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  Biological Significance of Osmosis

Biological Significance:

1) Crenation and Hemolysis. The physiological/isotonic saline is 0.9per cent NaCl. While red blood cells are suspended in 0.3% NaCl (hypotonic solution), water will come into the cells and the cell will burst releasing all the contents of it. This type of lysis is known as hemolysis. The resultant membranes are known as ghosts. Alternatively, while the cells are placed in 1.5% NaCl, water emerges of the cell, leading to shrinkage of cells. The procedure is known as crenation.

2) The erythrocyte fragility test is relies on the osmotic diffusion property. The capability of the membrane to withstand hypotonic solution relies on the integrity of the membrane. Specific genetic disorders such as sickle cell anemia and deficiency of vitamin E make the erythrocyte membrane much more fragile.

3) Osmotic pressure of blood is mainly because of its mineral ions like sodium, potassium, chloride, calcium and protein. The osmotic pressure exerted through proteins is of substantial biological importance owing to the impermeability of the plasma membrane to the colloidal particles.

4) Water’s absorption of water in the intestine is because of osmosis. Creation of urine in the kidneys might be credited to osmotic pressure. The net dissimilarity in the hydrostatic pressure and osmotic pressure is accountable for the filtration of water at the arterial end of the capillary and the re absorption of similar at the venous end. The hydrostatic pressure is 22 mmHg and the osmotic pressure is 15 mm Hg, at the arterial end. The pressure to eject the fluid is 7 mm Hg. At the venous end, the osmotic pressure is 7 mm Hg and hydrostatic pressure is 15 mm Hg. The gross absorption pressure to draw water back in the capillaries is 15 – 7 = 8 mm Hg. This is known as Starling's hypothesis.

5) The water’s renal excretion is regulated partly through the osmotic pressure exerted through the colloids in the blood plasma. Raised urination (polyuria) taking place in diabetes patients is because of the increased water retention through the urinary glucose.

6) Donnan Membrane Equilibrium Let us refer two compartments separated through a semi permeable membrane that is permeable to water and crystalloids, but not to colloidal particles. One of the compartments (A) is filled with moles of NaCl, and the another compartment (B) is filled with b moles of NaR, where R occurs to be a non diffusible ion.

(A)              (B)

a   Na+     Na+    b

a   Cl-       R-    b    …1


NaCl diffuses from (A) to (B) and after a few time, the system acquires equilibrium. At equilibrium, let us refer that x moles of NaCl have diffused from (A) to (B). Thus, the ionic concentration at equilibrium in both the compartments will be like this:

(A)                      (B)

a-x    Na+         Na+    b + x

a-x    Cl-           R-    b    ...2

Cl-    x

At equilibrium, the amount of ions which move from one compartment to another will be equal, and this will take place only, while the ionic products of the concerned ions are equal.

Hence, [Na+][Cl-] in both the compartments at equilibrium should be equivalent.

(a-x) (a-x) = (b+x) x
(a-x)2 =  bx +x2
a2 – 2ax + x2 = bx + x2
a2 – 2ax = bx
a2 = bx + 2ax
a2 = (b + 2a) x
x = a2 / (b+2a)

On substituting numerical values for a and b like 2 and 1 moles correspondingly,

X = 22    
    1+ (2*2)

 = 4/5
 = 0.8

Computing the total moles present in compartment (A) and (B) at equilibrium.

(A)                                 (B)

2 - 0.8 = 1.2        Na+    Na+    1 + 0.8

2 - 0.8 = 1.2        Cl-    R-    1

Cl-    0.8

2.4    3.6


From this we can obtain that:

(i) The concentration of solutes in the non-diffusible ion side (B) is greater as compared to the other.
(ii) There will be accumulation of the ion that is oppositely charged that is ion (Na+) in the side consisting of the non-diffusible ion (R-).

In biological systems, Donnan membrane equilibrium prevails because of the non-diffusible proteins and is as well important for the functional issues of the cell.

If the non-diffusible ion occurs to be R and one of the diffusible ion H+, after that there will be a alteration in the pH.  Because of imbalance in the electrolytes, swelling of proteins takes place that is known as Donnan osmotic effect.

 

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