You have landed on the right page to learn about the Mechanism of Absorption of Elements. There are more than 105 elements discovered so far. Only about 20 has been found to be essential for plant growth and metabolism. Deficiency of any element in plants may lead to symptoms such as chlorosis, necrosis, stunted growth, etc. To avoid these deformities in plants, efficient absorption of the mineral elements by plants is of great importance.

Earlier, It was a misconception that plants absorb minerals from the soil along with the absorption of water. Further studies about plant functions ensured that water absorption and mineral element absorption are two different processes. The mechanism of absorption of elements in plants is carried in two different ways. These include passive and active absorption. This article is the core study about the different types of elements and their absorption.

Essential Element and Mineral Absorption

An essential element is the one without which the plant cannot complete its life cycle, and it has an important physiological role in the plant life. These elements are directly involved in the metabolism of the plant and cannot be replaced by another element.

The process of intake of nutrients from the soil is called mineral absorption. It takes place with the intimate contact of the root system with the soil solution. The root hairs are the projections of the root epidermis that remain in direct contact of soil water and minerals and initially absorb the water. Therefore they are also named absorbent hair.

Sources of Essential Elements for Plants

Carbon, hydrogen, and oxygen serve as building blocks of macromolecules that make the main bulk of the plant body. Though these are not absorbed from the soil, hence are not considered mineral elements. Likewise, nitrogen is however essential for plants but not considered a mineral element because plants utilize the atmospheric nitrogen fixed by soil bacteria in the form of ammonium and nitrate ions.

As a whole, the soil is the main source of different types of nutrients such as phosphorus, sulphur, magnesium, calcium, potassium, etc. Plants absorb these mineral elements in their ionic forms. It can be said that mineral elements are derived from the weathering of parent rock and held by the soil.

Classification of Mineral Elements

Mineral elements can be classified into two main categories based on their amount of requirements. Macronutrients are the ones that are required in relatively large amounts by plants. The elements required in the small quantities are called micronutrients.

1. Macronutrients: Carbon, Hydrogen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sulphur, etc.
2. Micronutrients: Iron, Manganese, Zinc, Copper, Boron, Molybdenum, Chloride, etc.

Role of Essential Elements in Plants

The essential elements carry out the following functions in the plants:
1. Elements are the components of the plant body. Carbon, hydrogen, oxygen, nitrogen are found as a part of biomolecules.
2. Calcium, magnesium, sodium, chlorine, manganese, chlorine, and potassium serve as cofactors of enzymes and are called catalytic components.
3. The osmotic potential of the plant, pH of cell sap depends on the concentration of mineral elements and organic compounds present in the cell sap.
4. Mineral elements like Na, K, Ca, and Mg found in the cells help in maintaining the electrostatic neutrality of the cells.
5. Mineral elements also influence the absorption of water by cells and affect the degree of permeability of cell membranes.

Mechanism of Absorption of Elements

Minerals are either in dissolved form or absorbed from. Minerals are absorbed by the cells of epiblema in the maturation and elongation zone of roots. The absorption of minerals occurs in the following two phases:
1. Passive Absorption
2. Active Absorption

1. Passive Absorption

The absorption of minerals by physical processes such as diffusion without the expenditure of metabolic energy is known as passive absorption. An ion moves passively from the region of its higher concentration or higher electrochemical potential to that of lower electrochemical potential. The passive movements of ions usually occur through ion channels. The ion channels are transmembrane proteins that function as selective pores. Various hypotheses have been proposed to explain the movement of ions against an ECP gradient. These can be described as follows:

1. Mass flow hypothesis: According to this hypothesis, an increase in the water flow in the plant due to the transpiration pull also increases the total uptake of ions by roots without utilizing energy. The ions move in a mass flow with water from the soil solution through the root and eventually to the shoot. This theory was supported by Kramer, Russel, Barber and Lopushinsky.

2. Simple Diffusion hypothesis: The minerals from the soil are absorbed through the roots of the plant by the process of diffusion along the concentration gradient. Mineral uptake is facilitated by transpiration, rapid assimilation, and compartmentalization in the vacuole.

3. Facilitated Diffusion hypothesis: It is the mechanism in which the ions are absorbed and further transported across the membrane through a carrier protein. The ions move into the cell through protein channels present in between the lipid bilayer.

4. Ion Exchange hypothesis: Ions, both cations and anions, have the tendency to get absorbed on the surface of the cell and exchange with the ions present in the soil solution. According to this theory, ions absorbed to the surface of the cell wall or membrane of tissue may exchange with the ions from the external solution in which the tissue is immersed.

