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.

Essential Elements and Mineral Absorption

Essential elements are nutrients required by plants in specific amounts for proper growth, development, and physiological functions. These elements are directly involved in plant metabolism and cannot be replaced. This shows how important they are for plants in their life cycle.

These mineral elements are found scattered in the soil. Plants absorb essential elements from the soil through their roots, utilizing both active transport and passive diffusion. Root hairs increase the surface area for nutrient uptake, and once absorbed, these elements are transported through the xylem to various parts of the plant. Soil composition and pH significantly influence the efficiency of this absorption process.

Region of Root

Classification of Elements

Mineral elements are classified into two main groups based on the quantity required by plants:

• Macronutrients: These are needed in large amounts and include elements like nitrogen, phosphorus, and potassium. They are essential for basic plant functions and growth.
• Micronutrients: Required in smaller quantities, micronutrients include elements like iron, zinc, and manganese. Despite their lower quantity, they are crucial for specific metabolic processes in plants.

Also Read: Difference Between Micronutrients and Macronutrients

Mechanism of Absorption of Elements

Minerals are taken up by plants either in their dissolved form or from the soil. This absorption occurs in the cells of the epiblema located in the root zones of maturation and elongation. The process of mineral absorption happens in two main phases:

• Passive Absorption
• Active Absorption

Now, let’s study each of them in detail.

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

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.

Facilitated Diffusion

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

Active Absorption of Elements

Active absorption of mineral elements in plants involves the utilization of energy to transport ions against their concentration gradient, ensuring the uptake of essential nutrients even when they are scarce in the soil. This process is crucial for maintaining optimal levels of nutrients within the plant. Here’s how active absorption works:

• ATP-Dependent: Active absorption relies on adenosine triphosphate (ATP) for energy, enabling carrier proteins to transport ions against their concentration gradient.
• Ion Channels and Pumps: Carrier proteins in the cell membrane facilitate selective transport of ions, ensuring only specific ions pass through.
• Uptake of Scarce Nutrients: Active absorption allows plants to absorb mineral ions from the soil even when their concentration is lower outside root cells.
• Controlled Process: The activity of carrier proteins is regulated by factors such as metabolic status, hormonal signals, and environmental conditions.
• Selective Uptake: Carrier proteins exhibit specificity for particular ions, allowing plants to absorb essential nutrients efficiently while excluding harmful ions.

Difference Between Passive and Active Absorption

The difference between passive and active absorption is given below: