Osmosis and osmoregulation
Lecture
Osmoregulation (or hydromineral regulation) encompasses the processes
that regulate the water- and ion balance of an organism or cell.
The total concentration of osmotically active particles per litre of water is
expressed in Osmoles (Osmol) and is referred to as osmolarity.
Osmoles per kg water is referred to as osmolality. Under normal
circumstances, a litre of water weight 1 kg, which means there is no
practical difference.
Osmotic ‘problems’ are directly related to the ion concentrations outside
(environment) and inside (bodily fluids) of the animal. If there is no
difference, there is also no problem. If this difference is there, we speak of
an osmotic gradient; the osmotic gradient is the difference between the
two.
Diffusion
Diffusion is the random movement of particles as result of their thermal
energy. The diffusion can be calculated with:
∆C
J=D ∙
X
(It is not necessary to learn this formula by heart)
J = Net diffusion speed (amount per unit of time),
D = Diffusion coefficient (includes features such as permeability of the
membrane and
ΔC = Concentration gradient across distance X
If there is a difference in concentrations, or a gradient, there will be
diffusion. The larger this gradient, the more diffusion. If there is no
difference, when ΔC = 0, there is no net diffusion.
Osmosis
Osmosis is the spontaneous net movement of solvent molecules (such as
water) through a selectively permeable membrane from a region with high
water potential to a region with low water potential, in a direction that
tends to equalize the solute concentration on both sides. This solute can
be anything; Na, K, Cl, glucose, etc. They all count towards osmolarity.
Simply said, water moves towards the side
where the concentration of solutes is higher. If
this concentration is higher inside a cell (or
organism), osmosis will cause water to move
inside the semipermeable membrane of the cell.
If this is not regulated, the cell will swell and
eventually burst.
,The osmolarity inside our body is roughly 300 mOsm (300 miliosmol; 300
milimolar of osmotically active particles).
If such a cell with an osmolarity of 300 mOsm is placed in an environment
of the same osmolarity, there will be no net movement of water, and thus
no change in cell volume. We call this isotonic conditions.
If such a cell is placed in an environment with 200 mOsm – a lower
concentration – water will move towards the
higher concentration; inside the cell. The cell will
swell (and potentially burst). We call these
hypotonic conditions.
If a cell with an osmolarity of 300 mOsm is
placed in an environment with a much higher
osmolarity, the effect goes the other way; water
would rush out of the cell. These are hypertonic conditions. The cell
would lose water and shrink. This often results in a spiky phenotype of the
cell; this is because the cell would collapse onto the cytoskeleton.
The movement of water can be measured as osmotic pressure, named π.
The osmotic pressure (π) is essentially the osmotic difference in pressure
(Δπ) relative to the surrounding pressure. The Van ‘t Hoff equation:
π=R ∙ T ∙(n /V )
π = Osmotic pressure (in Pascal = N∙m2).
R = Gas constant (8.314 J∙mol-1K-1).
T = Temperature in Kelvin (K = ⁰C + 273.15).
n/V = Number of particles (mol) per unit of volume (m3) (1 mol L-1 = 1000
mol m-3) (mOsm/1000 = Osm) (If using mol L-1 directly, the answer will be
in kPa).
Amount of mol can be found by: mol = grams in solution/molar mass of
molecule.
, Now the osmotic pressure of a cell (or organism)
can be calculated, and the osmotic pressure of
the environment.
With those, the Δπ or gradient can be
determined.
These osmotic gradients contain pressure, and
thus energy. Enormous amounts of energy are released when fresh water
meets salt water, such as where the river turns into the sea. This energy
can be used to generate power through osmosis.
The reverse is also true; making fresh water from seawater is an energy-
demanding process.
Osmolarity and tonicity
Osmolarity and tonicity are not synonymous.
When two solutions exert similar osmotic pressure, they as isosmotic.
This is when they contain equal concentrations of osmotically active
particles.
A solution is isotonic relative to a cell, when that cell does not change it’s
volume in the solution. Tonicity is, therefor, defined based on the response
of the cell in the solution.
Isosmotic is not synonymous to isotonic.
In a situation with two compartments, with compartment
A having 0.5 m/L sucrose and urea, and compartment B
having 1 m/L sucrose, separated by a semipermeable
membrane (permeable to urea), the two compartments
are isosmotic to each other; they contain an equal
concentration of osmotically active particles.
They are, however, not hypotonic. This is because of the
membrane, which is permeable to
urea but not sucrose. As time
goes on, urea will move from A to B, changing the
concentrations. Sucrose will not move, as it cannot
cross the membrane. When the concentration in
compartment B rises, water will follow, and
compartment B will increase in volume.
Compartment B is hypertonic (increasing) and
compartment A is hypotonic.
Because the water follows to compartment B, the compartments are still
isosmotic; compartment B has a higher concentration solutes, but also a
higher amount of water.
Simply said: Osmolarity is the total
concentration of osmotically active
particles (n/V). Tonicity is that part of the
, osmotic pressure caused by particles to which the cell membrane is
impermeable.
Hyper: High concentration,
Hypo: Lower concentration.
Osmoregulation
Typically, the two environments
encountered for life are fresh water
or seawater. For seawater, the
osmolarity is set at 1000 mOsmol.
Freshwater can be anything below 20 mOsmol. Dealing with these two
different environments, requires totally different solutions.
Animals have to osmoregulate. If they did not, they would either
experience hypotonicity, causing cells to swell and burst, or hypotonicity,
which would cause the cells to decrease in volume; this would cause an
increase in ion strength (the cytoplasm would become more
concentrated). This then leads to macromolecular crowding (aggregation
and accumulation leads to precipitation of proteins, RNA/DNA molecules,
etc), but as the ionic strength increases, there is also disruption of protein
folding. Protein folding makes use of linking positive
and negative charges; with a very high ionic strength
in the cytoplasm, these charges are shielded and the
proteins cannot fold anymore.
Human beings consist of 60% water. This mostly
consists of intracellular fluid. The rest is extracellular
fluid; this consists of interstitial fluids (fluids between
cells) or blood plasma.
It is the extracellular fluid that is regulated by the
kidneys.
By regulating the osmolarity of the interstitial fluid
and blood plasma, the intracellular fluid is also
regulated as it is in contact with the cells.
Although the intracellular fluid has the same osmolarity as
the extracellular fluid, the composition is very different. In
extracellular fluid, such as blood plasma, there is a lot of
sodium, hardly any potassium, a lot of chloride and not
many proteins. Inside the cell, the concentration of sodium
is quite low, there is a lot of potassium and anionic
proteins.
Lecture
Osmoregulation (or hydromineral regulation) encompasses the processes
that regulate the water- and ion balance of an organism or cell.
The total concentration of osmotically active particles per litre of water is
expressed in Osmoles (Osmol) and is referred to as osmolarity.
Osmoles per kg water is referred to as osmolality. Under normal
circumstances, a litre of water weight 1 kg, which means there is no
practical difference.
Osmotic ‘problems’ are directly related to the ion concentrations outside
(environment) and inside (bodily fluids) of the animal. If there is no
difference, there is also no problem. If this difference is there, we speak of
an osmotic gradient; the osmotic gradient is the difference between the
two.
Diffusion
Diffusion is the random movement of particles as result of their thermal
energy. The diffusion can be calculated with:
∆C
J=D ∙
X
(It is not necessary to learn this formula by heart)
J = Net diffusion speed (amount per unit of time),
D = Diffusion coefficient (includes features such as permeability of the
membrane and
ΔC = Concentration gradient across distance X
If there is a difference in concentrations, or a gradient, there will be
diffusion. The larger this gradient, the more diffusion. If there is no
difference, when ΔC = 0, there is no net diffusion.
Osmosis
Osmosis is the spontaneous net movement of solvent molecules (such as
water) through a selectively permeable membrane from a region with high
water potential to a region with low water potential, in a direction that
tends to equalize the solute concentration on both sides. This solute can
be anything; Na, K, Cl, glucose, etc. They all count towards osmolarity.
Simply said, water moves towards the side
where the concentration of solutes is higher. If
this concentration is higher inside a cell (or
organism), osmosis will cause water to move
inside the semipermeable membrane of the cell.
If this is not regulated, the cell will swell and
eventually burst.
,The osmolarity inside our body is roughly 300 mOsm (300 miliosmol; 300
milimolar of osmotically active particles).
If such a cell with an osmolarity of 300 mOsm is placed in an environment
of the same osmolarity, there will be no net movement of water, and thus
no change in cell volume. We call this isotonic conditions.
If such a cell is placed in an environment with 200 mOsm – a lower
concentration – water will move towards the
higher concentration; inside the cell. The cell will
swell (and potentially burst). We call these
hypotonic conditions.