For instance, the cation K⁺ of the external solution is exchanged with a hydrogen ion (H⁺) absorbed through the surface membrane. The anions could possibly exchange with the hydroxyl ions (OH^–) in the same manner. The theory of ion exchange consists of the following two aspects:

I. Contact exchange theory: According to the contact exchange theory, ions the exchange takes place through surface contact.
i. The ions are not held tightly with the clay particles (clay micelles) but can move or swing within a small volume of space.
ii. An ion is absorbed electrostatically to a plant root and exchanged with an ion held by clay micelle.
iii. The exchanged cations and anions further moved into the roots by simple diffusion.

Fig: Contact ion-exchange theory

II. Carbonic acid exchange theory: This theory states that ions are exchanged in the dissolved form. It can be discussed as follows:
i. The carbon dioxide released during respiration combines with the water to form carbonic acid in the soil solution.
ii. The carbonic acid breaks into cations and anions (H⁺ and HCO₃^–).
iii. Hydrogen ions exchange themselves with the cations adsorbed on the clay particles
iv. The bicarbonate ions release the adsorbed anions to supply both anions and cations.

Fig: Carbonic acid ion-exchange theory

5. Donnan Equilibrium: This theory is proposed by F.G.Donnan. It explains the passive accumulation of non-diffusible ions against the ECP gradient. The cell membrane is not permeable to non-diffusible ions. Therefore, these ions are termed fixed ions. They may be anions or cations. The fixed ions may be present on one side of the membrane. The side or region which contains the fixed or non-diffusible ions is called the Donnan phase.  According to the equilibrium concept proposed by Donnan, more ions of the opposite charge are being absorbed to balance the charge of fixed ions.
i. If anions serve as non-diffusible fixed ions, then cations of equal charge will be absorbed.
ii. If cations serve as fixed ions, then anions of equal charge will be absorbed.

The Donnan equilibrium can be represented by the following mathematical equation:

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

Fig: Donnan equilibrium

2. Active Absorption

The movement of ions against the concentration or ECP gradient is called active absorption. This movement requires the expenditure of energy. Hogland studied the active absorption and accumulation of ions against the concentration gradient by utilizing energy in green algae Nitella and Valonia. The cell of these algae keeps absorbing the K⁺ and phosphate ions to such an extent that their concentration becomes hundreds or thousands of times greater than the concentration of ions in the pond water. The following hypotheses have been put forward to explain the process of active absorption:

1. Carrier Concept Theory: This theory was proposed by Van den Honert.
i. The space in the cell or tissue to which ions penetrate through metabolic energy consumption is said to be the inner space.
ii. The demarcation between the inner space and the outer space lies somewhere in the middle of the cytoplasm. This middle area is found to be impermeable to ions.
iii. Ions can be carried through the inner space by specific careers. There are separate carriers for cations and anions.
iv. The specific carriers form the intermediate ion-carrier complex with ions.
v. The ion-carrier complex can move through the impermeable membrane from the outer to the inner space.
vi. This complex breaks up and releases ions into the inner space. This process is known to be mediated by the enzyme phosphatase and ATP.
vii. The inactivated carrier is again activated by enzyme kinase or phosphokinase.
viii. The ATP molecule (energy) combines with the carrier molecules and allows passage of the substances against the concentration gradient.
ix. The activated career further combines with the new ions, and the entire cycle is repeated.

The process can be summarized in the following word equations:

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Fig: Carrier concept theory

2. Protein-Lecithin Theory: This theory was proposed by Bennet and Clark.
i. The scientists suggested that the carrier could be an amphoteric molecule that can carry cations as well as anions.
ii. In protein-lecithin theory, a carrier protein is associated with lecithin.
iii. Lecithin is a phosphatide that consists of a phosphate group that acts as a cation binding site.
iv. Choline is also present, which acts as an anion binding site.
v. The ions are therefore picked up by lecithin and form the ion-lecithin complex.
vi. Ions release on the inner surface when lecithin is hydrolyzed by the enzyme lecithinase into phosphatidic acid and choline.
vii. The enzyme choline acetylase and choline esterase resynthesized lecithin from its components.

