Question

Analyze what is meant by "the transport of an ion across a membrane is determined by its electrochemical potential." In doing this, describe the factors that must be taken into account when considering whether the transport of a charged solute across a membrane is endergonic or exergonic. Consider how this compares to uncharged solutes.

Compare simple diffusion and osmosis

Given that most cells are hypertonic relative to their environment, describe how this in turn affects the movement of water into cells and the adaptations of animal and plant cells.

Discuss the reasons for the differences in transport rate observed for facilitated carriers compared to channels. Consider how the mechanism via which these proteins allow the movement of solutes will in turn affect the rate at which the solutes are transported.

Answer 1

Such transport is governed by delta G, or change in Gibbs free energy, if that is negative the transport is favorable. Now the formula for delta G for charged and uncharged solutes are

Charged

Delta G inward = RT ln ( [S]_INSIDE / [S]_OUTSIDE ) + zFV, where R is gas constant, T = temperature in kelvin, [S] is concentration, z is charge of the substrate, F is Faraday constant, and V is membrane potential

Uncharged zFV term is absent

Delta G inward = RT ln ( [S]_INSIDE / [S]_OUTSIDE )

thus we see membrane potential does not affect the transport of uncharged solutes, whereas transport of charged solutes is affected by both the membrane potential and charge on the solute itself.

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Simple diffusion is the transport of solute from its higher to lower concentration whereas osmosis is the transport of solvent molecules from its (solvent's) higher to lower concentration across a semipermeable membrane.

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Because of hypertonicity of cells water has a tendency to move in via endosmosis but animal cells' phospholipid bilayer offers resistance to it save by the presence of special proteins called aquaporins which allow the movement of water inside thereby regulating it. Plant cells have and extra cell wall that provides some protection against bursting of cells. Certain animal cells like that of an amoeba have contractile vacuoles which pump water out to osmoregulate.