Mechanism of Mineral Salt Absorption in Plants

The absorption of mineral salts by plants is now understood to be a separate process from water absorption. Initially, it was believed that mineral salts were absorbed along with water, but research has shown that the two processes occur independently. Mineral salts are absorbed from the soil solution in the form of ions, primarily through the meristematic regions of the roots, especially near the tips.

Selective Permeability of Root Cells

The plasma membrane of root cells is selectively permeable, meaning it does not allow all ions to pass through at the same rate. This selective permeability results in unequal absorption of ions. The first step in mineral salt absorption is the process of ion exchange, which does not require metabolic energy.

Types of Absorption Mechanisms

There are two main types of absorption mechanisms:

1. Passive Absorption
2. Active Absorption

1. Passive Absorption

Passive absorption occurs when the concentration of mineral salts is higher in the soil solution than in the cell sap of the root cells. The mineral salts are absorbed by simple diffusion according to the concentration gradient. Since this process does not require metabolic energy, it is referred to as passive absorption.

Ion Exchange Ion exchange plays a crucial role in mineral salt absorption. Ions adsorbed on the surface of the plasma membrane of root cells can be exchanged with ions from the external solution. For example:

• Cation Exchange: Potassium ions (K+) from the soil solution may be exchanged with hydrogen ions (H+) from the root cell surface.
• Anion Exchange: Anions from the soil solution may be exchanged with hydroxide ions (OH-) from the root surface.

Two major theories explain the mechanism of ion exchange:

1. Contact Exchange Theory: In this theory, ions adsorbed on the surface of root cells and clay particles are not tightly bound and oscillate within a small space. If the roots and clay particles are in close contact, ions from the clay particles may be exchanged directly with those on the root surface, without first being dissolved in the soil solution.
2. Carbonic Acid Exchange Theory: According to this theory, CO2 released during respiration combines with water to form carbonic acid (H2CO3), which dissociates into H+ and bicarbonate ions (HCO3-). The H+ ions are exchanged with cations adsorbed on clay particles, and the resulting cations are absorbed by the root cells.

2. Active Absorption of Mineral Salts

Active absorption refers to the accumulation of mineral salts against a concentration gradient, a process that requires metabolic energy through respiration. In this mechanism, a carrier compound located in the plasma membrane helps facilitate the absorption of mineral ions.

The Carrier Concept The plasma membrane is impermeable to free ions, but certain compounds (carriers) present in the membrane bind with ions to form a carrier-ion complex. This complex moves across the membrane, where the ions are released inside the cell, and the carrier returns to the surface to pick up more ions.

There are two main hypotheses based on the carrier concept:

1. Lundegardh’s Cytochrome Pump Theory: According to this theory, anions are actively absorbed through the cytochrome chain, while cations are absorbed passively. The theory suggests that respiration leads to the generation of protons (H+) and electrons (e-), which drive the active transport of anions. Cations move passively to balance the anion absorption.
2. Bennert-Clark’s Lecithin Theory: This theory proposes that the carrier is a protein associated with phospholipids (lecithin) in the plasma membrane. Lecithin binds with cations and anions, and through enzymatic action, the ions are transferred across the membrane.

Donnan’s Equilibrium

Donnan’s equilibrium can explain the passive accumulation of ions inside plant cells. The theory suggests that some ions (called fixed ions) are present inside the cell and cannot diffuse outside through the membrane. If these fixed ions are anions, more cations from the surrounding solution will diffuse into the cell to balance the charge, resulting in an accumulation of cations inside the cell. Conversely, if there are fixed cations inside the cell, anions will accumulate to balance the charge.

In summary, the absorption of mineral salts by plants is a complex process involving both passive and active mechanisms. The active absorption process requires metabolic energy, while passive absorption relies on concentration gradients and ion exchange.

Types of Transport Systems for Nutrient Uptake

1. Symport (Co-transport): In this system, two different ions or molecules are transported in the same direction across the membrane. For example, nitrate (NO₃⁻) is often transported along with protons (H⁺).
2. Antiport (Counter-transport): In this system, one ion is transported into the cell while another is moved out. For example, potassium (K⁺) ions are often transported into the cell while sodium (Na⁺) ions are pumped out.
3. Uniport: This system involves the movement of a single type of ion across the membrane, typically against its concentration gradient, using energy. For example, calcium (Ca²⁺) uptake often uses this type of transport.

Factors Affecting Nutrient Uptake

Several environmental and physiological factors influence the efficiency and effectiveness of nutrient uptake:

1. Soil pH: The availability of nutrients can change with soil pH. For example, certain nutrients like phosphorus (P) become less available in acidic soils, while others, like iron (Fe), are more available in slightly acidic conditions.
2. Soil Moisture: Adequate soil moisture is essential for dissolving nutrients and allowing their movement into plant roots.
3. Root Surface Area: The larger the root surface area, the greater the capacity for nutrient absorption. Root hair development increases the surface area for nutrient uptake.
4. Soil Type: Loamy soils with good texture and drainage tend to facilitate better nutrient uptake, while clay soils may have high nutrient retention but may limit root penetration.
5. Temperature: Warmer temperatures generally increase metabolic activity, which can enhance both passive and active nutrient uptake.