Movement of Solutes in the Cells

As pointed out earlier, the soil solution diffuses freely into the cortical part of the apoplast. The ions present in the solution then move into the symplast. During this transport the ions have to pass through the plasmodesmata also.

The diffusion of ions depends not only on the osmotic or chemical potential gradient but on the electrical potential also, because ions are particles which have electric charges on them. The chemical potential gradient is produced if the concentration of an ion on one side of the membrane is higher than the other side. An electrical potential gradient may result from the presence of charged particles or ions on both sides of the membrane or by the charges associated with the surface of the membrane on both sides. An example will explain the difference between electrical and chemical potential gradients. If a cation (positively charged ion) is more concentrated inside the cell than outside but inside of the cell is negatively charged with respect to out side, the cation will tend to diffuse out of the cell down the chemical potential gradient, but it will tend to diffuse into the cell down the electrical potential gradient. The final direction of movement of the solution will be determined by the gradient (electrical or chemical), which is the steepest.

The relationship between electric potential and chemical potential is shown by Nernst Equation, which has been derived from physical-chemical laws.

Δ E = – 2.3 RT / ZF log aⁱ / a^o

Where is the electro-potential difference across a membrane and aⁱ / a^o is the chemical potential difference, being the ratio of activities inside (aⁱ) and outside (a^o), depending on molar concentration of ions, R is the gas constant, F the Faraday constant and Z is the charge per ion or valency. T stands for absolute temperature. If the temperature remains constant, the electro-potential difference across a membrane for any ion will be:

Δ E = K log ion concentration inside / ion concentration outside

So, the electro-potential across a membrane varies as the log of ion concentrations on either side of the membrane.

Ion Accumulation

The penetration of ions of mineral salts into the cells continues even if the concentration of ions inside is more than in the external solution. This phenomenon is called accumulation. It follows no rules of diffusion.

Selective Uptake of Ions

Algal cells have been observed to accumulate larger amounts of K⁺ ions and to reject other ions such as Na⁺. Root cells of higher cells also behave in the same way. Monovalent cations, such as K⁺ are taken up more readily than divalent cations, like Ca⁺⁺ or polyvalent cations. Similarly, monovalent anions, like Cl^-4, Br^- or NO^-3, accumulate more in the cells than divalent (SO^-4) or polyvalent anions. Certain plants, like halophytes, accumulate large quantities of Na⁺ ions. This is why such plants can survive in saline conditions.

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