Osmotic pressure is determined by the number of solute molecules dissolved in a solution, not by factors such as molecule size, mass, or chemical nature. It is measured in atmospheres (atm) and can be calculated using van't Hoff's law: π = nCRT, where n represents the number of dissociable particles per molecule, R is the gas constant, T is the temperature in Kelvin, and C is the total solute concentration (0.001 mol/L).

Fig. 1.8 demonstrates the concept of osmosis and how osmotic pressure is measured. In Fig. 1.7, several membrane transporters are illustrated, including primary active, passive, and secondary active transport. These transporters play a role in processes such as the secondary active transport of calcium (Ca++) and the primary active transport of sodium (Na+) and potassium (K+).

The transportation of glucose is carried out by epithelial cells in the gastrointestinal tract (small intestine) and proximal tubules of the kidneys. In the gastrointestinal tract, glucose is absorbed from ingested food, while in the kidneys, the proximal tubules reabsorb glucose to prevent its loss in the urine. The uptake of glucose in these processes involves sodium-glucose-linked transporters SGLT-1 and SGLT-2, which utilize the energy from the electrochemical gradient of Na+ to transport glucose.

SGLT-2 transports one glucose molecule with one Na+ ion, and based on the equation provided, an approximate 100-fold glucose gradient could be generated by SGLT-2 if the intracellular glucose concentration is 2 mmol/L. Similarly, by increasing the number of Na+ ions transported with glucose from one to two, SGLT-1 can generate an even larger glucose gradient of nearly 10,000-fold.

Therefore, if the intracellular glucose concentration is 2 mmol/L, SGLT-1 could effectively remove almost all glucose from either the lumen of the small intestine or the lumen of the proximal tubule, resulting in a luminal glucose concentration of approximately 0.0002 mmol/L.
Fig. 1.8 illustrates the concept of osmosis and the measurement of the osmotic pressure of a solution.
Osmotic pressure is determined by the number of solute molecules dissolved in the solution. It is not dependent on such factors as the size of the molecules, their mass, or their chemical nature (e.g., valence). Osmotic pressure (π), measured in atmospheres (atm), is calculated by van’t Hoff’s law as follows:
Equation 1.7 ˜= nCRT,
CHAPTER 1 Principles of Cell and Membrane Function
K+ Na+ 3Na+ Na+, K+ -ATPase 3Na+-Ca++ antiporter IntracellularconcentrationsExtracellularconcentrations3Na+ Na+: 145 mEq/L K+: 4 mEq/L Glucose: 5 mmol/L Ca++: 2.5 mEq/L (ionized) Na+: 12 mEq/L K+: 120 mEq/L Glucose: 2 mmol/L Ca++: 0.001 mEq/L (ionized) Ca++ Secondary active transport of Ca++ Primary active transport of Na+ and K+ Passive transport Na+ channel K+ channel Glucose uniporter Glucose 2K+ ATP •Fig. 1.7 Examplesofseveralmembranetransporters,illustratingprimaryactive,passive,andsecondaryactivetransport.Seetextfordetails.ATP,adenosinetriphosphate.
Glucoseistransportedbytheepithelialcellsthatlinethegastrointestinaltract(smallintestine),andbycellsthatformtheproximaltubulesofthekidneys.Inthegastrointestinaltract,theglucoseisabsorbedfromingestedfood.Inthekidney,theproximaltubulereabsorbstheglucosethatwasfilteredacrosstheglomerularcapillariesandtherebypreventsitfrombeinglostintheurine.Theuptakeofglucoseintotheepithelialcellfromthelumenofthesmallintestineandfromthelumenoftheproximaltubuleisasecondaryactiveprocessinvolvingthesodium-glucose–linkedtransportersSGLT-1andSGLT-2.SGLT-2transportsoneglucosemoleculewithoneNa+ ion,andtheenergyintheelectrochemicalgradientforNa+ (intothecell)drivesthesecondaryactiveuptakeofglucose.Accordingtothefollowingequation,forcalculatingtheelectrochemicalgradient,andifthemembranepotential(Vm)is−60mVandthereisa10-fold[Na+]gradientacrossthemembrane,anapproximate100-foldglucosegradientcouldbegeneratedbySGLT-2:[Glucose]i [Na+ ]o V 615. mV
Thus,iftheintracellularglucoseconcentrationwas2mmol/L,thecellcouldlowertheextracellularglucoseconcentrationtoapproximately0.02mmol/L.However,byincreasingthenumberofNa+ ionstransportedwithglucosefromonetotwo,SGLT-1cangenerateanearly10,000-foldglucosegradient: 615. mV
Again,iftheintracellularglucoseconcentrationis2mmol/L,SGLT-1couldremovevirtuallyallglucosefromeitherthelumenofthesmallintestineorthelumenoftheproximaltubule(i.e.,theluminalglucoseconcentration≅ 0.0002mmol/L).
where molecules at 37°C can exert an osmotic pressure of 2.54 n = number of dissociable particles per molecule × 10−2 atm, as calculated with eq. 1.7 and the following C = total solute concentration values: R = gas constant n = 1 T = temperature in degrees Kelvin C = 0.001 mol/L