9.19 Osmosis and Osmotic Pressure
375
Divers have learned to ascend slowly, giving time for the blood to release its
dissolved nitrogen to the breathing air, or, on ascent, before symptoms show, to
quickly enter a hyperbaric chamber, which slowly (over hours) brings the air from
the diving pressure down to the normal one atmosphere of pressure.
Astronauts breathe air at the same pressure as at the surface of the Earth.
Pressures below about 20% sea-level will cause hypoxia in humans, even if they
breathe 100% oxygen. The cabins of commercial passenger airplanes flying at 10 km
(33,000 ft), where the outside pressure is just 26% of that at sea level, are pressurized
to about 75% of sea-level pressure.
9.19 Osmosis and Osmotic Pressure
If a biological cell is immersed in a solution not matching its interior fluids, the
concentration gradients across the cell wall will cause a flow of material into or out
of the cell, for all substances which can pass through the cell-wall. This passage
might be through pores and/or by diffusion through the wall. The concentrations
of these substances (number of molecules per unit volume) on either side of the
wall will tend to equalize, assuming no other processes are involved. However, a
flow of material into a cell, acting alone, will expand the volume of the cell. The
cell walls respond by pressing back on the interior fluid, creating added interior
pressure. That added pressure pushes material out of cell, perhaps through the same
pores. Equilibrium can be established when the flow in by a concentration gradient
matches the flow out by a pressure gradient.
Imagine a cell with a solution in its interior, and with a cell wall permeable only
to water, 51 i.e. the cell wall is a ‘semi-permeable membrane’. Assume also that the
cell wall has some elasticity and strength. If we put such a cell into a larger volume
of water, water will diffuse into the cell because the substances dissolved in the
interior water makes that water less concentrated than the water on the exterior.
With flow in, the cell will swell in volume. The back pressure from the cell walls
will act to push water molecules out of cell. 52 We should expect that the osmotic
pressure will be proportional to the concentration gradient. Let’s see if this follows
from thermodynamical considerations.
We will use Gibbs’ balancing of chemical potentials to find a relationship
between the extra pressure across the cell wall, called the ‘osmotic pressure’, to
the concentration gradient.
51 For biological cells, with their bilayer-phospholipid walls, water can slowly diffuse through, but
the walls also have aquapin pores that pass water molecules more rapidly than diffusion.
52 The water molecules moving into the cell due to the differences in concentration of water act like
an expanding gas, and will tend to cool the interior, but the molecules being pushed out by pressure
generate heat by compression. The two effects mean the combined processes can take place at a
fixed temperature.
375
Divers have learned to ascend slowly, giving time for the blood to release its
dissolved nitrogen to the breathing air, or, on ascent, before symptoms show, to
quickly enter a hyperbaric chamber, which slowly (over hours) brings the air from
the diving pressure down to the normal one atmosphere of pressure.
Astronauts breathe air at the same pressure as at the surface of the Earth.
Pressures below about 20% sea-level will cause hypoxia in humans, even if they
breathe 100% oxygen. The cabins of commercial passenger airplanes flying at 10 km
(33,000 ft), where the outside pressure is just 26% of that at sea level, are pressurized
to about 75% of sea-level pressure.
9.19 Osmosis and Osmotic Pressure
If a biological cell is immersed in a solution not matching its interior fluids, the
concentration gradients across the cell wall will cause a flow of material into or out
of the cell, for all substances which can pass through the cell-wall. This passage
might be through pores and/or by diffusion through the wall. The concentrations
of these substances (number of molecules per unit volume) on either side of the
wall will tend to equalize, assuming no other processes are involved. However, a
flow of material into a cell, acting alone, will expand the volume of the cell. The
cell walls respond by pressing back on the interior fluid, creating added interior
pressure. That added pressure pushes material out of cell, perhaps through the same
pores. Equilibrium can be established when the flow in by a concentration gradient
matches the flow out by a pressure gradient.
Imagine a cell with a solution in its interior, and with a cell wall permeable only
to water, 51 i.e. the cell wall is a ‘semi-permeable membrane’. Assume also that the
cell wall has some elasticity and strength. If we put such a cell into a larger volume
of water, water will diffuse into the cell because the substances dissolved in the
interior water makes that water less concentrated than the water on the exterior.
With flow in, the cell will swell in volume. The back pressure from the cell walls
will act to push water molecules out of cell. 52 We should expect that the osmotic
pressure will be proportional to the concentration gradient. Let’s see if this follows
from thermodynamical considerations.
We will use Gibbs’ balancing of chemical potentials to find a relationship
between the extra pressure across the cell wall, called the ‘osmotic pressure’, to
the concentration gradient.
51 For biological cells, with their bilayer-phospholipid walls, water can slowly diffuse through, but
the walls also have aquapin pores that pass water molecules more rapidly than diffusion.
52 The water molecules moving into the cell due to the differences in concentration of water act like
an expanding gas, and will tend to cool the interior, but the molecules being pushed out by pressure
generate heat by compression. The two effects mean the combined processes can take place at a
fixed temperature.