Mechanism of absorption of elements
The studies on mechanism of absorption of elements by plants has been carried out in isolated cells, tissues or organs. These studies revealed that the process of absorption can be demarcated into two main phases.
In the first phase, an initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells (apoplast).
In the second phase of uptake, the ions are taken in slowly into the ‘inner space’ of the cells (symplast).
The passive movement of ions into the apoplast usually occurs through ion-channels, the trans-membrane proteins that function as selective pores.
The entry or exit of ions to and from the symplast requires the expenditure of metabolic energy which is an active process.
The movement of ions is usually called flux. The inward movement into the cells is influx and the outward movement is efflux.
| ... | Active absorption | Passive Absorption |
|---|---|---|
| Energy | It involves the expenditure of energy. | It does not require energy. |
| Movement of ions | The movement of ions takes place against the concentration gradient. | The movement of ions occurs in the direction of lower concentration to higher concentration (along the concentration gradient. |
| Rate of absorption | Depends on the DPD (diffusion pressure deficit). | Depends mainly on the transpiration pull. |
| Type of absorption | Active absorption follows the symplast pathway. | Passive absorption follows the apoplast pathways. |
| Rate of absorption | Slow. | High. |