Absorption and Translocation of Solutes

Introduction:

• All of the essential elements a plant needs are present within the soil.?
• The elements are dissolved in water inside the soil and are absorbed from the soil via roots.
• Root hairs are present in the maturation zone which increases the surface area of absorption.
• The rate of mineral absorption is generally independent of its concentration in the soil.
• Minerals are mostly taken up in the ionic form which gets accumulated by the plants against their concentration gradient in the soil.
• Absorption is followed by translocation of mineral salts through xylem along with the ascending stream of water, which is pulled up through the plant by transpirational pull.

Detailed Explanation:

A. Absorption-

• The mechanism of absorption and solute translocation is studied with the help of isolated cells, tissues and organs which were placed in the mineral solutions and observed.
• These studies revealed that the process of mineral absorption takes place in two phases, i.e. initial and metabolic.

i) Initial Phase

• In the first phase, there is a passive and rapid uptake of the essential elements within the free spaces present in between cells which is the area of apoplast.
• The free space comprises intercellular spaces and cell walls. Ions which get absorbed in these free spaces are freely exchangeable.
• This is a non-selective phase and does not require ATP usage.
• In the passive movement of ions, ion channels and the transmembrane proteins function as selective pores.
• The minerals can adapt two pathways, i.e. simple diffusion in which minerals diffuse into the root cells from the soil solution.
• And mass flow, in which the minerals follow the bulk flow of water due to high transpiration rates.

ii) Metabolic phase

• In the second phase, there is an active uptake of the ions and are slowly taken into the inner space which is called the area of symplast.
• This is a very selective phase.
• The inner space consists of cytoplasm and vacuole.
• In this space, ions cannot be freely exchanged with the ions in the external medium.
• The movement of ions to and from the symplast utilizes metabolic energy. And hence, is an active process.
• Proteins found within the root hair cells cell membrane actively pump the ions from the soil right into the cytoplasm of the epidermal cells.
• When the ions move into the cells or free spaces it is known as inward movement or influx and the outward movement or movement of the ions out of the cells is called the efflux.

B. Translocation-

• Once the absorption from the soil by epiblema cells is completed the absorbed salts move radially inwards through the tracheary elements.
• The translocation of solutes is known to be following the xylem path.
• The mineral salts are present in the xylem sap.
• An increase in the salt uptake occurs with increase in the rate of transpiration and transport of water.
• The ringing and hollowing experiments disrupt the continuity of the cortex, phloem and pith does not have any effect upon the rate of mineral uptake until the roots are metabolically active.
• The analysis of bark and wood gives the inference that bark (including phloem) contains more minerals compared to wood but phloem does not transport minerals.
• Stout and Hoagland in the year 1939 did an experiment by inserting a paraffin paper between xylem (wood) and phloem (bark) upto a length of 23 cm.
• The greater amount of mineral which is found in the bark above and below the strip is due to the local absorption of minerals from the xylem to the living cells of the bark.
• All the essential elements or minerals are moved using the ascending water through the xylem elements.
• The sap analysis has shown the presence of minerals or essential elements.
• Radioisotope tagged minerals have proven this fact with utmost clarity.
• Radioactive potassium or phosphorous was supplied in the rooting solution. The concentration of radioactive minerals was seen at various regions of the stem near and in the area where the xylem and phloem separate.
• During the ascent of sap essential elements are also moved because of transpiration pull through the xylem.
• Once the absorbed minerals reach the xylem they are passed into leaves.

Frequently Asked Questions (FAQs)

Q1. What is the process of absorption of essential elements?
Answer:

• All of the essential elements a plant needs are present within the soil. The elements are dissolved in water inside the soil and are absorbed from the soil via roots. Root hairs are present in the maturation zone which increases the surface area of absorption. The rate of mineral absorption is generally independent of its concentration in the soil. Minerals are mostly taken up in the ionic form which gets accumulated by the plants against their concentration gradient in the soil.
• The mechanism of absorption and solute translocation is studied with the help of isolated cells, tissues and organs which were placed in the mineral solutions and observed.
• These studies revealed that the process of mineral absorption takes place in two phases, i.e. initial and metabolic.

i) Initial Phase

• In the first phase, there is a passive and rapid uptake of the essential elements within the free spaces present in between cells which is the area of apoplast.
• The free space comprises intercellular spaces and cell walls. Ions which get absorbed in these free spaces are freely exchangeable.
• This is a non-selective phase and does not require ATP usage.
• In the passive movement of ions, ion channels and the transmembrane proteins function as selective pores.
• The minerals can adapt two pathways, i.e. simple diffusion in which minerals diffuse into the root cells from the soil solution.
• And mass flow, in which the minerals follow the bulk flow of water due to high transpiration rates.

ii) Metabolic phase

• In the second phase, there is an active uptake of the ions and are slowly taken into the inner space which is called the area of symplast.
• This is a very selective phase.
• The inner space consists of cytoplasm and vacuole.
• In this space, ions cannot be freely exchanged with the ions in the external medium.
• The movement of ions to and from the symplast utilizes metabolic energy. And hence, is an active process.
• Proteins found within the root hair cells cell membrane actively pump the ions from the soil right into the cytoplasm of the epidermal cells.
• When the ions move into the cells or free spaces it is known as inward movement or influx and the outward movement or movement of the ions out of the cells is called the efflux.

Q2. Name the two phases of essential element uptake?
Answer: The process of absorption of solutes follows two successive steps. These two phases are the initial phase and the metabolic phase.

Q3. How does element uptake happen in the initial phase?
Answer:

• In the first phase, there is a passive and rapid uptake of the essential elements within the free spaces present in between cells which is the area of apoplast.
• The free space comprises intercellular spaces and cell walls. Ions which get absorbed in these free spaces are freely exchangeable.
• This is a non-selective phase and does not require ATP usage.
• In the passive movement of ions, ion channels and the transmembrane proteins function as selective pores.

Q4. Why is the second phase of the essential element active phase?
Answer:

• In the second phase, there is an active uptake of the ions and are slowly taken into the inner space which is called the area of symplast.
• This is a very selective phase.
• The inner space consists of cytoplasm and vacuole.
• In this space, ions cannot be freely exchanged with the ions in the external medium.
• The movement of ions to and from the symplast utilizes metabolic energy. And hence, is an active process.

Q5. What is flux and describe its types?
Answer: The movement of ions is usually called flux. When the ions move into the cells or free spaces it is known as inward movement or influx and the outward movement or movement of the ions out of the cells is called the efflux.