Uptake of Mineral Nutrients in Plants:
The plants uptake their minerals from the soil with the help of roots. This process of uptake of mineral nutrients is known as the absorption of minerals. The mineral absorption takes place by two methods-
- Passive Absorption.
- Active Absorption.
Passive Absorption of Minerals:
It is the spontaneous process in which there is no use of ATP for energy. During this absorption, the minerals themselves move from the region of higher concentration to their region of low concentration i.e. from the soil into the root by diffusion. This type of passive absorption is known as simple diffusion.
The passive absorption of solute ions or molecules mediated by carriers is called facilitated diffusion. In this process certain carriers (protein molecules) help in the transport of molecules across the membranes. This type of diffusion does not make use of energy and takes place along the concentration gradient only, till the equilibrium stage is reached.
Ion Exchange Mechanism– Mineral elements are absorbed by the plant either in the form of molecules or mostly in the form of ions. But the ions are charged either positively or negatively. Therefore, when a positively charged ion is absorbed by the root cells from the soil, then the root cells have to pass out the same number of positively charged ions or every positively charged ion must be accompanied by the negatively charged ion into the cells. Thus, the positively charged ions (cations), such as Ca2+, K1+, Mg2+ and Fe2+ etc. of external soil solution exchange with H+ ions adsorbed to the surface of the cell wall or membranes of the tissue. Similarly, anions like NO3–, Cl–, SO42-, etc. exchange with the free OH– (Hydroxyl ions).
Donnan Equilibrium Theory- According to this theory there are present some non-diffusible anions along the inner surface of outer cell membrane. They remain fixed here because the outer membrane is impermeable to them. But the cell membrane is permeable to cations and anions present in the outer solution. Normally, the anions and cations diffuse into the cell in equal number till equilibrium between cell sap and the external medium is reached.
But for real electrical balancing, more cations from the external solution would be needed to electrically balance the fixed anions present in the cells. Thus, the cation concentration would be greater in the internal solution than in the external solution. This type of electrical balance or equilibrium is known as Donnan Equilibrium.
The Donnan Equilibrium theory and ion-exchange mechanism, explains the accumulation of ions against the concentration gradient without the use of metabolic energy.
Mass Flow- It is also believed that ions move through the roots along with the mass flow of water under the influence of transpiration.
Active Absorption of Minerals:
It has been observed that the plant cells normally have larger quantities of minerals as compared to the outside medium. That means the minerals are accumulated inside the roots from the soil, against the concentration gradient. The absorption of minerals against the concentration gradient is done by using energy (ATP). This type of absorption is known as active absorption. During this process, the ions/minerals are taken from outside (soil) where their concentration is less to inside the cell where their concentration is higher by the use of energy.
Carrier Concept- According to this concept there are present certain specific carriers (compounds) on the plasma membrane. There are specific carriers for different ions. On the outer surface, their carriers combine with the ions to be absorbed. The plasma membrane is permeable to these ion-carrier complexes. These complexes move into the cell and on the inner face of the membrane the complexes break down to release the ions. Later the carrier compound returns to the outer face of the membrane to repeat the process.