OSMOSIS

OSMOSIS

• Osmosis is passive transfer of solvent, especially water, across the membrane.
  • Also, there is diffusion of solvent towards an area of higher solute concentration.
  • Occurs if a semi-permeable (partially permeable) membrane is permeable to water but not to a particulate solute.
  • Is second major passive transport mechanism after diffusion.

OSMOTIC PRESSURE:

• Minimum pressure required to prevent diffusion of solvent is “Osmotic pressure”.
• Depends on the number of osmotically active particles rather than particle type present in solution. Given by “Van’t, Hoff relationship”,π=nCRT, where
  • n = number of dissociable particles per molecule
  • C – total solute concentration
  • R = gas constant
  • T = temperature in degrees Kelvin.

Contributors:

• Plasma proteins have lower molar mass (weight) & high plasma molar concentration (number).
• Hence, contribute greatly to plasma osmotic pressure.
• Albumin with low molecular weight contributes more.

IMPORTANT & RELATED FACTORS:

MOLE:

• A mole (gram molecular weight) is molecular weight of a substance in grams.

OSMOLE:

• One mole of osmotically active particles is called “One osmole”.
  • 1 osmotically active molecule (1 osmole) of a substance exerts osmotic pressure equal to 22.4 atmospheres.
  • Major contributor to body fluid (ECF) osmolarity is Na+ followed by Cl-.

Types:

Ineffective osmoles –

• Molecules that are freely diffusible across the membrane do not generate osmotic driving force. 
• Eg: Glucose & urea.

Effective osmoles – 

• Molecules that are not freely diffusible (non-diffusible ions) across the membrane generate osmotic driving force. 
• Eg: Sodium & chloride ions.
  • Causes “tonicity & osmosis of water”.

• The osmolar concentration of a solution in osmole/ liter is called “Osmolarity”.
• When expressed in osmole/Kg of solution is called “Osmolality”.
• Normal serum osmolality is about 290mOsm/Kg.
• ECF osmolarity is measured by osmometer.

Iso-osmolar:

• Two solutions having identical osmolarity. 
• Exert same osmotic pressure & hence, iso-osmotic.

Hyper-osmolar/hyperosmotic:

• When one of the two solutions has greater osmolarity.

SOLVENT DRAG/BULK TRANSPORT:

• Refers to influence exerted by, allowing solvent through membrane on simultaneous solute movement through membrane.
• Important in renal & GI transport mechanisms.

Don’t Forget to Solve all the previous Year Question asked on OSMOSIS
Click Here to Start Quiz