Ions may be taken up by the plant cells by two methods:

1. Passive Absorption.
2. Active Absorption.

Passive Absorption of Ions

Absorption of ions by the cells may occur without spending metabolic energy by the process of diffusion but accumulation of large quantities of ions in the cells against concentration gradient shows that it is not merely a process of diffusion. Such a passive accumulation of ions can be partly explained on basis of electro-chemical potential difference. Two explanations are given in the following paragraphs:

1. Ion Exchange Mechanism.
2. Donnan Equilibrium.

Ion Exchange Mechanism

Ions adsorbed on the surfaces of the cell membrane may exchange with ions present in the external solution. For example, the cation (K⁺) is present in the external solution and hydrogen ion (H⁺) is adsorbed to the surface of the cell membrane. The cation K⁺ will be exchanged for H⁺. K⁺ will thus be adsorbed to the membrane surface and will become osmotically inactive. So, large amounts of K⁺ can be accumulated in the cell by this process, although their concentration in the external solution may be less than that in the cell. Anions can also accumulate in the cells by exchange with free hydroxyl ions (OH^-) in the same manner.

Donnan Equilibrium

Cell membrane is composed of macromolecules of proteins and lipids that have many carboxyl groups (-COOH) and phosphate (HPO₃) groups, from which positively charged particles like protons of hydrogen (H⁺) can dissociate, leaving the macromolecules with negative charge. Thus the membrane is usually negatively charged. The negative charges are not diffusible because they are within the membrane structure. These negatively charged ions on the membrane called fixed ions. The negatively charged membrane is called Donnan phase, after F. G. Donnan, who discovered this property of the membrane.

Now, suppose that a solution of potassium chloride (KCl) is present outside the Donnan phase. The cations (K⁺) will tend to diffuse through the membrane because of electronic potential difference. The cations will finally come to equilibrium with fixed negative charges of the membrane. The Cl^- ions with negative charge will not move into the cell because of this electronic potential. They can, however, diffuse by chemical potential difference or difference of concentration of Cl^- on both sides of the membrane. Some K⁺ ions will also move into the cell by chemical potential gradient. The Cl^- ions which remain outside the membrane will set up an electric potential difference at the membrane surface which is negative, on account of Cl^- ions, compared to the external solution. Such an electric potential is called Donnan potential. The Donnan potential will allow K⁺ to diffuse through the membrane and repel the Cl^- ions (opposite charges repel each other). At equilibrium, Donnan potential (Δ E) for K⁺ ions can be expressed according to Nernst equation as follows:

Δ E = – K log K⁺ (inside) / K⁺ (outside)

Similarly, Donnan potential for Cl^- ions at equilibrium will be:

Δ E = -K log Cl^- (outside) / Cl^- (inside)

Combining both equations:

K⁺ (inside) / K⁺ (outside) = Cl^- (outside) / Cl^- (inside)

If at equilibrium the chemical potential of K⁺ and Cl^- is one,

K⁺ (inside) × Cl^- (inside) = K⁺ (outside) × Cl^- (outside)

This equilibrium is called Dannan Equilibrium. In general, Donnan equilibrium may be expressed in the following equation:

concentration of positive ions (inside) / concentration of positive ions (outside) = concentration of negative ions (outside) / concentration of negative ions (inside)

As at Donnan equilibrium, more cations (positively charged ions) tend to pass through the membrane the cations will accumulate in the cell against diffusion gradient.

The above discussion may be explained further by taking an example. Suppose there are six fixed charges on the inner surface of a membrane. These are balanced by 6 K⁺ ions as shown in the diagram (A). Then two more cations of K⁺ from KCl solution outside, pass through the membrane by Donnan potential and two Cl^- anions also go in by chemical potential gradient as shown in the diagram (B). Donnan equilibrium established and eight K⁺ ions are present inside the membrane as against 4 K⁺ ions remaining outside as shown in the diagram (C).

K⁺ (inside) / K⁺ (outside) = Cl^- (outside) / Cl^- (inside)

or, 8 / 4 = 4 / 2

Thus potassium ions accumulate inside. Such a Donnan type equilibrium may accumulate anions inside the membrane, if the membrane is positively charged. The ions may accumulate inside cells up to 30 times their concentration in outer solution by Donnan equilibrium.

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