Driving Force - Mineral Nutrition
Let us now find out what is the driving force involved in protein mediated transport. Many membrane transport proteins allow specific solutes to move across the lipid bilayers. If the transported molecule is uncharged, then the difference in its concentration on the two sides of the membrane, that is its concentration gradient - determines the direction of transport. However, if the solute to be transported carries a net charge, then both its concentration gradient and the total electrical gradient across the membrane influence its transport. For instance, an ion will move across a membrane if there is sufficient electrical gradient across the membrane even if the concentration gradient does not favour such a movement.
In other words, the direction of movement is decided by which of the two forces is steepest. The two gradients together constitute the electrochemical gradient. The gradient can develop in part due to the selective permeability of the membrane. So the related diffusion of cations may be more than anions or vice versa. For example, K+ diffuses out mort rapidly due to differences in electrical gradient than Cl- in the immediate exterior and hence excess of Cl- in the cell gives it a negative charge. In fact, all plasma membranes have electric potentials (Transmembrane potential) across them with inside of the cell more negative compared to the outside. This is due to active transport of ions particularly H+ ions out of the cell. This potential difference allows the entry of positively charged ions into the cell but opposes the entry of negatively charged ions.