Fig: Protein-Lecithin theory

3. Cytochrome Pump Theory: This theory is put forward by Lundegardh and Burstrom.
i. The anion absorption is independent of the cation absorption, and the cytochrome acts as an anion carrier.
ii. In this process, an oxygen concentration gradient exists from the outer surface to the inner surface of a membrane. This favours the oxidation at the outer surface and reduction at the inner surface.
iii. The dehydrogenase reactions on the inner surface produce protons and electrons.
iv. Electrons move outward through a cytochrome chain, and an anion moves inward, and the cytochrome is reduced.
v. At the outer surface, the reduced cytochrome is oxidized due to losing an electron and picking up an anion.
vi. The released electron unites with proton (H⁺) and oxygen to form water.
vii. However, on the inner surface, the oxidized cytochrome becomes reduced by the addition of an electron released in the dehydrogenase system.
viii. Cations are absorbed passively to balance the potential difference caused by the accumulation of anions on the inner barrier surface.

This theory can be correlated with ion absorption and respiration. The rate of respiration increases when a plant is transferred from water to a salt solution. This increase in the rate of respiration has been called anion respiration or salt respiration.

Fig: Cytochrome pump theory

4. Electrochemical gradient hypothesis: This theory is proposed by Peter Mitchell. The following concepts of this theory have been assumed:
i. The transmembrane potential exists across all the plasma membranes.
ii. The inner side of the cell is more negative than the other side.
iii. It causes the absorption of cation by utilizing energy in order to maintain an equilibrium.

Difference between Active and Passive Absorption

Active absorption	Passive Absorption
Active absorption involves the expenditure of energy.	Passive absorption of mineral nutrients does not require energy.
The movement of ions takes place against the concentration gradient.	The movement of ions occurs in the direction of lower concentration to higher concentration, i.e. along the concentration gradient
The rate of absorption depends on the DPD (diffusion pressure deficit).	The rate of absorption depends mainly on the transpiration pull.
Active absorption follows the symplast pathway.	Passive absorption follows the apoplast pathways.
The rate of absorption is slow.	The rate of absorption is high.

Translocation of Mineral Elements or Ions

Transportation from the epidermal cell to the root xylem: After the ions have reached the epidermal cells of the root, the mineral elements move from one cell to another cell of the root in the following ways:

1. Apoplast pathway (movement through the cell wall and intercellular spaces)
2. Transmembrane pathway (movement across the membranes)
3. Symplast pathway ( movement through the plasmodesmata)

Fig: Translocation of mineral elements

The mineral ions ultimately reach the root xylem through any of the pathways.

Translocation from roots to stem and other parts of the plant: After the ions have reached the xylem, the further transport up the stem to all parts of the plant takes place through a transpiration stream. The chief sinks of the mineral elements are apical and lateral meristems, young leaves, and developing flowers and fruits.

Factors Affecting Mineral Absorption

Mineral absorption is affected by certain external and internal factors.
I. Temperature, light, pH of the soil, the concentration of mineral ions in soil solution, and oxygen are some internal factors.
II. Age of the plant, competition, interaction, and mycorrhizal association are some internal factors.
III. Salt absorption increases with an increase in temperature.
IV. Light affects the absorption of minerals indirectly. For instance, Nitrogen uptake is increased due to the process of photosynthesis in the presence of light.
V. The pH of the soil solution affects the ionization of electrolytes.
VI. Young roots absorb minerals more efficiently than the older suberized and lignified roots.
VII. There is a competition between ions for common binding sites.
VIII. Mycorrhizal association increases the absorbing surface for water and minerals. It also takes part in the solubilization and mobilization of nutrients in the soil.

Summary

Like humans, plants also require minerals. The study of mineral absorption and its utilization for growth and development is called mineral nutrition. The lack of minerals in plants causes several deformities. Therefore minerals are essential constituents of plant life. Plants absorb the minerals from the soil in the form of ions. The process of absorption may or may not require the expenditure of energy, hence divided into active absorption and passive absorption, respectively.

Different theories have also been proposed with respect to the active and passive absorption to determine the maximum possible methods for the absorption of mineral elements. Some of the passive absorption theories include mass flow theory, ion exchange theory, Donnan equilibrium, whereas carrier concept theory, protein-lecithin theory, and cytochrome pump theory are based on the concept of active absorption.

Frequently Asked Questions

Q.1. Which zone of the root is mainly associated with mineral absorption?
Ans: Root hairs are the epidermal extension of the root that remains in the direct contact of the soil particles and therefore are mainly associated with mineral absorption.

Q.2. What is the other name of root hairs?
Ans: Root hairs are also named absorbent hairs since they absorb water and mineral nutrients from the soil.

Q.3. What is active transport?
Ans: The transport of substances against the concentration gradient by utilizing energy is called active absorption.

Q.4. What is mass flow in plants?
Ans: Mass flow refers to the passive absorption of different types of nutrients in the form of ions that remain dissolved in the soil water.

Q.5. Who proposed protein lecithin theory?
Ans: Bennet and Clark proposed the protein lecithin theory.