MECHANISMS OF NUTRIENT UPTAKE

MECHANISMS OF NUTRIENT UPTAKE

In plants, nutrients are absorbed mainly by the roots and must get contact with root surface. However, the structural complexity of root epiblema is the main barrier for entry of nutrients into cortex. 

Nutrients are not absorbed uniformly throughout roots as roots have three different regions viz. root hairy, elongation and meristematic zones, and radioactive studies showed that meristematic zone resulted in more nutrients uptake than others. Plants are also capable to absorb nutrients through aerial parts via stomata and cuticle.

In order for mineral nutrients to be taken up by a plant, they must enter the root by crossing the plasma membranes of root cells. From there they can be transported through the symplast tothe interior of the root and eventually find their way into the rest of the plant. 

Within the soil nutrient can move to the root surface both by bulk flow and by diffusion. 

Mechanism of absorption

Absorption of ions by plants requires transport of the ions across plasma membrane of an epidermal cell, or it may enter the apoplast and diffuse between epidermal cells through the cell walls. Plant nutrients are taken up by following two processes based on the requirement of

energy:

1. Passive uptake

2. Active uptake

Passive transport

Passive transport occurs when a solute molecule diffuses across a membrane down a concentration gradient without expending energy by the cell. Transport protein may facilitate diffusion by serving as carrier proteins or forming selective channels. Carrier protein bind selectively to a solute molecule on one side of the membrane, undergo a conformational change and release the solute molecule on the opposite side of the membrane. Similarly, transport channel selectively allows solute molecule to enter inside the cells. 

Transport through channel is always passive, and because specificity of transport depends on the pore size and electric change more than on selective binding, channel transport is limited mainly to ions or water.

Active Transport

When solute molecule is move across a membrane against a concentration gradient with thehelp of carrier proteins. Active transport may be primary or secondary active transport. Primaryactive transport is coupled directly with a metabolic source of energy such as ATP hydrolysis.

Or absorption of light by the carrier protein. Secondary active transport uses the energy stored in electrochemical-potential gradient. Proton pump play a central role in secondary activetransport across membrane.

Site of maximum absorption of solute in root

Nutrients are not absorbed uniformly throughout roots as roots have three different regions viz.root hairy, elongation and meristematic zones, and radioactive studies showed that meristematic zone resulted in more nutrients uptake than others. 

In many cases, mycorrhiza increases water and solutes absorption. Capacity of the root system to absorb nutrient is improved by the presence of fungal hyphae that are much finer than plant roots and can reach

beyond the areas of nutrient-depleted soil near the roots. Little is known about the mechanism by which the mineral nutrients absorbed by mycorrhizal fungi are transferred to the cells of plant roots.

Radial movement of ions from root surface to the tracheary element Radial movement of ions dissolved in water occurs through both apoplastic and symplastic pathways. Apoplastic pathway involves transport of dissolved solute through intercellular spaces between cells. 

However, water and mineral can not cross the endodermis apoplastically due to presence of casparian strip. Casparian strip forces minerals that are passively moving through the apoplast to cross the plasma membrane of an endodermal cell and enter the xylem via the symplast. 

Symplast pathway involves the movement of dissolved solutes from cell to cell through the plasmodesmata. It is the major pathway for transport of ions from the epidermis to a tracheary element of root. Xylem tracheary element are dead cells. To enter the tracheary element, the ions must exit the symplast by crossing a plasma membrane. The process where ions exit the symplast and enter the conducting cells of xylem is called xylem loading.