Mineral nutrients are found either as soluble fractions of soil solution or as adsorbed ions on the surface of colloidal particles. Various theories proposed to explain the
mechanism of mineral salt absorption can be placed in two broad categories:
I) Passive Absorption
II) Active Absorption
Ion uptake is both active and passive :
After several decades of research on this process of ion uptake it is now believed that the process involves both passive and active uptake mechanisms.
Whether a molecule or ion is transported actively or passively across a membranem (casparian band, plasma membrane or tonoplast) depends on the concentration and charge of the ion or molecule, which in combination represent the electrochemical driving force.
Passive transport across the plasma membrane, occurs along with the electrochemical potential. In this process ions and molecules diffuse from areas of high to low concentrations. It does not require the plant to expend energy.
Active transport, (in contrast, to passive transport) energy is required for ions diffusion against the concentration gradient (electro chemical potential). Thus, active transport requires the cell to expend energy.
Passive transport mechanism:
A) Diffusion: Simple diffusion to membranes occurs with small, non-polar molecules (i.e. O2, CO2). In this process ions or molecules move from the place of higher concentration to lower concentration. It needs no energy.
B) Facilitated diffusion: For small polar species (i.e. H2O, Ions and amino acids) specific proteins in the membrane facilitate the diffusion down the electrochemical gradient. This mechanism is referred to as facilitated diffusion. Eg.
a) Channel proteins: The specific proteins in the membrane form channels (channel proteins), which can open and close, and through which ions or H2O molecules pass in single file at very rapid rates. A K+ and NH4+ channel also operates by the same process of facilitated diffusion. In addition, Na+ can also enter the cell by this process.
b) Transporters or Co-transporters or carriers: Another mechanism involves transporters or co-transporters responsible for the transport of ions and molecules across membranes. Transporter proteins, in contrast to channel proteins, bind only one or a few substrate molecules at a time. After binding a molecule or ion, the transporter undergoes a structural change specific to a specific ion or molecule. As a result, the transport rate across a membrane is slower than that associated with channel proteins.
Three types of transporters have been identified:
1. Uniporters: transport one molecule (i.e. glucose, amino acids) at a time down a concentration gradient.
2. Antiproters: catalyze movement of one type of ion or molecule against its concentration gradient. This is coupled with the movement of a different ion or molecule in the opposite direction. Examples of antiporter transport are H+ /Na+ and H+ /Ca+2 transport into the vacuole.
3. Symporters: catalyze movement of one type of ion or molecule against its concentration gradient coupled to movement of a different ion or molecule down its concentration gradient in the same direction. The high H+ concentration in the apoplast provides the energy for symporter transport of NO3- and the other anions.
Therefore, the energy for antiporter and symporter transport originates from the electric potential and/or chemical gradient of a secondary ion or molecule, which is often H+.
Active transport mechanism:
Larger or more-charged molecules have great difficulty in moving across a membrane, requiring active transport mechanisms (i.e., sugars, amino acids, DNA, ATP, ions, phosphate, proteins, etc.). Active transport across a selectively permeable membrane occurs through ATP-powered pumps that transport ions against their concentration gradients. This mechanism utilizes energy released by hydrolysis of ATP.
The (Na+) – (K+) ATP pump transports K+ into the cell and Na+ out of the cell, another example is the (Ca+2) – ATP pump. Thus, it can be understood from the above discussion that the ion transport mechanisms operate both actively and passively. For some of the ions the uptake mechanism is active and for some others it is passive.