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Until now, we’ve talked about how herbicides can be passively absorbed into plant cells, where absorption is driven by passive diffusion and occurs along a concentration gradient. The majority of herbicides are absorbed in this manner. However, a few, such as 2,4-D, glyphosate, and paraquat, are absorbed via active transport mechanisms.
Active transport is the movement of a molecule across a membrane against the gradient of its concentration. This type of transport typically involves a protein carrier on the membrane that uses energy to transport the herbicide from one side of the membrane to the other. The protein can transport the molecule into the plant cell, increasing its concentration within the cell to levels significantly higher than those found outside the cell.
Active Absorption
Active absorption is the absorption of water by roots using adenosine triphosphate produced by root respiration: it is called active absorption because the root cells actively participate in the process. Jenner claims that active absorption occurs in low transpiring and well-watered plants, and that it accounts for 4% of total water absorption. Two theories govern active absorption: active osmotic water absorption and active non-osmotic water absorption. There is no need for energy in this process. Active absorption is critical for plants.
Active Absorption — Three Herbicide Examples
2,4-D
In addition to the simple, nonionic diffusion of the 2,4-D acid discussed previously, carrier-mediated transport of the 2,4-D anion has been proposed. At increasing external concentrations, 2,4-D absorption became saturable and competed with IAA absorption, implying carrier-mediated absorption. According to additional evidence, 2,4-D absorption may also involve facilitated diffusion, which is an increase at the rate of diffusion down an electrochemical gradient through a protein in the cell membrane. It is thought that 2,4-D enters the cell via a protein carrier that normally recognises the plant hormone auxin.
Glyphosate
Glyphosate absorption was nonlinear over time, with a rapid initial phase followed by a slower, steady-state phase. Phosphate competitively inhibited the saturable component of glyphosate transport, implying the involvement of a phosphate carrier that recognises the phosphate group on the glyphosate molecules. Glyphosate is thought to be taken up by the cell via a protein carrier that normally recognises the mineral phosphate.
Paraquat
The divalent cation paraquat was also absorbed via active transport and was concentration dependent. The kinetics of putrescine absorption, a divalent polyamine with a charge distribution similar to paraquat, were similar to paraquat absorption. Furthermore, putrescine inhibited competitively the saturable component of paraquat absorption. Paraquat is thought to be taken up by the cell via a protein carrier that normally recognises the polyamine putrescine.
Active Osmotic Water Absorption
The root cells function as an ideal osmotic pressure system, allowing water to move up from the soil solution to the root xylem along an increasing D.P.D gradient (suction pressure, which is the real force for water absorption). Water can enter the root cells from the soil via endosmosis if the solute concentration is high and the water potential is low. Because of the use of adenosine triphosphate, mineral nutrients are actively absorbed by the root cells (ATP). As a result, the ion concentration (osmotica) in the xylem vessels is higher than in the soil water. Between the root and the soil water, a concentration gradient is formed. The solute potential of xylem water is greater than that of soil water, and thus its water potential is lower. If stated, the water potential in soil water is relatively positive. Endosmosis is caused by this gradient of water potential. Water endosmosis continues until the water potential in the root and soil is equal. Mineral absorption, but not water absorption, consume metabolic energy. As a result, water absorption is an indirect active process in the life of a plant. Active transport moves in the opposite direction of diffusion.
Active Non-osmotic Water Absorption
Water is sometimes absorbed against a concentration gradient. This necessitates the use of metabolic energy generated by root cell respiration. There is no direct evidence, but some scientists believe that energy from respiration is involved. Finally, it is claimed that the evidence supporting active water absorption is weak.
This mechanism operates without the use of metabolic energy. Only the roots serve as an absorption or passage organ in this case. As a result, water absorption is sometimes referred to as ‘through roots’ rather than ‘by roots.’ It occurs during the day in rapidly transpiring plants due to stomatal opening and atmospheric conditions. The transpiration pull creates the force for water absorption at the leaf end. The main cause of this transpiration pull is water being lifted up in the plant axis like a person lifting a bucket of water from a well. Transpiration pull is responsible for dragging water at the leaf end; the pull or force is transmitted down to the root via the xylem elements’ column of water. Because of the cohesion between the molecules, the water column maintains its continuity and acts as a rope. Roots merely serve as a passive absorption organ. Water absorption occurs concurrently with transpiration to compensate for water loss from the leaf end. The majority of water entering plants is absorbed passively. Passive transport is similar to diffusion in that it does not require any effort.
Conclusion
Until now, we’ve talked about how herbicides can be passively absorbed into plant cells, where absorption is driven by passive diffusion and occurs along a concentration gradient. The majority of herbicides are absorbed in this manner. Active transport is the movement of a molecule across a membrane against the gradient of its concentration. Active absorption is the absorption of water by roots using adenosine triphosphate produced by root respiration: it is called active absorption because the root cells actively participate in the process. In addition to the simple, non-ionic diffusion of the 2,4-D acid discussed previously, carrier-mediated transport of the 2,4-D anion has been proposed. Glyphosate absorption was nonlinear over time, with a rapid initial phase followed by a slower, steady-state phase. Glyphosate absorption was nonlinear over time, with a rapid initial phase followed by a slower, steady-state phase. The divalent cation parquet was also absorbed via active transport and was concentration dependent. . Mineral absorption, but not water absorption, consumes metabolic energy. As a result, water absorption is an indirect active process in the life of a plant. Active transport moves in the opposite direction of diffusion. . The transpiration pull creates the force for water absorption at the leaf end. The main cause of this transpiration pull is water being lifted up in the plant axis like a person lifting a bucket of water from a well.
