Biology Reference
In-Depth Information
FIGURE 14.3 Osmosis and osmotic pressure. Water is placed in a U-shaped tube where each of the tube arms is
separated by a semi-permeable membrane with pores of a size that water can easily pass through but a solute
cannot. Upon addition of the solute to the tube's right arm, water diffuses from left to right (high water potential to
low). The column of water in the tube's right arm (the one containing the solute) rises until the extra weight of the
column equals the osmotic pressure caused by the solute. A pump could then be used to counter the osmotic
pressure whereupon the solution columns in the right and left arms of the tube are made the same. The pump
pressure required to equalize the height of the two columns is the osmotic pressure [4] . Note a small amount of
solute leaks from right to left since no filter is perfect.
can be easily followed by changes in absorbance due to light scattering using a simple spec-
trophotometer. Therefore, osmosis has been investigated for many years using common
and inexpensive methodologies and a lot is known about the process.
Membranes are rarely, if ever, perfectly semi-permeable. Deviation from ideality is defined
by a reflection coefficient (
s
). For an ideal semi-permeable membrane where a solute is totally
impermeable,
s ¼
1. If a solute is totally permeable (its permeability is equal to water),
s ¼
0.
Biological membranes are excellent semi-permeable barriers with
s ¼
0.75 to 1.0.
B. SIMPLE PASSIVE DIFFUSION
Movement of solutes across membranes can be divided into two basic types, passive diffu-
sion and active transport [7] . Passive diffusion requires no additional energy source other
than what is found in the solute's electrochemical (concentration) gradient and results in
the solute reaching equilibrium across the membrane. Passive diffusion can be either simple
passive diffusion, where the solute crosses the membrane anywhere by simply crossing the
lipid bilayer, or facilitated passive diffusion, where the solute crosses the membrane at
specific locations where diffusion is assisted by solute-specific facilitators or carriers. Active
transport requires additional energy, often in the form of ATP, and results in a non-
equilibrium, net accumulation (uptake) of the solute on one side of the membrane. The basic
types of membrane transport, simple passive diffusion, facilitated diffusion (by channels and
carriers) and active transport are summarized in Figure 14.4 [8] . There are countless different
examples of each type of membrane transport process [7] . Only a few representative exam-
ples will be discussed here.
Search WWH ::




Custom Search