Passive diffusion of molecules including H2O, CO2, and O2 across a cell membrane
ID: 908499 • Letter: P
Question
Passive diffusion of molecules including H2O, CO2, and O2 across a cell membrane results in movement from an area of high concentration to an area of low concentration until equilibrium is reached. Chloride ions, on the other hand, are able to move up a concentration gradient as they rely on passive transport through ion channels to maintain a lower concentration within the cytoplasm than the extracellular environment. In no more than 7 sentences, use your understanding of thermodynamics to explain to a college student with a strong grasp of thermodynamics and has completed general and organic chemistry (but has not taken biochemistry) how/why this is possible. A complete answer will include an explanation that ties the Cl- transport to the mathematical thermodynamics explanation.
Explanation / Answer
ANSWER: Neutral molecules like oxygen, water etc move along concentration gradient in the cells and is governed by following thermodynamic equation.
G = RTln[C]in/[C]out R = gas constant, [C]in = cooncentration nside cell and [C]out = concentration outside cell. If [C]in < [C]out , G will be negative hence molecules will diffuse from outside to inside and vice versa.
Chrged species however mo along the electrochemical gradient (combining effect of concentration gradient and charg grdient). G is given as
G = RTln[C]in/[C]out + FVm
F = faraday constant Vm = memebrane potential, its value ranges from -60 to -90 volts It implies there is accumulation of positive charge out side the cell hence negative charge (eg Cl-) will move from inside to out side up the concentration gradient but along the electrochemical gradient