Plant Nutrition: Mineral Absorption | Part 3

Special animated video made by our studio to explain the Mineral Uptake by Plants. This is Part 3.

Like all living organisms, plants need nutrients for their proper growth and development. They get carbon, oxygen and hydrogen from air and water to carry out photosynthesis which is a very important process for plants. But what about mineral nutrients? Where do they get their mineral nutrients from? The answer is soil.

The presence of minerals in soil is a result of a) weathering of parent rocks. This means the parent rocks undergo various chemical and physical/ mechanical changes leaving behind these mineral elements in the soil as a result. b) by decomposition of organic matter in the soil. These minerals in soil are either in adsorbed (onto the soil particles) form or in dissolved form, so to absorb the minerals from the soil, plants use their roots, and in some cases their leaves.

The mineral nutrients absorbed have many functions and roles to play in the body of plants. They are constituents of various proteins, pigments, nucleic acids and enzymes. They also play roles in cell signaling and metabolism. Plants take up these mineral nutrients in the form of ions.

This uptake of minerals can be either a passive process termed as “passive transport or absorption” or an active process termed as ‘active transport or absorption”. Passive absorption takes place along the concentration gradient and usually requires no energy. Whereas active absorption occurs against the concentration gradient and requires energy.

The passive process of absorption can be explained by theories Diffusion theory, the ion exchange theory and Donnan’s Equilibrium.

DIFFUSION:

Diffusion is more thoroughly discussed in our video titled “What is diffusion.” You may want to check it out first. Here’s a quick summary.

When the concentration of mineral ions is higher in the external soil solution than the root, then the ions in the soil solution will move from a region of higher concentration, in this case the soil solution, to a region of lower concentration that is the roots. This movement of molecules or ions is called diffusion.

The ions keep moving inside until an equilibrium is reached. The reason that the ions are able to diffuse from the outside environment to the root cells is because of the apparent free space. Because of this free space, the ions move freely in and out by diffusion.

However, when the ion concentration is low outside that is in the soil solution than the roots then proteins called carrier proteins facilitate movement of ions from the soil to the roots. This is referred to as facilitated diffusion.

Ion Exchange Theory

The surface of plant roots mainly carries a negative charge because of the carboxyl group of pectin and hemicellulose contained in their cell walls (Carpita and McCann 2000), Other compounds like proteins and phenols also contribute in formation of cell wall structure, due to presence of all these compounds ions can get adsorbed onto the root surface.

The ions (mainly cations) that are adsorbed on to the root surface are exchanged with the ions present in soil. The positively charged hydrogen ions adsorbed onto the surface of roots can easily be exchanged with cations of sodium and potassium present in the soil. Similarly, an anion like hydroxyl ion from the root cell can be exchanged with anion present in the soil.(See Overstreet and Jacobson, 1952).

Ion Exchange can be explained by Contact exchange theory and carbonic acid Exchange theory.

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