Plant Nutrition: Mineral Absorption | Part 1

Plant Nutrition: Mineral Absorption | Part 1


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. Contact exchange theory:
According to contact exchange theory, ions can be taken up the plants without being dissolved
in soil.The ions adsorbed onto the root surface and the ions attached to clay particles are
not entirely static, they oscillate in a small volume of space and if the roots of plant
and the soil particles, the ions are attached to, are in close proximity that the oscillation
volume of the ion adsorbed to the surface of root and the ions present in soil overlap,
ion exchange can take place. The exchanged ions enter the root by diffusion. Carbonic Exchange theory:
Roots of the plants continuously respire, which results in production of carbon dioxide. The carbon dioxide produced during respiration
in roots combines with water to form carbonic acid which dissociates into hydrogen and bicarbonate
ion. The hydrogen ions produced as a result are
exchanged with cations in soil. This is the carbonic acid theory. Donnan’s Equilibrium
Another theory of passive absorption is Donnan’s Equilibrium theory which states that, there
are some ions inside the root cells that are in diffusible across the membrane due to the
selective nature of the cell membrane. If the root cells have cations accumulated
inside as indiffusible ions, then anions present in soil enter the membrane without expenditure
of energy to maintain a balanced electric potential. This movement of anions (in this example)
will result in an equilibrium in the cell, termed as Donnan’s equilibrium. This enables the plants to take up minerals
from the soil to ensure their proper growth.

5 thoughts on “Plant Nutrition: Mineral Absorption | Part 1

  1. Awesome video! I had never heard of anything other than cation exchange (and in the context of CEC) nor did I realize that it was only theoretical. Mind. Blown.

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