cellu-
lar systems the solutions can be classified with respect to normotonic solution of
the cell (isotonic, hypotonic, and hypertonic) solutions. There are a couple laws
that can be applied to the solutions in the cell, such as:
•
Fick
'
s first Low of Diffusion
:
Jx Ddcdx
=
/
where
Jx
is the rate of diffusion, D is difference in concentration, and
dc
/
dx
is a
membrane thickness.
•
Va n
'
t Hoff
'
s equation for osmotic pressure
:
=
RT c
i
where the osmotic pressure, ʠ, in dilute solutions is ʠ =
RT
ʣ
c
i
,
R
is the universal gas
constant,
T
is the absolute temperature, and
c
i
is the molar concentration of solute
i
.
Many molecules do not diffuse through lipid bilayers. There are accessory mol-
ecules (constitutive molecules) in membrane bilayer that regulate the transport of
the molecules that do not pass freely through the lipid bilayer [
4
].
PPS
Facilitated Transport via Transporters
Glucose transport protein shuttles glucose molecule through the hydrophobic
membrane bilayer which is impermeable for glucose. This is possible through con-
formational changes of the protein stimulated by binding of glucose to the protein.
Facilitated transport protein as glucose transporter is present in the cell membrane
in limited number. Both facilitated transport and simple diffusion depend on the
concentration gradient; net solute transport always occurs from high to low concen-
tration [
4
,
5
].
Active transport systems
are similar to facilitated transport systems; both
involve the participation of trans-membrane proteins that bind a specific solute.
In primary active transport systems, however THE ENERGY IS PROVIDED (most
often by the hydrolysis of ATP) to drive a conformational change in the transporter
that leads to solute transport. (Na
+
/K
+
pump or ATP-ase activity is moving both
anions “up” their gradients). Na
+
is extracellular and K
+
intracellular cation [
4
,
5
]
(Fig.
3.2
).
Secondary active transport systems also move solutes “up” their concentration
gradients, but they gain the energy from the different source (co-transporters and
exchangers) (Fig.
3.3
).
These conditions establish membrane potential, which a the systems start work-
ing create action potential, characteristic with Na ions entering the cell and K ions
out the cell. High Na influx will cause the spike, which will after repolarization
establish the resting potential again. Action potentials are very important in excit-
able tissues such as heart and brain [
4
,
5
].
