Mineral nutrients can be found as soluble portions of soil solution or as adsorbed ions on colloidal particle surfaces. There are two basic groups of ideas attempted to explain the process of mineral salt absorption:
I) Passive Absorption
II) Active Absorption
Ion uptake is both active and passive :
After decades of research, it is now thought that the process of ion uptake involves both passive and active absorption systems.
The concentration and charge of an ion or molecule, which together provide the electrochemical driving force, determine whether it is carried actively or passively across a membrane (casparian band, plasma membrane, or tonoplast).
Along with the electrochemical potential, passive transport across the plasma membrane occurs. Ions and molecules spread from high to low concentration locations in this process. It does not necessitate that the plant use any energy.
For ions to diffuse against a concentration gradient, active transport (as opposed to passive transport) requires energy (electro chemical potential). As a result, active transport necessitates the cell’s expenditure of energy.
Mechanism of passive transport:
A) Diffusion: Small, non-polar molecules diffuse easily across membranes (i.e. O2, CO2). Ions or molecules travel from a greater concentration to a lower concentration in this process. It does not require any energy.
B) Facilitated diffusion: Specific proteins in the membrane assist the passage of tiny polar species (e.g., H2O, ions, and amino acids) down the electrochemical gradient.
Facilitated diffusion is the name for this process. Eg.
a) Channel proteins: The membrane’s specialised proteins form channels (channel proteins), which may open and close and through which ions or H2O molecules flow in single file at high speeds. The enhanced diffusion method is also used by A K+ and NH4+ channel. This mechanism can also allow Na+ to enter the cell.
b) Transporters or Co-transporters or Carriers: Another method is the transport of ions and molecules across membranes by transporters or co-transporters. In contrast to channel proteins, transporter proteins bind just one or a few substrate molecules at a time. The transporter undergoes a structural change unique to a certain ion or molecule after contacting a molecule or ion. As a result, the rate of transport across a membrane is slower than for channel proteins.
There are three different types of transporters:
1. Uniporters : Uniporters carry one molecule at a time along a concentration gradient (e.g., glucose, amino acids).
2. Antiproters: antiproters accelerate the transport of one kind of ion or molecule against a concentration gradient. This is accompanied by a separate ion or molecule moving in the opposite direction. H+/Na+ and H+/Ca+2 transfer into the vacuole are examples of antiporter transport.
3. Symporters: catalyse movement of one kind of ion or molecule against its concentration gradient in the same direction as movement of another type of ion or molecule down its concentration gradient. The apoplast’s high H+ content provides energy for symporter transport of NO3- and other anions.
As a result, the energy for antiporter and symporter transport is derived from the electric potential and/or chemical gradient of a secondary ion or molecule, usually H+.
Larger or more charged molecules have a difficult time migrating across a membrane, necessitating active transport processes (e.g., sugars, amino acids, DNA, ATP, ions, phosphate, proteins, and so on). ATP-powered pumps move ions across concentration gradients through a selectively permeable membrane during active transport. This method makes use of the energy generated by ATP hydrolysis. The (Na+) – (K +) ATP pump transfers K+ into the cell and Na+ out; the (Ca+2) -ATP pump is another example.
As a result of the preceding explanation, it is clear that ion transport systems function both actively and passively. Some ions have an active uptake process, whereas others have a passive uptake mechanism.
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