Mineral absorption by plants

1. Subject Botany
Topic Mineral Absorption
By
Shivance Sharma

2. Mineral Absorption
Plants obtain minerals from soil where these are dissolved in soil or present.
Mineral salt absorption is independent of water absorption.
Maximum mineral salt absorption occurs by zone of elongation.

3. No mineral salt absorption occur by root hair zone.
Minerals salt absorption occurs directly by cells of epiblema not by root hairs.
Mineral salts are absorbed mostly in the form of ions, i.e., anions and cations.
Mechanism of Mineral absorption
Mineral salt absorption takes place by passive and active both types. To explain passive
and active absorption many theories given.
Passive absorption
Absorption of ions without the use of metabolic energy is known as passive absorption.
This is carried by purely physical forces.
Briggs and Robertson (1957) demonstrate the passive absorption of ions by root system .
They showed
I. Mineral salts are not affected by temp. and metabolic inhibitors.
II. Rapid uptake of ions occurs when the concentration is higher in outer solution than in the
cell sap of the root cells, the mineral salts are absorbed according to the concentration
gradient by simple process of diffusion.

4. Some theories explaining the mechanism of passive
absorption of minerals
• Mass Flow Theory
According to Hylmo (1953, 1955) the ion absorption
increase with increase in transpiration. The ions have
been considered to move in a mass flow with water
from the soil solution through the root and
eventually to the shoot.

5. • Ion exchange hypothesis
According to this theory absorbed ions are exchanged from the
external medium. A cation is exchanged for a cation and anion
for anion.
There are two theories to explain the mechanism of ion
exchange__
• Contact exchange theory
According to this theory, ions are not completely static, they are
always oscillating around their absorption area and when the
oscillating volume of the ions on the roots and on the colloidal
partials overlap each other, ions get exchange.

6. • Carbonic acid exchange theory
According to this theory, Co2 released by roots
during respiration reacts with water to produce
carbonic acid which dissociated into H+ ions and
bicarbonate ions.
H+ ion exchanges itself with the cations absorbed on
the colloidal particles and the bicarbonate (HCO-
3)
ions release the absorbed anions to supply both
anions and cations nearby.

7. • Donnan equilibrium
This mechanism given by F.G Donnan in 1927.
According to this mechanism there are no fixed ions
there are equal no of anions and cations on both sides
of the membrane at equilibrium.
Positive ions inside/Positive ions outside = Negative
ions outside/Negative ions inside
• Simple diffusion theory
• Facilitated diffusion hypothesis

8. Active absorption
The lipid protein membrane of a cell is largely permeable to free ion. The energy is
used to transport of such free ions across the membrane is known as active
absorption.
Energy which is used in this process comes from the metabolic activities like
respiration etc.
Mineral absorption is mainly active process. Hoagland (1944) indicated active ion
absorption and their accumulation against concentration gradient in green algae
Nitella and Valonia.
Evidences show the use of energy in the absorption of mineral salt
• Higher rate of transpiration increase the salt accumulation inside the cell
• Respiration inhibitor check the process of salt uptake
• By decrease the oxygen content in the medium the absorption also decrease

9. • Carrier concept
This concept was proposed by Vanden Honert (1937).
According to this theory the plasma membrane is
impermeable to free ions. But some compound
present in it acts as carrier and combines with ions to
form carrier-ion-complex which can move across the
membrane. On the inner surface of the membrane
this complex breaks releasing ions into the cell while
the carrier goes back to the outer surface to pick up
fresh ions.

10. Some observations support the carrier concept of
active absorption of mineral salts
(i) Isotopic Exchange
Actively absorbed radioactive ions (such as 35S04)
cannot diffuse back or be exchanged with other ions
in the outer solution indicating thereby that the
plasma membrane is not permeable to free ions.
(ii) Saturation Effects
After a limit, If we increase salts con. in outer solution
there is no increase in mineral salt absorption. It is
because the active sites on the carrier compound
become saturated with ions.

11. (iii) Specificity
Active sites on carrier compound may be specific
which can bind only some specific ions. This also
explains the selective and unequal absorption of ions
by the plants.
To explain mechanism of active salt absorption
through carrier concept there two hypothesis…..
1. Lundegardh’s Cytochrome pump theory
Lundegardh and Burstrom (1933) believed that when a plant is
transferred from water to a salt solution the rate of respiration
increases. Which is called as anion respiration or salt respiration.
The inhibition of salt respiration and the accompanying absorption of
anions by CO and cyanides,
later on led Lundegardh (1950, 54) to propose cytochrome pump theory
which is based on the following assumptions:

12. (i) The mechanism of anion and cation absorption is different.
(ii) Anions are absorbed through cytochrome chain by an active process.
(iii) Cations are absorbed passively.
According to this theory
(i) Dehydrogenase reactions on inner side of the membrane give rise to protons (H+) and
electrons (e–).
(ii) The electron travels over the cytochrome chain towards outside the membrane, so that the
Fe of the cytochrome becomes reduced (Fe++) on the outer surface and oxidised (Fe++) on
the inner surface.
(iii) On the outer surface, the reduced cytochrome is oxidised by oxygen releasing the electron
(e–) and taking an anion (A–).
(iv) The electron thus released unites with H+ and oxygen to form water.
(v) The anion (A–) travels over the cytochrome chain towards inside.
(vi) On the inner surface the oxidised cytochrome becomes reduced by taking an electron
produced through the dehydrogenase reactions, and the anion (A–) is released.
(vii) As a result of anion absorption, a cation (M+) moves passively from outside to inside to
balance the anion.

14. Main defects of the above theory are
(i) It envisages active absorption of only anions.
(ii) It does not explain selective uptake of ions.
(iii) It has been found that cations also stimulate respiration.
(2) Bennet-Clark’s Protein-Lecithin Theory
In 1956, Bennet-Clark suggested that because the cell membranes chiefly consist of phos-
pholipids and proteins and certain enzymes seem to be located on them, the carrier could be a
protein associated with the phosphatide called as lecithin.
According to this theory
(i) The phosphate group in the phosphatide is regarded as the active centre binding the
cations, and the basic choline group as the anion binding centre.
(ii) The ions are liberated on the inner surface of the membrane by decomposition of the
lecithin by the enzyme lecithinase.
(iii) The regeneration of the carrier lecithin from phosphatidic acid and choline takes place in
the presence of the enzymes choline acetylase and choline esterase and ATP. The latter acts
as a source of energy.