Passive Absorption of Elements

Passive absorption of mineral elements occurs without the expenditure of energy. Here, they rely on natural physical processes to move minerals into plant roots. This process is driven by the concentration gradients and other physical forces. The following hypotheses and mechanisms explain the passive absorption of minerals:

Mass Flow Hypothesis

According to mass flow hypothesis, minerals are transported to the roots along with the flow of water during transpiration. As water is absorbed by the roots and pulled upwards through the plant, it carries dissolved minerals with it. This process ensures a continuous supply of nutrients as long as the transpiration stream is active.

Also Read: The Pressure Flow or Mass Flow Hypothesis

Simple Diffusion Hypothesis

Simple diffusion involves the movement of mineral ions from an area of higher concentration in the soil solution to an area of lower concentration inside the root cells. This process is driven purely by the concentration gradient of the ions, without the involvement of energy or transporter proteins.

Facilitated Diffusion

Unlike simple diffusion, facilitated diffusion requires specific carrier proteins or channels in the cell membrane to help the movement of ions. These proteins help transport ions that cannot diffuse freely across the lipid bilayer, making the process more efficient and selective.

Also Read: What is Facilitated Diffusion?

Ion Exchange Hypothesis

In this hypothesis, the absorption of mineral ions occurs through the exchange of ions between the soil particles and the root surface. Here are two key theories that explain this process:

Contact Exchange Theory

• Mechanism: Ions are exchanged through direct contact between the root surface and soil particles.
• Cation Exchange: For example, potassium ions (K+) from the soil solution are exchanged with hydrogen ions (H+) absorbed on the root surface membrane.
• Anion Exchange: Similarly, anions in the soil can be exchanged with hydroxyl ions (OH-) from the root.
• Dynamic Movement: Ions are not tightly bound to soil particles (clay micelles) but can move within a small volume of space.
• Electrostatic Absorption: Ions are absorbed electrostatically to the plant root and exchanged with ions held by the clay micelles.
• Diffusion: The exchanged cations and anions are then moved into the roots by simple diffusion.

Carbonic Acid Exchange Theory

• Mechanism: Ions are exchanged in their dissolved form in the soil solution.
• Formation of Carbonic Acid: Carbon dioxide released during respiration combines with water to form carbonic acid in the soil solution.
• Dissociation: Carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).
• Cation Exchange: Hydrogen ions replace cations adsorbed on clay particles, releasing these cations into the soil solution.
• Anion Exchange: Bicarbonate ions release adsorbed anions, supplying both anions and cations to the plant roots.

Donnan Equilibrium

This concept, introduced by F.G. Donnan, highlights the passive accumulation of non-diffusible particles against the electrical potential gradient (ECP) angle. The cellular membrane selectively allows the passage of diffusible particles, while non-diffusible particles called fixed particles, remain constrained.

These fixed particles, which can be either anions or cations, are predominantly present on one side of the membrane, forming the Donnan phase. According to Donnan’s equilibrium principle, ions of opposite charge are attracted to balance the charge of fixed particles:

• If anions serve as non-diffusible fixed particles, cations of corresponding charge are absorbed.
• If cations serve as fixed particles, anions of corresponding charge are absorbed.

The Donnan equilibrium can be mathematically described by the following equation.

[Ci+] [Ai-] = [Co+] [Ao-]

Where Ci+ = Cations inside
Co+ = Cations outside
Ai^– = Anion inside
Ao^–= Anion outside

The mechanism of absorption of elements in plants involves the absorption of mineral elements from the soil and transporting them to different plant parts. This mechanism of absorption of elements or minerals by plants includes two phases: passive absorption and active absorption

The roots actively take up these minerals through specialized cells and translocate them to other plant parts. In this article, we will study the mechanism of absorption of elements in detail.