If a cell with an osmolarity of 300 mOsm is
placed in an environment with a much higher
osmolarity, the effect goes the other way; water
would rush out of the cell. These are hypertonic conditions. The cell
would lose water and shrink. This often results in a spiky phenotype of the
cell; this is because the cell would collapse onto the cytoskeleton.
The movement of water can be measured as osmotic pressure, named π.
The osmotic pressure (π) is essentially the osmotic difference in pressure
(Δπ) relative to the surrounding pressure. The Van ‘t Hoff equation:
π=R ∙ T ∙(n /V )
π = Osmotic pressure (in Pascal = N∙m2).
R = Gas constant (8.314 J∙mol-1K-1).
T = Temperature in Kelvin (K = ⁰C + 273.15).
n/V = Number of particles (mol) per unit of volume (m3) (1 mol L-1 = 1000
mol m-3) (mOsm/1000 = Osm) (If using mol L-1 directly, the answer will be
in kPa).
Amount of mol can be found by: mol = grams in solution/molar mass of
molecule.
, Now the osmotic pressure of a cell (or organism)
can be calculated, and the osmotic pressure of
the environment.
With those, the Δπ or gradient can be
determined.
These osmotic gradients contain pressure, and
thus energy. Enormous amounts of energy are released when fresh water
meets salt water, such as where the river turns into the sea. This energy
can be used to generate power through osmosis.
The reverse is also true; making fresh water from seawater is an energy-
demanding process.
Osmolarity and tonicity
Osmolarity and tonicity are not synonymous.
When two solutions exert similar osmotic pressure, they as isosmotic.
This is when they contain equal concentrations of osmotically active
particles.
A solution is isotonic relative to a cell, when that cell does not change it’s
volume in the solution. Tonicity is, therefor, defined based on the response
of the cell in the solution.
Isosmotic is not synonymous to isotonic.
In a situation with two compartments, with compartment
A having 0.5 m/L sucrose and urea, and compartment B
having 1 m/L sucrose, separated by a semipermeable
membrane (permeable to urea), the two compartments
are isosmotic to each other; they contain an equal
concentration of osmotically active particles.
They are, however, not hypotonic. This is because of the
membrane, which is permeable to
urea but not sucrose. As time
goes on, urea will move from A to B, changing the
concentrations. Sucrose will not move, as it cannot
cross the membrane. When the concentration in
compartment B rises, water will follow, and
compartment B will increase in volume.
Compartment B is hypertonic (increasing) and
compartment A is hypotonic.
Because the water follows to compartment B, the compartments are still
isosmotic; compartment B has a higher concentration solutes, but also a
higher amount of water.
Simply said: Osmolarity is the total
concentration of osmotically active
particles (n/V). Tonicity is that part of the
, osmotic pressure caused by particles to which the cell membrane is
impermeable.
Hyper: High concentration,
Hypo: Lower concentration.
Osmoregulation
Typically, the two environments
encountered for life are fresh water
or seawater. For seawater, the
osmolarity is set at 1000 mOsmol.
Freshwater can be anything below 20 mOsmol. Dealing with these two
different environments, requires totally different solutions.
Animals have to osmoregulate. If they did not, they would either
experience hypotonicity, causing cells to swell and burst, or hypotonicity,
which would cause the cells to decrease in volume; this would cause an
increase in ion strength (the cytoplasm would become more
concentrated). This then leads to macromolecular crowding (aggregation
and accumulation leads to precipitation of proteins, RNA/DNA molecules,
etc), but as the ionic strength increases, there is also disruption of protein
folding. Protein folding makes use of linking positive
and negative charges; with a very high ionic strength
in the cytoplasm, these charges are shielded and the
proteins cannot fold anymore.
Human beings consist of 60% water. This mostly
consists of intracellular fluid. The rest is extracellular
fluid; this consists of interstitial fluids (fluids between
cells) or blood plasma.
It is the extracellular fluid that is regulated by the
kidneys.
By regulating the osmolarity of the interstitial fluid
and blood plasma, the intracellular fluid is also
regulated as it is in contact with the cells.
Although the intracellular fluid has the same osmolarity as
the extracellular fluid, the composition is very different. In
extracellular fluid, such as blood plasma, there is a lot of
sodium, hardly any potassium, a lot of chloride and not
many proteins. Inside the cell, the concentration of sodium
is quite low, there is a lot of potassium and anionic
proteins.
