“Diffusion” sounds slow—and over everyday dis-tances it is—but on the scale of a cell it is very fast. The aver-age instantaneous velocity of a particle in solution—that is,the velocity between the very frequent collisions—isv = (kT/m)½where k = 1.38 × 10–16 g cm2/K sec2, T = temperature in K(37°C is 310 K), and m = mass in g/molecule.Calculate the instantaneous velocity of a watermolecule (molecular mass = 18 daltons), a glucose mol-ecule (molecular mass = 180 daltons), and a myoglobinmolecule (molecular mass = 15,000 daltons) at 37°C. Justfor fun, convert these numbers into kilometers/hour. Before you do any calculations, try to guess whether themolecules are moving at a slow crawl (<1 km/hr), an easywalk (5 km/hr), or a record-setting sprint (40 km/hr).
“Diffusion” sounds slow—and over everyday dis-
tances it is—but on the scale of a cell it is very fast. The aver-
age instantaneous velocity of a particle in solution—that is,
the velocity between the very frequent collisions—is
v = (kT/m)½
where k = 1.38 × 10–16 g cm2/K sec2, T = temperature in K
(37°C is 310 K), and m = mass in g/molecule.
Calculate the instantaneous velocity of a water
molecule (molecular mass = 18 daltons), a glucose mol-
ecule (molecular mass = 180 daltons), and a myoglobin
molecule (molecular mass = 15,000 daltons) at 37°C. Just
for fun, convert these numbers into kilometers/hour. Before you do any calculations, try to guess whether the
molecules are moving at a slow crawl (<1 km/hr), an easy
walk (5 km/hr), or a record-setting sprint (40 km/hr).
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