Previouse Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/gating-of-protein-channels.html

Osmosis Across Selectively Permeable Membranes

Osmosis is described as the net movement of water across a selectively permeable membrane caused by a concentration gradient of solutes. Under normal conditions, water molecules are constantly bouncing around and diffusing through cell membranes at a extraordinarily high rate.

 

When a solute like sodium or glucose is present in higher concentration on one side of a membrane that it cannot cross, it effectively "displaces" some of the water molecules on that side. This creates a situation where the concentration of pure water is higher on the side with fewer solute particles. As a result, more water molecules hit and pass through the membrane pores from the low-solute side to the high-solute side than in the reverse direction. This net movement of water continues until the solute concentration becomes equalized or until an opposing pressure known as osmotic pressure builds up enough to stop further net water transfer. It is precisely this simple physical process that prevents our body cells from either shrinking into oblivion or bursting open when the surrounding fluid composition shifts.

• Water is the most abundant diffusing substance.
• Equal inward and outward water diffusion prevents cell volume changes.
• Water concentration differences cause net water movement.
• Net movement of water caused by concentration differences is osmosis.
• Water diffuses easily; sodium and chloride ions diffuse with difficulty.
• Solutes displace water, reducing water concentration in solution.
• Water moves from higher water concentration toward lower concentration.

Osmotic Pressure:

• Pressure applied to stop osmosis is called osmotic pressure.
• No diffusible solutes create fluid level differences across membranes.
• Pressure difference balancing osmosis equals the solution's osmotic pressure.

Particle Number and Osmotic Pressure:

• Osmotic pressure depends on particle concentration, not particle mass.
• All particles exert equal average pressure regardless of mass.
• Larger particles move slower; smaller particles move faster.
• Average kinetic energy k =mv^22 is equal for all particles.
• Molar particle concentration determines osmotic pressure.

Osmolality and the Osmole:

• Osmole expresses solution concentration by number of particles.
• One osmole equals 1 gram molecular weight of no dissociated solute.
• 180 grams of glucose equals 1 osmole.
• Dissociating solutes produce multiple osmoles per gram molecular weight.
• 58.5 grams of fully dissociated NaCl equals 2 osmoles.
• Osmolality measures osmoles per kilogram of water.
• One million mole per kilogram equals 1/1000 osmole per kilogram.
• Normal body fluid osmolality is about 300 mOsm/kg.

Relation of Osmolality to Osmotic Pressure:

• At 37°C, 1 osmole/liter produces 19,300 mmHg osmotic pressure.
• One million mole/liter equals 19.3 mmHg osmotic pressure.
• Calculated osmotic pressure of body fluids is 5,790 mmHg.
• Measured body fluid osmotic pressure averages about 5,500 mmHg.
• Interior ice attractions prevent ions from exerting full osmotic potential.
• Actual osmotic pressure is about 0.93 times calculated value.

Osmolarity:

• Osmolarity measures osmoles per liter of solution
• Osmolality measures osmoles per kilogram of water.
• Difference between osmolarity and osmolality is under 1 percent.
• Osmolarity is standard practice due to practical measurement ease.

Active Transport:

• Moves molecules against concentration, electrical, or pressure gradients.
• Simple diffusion cannot maintain high intracellular concentration gradients.
• Requires an energy source to move substances "uphill."
• Transports ions, sugars, and most amino acids.

Types of Active Transport:

• Primary Active Transport:

• Uses energy directly from ATP breakdown.
• Employs transmembrane carrier proteins.
• Imparts energy to move substances against gradients.

Active Transport:

• Uses stored ionic concentration gradient energy.
• Energy originates from primary active transport.
• Employs transmembrane carrier proteins.

Primary Active Transport: Sodium-Potassium Pump:

• Pumps sodium outward and potassium inward in all cells.
• Maintains sodium and potassium membrane concentration differences.
• Establishes negative electrical potential inside cells.
• Formed by \alpha (100,000 MW) and \beta (55,000 MW) subunits.
• \alpha subunit has three intracellular sodium binding sites.
• \alpha subunit has two extracellular potassium binding sites.
• Intracellular portion has ATPase enzyme activity.

Mechanism of the Sodium-Potassium Pump:

• Binding of three sodium's and two potassium's activates ATPase.
• ATP splits into ADP, releasing high-energy phosphate.
• Liberated energy causes protein conformational change.
• Extrudes three sodium ions outside, two potassium inside.
• Pump can run in reverse to synthesize ATP.
• Requires 60 to 70 percent of nerve cell energy.

Cell Volume Control by the Na+-K+ Pump:

• Prevents cell swelling and bursting.
• Nonpermeable intracellular proteins attract positive ions.
• Intracellular ion buildup promotes inward water osmosis.
• Pumping three Na^+ out for two K^+ in causes net ion loss.
• Low membrane permeability keeps sodium outside cell.
• Net ion loss drives outward water osmosis.
• Cell swelling automatically activates the pump.

Electrogenic Nature of the Pump:

• Moves three positive ions out for two positive ions in.
• Creates net positive charge movement to cell exterior.
• Leaves a deficit of positive ions inside the cell.
• Generates an electrical potential across the cell membrane.
• Essential for transmitting nerve and muscle signals.