Passive Absorption: Definition, Mass Flow Hypothesis, Ion Exchange Theory, and, Facilitated Diffusion

Passive Absorption: Definition, Mass Flow Hypothesis, Ion Exchange Theory, and, Facilitated Diffusion


Minerals can be absorbed by physical processes called Passive absorption, which does not directly require metabolic activity. The concept of passive absorption is based on “outer space,” also known as “free space” or “diffusion space.” Ions will freely enter or exit the tissue if just a fraction of the tissue volume is accessible to free diffusion. After some time, the portion of the tissue that is experiencing free diffusion will achieve equilibrium with the external solution, resulting in the same ion concentration within the tissue as that of the exterior solution. The term “free space” or “outside space” refers to the region of a cell or tissue that permits ion diffusion freely. Usually, only 5% of the entire root volume is accessible for free space in the root tissue. For a particular volume of open space, the amount of solute flux depends on several variables, including the rate of transpiration, the solute concentration, and the production of root hair. Cortex cells can, however, directly take up solutes from the external solution to the existence of such a free area. Due to the presence of polygalacturonic acid [carboxylic groups (R. CHOO)] in the middle lamella, the cell walls have a negative charge. The apoplasm’s negative charges function as cation exchangers, causing cations to accumulate while repelling anions. As a result, charged solute access into the free space is limited. To define “free space,” Hope and Stevens (1952) used the term apparent free space (AFS). Water-free space (WFS), a component of AFS, is open to ions and both charged and uncharged molecules. Another term for the region where anion repulsion and cation exchange occur is Donnan free space (DFS).

Some  of the various theories of Passive Absorption

Mass Flow Hypothesis

This theory states that under the influence of transpiration, ions are absorbed by roots along with mass water flow. By eliminating ions after they were released into the xylem channel in detopped tomato plants, an increase in transpiration increased salt absorption. Therefore, the dilution resulted in increased ion absorption. As a result, plants accumulated a modest fraction of their overall salt uptake through passive absorption. However, the following could be used to explain this salt accumulation by passive process:

(1) Free diffusion of ions along concentration gradient into a tissue’s apparent free space

(2) ion exchange or Donnan equilibrium-driven ion buildup versus concentration gradient,

(3) It is also possible for ions to move in large quantities through the roots as a result of transpirational “pull.” None of these procedures require the use of metabolic energy.

Ion Exchange Theory

Both cations and anions tend to become adsorbed on the cell membranes and walls before exchanging with ions in the soil solution. A positively charged ion like H+ is either pushed from the cell (ion exchange) or a negatively charged ion enters the cell via the absorption of a positively charged ion like K+. Similar to hydroxyl (OH) ions, anions can exchange with them. Ion exchange is the process of exchanging ions in the solution with ions that have been adsorbed. In some cases, the roots’ uptake of nutritional cations from a soil solution outpaces their uptake of anion and vice versa. The neutrality is upheld in each instance. The OH- and bicarbonate (HCO3 ) ions are transferred from inside the cells into the open space when anion uptake exceeds cation uptake. Likewise, cells exchange some hydrogen ions if cation absorption outpaces anion uptake. The following describes the ionic exchange procedure:

1. Carbonic acid exchange theory

Continuous root respiration releases CO2 into the rhizosphere. This creates carbonic acid (H2CO3) when combined with water, which breaks down into the ions H+ and HCO3. Due to strong respiratory activity, root tips create a zone of carbonic acid. The H+ ions exchange with cations that have been taken up by the clay micelle, and the cations then diffuse onto the root surface when they enter the soil solution.

2. Contact exchange theory

 When root and soil colloids come into contact with one another directly, without first being dissociated in the soil solution, a comparable ion exchange occurs.

3. Donnan equilibrium

This theory takes the effect of fixed or nondiffusible ions into consideration. A membrane with variable levels of permeability separates the contents of the cell from the outside environment. This membrane is impervious to the cell’s anion concentration. The ions will diffuse over a potential gradient across the membrane if one side of the membrane has a negative nondiffusing charge. Until electrochemical equilibrium is attained, an equal amount of cations and anions diffuse through the membrane as a result of this. However, more cations will be needed to balance this charge due to the “fixed” negative (anion) charge on the inner side of the membrane. As a result, the internal solution (inside the cell) will have a higher cation concentration than the external solution. Additionally, due to the excess of negative charges brought on by “fixed” anions, the concentration of anions in the exterior solution will be lower than that of the external one. Consequently, Donnan equilibrium, a concept put out by F.G. If the internal solution’s anions and cations product equal the external solution’s anions and cations product, Donnan is reached.

Facilitated Diffusion

Carrier proteins located in the plasma membrane can also do passive transport, and they can transport a far greater variety of potential chemicals. Facilitated diffusion describes this passive movement that is carried out by the use of a carrier. But the only way it resembles diffusion is in the way that it moves things down their gradient of electrochemical potential without any additional energy input.

Frequently Asked Questions

Question: What is passive absorption?

Ans: Minerals can be absorbed by physical processes called Passive absorption, which does not directly require metabolic activity.

Question: What is passive nutrient absorption?

Ans: Nutrients are absorbed without using up any energy when they go from a higher concentration to a lower concentration.

Question: Is osmosis active or passive?

Ans: Passive

Question: What is meant by passive absorption of minerals by plants?

Ans: It is a passive process that doesn’t involve using energy. Through simple diffusion, ionic exchange, and Donnan equilibrium, it takes place along the concentration gradient, that is, from a region of higher concentration to a region of lower concentration.

Question: Is mineral uptake active or passive?

Ans: Active process

Question: Is water movement active or passive?

Ans: Passive

Question: Does passive absorption require energy?

Ans: No

Question: What are 3 examples of passive transport?

Ans: Simple diffusion, Osmosis, and Facilitated diffusion

Question: What are the 4 types of passive transport?

Ans: Simple Diffusion, Facilitated Diffusion, Filtration, Osmosis.

Question: What are the 3 types of osmosis?

Ans: Hypertonic, isotonic, and hypotonic.

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